Selective Static Stretching of Rectus Femoris Alters Motor Unit Firing Behaviors of Knee Extensors.

Static stretching immediately affects various neuromusculoskeletal components. Among quadriceps muscles, only the rectus femoris (RF) is stretched by hip extension and knee flexion. The aim of this study was to investigate the motor unit (MU) firing behaviors of two synergistic muscles after selective static stretching on only one side. Fourteen males (23.7 ± 2.4 years) performed knee extension tasks before and after the intervention: static stretching or control conditions. The static stretching protocol consisted of passive hip joint extension and knee joint flexion as selective stretching of the RF for 1 min, repeated for 6 sets, while a 6-min rest was applied as the control condition. MU firings of RF and the vastus lateralis (VL) were detected using high-density surface electromyography, and the MU recruitment threshold and firing rate were calculated during ramp-up contraction to 35% of maximal voluntary contraction. RF stiffness, evaluated by shear wave elastography, was significantly reduced after static stretching, and not VL stiffness (interaction: p = 0.037). There were main effects of time, but no interaction in the MU recruitment threshold of RF (p = 0.282), firing rates of either muscle (RF: p = 0.363, VL: p = 0.557), or maximal strength (p = 0.362), suggesting that these variables were changed after both conditions. However, an interaction was noted in the recruitment threshold of VL (p = 0.018), indicating that the decline in recruitment threshold of VL was greater in the static stretching (from 24.9 ± 6.3 to 21.7 ± 6.1 %MVC) than in the control condition (from 24.4 ± 6.7 to 22.3 ± 6.1 %MVC). These findings suggest that reducing muscle stiffness by static stretching in one muscle can enhance MU recruitment in the other synergist while the change was slightly small.