Increasing Robotic Horizontal Resistance Alters Pair-Specific Intermuscular Timing During Sprint Acceleration

Chang, F, Zhang, Y, Qian, D, Liu, T, and Marshall, P. Increasing robotic horizontal resistance alters pair-specific intermuscular timing during sprint acceleration. J Strength Cond Res XX(X): 000-000, 2026-The purpose of this study was to determine whether increasing horizontal resistance alters pair-specific intermuscular timing and coupling during sprint acceleration. Fifteen university sprinters, 8 men and 7 women, performed maximal 30-m accelerations under 4 randomized robotic horizontal resistance conditions of 0, 5, 10, and 15% body weight (BW). Surface electromyography (EMG) was recorded from gluteus maximus (GMax), biceps femoris (BF), and gastrocnemius medialis (GM). All EMG channels shared a common acquisition time base. Gluteus maximus onset was used to define a common 300-ms analysis window, and normalized cross-correlation quantified temporal lag at peak correlation and coupling strength for the GMax-BF and GMax-GM pairs within a fixed ±100-ms lag range. Increasing resistance produced a greater GMax-GM lag at 15% than at 0% BW (paired mean difference 29.6 ms, 95% confidence interval: 14.1-45.1 ms; Holm-adjusted p = 0.006; Hedges' g = 0.90). No corrected GMax-BF pairwise contrast was identified, and coupling strength was unaffected by load in either muscle pair. For practitioners, resisted-sprint load selection may therefore influence the acute intermuscular timing demands imposed during acceleration, although the present results do not identify an optimal load or establish transfer to unresisted sprint performance.