Rationale: Levosimendan is clinically used to improve myocardial contractility by enhancing calcium sensitivity of force generation. The effects of levosimendan on skeletal muscle contractility are unknown. Patients with COPD suffer from diaphragm weakness, which is associated with decreased calcium sensitivity. Objective: To investigate the effects of levosimendan on contractility of diaphragm fibers from COPD patients. Methods: Muscle fibers were isolated from diaphragm biopsies obtained from thoracotomized COPD and non-COPD patients (both groups n=5, 10 fibers per patient). Diaphragm fibers were skinned and activated with solutions containing incremental calcium concentrations and 10 :M levosimendan or vehicle (0.02% DMSO). Developed force was measured at each step and force vs. calcium concentration relationships were derived. Results were grouped per myosin heavy chain isoform, which was determined by SDS-PAGE. Main Results: At sub-maximal activation levosimendan improved force generation of COPD and non-COPD diaphragm fibers by ~25 %, both in slow and fast fibers. Levosimendan increased calcium sensitivity of force generation (p<0.01) in both slow and fast diaphragm fibers from COPD and non-COPD patients, without affecting maximal force generation. Conclusion: Levosimendan enhances force generating capacity of diaphragm fibers from COPD and non-COPD patients by increasing calcium sensitivity of force generation. These results provide a strong rationale for testing the effect of calcium sensitizers on respiratory muscle dysfunction in patients with COPD.