The Biomechanical Features Of The Lower Limb Muscles In Drop Jumping

Purpose: Drop jumping is a frequently used exercise to develop explosive muscle power of the lower leg. A research on the biomechanical features of the lower leg muscles in drop jumping not only enriches the theoretical system for power training but also satisfies the needs of practical training. Through an analysis and discussion on the kinematics, dynamics and the EMG activity of the eight muscles in the lower leg, set up a biomechanical features structure of the lower leg muscles in drop jumping.Methods: This study employed a 3D Kistler Force Measurer(500Hz), a TM-6710CL High-speed Video Camera(120f/s), and sixteen Wireless Telemetering EMG Testing Systems(1000Hz) to conduct the simultaneous testing on 12 third-class and 12 first-class athletes performing the drop jumping at different whereabouts heights, and in both normal and relative fatigue states (after heavy loading exercise).Results and conclusions:1. Before landing, the measured muscles in the lower legs have already been"pre-activated."In the normal state, the"pre-activation"strategies differ from person to person, and the individual's"pre-activation"modes of the lower legs do not change fundamentally with the change of whereabouts heights.2. In the landing stage, the buffer time is shorter than the extending time, and the change in hip angle is the minimum, whereas the range of ankle angle velocity change is the biggest. The knee-extending muscles in the lower legs play the leading role, and then come ankle muscles, hip stretching muscles. In the three muscles in the knee joints, the vastus medialis muscle and the vastus laterlis muscle play a more important role than the vastus intermedius muscle. At a moderate descending height, the buffer capacities and extending capacities give a fullest display, and the re-jump height reaches its peak value. At a higher whereabouts height, the buffer range of the joints in the lower legs increases and the buffer and coupling time prolongs, which affects the conversion velocity from centrifugal contraction to centripetal contraction and the smooth completion of the action.3. With the increase of the whereabouts height, the earlier the peak force value appears, the smaller the ratio to the time in the landing period. At a higher whereabouts height, the most conspicuous feature of the peak value change is that the higher peak value appears and declines rapidly with the force value. However, at a moderate height, the force value at the moment of buffer finishing, which reflects the buffer capacity and extending strength of the muscles in the lower legs, reaches its peak. The centrifugal contraction capacity of the lower legs plays a very important role in the stretch-shortened cycle of muscle. The key for the drop jumping training is to emphasize the"yielding"capacity of the lower leg muscles, and attach importance to the combination of the"yielding"capacity and high-speed extending capacity of the lower leg muscles in practical training.4. The RMS of EMG for most of the measured muscles of lower limb in the buffer period and the average IEMG of EMG are both bigger than those in the extending period. During the normal state, the same athlete's active mode of EMG of the muscles measured at different whereabouts height remains the same; it does not change essentially with the changes in whereabouts heights. When different athletes perform drop jumping at the same whereabouts height, the active strength of EMG and changing tendency differ, although the same muscle performs the same function in completing the same intensity work.5. During the relative fatigue state (after completion of heavy loading exercise), the touchdown time, buffer time and coupling time extend; re-jump height decreases tremendously. Compared with that in the normal state, the movement sequence of hip, knee and ankle changes, with the knee movement becoming the last. The MPF of EMG of the muscles measured in the lower limb declines. Changes also happen to the forcibly features, the"pre-activation"modes of the eight tested muscles, EMG movement modes, and the modes of working ratio in the course of movement, and the coordinating modes between muscles. The strength of the lower limb muscles decreases obviously.6. In terms of biomechanical features, the difference between the third-class and first-class athletes is caused by the fact that the third-class athletes'basic strength of the lower limb and the capacity of completing the movement are lower than those of the first-class athletes. The effective approaches to enhance the outcomes of drop jumping practice is to stress the basic strength exercise, and improve the reflective coordinating ability of the central nerves system on lower limb muscles, and to increase the whereabouts height gradually.7. The"moderate"whereabouts height for drop jumping should be determined by the training objectives and athletes'basic strengths of their lower limbs. Deciding on appropriate proportion of various whereabouts heights for drop jumping training, scientific arrangement of the amount and proper intervals between groups of movements as well as pre-training preparation and post-training recovery activities are crucial to drop jumping. Drop jumping in a relative fatigue state, especially after heavy workload, should be avoided.8. Moderate whereabouts height, the rapid and smooth connection between buffer and extending activities, and a high re-jump height are key criteria for assessing the quality of drop jumping performances.