Slow walking synergies reveal a functional role for arm swing asymmetry in healthy adults: a principal component analysis with relation to mechanical work.

Introduction: The purpose of this study was to reveal a functional role for arm-swing asymmetry during gait in healthy adults. The primary aim was to identify differences in propulsive and collision work between sides at either end of the double-support phase of slow-walking (WDS). The secondary aim was to identify differences between sides in propulsive and collision work done at either end of the single-support phase (WSS) and the effect of arm-swing asymmetry on this difference. It was hypothesized that differences between sides would be evident during the double-support phase and that these differences would be coherent with differences in single-support control symmetry. It was also hypothesized that left-side dominant arm-swing would reduce the collision work done on the dominant lower-limb side. Methods: A secondary analysis of slow-walking trials of 25 healthy, uninjured adults was undertaken where a principal component analysis of kinematic data was carried out to generate the movement synergies (PMk). Independent variables included the tightness of neuromuscular control (Nk) which was formulated from the first PMk and arm-swing asymmetry which was quantified using the directional Arm-swing asymmetry index (dASI). Dependent variables included the difference between double-support collision and propulsive work (WDS) and a ratio consisting of the difference between single-support collision and propulsive work of both sides (WSS). A linear mixed-effects model was utilized for aim 1 while a multiple linear regression analysis was undertaken for aim 2. Results: Healthy adult gait was accompanied by a left-side dominant arm-swing on average as seen elsewhere. For aim 1, Nk demonstrated a significant negative effect on WDS while sidedness had a direct negative effect and indirect positive effect through Nk on WDS. The most notable finding was the effect of a crossover interaction between dASI and Nk which demonstrated a highly significant positive effect on WSS. All main-effects in aim 2 were in the hypothesized direction but were insignificant. Interpretation: The aim 1 hypothesis was supported while the aim 2 hypothesis was not supported. Nk exhibited opposing signs between ipsilateral and contralateral WBAM regulation, revealing a differential control strategy while the effect of sidedness on WDS was evident. The findings from aim 2 describe a relationship between arm-swing asymmetry and the magnitude of lower-limb mechanical work asymmetry that is cohesive with the sidedness effect found in aim 1. Individuals with left-side dominant arm-swing had an increased collision work indicative of a lateralised preference for WBAM regulation. Evidence was therefore put forward that arm-swing asymmetry during gait is related to footedness. Future studies should look to formally confirm this finding. Implications for further research into dynamic balance control mechanisms are also discussed.