Skeletal Muscle Dysfunction in Chronic Obstructive Pulmonary Disease
A Statement of the American Thoracic Society and European Respiratory Society

	INTRODUCTION

TOP INTRODUCTION I. Normal Muscle Function A. OVERVIEW OF MOTOR... B. MUSCLE STRENGTH C. MUSCLE ENDURANCE II. Skeletal Muscle... A. PATHOPHYSIOLOGY OF SKELETAL... B. ETIOLOGY OF SKELETAL... C. FUNCTIONAL IMPACT ON... III. Effects of Interventions... A. EXERCISE TRAINING B. OXYGEN THERAPY C. NUTRITIONAL SUPPLEMENTATION D. ANABOLIC HORMONE... E. LUNG TRANSPLANTATION AND... Suggestions for Future Research A. MECHANISMS OF SKELETAL... B. CLINICAL DIAGNOSTIC TOOLS C. TREATMENT MODALITIES REFERENCES

Introduction

Chronic obstructive pulmonary disease (COPD) is almost always caused by cigarette smoking. Although medical science has developed no cures (1), several forms of treatment are of symptomatic benefit. Smoking cessation can slow the rate of deterioration of lung function. Bronchodilator and anti-inflammatory agents are of appreciable help to many patients, but the reduction in airway obstruction is generally modest. Oxygen supplementation prolongs survival and improves quality of life in severely hypoxemic patients (2, 3). Lung transplantation (4) and lung volume reduction surgery (5) may have a role in a highly selected minority of patients.

COPD frequently leads to significant debilitation. Patients often come to medical attention only after severe damage to the lung's function has been induced. The most frequent complaint of these unfortunate patients is exercise intolerance (6). Patients often become homebound, isolated, and depressed as they seek to avoid the dyspnea that everyday activities produce.

Pulmonary rehabilitation has been touted by some as the standard of care for patients chronically disabled by their lung disease (7), and it is commonly prescribed. However, a skeptic might view it as a therapy without a clearly established rationale in the context of improving exercise tolerance. Current concepts involving the pathophysiologic mechanisms of exercise intolerance focus on the lung, where abnormal lung mechanics (leading to mechanical disadvantage of the respiratory musculature), impaired gas exchange, and destruction of the pulmonary vascular bed directly impact the ability to sustain exercise. Clearly, pulmonary rehabilitation does nothing to improve lung function. Nevertheless, there are many reports of improvements in exercise tolerance after a program of rehabilitation (6, 8). This has been ascribed to psychological factors, including improved motivation and decreased sensitivity to dyspnea (9); these factors are undoubtedly important.

There is a growing realization that COPD is a multi-organ- system disease. In particular, there is accumulating evidence that the skeletal muscles do not function normally and that this contributes to exercise intolerance. This is important because skeletal muscle dysfunction may well be a remediable source of exercise intolerance. Moreover, programs of pulmonary rehabilitation are a natural setting to attempt to apply remediation. It may, therefore, be appropriate to reorient pulmonary rehabilitation to focus on strategies to improve skeletal muscle function. Pulmonary rehabilitation might be established as a modality that treats (and perhaps cures) skeletal muscle dysfunction.

To further this end a workshop group was established under the auspices of the Respiratory Structure and Function Assembly of the American Thoracic Society. Sponsorship of both the American Thoracic Society and the European Respiratory Society was obtained. An international group of 16 scientists was recruited. Included in this group were experts in the basic science of muscle biology, exercise physiologists, and rehabilitation specialists. This group met twice in Miami, Florida, in December 1997 and in Chicago, Illinois, in April 1998. Each participant composed a section of the manuscript; all members reviewed and commented on the resulting document. Among the group's earliest decisions was to restrict this document to discussion of the limb musculature. Respiratory muscle abnormalities are undoubtedly present; this topic has been thoroughly reviewed (e.g., 10). However, the group felt that fundamental differences between these two types of skeletal muscles were present (among them that, in most patients with COPD, the respiratory muscles are chronically overworked and the limb muscles are chronically underworked) and justified the division. Another decision was to compare and contrast the role of strength versus endurance characteristics of the limb muscles. To date, the COPD literature has almost exclusively focused on endurance activities; recent literature has stressed the importance of muscle strength in elderly subjects (11).

This position paper is divided into three sections. The first is a primer on muscle biology in which the characteristics of skeletal muscle that dictate the ability to perform both strength and endurance activities are discussed. The second section summarizes the state of current knowledge concerning the mechanisms of skeletal muscle dysfunction in COPD. The third section describes strategies for treating skeletal muscle abnormalities. Finally, a short section concerns possible directions for future research.

The writers are grateful to Glaxo Wellcome Pharmaceuticals for supporting the publication of this report as a supplement to the American Journal of Respiratory and Critical Care Medicine.

	I. Normal Muscle Function

	A. OVERVIEW OF MOTOR CONTROL

TOP INTRODUCTION I. Normal Muscle Function A. OVERVIEW OF MOTOR... B. MUSCLE STRENGTH C. MUSCLE ENDURANCE II. Skeletal Muscle... A. PATHOPHYSIOLOGY OF SKELETAL... B. ETIOLOGY OF SKELETAL... C. FUNCTIONAL IMPACT ON... III. Effects of Interventions... A. EXERCISE TRAINING B. OXYGEN THERAPY C. NUTRITIONAL SUPPLEMENTATION D. ANABOLIC HORMONE... E. LUNG TRANSPLANTATION AND... Suggestions for Future Research A. MECHANISMS OF SKELETAL... B. CLINICAL DIAGNOSTIC TOOLS C. TREATMENT MODALITIES REFERENCES

Early in this century, Sherrington (12) recognized that motoneurons constitute the final common pathway for the central nervous system control of muscles. The motor unit, comprising a motoneuron and the muscle fibers it innervates, was identified as the basic functional element (or quantum) of neural control of muscles (13). Sherrington also introduced the concept of an orderly recruitment of motor units to accomplish different motor behaviors (13, 14). Accordingly, in this model, the total force generated by a muscle depends on the numbers of motor units activated and the quantal amount of force contributed by each motor unit. Because the force generated by each motor unit depends on the rate of neural activation (i.e., the force/frequency relationship), the central nervous system has two levels of control of the force generated by the muscle: (1) the number of activated motor units (i.e., recruitment), and (2) the action potential discharge frequency of motoneurons (frequency modulation). This simple model assumes that all muscle fibers within a motor unit are activated by the motoneuron in an all-or-none fashion, and that each muscle fiber belongs to only one motor unit. These assumptions have been shown to be essentially correct in normal healthy adults.

Within a muscle, motor units can vary considerably in their mechanical, histochemical, and biochemical properties (15- 24), and it is this variety that provides the basis for a range of motor control via the selective recruitment of different motor unit types. In the early 1970s, Burke and colleagues (15, 16) introduced standardized criteria for the classification of different motor unit types based on mechanical and fatigue properties. Accordingly, motor units were classified as (1) slow-twitch, fatigue resistant (S), (2) fast-twitch, fatigue resistant (FR), (3) fast-twitch, fatigue intermediate (FInt), and (4) fast-twitch, fatigable (FF). With few exceptions, it has been shown that each motor unit type comprises muscle fibers of a single histochemical type, regardless of the classification scheme used (Table 1) (15, 16, 24).

