Sprint Acceleration Mechanics: The Major Role of Hamstrings in Horizontal Force Production

Abstract

Recent literature supports the importance of horizontal ground reaction force (GRF) production for sprint acceleration performance. Modeling and clinical studies have shown that the hip extensors are very likely contributors to sprint acceleration performance. We experimentally tested the role of the hip extensors in horizontal GRF production during short, maximal, treadmill sprint accelerations. Torque capabilities of the knee and hip extensors and flexors were assessed using an isokinetic dynamometer in 14 males familiar with sprint running. Then, during 6-s sprints on an instrumented motorized treadmill, horizontal and vertical GRF were synchronized with electromyographic (EMG) activity of the vastus lateralis, rectus femoris, biceps femoris, and gluteus maximus averaged over the first half of support, entire support, entire swing and end-of-swing phases. No significant correlations were found between isokinetic or EMG variables and horizontal GRF. Multiple linear regression analysis showed a significant relationship (P = 0.024) between horizontal GRF and the combination of biceps femoris EMG activity during the end of the swing and the knee flexors eccentric peak torque. In conclusion, subjects who produced the greatest amount of horizontal force were both able to highly activate their hamstring muscles just before ground contact and present high eccentric hamstring peak torque capability.

Keywords: isokinetics; muscle; neuromuscular; performance; sprint kinetics; surface electromyography.

Figure 1

Raw EMG signals for vastus lateralis (VL), gluteus maximus (Glut), biceps femoris (BF), and rectus femoris (RF) muscles, synchronized with vertical GRF during a typical sprint stride (7th stride of a maximal 6-s acceleration). The following phases for EMG analysis were determined from vertical GRF data (30 N threshold): entire swing, end-of-swing, entire stance, and first half of the stance.

Figure 2

Typical foot (circles) and knee (triangles) path diagram during an entire sprint treadmill acceleration of a rugby player. The stride at maximal running velocity (9th stride in this trial) is shown with black circles and triangles. Positions are displayed with reference to the treadmill belt frame (vertical axis) and the subject's resting standing position was used for calibration (femoral marker in the horizontal axis). Position 1 indicates the maximal forward position of the foot and knee as retained for analyses. Position 2 indicates the initial foot-ground contact. The path of the foot marker from position 1 to position 2 is considered as the “pawing” backward motion of the foot at the end of the swing phase.

Figure 3

Average EMG activity of the VL, RF, BF, and Glut muscles over the entire sprint acceleration (all steps), for the different phases of the step cycle. Error bars indicate standard deviation. VL, vastus lateralis; RF, rectus femoris; BF, biceps femoris; Glut, gluteus maximus; MVIC, maximal voluntary isometric contraction. The muscle effect on EMG activity was tested by One-way analyses of variance (^*, ANOVA significant main effect with P < 0.05; ns, not significant) and Newman Keuls post-hoc tests (a, significantly different from RF; b, significantly different from BF; c, significantly different from Glut).

Figure 4

Average EMG activity (±SD) of the VL, BF, and Glut muscles during the first half of the stance and the end-of-swing phase for all the right leg steps of the sprint acceleration. VL, vastus lateralis; RF, rectus femoris; BF, biceps femoris; Glut, gluteus maximus; MVIC, maximal voluntary isometric contraction. The effect of “step number” over the entire acceleration were tested by One-way analyses of variance: ^*, ANOVA significant main effect (P < 0.05); ns, not significant.