Influence of Maximal Strength on Joint-Specific Function and Coordination Across Stretch-Shortening Cycle Task Demands
Abstract
Jumping involves the stretch-shortening cycle (SSC) and is a familiar movement in many sports. More explosive jumping may provide a competitive advantage, but the biomechanics that relate to optimal SSC utilization for performance are not well understood. Furthermore, while strength relates to better jump performance and may facilitate mechanisms like joint stiffness that benefit SSC utilization, how strength impacts adjustments to biomechanics and locomotor functional roles of the lower extremity joints under different SSC demands has not been established. Therefore, the purpose of this dissertation was to investigate how strength influences biomechanics and functional roles of the ankle, knee, and hip in male and female athletes across jumping tasks. Forty-five NCAA and collegiate club level athletes, including 23 males and 22 females, performed drop jumps (DJ) from four different box heights while movement and ground reaction force data were collected with three-dimensional motion capture and force plates, respectively. Athletes’ peak vertical ground reaction force was also tested during an isometric midthigh pull as the measure of strength. In the first study, locomotor functional roles at the three lower extremity joints during a 12-inch DJ were described, related to strength, and assessed for their ability to predict DJ performance using cross-validated elastic net regression. In the second study, cluster analysis of functional roles across three standardized box heights (i.e., 12, 18, 24 inches) was performed to identify subgroups of athletes with similar joint function profiles and better understand how strength drives performance and biomechanical adjustments to SSC task demands. In the last study, the influence of strength on sex differences in performance, biomechanics, and joint functional roles was examined using a DJ box height matched to each athlete’s maximal vertical jump height. The key findings of this dissertation were that 1) strength facilitates motor-like joint function and the generation of mechanical energy is a primary determinant of DJ performance, 2) greater strength helps joints maintain strut-like stiffness and jump performance with increased SSC task demands, and 3) sex differences in size and strength underpin sex differences in DJ performance and biomechanics but do not encourage distinct functional roles at the lower extremity joints.