In one histochemical technique, muscle fibers were classified as slow-twitch oxidative (SO), fast-twitch oxidative, glycolytic (FOG), or fast-twitch glycolytic (FG) based on differences in staining for myofibrillar ATPase and subjective differences in staining for glycolytic and oxidative enzymes (27). In another histochemical classification scheme, differences in the pH lability of staining for myofibrillar ATPase were used to classify fibers as type I, IIa, IIb, or IIx (28, 29). Finally, in most recent studies, fiber types have been classified based on differences in immunoreactivity for antibodies specific to different myosin heavy chain (MHC) isoforms (30). Several studies have confirmed a general correspondence between the histochemical classification of fibers as type I, IIa, IIb, and IIx and immunoreactivity for MHC _Slow, MHC _2A, MHC _2B, and MHC _2X isoforms, respectively (31). Indeed, this relationship formed the basis for the identification of these MHC isoforms in skeletal muscle. In contrast, it has been shown that there is only a very poor correspondence between the classification of SO, FOG, and FG fiber types and the classification of type I, IIa, IIb, and IIx fibers (35). Thus, there is general agreement that the classification of S, FR, FInt, and FF motor unit types corresponds with the expression of MHC_Slow, MHC _2A, MHC _2X, and MHC _2B isoforms in muscle fibers and the histochemical classification of type I, IIa, IIx, and IIb fibers (24).

Four decades ago, Henneman (38) suggested that motoneuron size was the major determinant of motoneuron excitability and susceptibility to discharge. He demonstrated that motoneurons with slower conduction velocities, a correlate of motoneuron size, were recruited first during most motor behaviors. This "size principle" predicted that the orderly recruitment of motor units was related to the intrinsic membrane properties of motoneurons (38, 39). Smaller motoneurons, with smaller axonal diameters and conduction velocities, have high input resistance, providing them with a more negative threshold for discharge. In contrast, larger motoneurons, with larger axons and faster conduction velocities, have lower input resistance and, thus, a less negative recruitment threshold. Based on a comparison of axonal conduction velocities and motor unit recruitment thresholds, the size principle has been confirmed in a number of mammalian motor systems (17, 39).

Models have also been proposed based on an orderly recruitment of different motor unit types, with S units recruited first followed by FR, FInt, and FF units (e.g., 23, 45). Based on the average force contributed by each motor unit type and the proportion of each type within a muscle, the recruitment requirements to accomplish different motor behaviors can be predicted. For example, in the cat medial gastrocnemius muscle, it has been estimated that only type S motor units need to be recruited during standing, that additional recruitment of type FR motor units is required during walking and running, and that FInt and FF motor units are recruited only during behaviors such as jumping, which require maximal efforts for short durations (45). Similarly in the cat diaphragm muscle, eupneic breathing can be achieved by the recruitment of only type S motor units, whereas more forceful ventilatory behaviors (e.g., during hypoxia and hypercapnia) would require the recruitment of FR and some FInt units (23). Recruitment of the more fatigable FInt and FF motor units in the diaphragm muscle would be required only during more forceful nonventilatory behaviors (23). These motor unit recruitment predictions hold even assuming submaximal motoneuron discharge rates in the range of 10 to 30 Hz (e.g., 46). It should be noted that motor unit activation rates represent the steepest portion of motor unit force/frequency curves, and thus allow for optimal frequency coding of motor units for rapid adjustments in force generation (46).

	B. MUSCLE STRENGTH

TOP INTRODUCTION I. Normal Muscle Function A. OVERVIEW OF MOTOR... B. MUSCLE STRENGTH C. MUSCLE ENDURANCE II. Skeletal Muscle... A. PATHOPHYSIOLOGY OF SKELETAL... B. ETIOLOGY OF SKELETAL... C. FUNCTIONAL IMPACT ON... III. Effects of Interventions... A. EXERCISE TRAINING B. OXYGEN THERAPY C. NUTRITIONAL SUPPLEMENTATION D. ANABOLIC HORMONE... E. LUNG TRANSPLANTATION AND... Suggestions for Future Research A. MECHANISMS OF SKELETAL... B. CLINICAL DIAGNOSTIC TOOLS C. TREATMENT MODALITIES REFERENCES

1. Determinants of Muscle Strength

As mentioned above, the force generated by a muscle is determined by the number and type of motor units recruited (17, 23). In most muscles, it has been generally reported that FInt and FF motor units generate greater forces than do S and FR units (17, 19, 23, 47, 48). The force generated by a motor unit depends on three interrelated factors: (1) the innervation ratio of the unit, i.e., the number of muscle fibers innervated by a motoneuron; (2) total functional cross-sectional area of all muscle fibers within the unit; and (3) specific force of the muscle fibers, i.e., the force per cross-sectional area (17). In 1968, Edstrom and Kugelberg (49) introduced the method of glycogen depletion by which muscle fibers making up a single motor unit can be identified. Subsequently, a number of studies have characterized the importance of these three factors in the forces generated by different motor unit types. Generally, the greater force generated by FInt and FF motor units has been attributed to a combination of all three factors (17, 19, 47, 48).

The force generated by single skeletal muscle fibers is determined primarily by the level of activation (e.g., intracellular Ca²⁺ concentration, [Ca²⁺]_i) and the force per cross-sectional area of muscle (specific force). As the frequency of neural activation increases, the force generated by muscle fibers increases in a sigmoidal fashion. In motor unit studies, it has been shown that the force/frequency relationship of slow-twitch motor units is shifted leftward compared with fast-twitch motor units (23). Thus, at a given frequency of submaximal neural activation, slow-twitch motor units generate a greater percentage of their maximal force. This difference in the force/frequency relationship of motor units could relate either to the amount of Ca²⁺ released from the sarcoplasmic reticulum at a given frequency of activation (differences in excitation-contraction coupling) (50), differences in sarcoplasmic reticulum Ca²⁺ reuptake (54), or differences in the Ca²⁺ sensitivity of myofibrillar proteins (55).

A number of studies have examined the force/Ca²⁺ relationship in single permeabilized skeletal muscle fibers, where [Ca²⁺]_i can be clamped at different levels. Generally, it has been reported that muscle fibers expressing the MHC_Slow isoform have greater Ca²⁺ sensitivity than do fibers expressing fast MHC isoforms, so that slow fibers generate a greater fraction of their maximal force for a given [Ca²⁺]_i. Accordingly, the force/Ca²⁺ relationship of slow muscle fibers is shifted leftward compared with fast fibers (57).

Fiber type differences in specific force have been reported (60), but such differences remain controversial (65). Differences in mitochondrial volume densities may contribute, at least in part, to fiber type differences in specific force. Generally, fibers expressing the MHC_Slow and MHC _2A isoforms have significantly higher mitochondrial volume densities and oxidative capacities than do fibers expressing the MHC _2X and MHC _2B isoforms (33). Indeed, it is likely that the higher oxidative capacities of fibers expressing the MHC_Slow and MHC _2A isoforms may at least partially account for the greater fatigue resistance of these fibers as compared with fibers expressing the MHC _2X and MHC _2B isoforms (18, 66). The higher mitochondrial volume densities of fibers expressing the MHC_Slow and MHC _2A isoforms would presumably be at the expense of a corresponding lower myofibrillar volume density, lower MHC content, and, thus, fewer cross-bridges in parallel for a given fiber cross-sectional area.

