Peripheral Neuromuscular Fatigue Responses of the Knee Extensors to Distinct Concurrent Training Protocols: A Preliminary Study

Background: This study aimed to investigate the extent and time course of peripheral neuromuscular fatigue of the knee extensors following different concurrent training protocols in recreationally active men. Methods: In a randomized, counterbalanced, crossover design, ten participants completed one exercise session of three concurrent exercise protocols in consecutive weeks and in similar resting conditions: traditional concurrent training (TCT), sprint interval training (SIT), and high-intensity resistance circuit training (HRC). Maximal voluntary isometric contraction (MVIC) and electrically evoked tetanic force of the knee extensors were assessed before, immediately after, and at 24 and 48 h following each exercise session. Linear mixed models were used to examine the differences among exercise modalities and time points. Results: No significant changes were found in MVIC force following HRC and TCT at any time point (p > 0.05), while significant declines were observed post-exercise (p = 0.015), 24 h (p = 0.001) and at 48 h (p = 0.003) after SIT. Moreover, MVIC force was significantly lower for SIT than HRC at 48 h (p = 0.001). Tetanic force significantly declined in SIT from pre-exercise to post-exercise (p = 0.034), with significant differences when compared to HRC (p = 0.003) and TCT (p = 0.003). HRC and TCT induced no knee extensor fatigue, contrary to a single session of SIT. Conclusions: Peripheral fatigue seemed to prevail following SIT in comparison with HRC and TCT, as seen by the decreased tetanic force in the former only. From an applied perspective, practitioners should carefully plan training activities to be performed the days following a SIT session, as force-generating capacity may be impaired for up to 48 h.