Is Lower Limb Movement Enough? Quantifying Overground Arm Swing Kinematics and Coordination to Assess Ageing Decline in Older Adults

Traditional clinical gait assessments focus on lower-limb kinematics and overall walking speed, often overlooking the upper-limb dynamics and inter-limb coordination that matter for real-world ambulation. Measuring these movements outside the laboratory is difficult, and this study presents and validates a wearable sensor algorithm to quantify arm swing kinematics and arm-leg coordination (Phase Locking Value, PLV) during overground walking in the home. Validated against optical motion capture, the algorithm detected swing events reliably and with negligible temporal bias. In home-based gait recordings from nearly 1500 community-dwelling older adults, arm-leg coordination was the strongest arm swing predictor of rhythmic gait stability once walking speed was accounted for. Exploratory factor analysis separated upper-limb function into distinct "coordination and stability'" and "pace and capacity'' axes, identifying arm swing as an independent dimension of the gait profile. Analysis of dynamic resilience during turns showed that frail older adults have slower recovery of arm-leg coordination, pointing to a loss of motor automaticity. Across a 30-year age span, arm swing amplitude declined with age while arm-leg coordination did not change detectably. In a separate laboratory cohort, coordination was also reduced in Parkinson's disease, indicating that the measure responds to neurological impairment as well as to frailty. This algorithm offers a scalable way to assess upper-limb gait dynamics in daily life. Shifting the clinical focus from walking speed alone to full-body movement may help detect instability early.