No demonstrable ultrastructural adaptation of the human myotendinous junction to immobilization or 4 weeks of heavy resistance training

The myotendinous junction (MTJ) connects muscle and tendon and is subject to strain injuries. These injuries can be prevented by heavy resistance training of the hamstrings. In animal MTJ, the ultrastructural foldings of tendon into muscle increase with training and decrease with immobilization, but whether this happens in the human MTJ is unknown. Therefore, the aim of this study was to investigate by electron microscopy the effect of 4 weeks heavy resistance training (6 patients) and 5 weeks immobilization (3 patients) on the ultrastructure of the human MTJ, compared to controls (7 patients). Specimens of MTJ were obtained from the semitendinosus and gracilis muscle in connection with harvest of the tendon for reconstruction of the anterior cruciate ligament. We registered the number of branches from the MTJ into muscle, interface length/baseline, length and width of the MTJ unit, and area of the foldings and number of foldings per MTJ unit. There were no significant differences in any measures between the three groups. Adaptations of the human MTJ in connection with heavy resistance training are smaller than the changes reported in animals. Perhaps 4 weeks is too short for the human MTJ to react structurally to loading.