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

## TL;DR

Swinging arms during a jump improves performance by changing how joints extend, allowing for more forceful and efficient movement.

## Contribution

The study reveals how arm swings influence joint coordination and reduce control demands during jumps.

## Key findings

- Arm swings increase jump performance by enhancing hip and ankle vertical impulses.
- Arm swings introduce an additional functional degree of freedom, indicating less mechanical coupling with lower limbs.
- Hip extension is delayed, allowing higher force generation and more efficient movement patterns.

## 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 (CMJNoArms) and with (CMJArms) 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 CMJArms 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 CMJNoArms, while four fDOF were present for CMJArms. This suggests that the underlying anatomy provides mechanical constraints during a CMJ, reducing the demand on the control system. The additional fDOF present in CMJArms suggests that the arms are not mechanically coupled with the lower extremity, resulting in additional variation within individual motor strategies.