Am. J. Respir. Crit. Care Med.,
Volume 157, Number 3, March 1998, 833-839
Low Load Inspiratory Muscle Training Increases
Diaphragmatic Fiber Dimensions in Rats
HÉLÈNE
ROLLIER,
ANJA
BISSCHOP,
GHISLAINE
GAYAN-RAMIREZ,
RIK
GOSSELINK,
and
MARC
DECRAMER
Respiratory Muscle Research Unit, Laboratory of Pneumology, Katholieke Universiteit Leuven, Leuven, Belgium
The effects of 8 wk of inspiratory resistive loading (30 min/d, 3 ×/wk) on diaphragm mass, contractile properties, fatigue, and fiber dimensions were studied in 10 male Wistar rats. They were conditioned to breathe through a Hans-Rudolph device. Half of them had to overcome a moderate inspiratory resistance (MR; n = 5), whereas the others only had to overcome the small resistance (SR; n = 5)
of the inspiratory valve of the device. Results were compared with control rats (C; n = 5) moving and
breathing freely. At the end of training, animals submitted to MR and SR generated mean inspiratory
pressures of 
2.5 ± 1.1 and 0.2 ± 0.05 cm H2O, respectively. TI/Ttot was 0.60 ± 0.06 and 0.57 ± 0.05, respectively. Body and diaphragm weight were unaffected by loading. Little or no change in in
vitro diaphragmatic twitch kinetics, force generation, and fatigability was found between the three
groups. Nevertheless, cross-sectional area of all fiber types increased in the two loaded groups compared with control animals. This increase reached statistical significance for type I fibers in the MR
group (846 ± 74 µm2) compared with the C and SR groups (589 ± 32 and 683 ± 96 µm2, respectively, p < 0.05). For IIa fibers both training groups were significantly different from the control
group (SR: 768 ± 99 and MR: 790 ± 108 versus C: 592 ± 37 µm2, p < 0.05). A hypertrophy of type
IIx/b fibers was seen in MR compared with control animals (C: 1,555 ± 136, SR: 1,845 ± 338, MR:
2,053 ± 326 µm2, p < 0.05). No differences were present in fiber type proportions between the three
groups. We conclude that in our training setup, 8 wk of intermittent long-term inspiratory loading
stressed the diaphragm already with a small resistance resulting in hypertrophy of predominantly
type IIa fibers. A higher resistance resulted in hypertrophy of all fiber types.