2. Protein Turnover

Protein turnover refers to a continuous and dynamic flux in protein metabolism whereby all proteins are constantly being synthesized and degraded (Figure 1) (67). Although this process incurs a significant metabolic cost, it permits the ability to finely regulate specific protein pools (68, 69). Thus, turnover rates for specific proteins may differ considerably depending on function and need (69). Amino acids, the building blocks of proteins, are derived from an active metabolic pool, 40% of which originates from endogenous protein breakdown, with the remainder being derived from dietary protein sources (69). The regulation of protein turnover in skeletal muscle is of considerable importance in view of the fact that skeletal muscle mass makes up about 40 to 45% of body weight and about 60 to 80% of body cell mass. As such, skeletal muscle serves as an important reserve system that, in conditions of need, maintains supplies of essential amino acids for protein synthesis and energy metabolism (70).

View larger version (41K): [in this window] [in a new window]   Figure 1.   Pathways mediating protein turnover (see text).

a. Muscle Protein Degradation. Although numerous cellular proteolytic systems have been described, degradation of muscle contractile proteins, particularly in catabolic states, is likely mediated via the ubiquitin-proteasome pathway (Figure 1) (71, 72). This involves activation of small peptide cofactors (ubiquitin) and their transfer and covalent linkage to the specific target proteins to be degraded. The ubiquitin-protein conjugate binds to an activating protein complex, which releases the ubiquitin chain, catalyzes conformational change in the protein, and promotes entry into the 26S proteasome (a barrel-shaped protein structure composed of several rings and subunits surrounding a central cavity), where the protein is degraded by multiple proteolytic sites within the inner rings of the proteosome. The degraded products (i.e., peptides) are subsequently released into the cytoplasm, where they can be rapidly metabolized to amino acids (71, 72). The ubiquitin pathway has been implicated in mediating enhanced proteolysis in a number of different catabolic states, in both animal models and in humans. For example, varying increments in messenger RNA (mRNA) for ubiquitin, proteasome subunits, and/or key pathway enzymes have been reported in a variety of models. These include fasting (73), acidosis (74), denervation (75), sepsis/endotoxemia (76, 77), tumor necrosis factor (TNF) administration (78), diabetes mellitus (79), and neoplasia (80). There is also evidence for activation of the ubiquitin pathway in rat limb muscles with the use of high dose corticosteroids (81). By contrast, insulin and insulinlike growth factor-1 (IGF-1) have been shown to suppress E2 (a ubiquitin conjugating enzyme) in muscle cell cultures, and thus could in part suppress proteolysis via influences on the ubiquitin pathways (82, 83).

b. Hormonal and Mediator Influences on Muscle Protein Turnover.

(1) Anabolic influences. The growth-promoting and anabolic actions of growth hormone (GH) are thought to be mediated via polypeptide growth factors (somatomedins) produced in the liver and in other tissues, including skeletal muscle (84). IGF-1 is the principal somatomedin, and it can exert both hormonal and local effects via the IGF-1 receptor (85). The actions of IGF-1 in either the bloodstream or within specific tissues can be modulated by various binding proteins (IGFBP). In animal and in vitro studies, IGF-1 produces increments in protein synthesis and usually diminishes protein degradation (86). In humans, both GH and IGF-1 enhance local skeletal muscle protein synthesis when administered directly (87, 88). GH administered systematically or locally has no impact on protein degradation, whereas IGF-1 at high dosages suppresses protein breakdown, possibly via effects on the ubiquitin pathways (82, 87, 88). In skeletal muscle, the anabolic effects of IGF-1 on muscle fibers may be mediated in part by stimulatory effects on satellite cells (89). The latter are myogenic cells that retain mitotic activity and serve as a source of myonuclei to growing, enlarging, and regenerating fibers (90). Testosterone, an anabolic/androgenic steroid, has distinct influences on skeletal muscle protein turnover in animals as well as in humans (91). In humans, testosterone administered in replacement or pharmacologic doses increases muscle protein synthesis (93). It is likely that the anabolic effects of testosterone may be mediated at least in part by increased intracellular production of IGF-1. For example, after administration of testosterone to elderly men, several changes were found in vastus lateralis muscle biopsy specimens (95). IGF-1 mRNA was increased and IGFBP-4 was decreased, presumably leading to decreased IGF-1 binding. In cell cultures, in vitro muscle preparations and in animals, insulin promotes anabolism by promoting amino acid uptake with increment in protein synthesis and inhibition of protein degradation (96). Studies in humans, however, indicate that the predominant anabolic action of insulin is mediated primarily by a reduction in protein breakdown (97). It is postulated that the reduction in serum levels of amino acids induced by insulin may limit enhanced protein synthesis.

(2) Catabolic influences. In animal studies, corticosteroids have demonstrated catabolic influences mediated predominantly by inhibition of protein synthesis, with or without enhanced protein degradation (96). The latter effect may in part be due to activation of ubiquitin-mediated proteolysis (81). Insufficient data on protein turnover in skeletal muscle are available in human studies to draw parallel conclusions to those in animal models (101). Studies in patients with rheumatoid arthritis treated with corticosteroids, however, suggest reduced skeletal muscle protein synthesis (102). Although release of stress hormones (corticosteroids, catecholamines, glucagon) may result in protein catabolism, glucagon has not been shown to effect protein metabolism in human skeletal muscle (103). In addition, variable results have been reported for the influences of epinephrine on skeletal muscle in humans (104). Excess thyroid hormone may produce a negative protein balance by influences on both protein degradation (enhanced) and protein synthesis (suppressed) in skeletal muscle (105). A variety of cytokines, including tumor necrosis factor (TNF) and interleukins (e.g., IL-1) have been reported to produce muscle wasting, mediated in part by activation of ubiquitin pathways with enhanced proteolysis (78). Furthermore, various prostaglandins (PG) may also exert positive (e.g., PG F2, E1) or negative (e.g., PG E2) influences on protein turnover in skeletal muscle (106, 107).

c. Nutrient Influences on Muscle Protein Turnover. Malnutrition is an important clinical state exerting a negative influence on protein balance, particularly within muscle. Significant degrees of malnutrition may complicate clinical illness in unstressed or stressed conditions. The main difference between unstressed and stressed states of malnutrition (e.g., associated with sepsis, major trauma, etc.) is the inability to suppress amino acid release from muscle with stress despite nutritional supplementation (70). Furthermore, much greater urinary losses of nitrogen are observed with states of stressed starvation (e.g., 15 to 50 g/d) compared with the unstressed state (3 to 12 g/d). Most of the nitrogen lost during acute stress derives from skeletal muscle sources and, in particular, contractile proteins (108, 109). Among the amino acids mobilized during stress are substrates for synthesis of essential proteins, those involved in gluconeogenesis, and fuel substrates such as branched chain amino acids (70). Malnutrition with or without associated stress is associated with a biochemical milieu that favors a negative protein balance. This includes reduced serum levels of IGF-1 and insulin, coupled with elevated levels of stress hormones and other mediators (e.g., TNF).

d. Influence of Aging on Muscle Protein Turnover. With advancing age, reductions in skeletal muscle mass parallel loss of total muscle force (11, 110). In both elderly animals and humans, reductions in muscle protein synthesis have been documented. For example, Welle and colleagues (111) reported a significant (~ 28%) reduction in the rate of myofibrillar protein synthesis in elderly subjects compared with that in young subjects. Hormonal changes noted with aging may be partly responsible. For example, in the elderly, GH is less responsive to GHRH while levels of the inhibitory somatostatin increase (112). The above events result in a decrease in circulatory and possibly tissue levels of IGF-1. In addition, in elderly male subjects, significant reductions in total and free testosterone are noted (113, 114). It is currently unclear to what extent reduced estrogen levels in postmenopausal women contribute to reduced muscle bulk and protein turnover. Finally, age-related changes in cytokine production may also produce altered protein turnover in the elderly (e.g., enhanced IL-1J production in vitro in blood cells from elderly subjects compared with those from young subjects) (115).

