Lower extremity control and dynamics of landings with horizontal momentum redirection

During landings, task-specific multi-joint control strategies are implemented to regulate the magnitude and direction of the reaction forces relative to the total body center of mass (McNitt-Gray et al., 2001; Edwards et al., 2009). In athletic movements, landings are often followed by horizontal translation in sport-specific directions, i.e. land-and-go tasks. Landing tasks involving a rapid reduction in velocity in conjunction with a change in direction are often associated with injury of the lower extremity (Boden et al., 2000; Krosshaug et al., 2007). The current body of experimental knowledge on the control and dynamics of landings is primarily based on comparisons of drop landings or drop vertical jumps between gender, height (McNitt-Gray, 1991, 1993; Dufek and Bates, 1990), fatigue conditions (Gehring et al., 2009), and technique (Dufek and Bates, 1990; DeVita and Skelley, 1992), with a focus on only the impact phase. Landings involving horizontal momentum redirection have been studied using stop jumps (Yu et al., 2006; Chappell et al., 2002, 2005, 2007) and hopping tasks. Results of these studies are used to develop training programs and performance screening protocols aimed at identifying athletes whose landing strategies place them at a high risk of injury. However, while these studies provide valuable insight into control strategies during tasks involving large impact forces, there is mounting evidence that the landings investigated do not reflect the task objectives and load imposed during land-and-go tasks (Edwards et al., 2009). Additionally, while much is known about lower extremity control of the sagittal plane during landings, there is a paucity of research investigating how the ankle, knee, and hip work together to control lower extremity frontal plane dynamics. ❧ The over-arching goal of this research is to identify lower extremity control strategy modifications implemented by the neuromuscular system to accomplish different mechanical objectives during realistic landings. Volleyball block-and-go landings performed by skilled athletes provide a set of repeatable, well-practiced, goal-directed tasks that allow us to gain insight into the biomechanics of ecologically-valid landings with different directional contexts. To date, no investigation into the biomechanics of vertical to horizontal momentum redirection has been reported. ❧ The purpose of this study was to determine how and when subjects modify control and dynamics at the lower extremity subsystem level in order to accomplish different subsequent horizontal task requirements at the whole body level during landing. Six female volleyball players currently competing on a varsity team ranked in the top ten of Division I colleges participated in this study. Subjects performed a series of blocking tasks including block-and-stop, block-and-go-left, block-and-go-diagonal, and block-and-go back. Three-dimensional ground reaction forces, lower extremity kinematics, and lower extremity kinetics were collected to perform a within-subject analysis. We hypothesized that systematic modifications in horizontal target direction would result in modifications that were initiated prior to and during ground contact. Between-task differences in a) lower extremity kinematics prior to and during ground contact, b) ground reaction force magnitude, orientation, and between-leg distribution during impact, and c) distribution of mechanical demand in the frontal and sagittal planes during impact and push suggest that subjects modify feed-forward control strategies during flight based on subsequent horizontal task requirements. Similarities in ground reaction force magnitude between the impact and push phases indicate that realistic landings have two periods of high loading that occur within different kinematic and kinetic contexts. Reorientation of the ground reaction force in global space and increased flexion of the lower extremity during the push phase of block-and-go tasks result in increased mechanical demand in the frontal and sagittal planes as compared to the impact phase of block-and-stop. For all tasks, the frontal plane motion of the lower extremity is controlled primarily by the hip joint with minimal assistance from the ankle. The addition of a horizontal momentum redirection requirement resulted in increased out-of-plane knee motion during the block-and-go-diagonal and block-and go back tasks which included whole body rotation. ❧ Identification of strategies used to simultaneously regulate impact forces and generate horizontal momentum provides insight into how the nervous system prioritizes multi-joint control during complex, realistic landings. Future research should take advantage of the robust set of information to be garnered from these challenging land-and-go tasks in order to improve our understanding of lower extremity control during landing. A better understanding of hierarchical control strategies will be useful in creating successful targeted evaluation criteria and training programs for athletes who perform landings.