Reaction time and electromyographic activity during a sprint start

Summary

Eight male sprinters were filmed running three maximal starts over 3 m on a long force platform. The subjects were divided into two groups (n=4) according to the leg on which the electromyograph (EMG) electrodes were fixed. When in the set position one group had electrodes on the front leg (FLG) and the other group on the rear leg (RLG). The EMG activities of the gastrocnemius caput laterale muscle (GA), vastus lateralis muscle (VL), biceps femoris caput longum muscle (BF), rectus femoris muscle (RF) and gluteus maximus muscle (GM) were recorded telemetrically using surface electrodes. Total reaction time (TRT) was defined as the time from the gun signal until a horizontal force was produced with a value 10% above the base line. Pre-motor time was defined as the time from the gun signal until the onset of EMG activity and motor time (MT) as the time between the onset of EMG activity and that of force production. Reproducibility of the reaction time variables was satisfactory (r=0.792–0.89; coefficient of variation= 8.8%–11.6%). The TRT was 0.121 s, SD 0.014 in FLG and 0.119 s, SD 0.011 in RLG. The MT ranged from 0.008 s, SD 0.009 (GM) to 0.057 s, SD 0.050 (GA) in FLG and from 0.018 s, SD 0.029 (GA) to 0.045 s, SD 0.009 (GM) in RLG. In some individual cases there were no MT values before horizontal force production. Significant positive correlations were observed between MT and maximal horizontal force and the velocity of the centre of gravity during the last observable contact on the blocks (P<0.05–0.01). The EMG activities of the muscles analysed demonstrated large individual variations until the end of the first contact after the blocks. This resulted in non-observable MT in some individual cases. In general, however, despite the complex multijoint character of TRT, its fractions could be analysed during the early phases on the blocks. To optimize starting action it is desirable that all the important muscles should be activated before any force can be detected against the blocks.