Effect Of Countermovement Depth On The Neuromechanics Of A Vertical Jump

Abstract

PURPOSE: The purpose of this study was to examine kinematic, kinetic and muscle activation metrics during countermovement jumps (CMJs) with varying countermovement depths. The hypothesis was that a shallow countermovement depth would compromise jump height by disrupting neuromechanical control. METHODS: Ten healthy men (age 26±8 yr, height 1.81±0.08 m, mass 83.5±9.0 kg) performed maximal CMJs at self-selected countermovement depth (self-selected CMJ), at reduced countermovement depth (shallow CMJ) and at increased countermovement depth (deep CMJ). Three jumps were performed in each condition on force plates with ankle, knee and hip motion recorded and electromyograms (EMG) recorded from the gluteus maximus (GM), vastus lateralis (VL) and medial gastrocnemius (MG) muscles. During CMJs knee flexion angle was recorded with an electrogoniometer. Jumpers were instructed to flex 15% less (shallow CMJ) and 15% more (deep CMJ) than the self-selected CMJs. Kinematic, kinetic and EMG metrics were compared between the different CMJ depths using repeated measures ANOVA. RESULTS: Compared with self-selected CMJs, shallow CMJs had 26% less countermovement depth (9.5 cm less, P<0.001) and the deep CMJs had 28% greater countermovement depth (10.2 cm more, P<0.001). Jump height was 8% less for the shallow vs. self-selected CMJs (0.344 m vs. 0.373 m, diff 2.9±2.5 cm, P=0.007) but not different between self-selected and deep CMJs (0.373 m vs. 0.378 m, diff 0.05±0.1.9 cm, P=0.254). Shallow CMJs differed from self-selected CMJs at the initiation of the countermovement (unweighting phase). For self-selected CMJs force dropped to 43% of body weight during unweighting but only to 58% for shallow CMJs (P=0.015). During unweighting VL EMG averaged 5.5% of MVC during self-selected CMJs versus 8.1% for shallow CMJs (P=0.014). Percent decline in jump height with shallow versus self-selected CMJs was correlated with the difference in VL EMG during unweighting between shallow and self-selected CMJs (r=0.651, P=0.041). CONCLUSIONS: Failure to deactivate the quadriceps during the unweighting phase of shallow CMJs compromised unweighting and resulted in a reduced jump height. These findings highlight the importance of unweighting at the initiation of the countermovement in dictating subsequent jump performance.