Effects of drop jump height and technique on ground reaction force with possible implication for injury

The purpose of this study was to quantify peak vertical ground reaction force (VGRF) and loading rate during performance of a drop jump when (a) attempting to jump for maximum height (DJ‐H) and (b) attempting to jump for maximum height and minimum ground contact time (DJ‐H/t), both common training techniques for athletes in a variety of sports. Peak VGRF's were between 7.59 and 9.37 times bodyweight for the DJ‐H/t technique, which was approximately twice the magnitude of the DJ‐H technique. The maximum rate of loading was also greater for DJ‐H/t. In an earlier related study it was observed that 3 subjects who trained with the DJ‐H/t method experienced lower limb pain, which prevented them from further training. These dropouts were compared to non‐dropouts on factors related to their pre‐training level of leg strength and training drop height. An index of drop height divided by leg strength was found to be significantly greater (p<0.05) for the dropouts. It was concluded that training to improve the DJ‐H/t technique imposes a relatively high stress on the leg musculo‐skeletal system. Alternatively, using a relatively high drop height in conjunction with a low pre‐training leg strength level also increases the risk of injury. Requiring an athlete to qualify for drop jump training by exceeding a standard passing score on the drop jump/leg strength index might enable strength and conditioning coaches to limit the injury risk during such plyometric training.