e. Measurement of Muscle Protein Turnover. A variety of indirect and direct measures may be employed in humans to evaluate aspects of protein turnover. These include urinary creatinine excretion (116), urinary 3-methylhistidine measurement (derived from actin and myosin proteolysis) (117), muscle protein analysis, amino acid balance studies (e.g., across forearm) (99), and estimates of protein turnover using amino acid incorporation techniques (118, 119) or arteriovenous amino acid balance techniques. As myofibrillar proteins comprise several different constituent proteins (e.g., myosin, actin, mitochrodrial proteins, and sarcoplasmic reticulum proteins), new techniques have been developed to determine myosin heavy chain synthesis rates in skeletal muscle samples using incremental enrichment of both radiolabeled (120, 121) and stable isotope labeled (122) leucine into myosin heavy chains. Using these techniques, it has been shown that the synthesis rate of slow myosin isoenzyme is faster than that of the fast myosin isoenzyme (121). Recently, techniques have been developed aimed at determining the fractional synthesis rate of individual myosin heavy chain isoforms extracted from single skeletal muscle fibers (G. C. Sieck, K. S. Nair, P. Balagopal, unpublished data).

3. Measurements of Muscle Strength

a. Morphologic. The overall mass of functioning muscle tissue and the size and type of its constituent muscle fibers provide an index of predicted muscle force. A number of direct and indirect approaches have been used to assess muscle mass (123). These include measurements of total body potassium and nitrogen (124), estimates of fat-free mass, e.g., anthropometry such as four-site skinfold thickness (127, 128), deuterium dilution (129), underwater weighing (130), bioelectrical impedance (129, 130), biochemical estimates of total skeletal muscle mass, e.g., creatinine height index (116), measures of regional limb skeletal muscle mass, e.g., dual energy X-ray absorptometry (DEXA) scanning (131) and ultrasonography (132), and anatomic delineation of the mass of specific muscles, e.g., magnetic resonance imaging (MRI) (133), computed tomography (CT) (134).

Although these methods offer noninvasive approaches to the estimation of muscle mass, a number of important limitations should be highlighted. First, most of these methodologies provide only gross whole-body or regional estimates and are not specific for individual muscles or muscle groups. Anthropometric indices may be more prone to error in the elderly because of alterations in the distribution of body fat (135). In addition, an increase in the extravascular fluid compartment can be mistaken for an increase in muscle mass. More recently, comparisons between anthropometric estimates of limb muscle mass in the elderly with either CT or MRI have revealed significant inconsistencies (133, 136). Similarly, total body potassium and nitrogen assays were reported to underestimate skeletal muscle mass using CT as the "gold standard" (137). Bioelectrical impedance has not been fully validated in the elderly and its interpretation may be limited by the presence of edema and skin changes that affect impedance (138). Although total body skeletal muscle mass determined by DEXA scanning has correlated well with estimates obtained by CT, the former tended to overestimate muscle mass (137). Lastly, although CT and MRI appear to be emerging as reference measurements, they are expensive tests. Furthermore, logistic and cost considerations may preclude the performance of multiple serial studies.

Muscle biopsy is an extremely valuable tool as it affords a detailed assessment of morphologic and biochemical indices at molecular, cellular, or tissue levels (e.g., 139). Among the main indices that pertain to the determination of muscle strength, the most important include: (1) determination of fiber type proportions using myofibrillar ATPase histochemical staining techniques (28) or antibodies against myosin heavy chain (MHC) isoforms (32), (2) quantitative assessment of individual fiber cross-sectional areas, and (3) gel electrophoresis of muscle homogenates from whole biopsy specimens or single muscle fibers dissected from the biopsy to determine MHC proportions and/or their mRNA transcripts (55).

Despite the useful information obtained from muscle biopsy analyses, several limitations of the method should be highlighted. Although the needle biopsy technique is generally regarded as a minor procedure, potential complications include pain and discomfort, bleeding, infection, and scarring. A major limitation is the presence of sampling error as the biopsy represents only a tiny fraction of the whole muscle. The coefficient of variation for duplicate biopsies is in the region of 10 to 20% for fiber-type proportions and 15 to 20% for cross-sectional areas (140, 141). Errors relating to cross-sectional areas may also originate from fiber contraction, with sectioning or delays in freezing (142) and nonperpendicular orientation of the muscle fibers. In humans, considerable genetic variation in fiber types exists among subjects, thus mandating a larger number of subjects to account for this. Although the vastus lateralis represents the most commonly studied limb muscle, the choice of muscle for biopsy should take into account the intervention administered to the study group. In addition, recent molecular studies have demonstrated that human skeletal muscle fibers traditionally classified as type IIb fibers express MHC mRNA transcripts for the 2X isoform (143). Because there is no evidence that true type IIb fibers exist in human skeletal muscle, this may have implications for both accurate fiber typing and physiologic correlates of specific force.

b. Functional. Four different devices/methods have been employed in the measurement of limb muscle strength: cable tensiometer, dynamometer, motorized dynamometer, and repetition maximum (RM) tests. All these tests may be limited by volitional factors, learning effect, and non-muscle factors (e.g., arthritis in the elderly). The cable tensiometer can reliably measure isometric force only (144). Hand grip dynamometry evaluates a limited muscle group and there are few reference data applicable to the elderly (145). In addition, upper limb strength is better preserved than lower limb strength in the elderly (146). The Cybex II isokinetic motorized dynamometer is commonly used to assess limb muscle strength (e.g., quadriceps/knee extension). Artifacts caused by excess acceleration of the lever arm of the machine and problems incurred by gravitational effects with flexion testing can result in significant errors (147, 148). RM tests with free weights or exercise machines assess the maximum resistance that can be moved across a range of motion and correlate better with normal dynamic functional tasks (149), but they may require a greater learning period than other tests.

	C. MUSCLE ENDURANCE

TOP INTRODUCTION I. Normal Muscle Function A. OVERVIEW OF MOTOR... B. MUSCLE STRENGTH C. MUSCLE ENDURANCE II. Skeletal Muscle... A. PATHOPHYSIOLOGY OF SKELETAL... B. ETIOLOGY OF SKELETAL... C. FUNCTIONAL IMPACT ON... III. Effects of Interventions... A. EXERCISE TRAINING B. OXYGEN THERAPY C. NUTRITIONAL SUPPLEMENTATION D. ANABOLIC HORMONE... E. LUNG TRANSPLANTATION AND... Suggestions for Future Research A. MECHANISMS OF SKELETAL... B. CLINICAL DIAGNOSTIC TOOLS C. TREATMENT MODALITIES REFERENCES

1. Oxygen Transport to Mitochondria

In contrast to strength performance, endurance exercise depends on O₂ transport from the air to the muscle mitochondria. The O₂ transport pathway (150) begins with ventilation delivering air to the alveolar surface, principally by convection. Without adequate ventilation, in relation to metabolic rate, arterial PO₂ will fall and arterial PCO₂ will rise. O₂ crosses into pulmonary capillary blood by diffusion. Whether diffusion equilibration (equalization of alveolar and end-capillary PO₂) occurs or not depends on three factors: (1) the diffusing capacity of the lungs (DL), (2) blood flow (), and (3) the capacitance of blood for O₂ (, which is defined by the slope of the O₂Hb dissociation curve between arterial and venous PO₂ values and is thus equivalent to effective solubility of O₂). In fact, it is the ratio DL/() that determines degree of equilibration (151) and to the extent that DL is reduced and/or increased, arterial PO₂ may be less than alveolar PO₂.

