Changes in biomechanical properties during drop jumps of incremental height.

The purpose of this study was to investigate changing biomechanical properties with increasing drop jump height. Sixteen physically active college students participated in this study and performed drop jumps from heights of 20, 30, 40, 50, and 60 cm (DJ20-DJ60). Kinematic and kinetic data were collected using 11 Eagle cameras and 2 force platforms. Data pertaining to the dominant leg for each of 3 trials for each drop height were recorded and analyzed. Statistical comparisons of vertical ground reaction force (vGRF), impulse, moment, power, work, and stiffness were made between different drop jump heights. The peak vGRF of the dominant leg exceeded 3 times the body weight during DJ50 and DJ60; these values were significantly greater than those for DJ20, DJ30, and DJ40 (all p < 0.004). The height jumped during DJ60 was significantly less than that during DJ20 and DJ30 (both p = 0.010). Both the landing impulse and total impulse during the contact phase were significantly different between each drop height (all p < 0.036) and significantly increased with drop height. There were no significant differences in the takeoff impulse. Peak and mean power absorption and negative work at the knee and ankle joints during DJ40, DJ50, and DJ60 were significantly greater than those during DJ20 and DJ30 (all p < 0.049). Leg, knee, and ankle stiffness during DJ60 were significantly less than during DJ20, DJ30, and DJ40 (all p < 0.037). The results demonstrated that drop jumps from heights >40 cm offered no advantages in terms of mechanical efficiency (SSC power output) and stiffness. Drop jumps from heights in excess of 60 cm are not recommended because of the lack of biomechanical efficiency and the potentially increased risk of injury.