SummaryBackground: Skeletal muscle trauma leads to severe functional deficits, which cannot be addressed by current treatment options. Previous investigation could show the efficacy of a local transplantation (TX) of mesenchymal stroma cells (MSCs) for the therapy of muscle injury. Underlying mechanisms remain to be elucidated. The aim of the present work was to characterize the fiber composition changes following MSC-TX after open crush injury. Methods: 20 male SD rats received an open crush trauma of the left soleus muscle. 2.5 × 106 autologous MSCs were transplanted into the crushed soleus muscle of 10 animals 7 days after trauma (group 1, n = 10). Control animals received an injection of saline solution (group 2, n = 10). Histologic analysis of fibrosis, fiber type composition, and muscle force measurements were performed 28 days after trauma. Results: MSC-TX improved muscle force significantly (fast-twitch, treated: 0.76 (0.51-1.15), untreated: 0.45 (0.32-0.73); p = 0.01). Tetanic stimulation resulted in a significant increase of force development (treated: 0.63 (0.4-1.21), untreated: 0.34 (0.16-0.48); p = 0.04). Histological analyses showed no differences in the amount of fibrotic tissue (treated vs. untreated, p = 0.42). A shift towards fastMHC-positive fibers was observed following MSC-TX (treated vs. untreated; p = 0.01 (mm2) or 0.007 (%)). Conclusion: This study demonstrated an effect of locally administered MSCs in the treatment of skeletal muscle injuries on a structural level. For the first time a fiber type shift towards fastMHC following MSC-TX after crush injury could be demonstrated and related to MSC-TX. These results might open the discussion of an alternative mode of action of MSCs in tissue regeneration.