In addition to potential diffusion limitation, nonuniform distribution of inspired air and of pulmonary capillary blood occurs even in normal lungs, for a number of reasons. In pulmonary disease, such ventilation/perfusion inequality is usually exaggerated and may further lead to significant reduction in arterial PO₂ (152). The fourth factor that can reduce arterial PO₂ is the presence of right-to-left shunts. In health, these are negligible (on the order of 4%), but shunts can cause severe hypoxemia in certain diseases. Abnormalities in pulmonary gas exchange from inadequate ventilation, diffusion limitation, ventilation/perfusion inequality, or shunt may limit endurance performance by impairing O₂ transport to the muscles and by impairing CO₂ transport from the muscles to the lungs. Reduced O₂ and CO₂ transport in turn may contribute to uncomfortable leg and/or respiratory symptoms during exercise.

Blood containing O₂ combined with Hb as a result of the pulmonary gas exchange process is pumped to the muscles, and this requires adequate cardiovascular function. Cardiac output during exercise is very tightly coupled to O₂ consumption, even in COPD, both rising linearly with heart rate along approximately the same relationship as in healthy subjects (153). Although the mechanisms responsible for this coupling remain incompletely understood, the autonomic nervous system is centrally involved. Parasympathetic withdrawal and sympathetic activation are seen with exercise, increasing both heart rate and stroke volume (154). Venous return is enhanced by skeletal muscle contractions and by increased venous tone, and these appear to contribute to increasing cardiac output via the Starling effect (154).

Preferential distribution of blood flow to the contracting muscles is an important adjustment to exercise and requires vasodilatation in the active muscles (and vasoconstriction in other vascular beds). This occurs through a number of parallel mechanisms involving the autonomic nervous system as well as a host of local metabolic and neurologic factors. The end result is (at least in health and during submaximal exercise), appropriate delivery of O₂ for the metabolic needs of the muscle. O₂ delivery (O₂) to the muscle microcirculation is defined as:

(1)

where [Hb] is hemoglobin concentration, Sa_O2 is fractional arterial O₂ saturation, Pa_O2 is arterial PO₂ and _M is muscle blood flow. Except in extraordinary circumstances such as severe anemia and hyperbaric environments, the amount contributed by physically dissolved O₂ (0.003 × Pa_O2) can be neglected for simplicity since it accounts for less than 2% of the O₂ present in blood. Therefore, the implications of this equation are paramount to understanding the basis for endurance exercise: O₂ supply to the muscles depends on the blood ([Hb]), the lungs (Sa_O2), and the cardiovascular system (_M). The latter factor encompasses not only adequate cardiac function but also peripheral vascular structure and function to assure proper distribution of cardiac output to active muscle. Reduction in any one of these three variables ([Hb], Sa_O2, or _M) will reduce convective muscle oxygen transport and, consequently, endurance performance.

It is important to note that this discussion focuses on muscle blood flow, _M, and not on cardiac output. This is for two practical reasons. First, attempting to infer muscle blood flow from cardiac output is likely to produce significant errors in assessment of muscle O₂ delivery. Second, using cardiac output and O₂ at the mouth to calculate arteriovenous [O₂] difference and from this to estimate muscle O₂ extraction is equally uncertain when a significant proportion of cardiac output and O₂ is distributed to tissues other than actively contracting muscle. Thus, experimental methods should focus directly on the active muscle groups.

O₂ delivery to muscle (Equation 1) is by no means the end of the O₂ transport story. O₂ must be unloaded from Hb in the muscle microcirculation and diffuse through the red cell wall, plasma, and the capillary wall. It then must diffuse into the myocyte where it binds to myoglobin, which is believed to greatly facilitate its transport to the mitochondria for use in ATP generation (155). The intramuscular diffusion pathway is unique because of myoglobin. Although the distance O₂ must diffuse between Hb and the sarcolemma is just a few microns, and the distance O₂ must travel to the mitochondria may be some 50 microns, almost all of the impedance to O₂ diffusion is in the initial short distance from Hb to sarcolemma. This is shown by the large difference between mean capillary PO₂ (generally ~ 40 mm Hg) and average intracellular PO₂ (about 3 mm Hg) during exercise in humans (156). Several lines of evidence point to the critical importance of muscle capillary surface area in determining the impedance to O₂ unloading in muscle; diffusion distance may well be relatively unimportant (156). Thus, from a morphometric standpoint, the ability to move O₂ from Hb to the mitochondria is a function of how many capillaries surround each muscle fiber. Fiber cross-sectional area on the other hand appears unimportant (159). If in a given disease state there is a reduction in the number of capillaries around each fiber, one would expect impaired O₂ unloading; fiber size changes without alterations in capillary number would not be expected to reduce O₂ availability.

Just as described above for the lungs, the diffusive process of O₂ unloading depends on the muscle diffusing capacity for O₂ (D_M), the capacity of blood to hold O₂ (), and muscle blood flow (_M). Again, completeness of unloading depends on the ratio D_M/(_M) (153). Intuitively, the greater the D_M, the higher the rate of transport of O₂ between Hb and mitochondria. However, completeness of unloading will be reduced if there is a large amount of O₂ to be unloaded from the blood (large value for ) or if perfusion (_M) is high. When  is high, it simply takes longer to unload O₂ because there are more O₂ molecules in the blood; when Q_M is high relative to the muscle capillary volume, transit time is reduced and there is less time available for O₂ unloading.

Extraction is thus dependent on D_M/(_M). If one assumes that the relevant portion of the O₂Hb dissociation curve is linear, then muscle venous PO₂ (PV_O2) is a simple exponential function of D_M/(_M):

(2)

where Pa_O2 is arterial PO₂. Assuming a linear O₂Hb dissociation curve (approximately correct at low PO₂), fractional O₂ extraction, E (which is the ratio of arteriovenous [O₂] to arterial [O₂]) becomes:

(3)

The critical message of Equation 3 is the following: whereas D_M is reasonably considered as a "peripheral" factor in that it is mostly determined by muscle structure, both  and _M are largely determined by factors unrelated to muscle. They are functions of the heart and blood. Extraction of O₂ is thus not simply an index of peripheral (muscle) function but a complex end-product of the interaction between the peripheral factor D_M and the "central" factors  and _M. These concepts remain valid even if the O₂ dissociation curve is not linear. Quantitative but not qualitative differences in predictions from Equations 2 and 3 will be seen for the case of the real O₂ dissociation curve (160, 161).

Equally important is another concept: just as in the lungs, where O₂ diffusing capacity increases with pulmonary blood flow, so too will muscle diffusing capacity increase with increasing muscle blood flow: as exercise intensity increases, more muscle fibers and capillaries are recruited. Consequently, if overall exercise intensity in COPD is governed mostly by a low ceiling on ventilation, which in turn results in a limited increase in cardiac output and hence in _M, so too will muscle O₂ diffusing capacity not reach its potential maximal value during exercise.

