Effects of resistance on spatiotemporal variables and electromyographic activity of the hamstrings during maximal sprint acceleration.

This study aimed to examine how horizontal resistance affects spatiotemporal kinematics and the electromyographic (EMG) activity of the biceps femoris long head (BFlh) and semitendinosus (ST) in sprinting.Eleven trained male football players (n = 10) performed 40 m sprints under three resistance conditions (0, 10, and 20% decrements in maximum sprint velocity, %vdec). Spatiotemporal variables were captured using a photoelectric system, while high-density surface EMG recorded regional muscle activity in BFlh and ST. Analyses were conducted at both maximum speed and matched running speed across conditions to isolate the effects of resistance and velocity. Regional EMG distribution (centroid analysis) was evaluated in a subset of participants with sufficient signal quality (n = 5).At matched speed, spatiotemporal variables showed no differences across resistance levels, however ST EMG activity was higher at 0 %vdec condition compare to 10 %vdec condition during the swing phase (70.50-75.70% P = 0.001). At maximum speed, moderate differences were found in step length and frequency. ST EMG activity was higher at 0 %vdec compare to 20 %vdec during stance phase (10.22-18.02% P = 0.001) and 0 %vdec compare to 10 %vdec during swing phase (68.50-71.00% P = 0.004). Centroid analysis revealed individual-specific shifts in the region of the highest EMG amplitude within the muscles.Moderate horizontal resistance has no or small impact on global hamstring EMG activity or sprint mechanics at matched speed but may induce individual shifts in regional EMG activity. Future studies should examine how these acute neuromuscular responses translate into long-term neuromuscular adaptations.