The Effect Of Shank Additional Mass On The Recruitment Characteristic Of Lower Extremity Muscles During Maximum Speed Sprint

The study of sprint dates back to 1920,and with years of research the biomechanical factors that affect sprint,including the muscle activity,are increasingly clear.Previous studies have found that there is a close relationship between running speed and muscle activity,and the nervous system can change the performance by controlling the preactivity and co-contraction of muscles.These indicators can reflect the control adjustments made by the nervous system for changes in the environment(e.g.load).Resisted training is a common method to enhance the sprint speed.Adding a certain mass to the distal extremity as an external load is considered to be able to maximize the performance without changing the kinematics characteristics.But the changes of muscle activity in such training and how the nervous system will be adjusted remains to be studied.The purpose of this study was to investigate the effect of three additional mass conditions on the electromyography of lower extremity.Methods: Three-dimension motion capture system(200Hz)and force plate(1000Hz)were used to record the kinematics and kinetics data of 13 elite sprint athletes at their maximum speed.Electromyography of four muscles(Rectus Femoris,Vastus Lateralis,Vastus Medialis,Biceps Femoris)were recorded at the same time through a trigger synchronizer.Result: The results indicated that(1)With the increment of additional mass the step frequency and running speed were decreased(P<0.05),the time of support phase increased either.(2)In the pre-contact phases,the Root Mean Square EMG of Rectus Femoris increased which is contrary to the other three muscles.Besides,the cocontraction level enhanced during this phase.(3)The activity contribution ratio of Rectus Femoris decreased significantly(P<0.05)in the swing ahead phase between without and 10% additional mass conditions.To Biceps Femoris,the activity contribution ratio of propulsion phase increased both in two additional mass conditions(P<0.05,P<0.05).Conclusions: This resisted-training method may exercise the contraction ability of special muscle for sprint effectively,which turns out will advance athletic performances.Furthermore,Nervous system makes corresponding adjustments to the additional mass of lower limb.It presented as the increment of co-contraction level and the changes of muscle activity contribution ratio under different phase.