An inference may be drawn: attempting to study the skeletal muscles during exercise using large enough muscle groups to force the patient to have reached his low ventilation ceiling will not provide the answer to a basic question: are the skeletal muscles normal or not in COPD? To answer this question, it can be predicted that a helpful strategy will be to activate a relatively small muscle mass. Ideally, the amount of muscle involved should be small enough to permit it to reach this muscle's maximal O₂ utilization without the patient having reached his ventilatory limit. However, the muscle would need to ideally be large enough to allow measurement of muscle O₂ delivery (O₂) (Equation 1). O₂ delivery to the contracting muscle is a key measurement. If O₂ delivery is reduced, the immediate biochemical consequences might be interpreted as evidence of intrinsic muscle dysfunction instead of actually reflecting inadequate O₂ supply to otherwise normal muscle. The increase in arterial lactate concentration ([La]) with increasing O₂ is a case in point. Patients with COPD or congestive heart failure often exhibit an early rise in [La] that may be due to inadequate O₂ delivery. Enhancing O₂ delivery (and thereby normalizing [La] response) would indicate that the problem lay with O₂ delivery rather than with skeletal muscle dysfunction. The same holds for indices of bioenergic state measured by magnetic resonance spectroscopy such as phosphocreatine/inorganic phosphate ratios and intracellular pH. Unless O₂ delivery to the active muscle can be measured and matched to that of appropriate control subjects, differences in such indices of muscle function may well be incorrectly attributed to an intrinsic myopathy when all that they reflect is reduced O₂ delivery.

2. Mitochondrial Function and Cellular Energetics

a. Cellular Energetics. Skeletal muscle is capable of large changes in metabolic activity (162); relative to the basal rate, metabolism can be increased 20- to 500-fold (163). Several strategies are available to the muscle fiber for generating adenosine triphosphate (ATP), which is the common energetic currency of the cell. ATP provides the energy for cross-bridge cycling and for active transport. At the onset of contractile activity, muscle fibers initially draw on a small pool of existing high-energy phosphates (164). However, ATP levels do not change significantly because phosphocreatine provides the substrate for rapid resynthesis catalyzed by creatine kinase. Inorganic phosphate and creatine are the net products of this overall reaction; both metabolites accumulate within the tissue during the early phase of exercise and can contribute to fatigue (see below). Skeletal muscle contains high levels of phosphocreatine relative to other tissues (165), but these stores can supply cellular needs for only a few seconds of strenuous contractions. During this time, glycogenolysis accelerates and the rate of glycolysis increases to provide a more sustainable energy source. Cytosolic pathways metabolize glucose to pyruvate. Pyruvate is either reduced to lactic acid via lactate dehydrogenase, an anaerobic mechanism, or is transported into the mitochondrion. Within the inner mitochondrial matrix, pyruvate is converted to acetyl CoA and metabolized aerobically via the tricarboxylic acid cycle to yield CO₂ and water.

Compared to anaerobic glycolysis, aerobic metabolism requires longer to activate. Energy utilization during the first 2 to 3 min of exercise requires more ATP than can be synthesized from oxygen stores in the muscle; the time required for blood flow and oxygen delivery to increase to steady-state levels causes an "oxygen debt" to develop within the muscle (166) that must be repaid after exercise. Nevertheless, aerobic metabolism has two important advantages to working muscle fibers. It produces more energy, 38 ATP per mole of glucose versus 2 ATP from anaerobic glycolysis, and it enables the cell to utilize stored lipid as a fuel via fatty acid metabolism (167). These properties are critical for muscle endurance, and mitochondrial content is a major determinant of muscle performance during prolonged exercise.

b. Mitochondrial Metabolism. Mitochondria are syncytial organelles that are located in interfibrillar or subsarcolemmal regions of the muscle fiber (168). Muscle fibers differ profoundly in their mitochondrial content both within and among muscles. Within a given individual, mitochondrial content of muscle fibers is not tightly linked to myosin ATPase activity (i.e., fiber type) but is strongly influenced by the pattern of muscle use. Muscles that undergo chronic endurance activity have the highest mitochondrial content; these include the heart, diaphragm, and soleus muscles (169).

Mitochondrial respiration requires dehydrogenases within the mitochondrial matrix that metabolize pyruvate or free fatty acids to produce reducing equivalents, i.e., reduced nicotinamide adenine dinucleotide (NADH). In the presence of molecular oxygen, NADH functions as a substrate for oxidative phosphorylation with the following overall reaction:

(4)

Three factors determine the maximum rate of this reaction at the cellular level (170). First is the capacity of respiratory chain enzymes to process oxygen and NADH via oxidative phosphorylation. In the absence of pathology, this capacity is determined by the quantity of respiratory chain enzymes that is proportional to mitochondrial volume: more mitochondria equals a higher oxidative capacity. Second is the amount of NADH available to the respiratory chain. This is influenced by the activity of mitochondrial dehydrogenases and by enzymes of the metabolic pathways that supply substrate for these dehydrogenases. These are primarily located within the mitochondrial matrix and, like the respiratory enzymes, are increased in proportion to mitochondrial volume density. Third is delivery of oxygen to the mitochondria. As noted in the previous section, oxygen delivery depends on a complex array of factors, including arterial oxygenation, hemoglobin content, muscle blood flow, vascular regulation, capillary number, fiber size, and myoglobin content.

c. Metabolic Adaptation. The metabolic properties of skeletal muscle are highly adaptive (171). Endurance training stimulates an integrated response within the tissue that increases the overall capacity for oxidative metabolism. Each of the three factors listed aboveoxidative capacity, availability of reducing equivalents, and oxygen deliveryis affected by training. Proliferation of mitochondria in endurance-trained muscle simultaneously increase the capacity of the respiratory chain, the dehydrogenases that generate reducing equivalents, and the activities of metabolic pathways that supply the dehydrogenases. Oxygen delivery is increased by a pleiotropic response at the local and systemic levels: muscle adapts by increasing capillary number and myoglobin content. This is complemented by an increase in cardiac function that improves the peak capacity for blood flow and oxygen delivery to the muscle. The opposite changes are produced by chronic reductions in muscle activity, e.g., by denervation or inactivity, and aerobic capacity falls accordingly.

3. Determinants of Fatigue

Muscle fatigue can been defined as a loss in contractile functionforce, velocity, power, or workthat is caused by prolonged exercise and is reversible by rest (10). Studies of isolated muscle preparations have shown that fatigue directly compromises intracellular processes. Prolonged contraction at near-maximal activation frequencies causes fatigue because of electrical failure of the sarcolemma, so-called "high-frequency fatigue." Repetitive contraction using lower, quasi-physiologic activation frequencies causes loss of calcium homeostasis in muscle fibers, diminishes the calcium sensitivity of myofilaments, and can cause cross-bridge dysfunction. In contrast to isolated muscle preparations, human subjects may experience fatigue during volitional exercise because of cardiovascular limitations or central neural mechanisms. Thus, a variety of factors can limit endurance performance, and the actual cause(s) of fatigue depends on exercise conditions. This section provides an overview of the mechanisms that are most likely to contribute to fatigue in intact humans under physiologic conditions. Readers desiring more detail are referred to reviews by Westerblad and colleagues (167) and by Fitts (165).

