An arm swing enhances the proximal-to-distal delay in joint extension during a countermovement jump

Abstract An abundance of degrees of freedom (DOF) exist when executing a countermovement jump (CMJ). This research aims to simplify the understanding of this complex system by comparing jump performance and independent functional DOF (fDOF) present in CMJs without (CMJ NoArms ) and with (CMJ Arms ) an arm swing. Principal component analysis was used on 39 muscle forces and 15 3-dimensional joint contact forces obtained from kinematic and kinetic data, analyzed in FreeBody (a segment-based musculoskeletal model). Jump performance was greater in CMJ Arms with the increased ground contact time resulting in higher external ( p = 0.012), hip ( p < 0.001) and ankle ( p = 0.009) vertical impulses, and slower hip extension enhancing the proximal-to-distal joint extension strategy. This allowed the hip muscles to generate higher forces and greater time-normalized hip vertical impulse ( p = 0.006). Three fDOF were found for the muscle forces and 3-dimensional joint contact forces during CMJ NoArms , while four fDOF were present for CMJ Arms . This suggests that the underlying anatomy provides mechanical constraints during a CMJ, reducing the demand on the control system. The additional fDOF present in CMJ Arms suggests that the arms are not mechanically coupled with the lower extremity, resulting in additional variation within individual motor strategies.