Various therapeutic strategies that aim to influence clinical outcome after severe skeletal muscle trauma have been considered. One such method, the local transplantation of stem cells, has been shown to improve tissue regeneration. The number of cells required for successful regeneration, however, remains unclear. The aim of this study was therefore to examine the correlation between the number of transplanted bone marrow-derived mesenchymal stem cells (MSCs) and the resulting muscle function. One week after inducing an open crush trauma in 34 female Sprague Dawley rats, increasing quantities of autologous MSCs (0.1 x 10(6), 1 x 10(6), 2.5 x 10(6), and 10 x 10(6) cells) or saline solution (control group) were transplanted into the left soleus muscle of the rat hind limb. At 4 weeks posttrauma, the outcome was assessed by measuring muscle contraction forces following an indirect fast twitch and tetanic stimulation. A logarithmic dose-response relationship was observed for both maximum twitch and tetanic contraction forces (R(2) = 0.9 for fast twitch [p = 0.004]; R(2) = 0.87 [p = 0.002] for tetanic contraction). The transplantation of 10 x 10(6) cells resulted in the most pronounced improvement of muscle force. MSC therapy represents a promising new tool for the treatment of skeletal muscle trauma that shows potential for aiding in the prevention of severe functional deficiencies. The logarithmic dose-response relationship demonstrates the association between the number of transplanted cells and the resulting muscle forces, as well as the amount of MSCs required for promoting muscular regeneration.