a. Events Within the Muscle Fiber. Fatigue is generally associated with a metabolic imbalance in which metabolite concentrations change in order to maintain energetic supply. Such imbalance leads to depletion of intracellular energy stores such as glycogen, glucose, and phosphocreatine and accumulation of metabolic by-products within the cell. Under most conditions, muscle function declines prior to depletion of high- energy phosphate stores; ATP availability generally is not rate-limiting (172). In contrast, metabolic by-products directly inhibit contractile function and are thought to be primary mediators of fatigue. First among these are the hydrogen ions generated by glycolytic metabolism. Lactate production by muscle sharply increases when energetic demand exceeds 50 to 60% of maximal aerobic capacity (165). Lactate release from muscle has long been correlated with muscle fatigue (173), but lactate itself is unlikely to cause dysfunction (174). Lactate-derived hydrogen ions have been implicated; acidosis directly compromises myofilament function, decreasing both force and the velocity of shortening (175); however, the magnitude of this effect is small at physiologic temperatures (176). A second factor contributing to fatigue is the inorganic phosphate produced by ATP hydrolysis. When phosphorylation potential falls, as occurs in heavy exercise, inorganic phosphate accumulates in the muscle and decreases calcium sensitivity of the myofilaments. Force production falls as a result (177). A third factor is the magnesium produced by MgATP hydrolysis. During periods of heavy MgATP utilization, intracellular magnesium concentration rises (178). Elevated magnesium levels may contribute to fatigue by inhibiting calcium release from the sarcoplasmic reticulum and thereby depressing myofilament activation (179). Fourth, reactive oxygen species are produced at accelerated rates by heavily exercising muscle. Accumulation of reactive oxygen species causes oxidative stress, inhibiting the function of both myofilaments and the sarcoplasmic reticulum (180, 181) and contributing to loss of muscle function in fatigue (182).

Electrolyte shifts may disrupt sarcolemmal function during fatiguing exercise. Muscle activation causes potassium release and sodium influx with each action potential. During periods of intense activation, electrolyte flux rates can exceed the regulatory capacity of sarcolemmal pumps such that intracellular potassium concentration falls and intracellular sodium levels rise (183). Resting membrane potential therefore becomes less negative and the charge movement produced by action potentials is diminished. During intense activation, potassium accumulation within t-tubules can prevent action potential conduction and inhibit voltage-dependent calcium release (184). This phenomenon is the putative basis for high-frequency fatigue (167).

b. Mechanisms Beyond the Muscle Fiber. In intact humans, fatigue of exercising muscle may be influenced by other physiologic mechanisms. Blood flow to working muscle determines the rate of oxygen delivery and governs the clearance of deleterious metabolites (185). The process of exercise hyperemia is regulated by local mediators released from the working muscle (186), by vasoactive mediators produced locally (187), and by autonomic control (188). Loss of vascular control or peripheral vascular disease can accelerate fatigue by decreasing total blood flow to the muscle or, more subtly, by causing maldistribution of flow within the tissue. A global determinant is cardiac function. Insufficient cardiac output can limit blood flow to working muscle, especially in whole-body exercise tasks that involve a large muscle mass (189).

Neural mechanisms also are important. During volitional exercise, fatigue is most commonly reflected by "task failure," which is defined at the whole-body level as the inability to sustain a desired level of exercise. Task failure usually reflects two concurrent processes: contractile failure of the fatiguing muscle leading to loss of neural activation (190). Conscious activation of exercising muscle requires coordinated neural output from motor and premotor regions of the cerebral cortex; this is automatically accompanied by neural adjustments to cardiovascular function (191) and to respiration (192). Task failure may be caused by peripheral mechanisms. For example, neuromuscular transmission failure can be demonstrated in electrically stimulated neuromuscular preparations and is known to mediate fatigue in persons with neuromuscular disease such as myasthenia gravis; however, the physiologic importance of neuromuscular transmission failure during normal motor activity remains controversial (193). Task failure also may be accelerated by inhibitory spinal reflexes that are stimulated by afferent nerve endings in the working muscle (194). Alternatively, the subject may consciously decide to quit exercising because the task has become uncomfortable. Both neural mechanisms depend on afferent signals that can be used by the central nervous system to monitor changes in exercising muscle. The free nerve endings of group III and group IV afferents probably serve this purpose; they are sensitive to a variety of physiologic stimuli and can mediate either reflex or conscious adjustments to motor output (194). During day-to-day activities, the primary cause of task failure is that people quit when exercise becomes unpleasant. Highly motivated subjects and well-trained athletes can sustain motor output despite considerable discomfort. In these persons, it is controversial whether task failure directly reflects muscle fatigue or whether inhibitory reflexes accelerate task failure.

4. Measurements of Muscle Endurance

Endurance reflects the ability of muscle to sustain mechanical output during loaded contraction, i.e., resist fatigue. Measurement of endurance requires that a muscle or muscle group contract against a load, producing a time-dependent loss of mechanical function. The slower the loss, the greater the endurance. For detailed accounts of the techniques used to measure muscle endurance in humans, readers are referred to recent reviews of the field (195, 196). To briefly outline the concepts, measurements of muscle endurance have three basic components that can vary according to experimental design.

First is the strategy of muscle activation: volitional effort versus exogenous stimulation. Volitional activation is used for stereotypical exercise tasks performed using specific muscle groups, e.g., hand grip or knee extension. This strategy may be complicated by variations in motor output caused by dyscoordination, distraction, or lack of motivation, which cause the intensity of muscle activation to vary. Thus, it can be difficult to discriminate between decrements caused by muscle fatigue and decrements caused by a drop in motor output. In studies of individual muscles, the classic twitch interpolation technique (197) can be used to assess muscle activation; an electrical pulse is applied to the motor nerve while measuring the force of a volitional contraction; an abrupt increase in force upon stimulation demonstrates that volitional activation was submaximal. An alternative approach is to bypass the central nervous system altogether. Exogenous electrical stimulation can be used to induce fatigue of limb muscles from which mechanical output can be measured such as adductor pollicis or tibialis anterior. A limitation of electrical stimulation is that activation of the motor nerve is usually submaximal, recruiting only a subset of the motor units in the muscle.

Second is the exercise condition: isometric versus isokinetic. Under isometric conditions, muscle length is fixed and endurance is measured as the ability to maintain force or torque. During isokinetic exercise, the muscle undergoes loaded shortening; endurance is measured as the ability to maintain velocity, work, or power. Note that exercise condition is largely independent of activation strategy.

Third is the exercise pattern: timing and intensity. The exercise task may involve either a single prolonged contraction or a series of repetitive contractions. During volitional exercise, the subject usually is asked to maintain a submaximal exercise level. Endurance then is measured as the time over which the task is maintained, i.e., time to task failure. During electrical activation, exercise intensity is determined by the stimulus characteristics, which usually remain fixed during the exercise bout; endurance is assessed by the rate at which muscle function declines during continued stimulation.

