• António P. Veloso
  • P. Pezarat-Correia
  • P. Armada
Keywords: joint moments, drop jump (DJ), load, electromyogram (EMG), stretch reflex, muscle stiffness


INTRODUCTION: Drop jump (DJ) exercises from different dropping heights are often used to obtain important data about the degree of adaptability of the locomotor system to increasing amounts of mechanical load. The purpose of this study was to analyze the adaptation of the neuromuscular system to mechanical load increases. These neuromuscular adaptations where related to the joints’ moments of force and to relative changes in the length of the leg extensor muscles. METHODS: Ten elite sprinters (height: 182±5.0 cm, body mass: 75.3±4.5 kg, best performance over 100 meters: 10.4±0.2 s) performed 6 DJs from 25, 40, 55 and 70 cm. EMG signals and ground reaction forces were recorded at 1000 Hz. EMGs from the tibialis anterior (TA), soleus (SOL), gastrocnemius (GAS), rectus femoris (RF), vastus medialis (VM), biceps femoris (BF) and gluteus maximus GM muscles were full wave rectified and integrated (iEMG) across different functional phases: pre-activation (PRE) (100 ms prior to contact), reflex induced activation phase (RIA) (from 20 ms to 100 ms after contact) and late EMG response phase (LER) (from 100 ms to the finish). Vertical peak and vertical net impulses were calculated. Simultaneously, angular kinematic data on the ankle, knee and hip joints were calculated using a video analysis system (120 Hz). Joint net moments from the hip, knee and ankle were calculated using an inverse dynamics method. Student’s T-tests were used to analyze the effects of drop jump height. RESULTS: The stretching and shortening velocities increased in all muscles with an increase in DJ height. Nevertheless, no differences were found in vertical jumping performance between DJs executed from different heights. The subjects were able to reduce the increasing stretching load, producing higher vertical net impulses during the downward movement. The percentage of total length changes of GAS, RF, VM and SOL increased with stretching load, average values increasing from 4.7% at DJ25 to 7.8% at DJ70; these values are inside the short range elastic stiffness. The iEMG of the RIA phase increased slightly with the increase of dropping height for VM, RF, GAS, SOL. For these muscles, the RIA phase presented higher iEMG values when compared with LER. For the four jumping heights, the peak values of hip, knee and ankle joint moments were similar. Nevertheless, a different joint moment/time curve pattern was obtained. On DJ70, the peak values of joint moments of the three joints were obtained earlier, coinciding with the stretching phase of the muscles studied. For DJ70, a decline in the value of the moments of force on the three joints was observed during the ascending phase. CONCLUSION: The athletes studied were able to resist increasing stretching speeds and forces during the braking phase. This ability was related to the increased capability of maintaining high levels of stretch reflex during the RIA phase, revealing the importance of the stretch reflex effect on increasing muscle stiffness.