Differences in Lower Extremity Kinematics Based On Drop Vertical Jump Task Variations

Abstract

Background: More advanced measures of movement quality may be necessary to better estimate return-to-play readiness. This emphasizes the need for a standardized motion analysis protocol to capture patients’ biomechanics during dynamic tasks, such as the drop vertical jump (DVJ). Given that the DVJ elicits a dynamic, game-like movement, it is commonly used in return-to-play functional testing and injury-risk research. However, the task procedure is not consistent across treatment and/or research centers. Hypothesis/Purpose: The purpose of this study was to determine whether variations in jump distance, verbal instructions, or target use would influence mechanics during a DVJ task. Methods: Fourteen healthy subjects (7 female, aged 22.7±3.7 years) were asked to perform six DVJ tasks from a 31cm plyo-box. Lower extremity kinematics were collected during the first landing/takeoff phase. Three tasks varied by jump distance (distance from box to center of force plates): zero distance (DROP), one-third (THIRD) and one-half (HALF) subject height. Additional verbal instructions were provided for the DROP task. Specifically, subjects were asked to first “drop off the box onto the force plates” (DROP), and then to “slightly bend their knees to ‘pop-off’ the box with both feet at the same time” (POP). Lastly, a target was positioned above the force plates for the HALF (H-TAR). Wilcoxon signed-rank tests were performed to assess differences between tasks ( α=0.05). Results: At initial contact (IC), knee flexion increased in the DROP (24.0˚±18.4˚, p=0.011) and THIRD (16.3˚±7.9, p=0.005) compared to the HALF (12.0˚±6.8˚). During the landing phase, maximum hip flexion increased in the HALF (106.0˚±27.5˚) compared to the DROP (100.6˚±25.0˚, p=0.011). Average pelvic tilt decreased in the DROP (32.1˚±4.8) compared to both the POP (37.8˚±5.7˚, p=0.009) and THIRD (34.5˚±6.2˚, p=0.008). When using a target, knee flexion increased at IC during the H-TAR (H-TAR:18.8˚±8.8˚, HALF:12.0˚±6.8˚, p=0.004). Conclusion: These findings emphasize the importance of understanding movement strategies associated with different variations of the DVJ. Specifically, a shift in landing strategy was seen as box distance increased. Therefore, the DROP may be more appropriate early in rehabilitation given that maximum hip flexion was reduced. In contrast, the HALF may be more appropriate for return-to-sport testing given that patients would exhibit improved neuromuscular motor control. These findings also highlight the need for standardized instructions and task setup. While this pilot data was collected in young adults, future work will need to focus on evaluating these task variations in a youth population. [Table: see text]