	II. Skeletal Muscle Abnormalities in COPD

	A. PATHOPHYSIOLOGY OF SKELETAL MUSCLE DYSFUNCTION IN COPD

TOP INTRODUCTION I. Normal Muscle Function A. OVERVIEW OF MOTOR... B. MUSCLE STRENGTH C. MUSCLE ENDURANCE II. Skeletal Muscle... A. PATHOPHYSIOLOGY OF SKELETAL... B. ETIOLOGY OF SKELETAL... C. FUNCTIONAL IMPACT ON... III. Effects of Interventions... A. EXERCISE TRAINING B. OXYGEN THERAPY C. NUTRITIONAL SUPPLEMENTATION D. ANABOLIC HORMONE... E. LUNG TRANSPLANTATION AND... Suggestions for Future Research A. MECHANISMS OF SKELETAL... B. CLINICAL DIAGNOSTIC TOOLS C. TREATMENT MODALITIES REFERENCES

1. Structural Alterations of Skeletal Muscle in COPD

a. Muscle Mass. There are no quantitative data available concerning the mass of specific limb muscles in patients with COPD. However, indirect estimates of a reduced muscle mass in patients with COPD have been reported. These have included findings of a decreased fat-free mass assessed by bioelectrical impedance analysis (129, 198) and a significantly smaller (13%) cross-sectional area of the calf muscle evaluated by magnetic resonance imaging (MRI) compared with matched control subjects (200). Assessment of more specific regional bone, fat, and lean body mass by dual emission X-ray absorptometry (DEXA) scanning has recently been used in conjunction with limb muscle size measurement by MRI in a longitudinal study in patients with COPD (201). However, a matched healthy control group was not included in the study.

b. Muscle Fiber Types and Sizes. More direct evidence of structural alterations of skeletal muscle in patients with COPD comes from studies at the cellular level. Biopsy analyses from the quadriceps femoris of patients with moderate COPD revealed no change in fiber-type proportions but significant atrophy of type II fibers, with the degree of atrophy correlated to the amount of weight loss (202). Others reported a reduced proportion of type I fibers in the vastus lateralis of patients with advanced COPD compared with unmatched control subjects (203, 204). Compared with matched control subjects, this reduction in the proportion of type I fibers in patients with COPD was recently found to be accompanied by an increase in the proportion of type IIb fibers (205). Furthermore, atrophy of both types I and IIa fibers was reported for these patients (205). It has been suggested that hypoxemia (204) and long-term disuse (205) may contribute to the high proportion of type II fibers found in patients with COPD. In contrast to the lower limb, the proportion of type I fibers in the biceps of patients with severe COPD was found to be similar to that of age-matched control subjects (206). The diameter of type I and (to a greater degree) type II fibers were smaller than those of control subjects and correlated with amount of weight loss and reduction in percent predicted FEV₁ (206). Another study in patients with COPD reported fiber-type analysis by quantifying myosin heavy-chain (MHC) and myosin light-chain (MLC) isoforms determined by gel electrophoresis (207). A significantly greater proportion of MHC-2B was found in the vastus lateralis of patients with COPD than in control patients. Moreover, the pattern of distribution of MLC isoforms in patients with COPD was also significantly shifted towards fast isoforms (207). Diffusion indices, VC and FEV₁ were all positively correlated with the content of slow MHC. As reported above (204, 205), it was hypothesized that both reduced oxygen availability and muscle disuse probably determine muscle alterations in COPD (208). In this study, however, control patients were not age-matched.

c. Capillarity. An electron microscopic study reported that the number of capillaries per unit surface area in the vastus lateralis of patients with COPD was 53% lower than in age-matched normal subjects (209). Because the number of mitochondria per unit surface was unchanged in patients with COPD, their capillary/mitochondria ratio was also decreased by 59% compared with that in normal control subjects. Whereas the capillary/fiber ratio in the vastus lateralis of patients with COPD was not significantly lower than in control subjects, the number of capillary contacts for types I and IIa fibers were significantly lower in the patients with COPD (205). However, when normalized for fiber cross-sectional areas, the number of capillary contacts for each type of fibers were similar between patients with COPD and control subjects (205). This lower number of capillary contacts, along with a 25% lower myoglobin level found in the vastus lateralis of patients with COPD (209), may contribute to reduced oxygen delivery within muscle in these patients.

d. Metabolic Enzymes. Analyses from homogenates of muscle biopsy specimens from patients with severe COPD have demonstrated lower oxidative enzyme (citrate synthase, succinate dehydrogenase, and 3-hydroxyacyl-CoA dehydrogenase) capacities in the vastus lateralis compared with age- and height-matched healthy subjects (210, 211). No significant differences in glycolytic enzyme (phosphofructokinase, lactate dehydrogenase, and hexokinase) activities were detected between these two groups (210, 211), with the exception of phosphofructokinase activity, which was higher in the patients with COPD (210). It was suggested that both inactivity and hypoxemia contribute to these metabolic alterations in the COPD group. However, there were no reversal in the activities of any enzyme after 7 mo of long-term oxygen therapy (210). No studies have been performed to analyze enzyme activities within individual muscle fibers of identified type in limb muscles of patients with COPD.

2. Skeletal Muscle Function in COPD

a. Muscle Strength. Peripheral muscle weakness is common in patients with COPD (212, 213). In a large study, Hamilton and colleagues (213) found that approximately 70% of patients with chronic lung disease, some of them with restrictive disorders, had lower quadriceps strength than the mean value obtained in normal subjects of similar age. The reduction in muscle mass certainly contributes to peripheral muscle weakness in patients with COPD (199, 214), but it is still unclear if muscle weakness can be attributed entirely to muscle atrophy (215, 216).

Compared with normal subjects of similar age, the reduction in quadriceps strength averaged 20 to 30% in patients with severe to moderate disease (212, 213, 215), but, in general, their upper limb strength was relatively preserved compared to that of the lower limbs (212, 214, 217). The uneven distribution of muscle weakness between upper and lower limbs could be related to differences in accustomed level of activity between the different muscle groups. Compared with lower limb muscles, the upper limb muscles are probably more normally involved in activities of daily living. Furthermore, in COPD, the pectoralis major and the latissimus dorsi muscles may also act as accessory inspiratory muscles, another potential source of stimulation (218, 219).

b. Muscle Endurance. The information on limb muscle endurance in patients with COPD is conflicting. Endurance of the vastus lateralis muscle has been reported to be normal in hypoxemic patients with COPD (220). This finding is surprising when taking into account the morphologic and enzymatic deficiencies found in the vastus lateralis muscle of these patients (203, 211). In contrast, other investigators found a 50% reduction in the endurance of the vastus lateralis muscle in 17 patients with COPD when compared with age-matched normal control subjects (221). The effects of COPD on the endurance of the upper limb muscles is also unclear; both normal endurance of the elbow flexors (217) and reduced endurance of the adductor pollicis muscle have been reported (220). Discrepant results between studies may be related in part to differences in the methodology used to measure peripheral muscle endurance; further studies are required to resolve to what extent limb muscle endurance is altered in patients with COPD.

3. Muscle Bioenergetics

a. Oxygen Delivery and Utilization. Patients with COPD characteristically show poor exercise performance indicated by a marked reduction in both peak pulmonary O₂ uptake and work rate at peak exercise. However, the relationship between whole-body O₂ uptake and work rate is normal and the O₂ uptake for a given submaximal work rate in these patients is similar to that seen in healthy sedentary subjects (222).

Central pulmonary factors such as inability to adequately increase total ventilation because of the elevated work of breathing and disturbances of arterial respiratory blood gases (Pa_O2 and Pa_CO2) have been historically invoked as principal determinants of exercise intolerance in these patients (222, 225). Recently, other evidence (211, 212, 226) suggests that skeletal muscle dysfunction should also be considered as playing a role in the limitation of exercise tolerance in COPD.

At peak exercise, systemic O₂ delivery is clearly below normal levels (222). Pulmonary dysfunction is not the sole cause because A/ inequality does not lead to a large fall in O₂ content (Equation 1) in most patients (225). Because the linear relationship between cardiac output and O₂ is about the same in patients with COPD and in normal subjects, cardiac output at a given submaximal O₂ upt