Lack of Evidence for Crossover Fatigue with Plantar Flexor Muscles

The occurrence and mechanisms underlying non-local or crossover muscle fatigue is an ongoing issue. This study aimed to investigate crossover fatigue of the plantar flexor muscles. Sixteen recreationally active males (n = 6) and females (n = 10) visited the laboratory for four sessions and performed a single 5-s pre-test maximal voluntary isometric contraction (MVIC) with each plantar flexors muscle. Thereafter, the fatigue intervention involved two 100-s MVICs (60-s recovery) with their dominant plantar flexors or rested for 260-s (control). Subsequently, in two separate sessions, Hoffman reflexes (H-reflex) were evoked in the non-dominant, non-exercised, leg before and following the dominant leg fatigue or control intervention (Fatigue-Reflex and Control-Reflex conditions). MVIC forces and volitional (V)-waves were monitored in the non-dominant leg in the other two sessions (Fatigue-MVIC and Control-MVIC) before and after the intervention (fatigue or control) as well as during 12 repeated MVICs and immediately thereafter. Despite the force reduction in the dominant leg (42.4%, p = 0.002), no crossover force deficit with single (F_(1,9) = 0.02, p = 0.88, pƞ = 0.003) or repeated (F_(1,9) = 0.006, p = 0.93, pƞ) MVIC testing were observed. The H-reflex did not change after the fatigue (F_(1,7) = 0.51; p = 0.49; pƞ 0.06) or repeated MVICs (F_(1,8) = 0.27; p = 0.61; pƞ 0.03). There were also no crossover effects of fatigue on the V-wave with single (F_(1,8) = 3.71, p = 0.09, pƞ) or repeated MVICs (F_(1,6) = 1.45, p = 0.27, pƞ). Crossover fatigue was not evident with the plantar flexors nor any significant changes in H-reflex and V-waves in the soleus muscle. This finding suggests that crossover fatigue may not necessarily occur in slow-twitch predominant muscle groups.