Potentiation of maximal voluntary concentric torque in human quadriceps femoris

Purpose: Previous studies have shown that one bout of 6-s maximal voluntary contraction (MVC) can enhance subsequent dynamic joint performance with maximal voluntary effort. However, the conditioning contraction also induces central fatigue, which attenuates the enhancement of the subsequent voluntary joint performance. Here, as a modality for minimizing the fatigue while enhancing joint performance, we examined the effects of one bout of a short-duration isometric MVC and neuromuscular electrical stimulation (NMES). Methods: In the first experiment, the optimal duration of NMES for inducing twitch potentiation of the quadriceps femoris muscle was determined, which was found to be 5 s. Then in the second experiment, 13 subjects performed maximal voluntary isokinetic concentric knee extensions at 210°•s^-1 in the following sequence: before and immediately after each of the 3-, 5-, and 10-s MVCs and 5-s NMES of knee extension and 1, 3, and 5 min thereafter. Results: When the isometric MVC was used as a conditioning contraction, the maximal voluntary concentric torque was significantly enhanced at 1 and 3 min after MVC only in the 5-s MVC trial (106.6% ± 2.3% and 107.2% ± 2.6% of the initial value, respectively) but not in the 3-and 10-s MVC trials. In the 5-s NMES trial, the voluntary concentric torque was significantly increased immediately after the conditioning contraction (105.1% ± 2.2%) as well as 1 and 3 min thereafter (107.5% ± 3.3% and 107.8% ± 2.7%, respectively). Conclusions: These results suggest that conditioning contraction of around 5 s, performed with isometric MVC or NMES, can be a modality to enhance dynamic voluntary joint performance, with the latter having a more immediate effect.