The Biomechanical Insights Into Acceleration Phase And Maximum Velocity Phase Of Sprinting

Purpose: Sprinting is an event requiring athletes to cover a relatively short distance as quickly as possible and to develop the maximum strength and power of lower extremities. It is made up various phases. Sprinting success relies on initially performing a fast start during the acceleration phase and then achieving and maintaining the high velocity during the maximum velocity phase. The biomechanical parameters are different between phases, and thus the physical characteristics required are diverse. Most of previous studies focused on one phase of sprinting, only few study investigated differences between various phases. The purpose of this study is to investigate the differences in biomechanical parameters and neuromuscular control during the acceleration and maximum velocity phases of full-effort sprinting. It was hoped that the information gathered here would provide insights into the physical characteristics required of a successful athlete, and training programs that would foster these characteristics.Method:VICON motion analysissystem(sample rate: 200 Hz, 12 cameras), KISTLER force plates(1000Hz, 3 force plates) and DELSYS wireless system(4000Hz, 7sensors) were used to collect the kinematic, ground reaction force(GRF) and EMG data of 20 sprint athletes during the acceleration and maximum velocity phase. The starting line was 12 meter and 40 meter from the first force plate for acceleration phase and maximum velocity phase, respectively. Visual 3D software was used to filter kinematic and GRF data and build 15 segments human body model to compute the velocity of center of mass. The cut-off frequency of low-pass filter was 12 Hz and72 Hz for kinematic and ground reaction force, respectively. Intersegmental dynamics analysis(ISD) during one stride was performed using a custom-written program prepared by C#. EMG data were filtered and rectified in DELSYS software firstly,and then inputted into a custom-written program prepared by C# to compute root mean square(RMS) in various periods. Paired-sample t-tests were used to determine differences inkinematic, kinetic and EMG variables between the accelerationandmaximum velocity phases.The level of significance level was set at p < 0.05, and was adjusted applying a Bonferroni correction for different data sets.Results: There were significant differences in running velocity, stance duration and stride length between the two sprint phases. For GRF variables, the ratio of braking impulse and propulsive impulse was 1:4 during the acceleration phase. However,propulsive impulse was slightly greater than braking impulse during the maximum velocity phase. The peak braking force was significantly smaller during the acceleration phase than maximum velocity phase. No significant differences were observed in peak propulsive force. The timing of force peaks were similar between the two sprint phases. Peak braking force occurred at about 10% of stance stage, and peak propulsive peak occurred at 72% of stance stage. The peak vertical force was greater during the maximum velocity phase than acceleration phase, but the vertical impulse was not significantly different between the two phases. Timing of peak vertical force was significantly different. The timing was 31% of stance stageduring the maximum velocity phase, and 37% of stance stage during the acceleration phase.For intersegmental dynamics, the muscle torque(MUS) of lower extremity mainly counteracted external force torque(EXT) during the stance stage; MUS of lower extremity mainly counteracted internial torque(INT) during the swing stage. The hip flexion and knee extension MUS peak at 10% of stance stage, knee extension and ankle plantarflexion MUS peak at 30-40% of stance stage and hip extension MUS peak at the end of swing stage were significantly different between the acceleration and maximum velocity phase. The MUS peak values were greater during the maximum velocity phase than acceleration phase. The RMS of mainly contributing muscles of various stages were significantly different between the acceleration and maximum velocity phase: medial head of gastrocnemius at the stance stage, rectus femoris and tibialis anterior at the forward swing stage, and biceps femoris at the backward swing stage.Conclusions: The acceleration in center of mass of body during the acceleration phase was caused by smaller braking force, not greater propulsive force. This indicated that the technology training aimed at enhancing the accelerationperformance should focus on reducing horizontal braking force during the acceleration phase. For motor control, MUS mainly counteracted EXT caused by GRF.Differences in the MUS peak values at 10% and 30-40% of stance stage between the two sprint phases were related to differences in horizontal braking force peak value and vertical force peak value, respectively. At stance stage of the maximum velocity phase, the activation of gastrocnemius was greater to counter stronger landing impact caused by larger vertical force peak. At the forward swing stage of the maximum velocity phase, the greater activation of rectus femoris made greater hip flexion MUS to counteract greater hip extension EXT. At the backward swing stage of the maximum velocity phase, the greater activation of biceps femoris made greater hip extension MUS to counteract greater hip flexion EXT. All these findings had guiding meaning for sprint running training.