The effect of arm swings on lower limb kinetics during single-leg forward, vertical, and backward hopping

Single-leg forward hopping was commonly used to evaluate knee function and quadricep strength deficits following anterior cruciate ligament (ACL) injuries. Achieving similar hopping performance between legs was insufficient to support symmetrical knee function and quadricep strength in patients following ACL injuries, likely due to movement compensation. This study aimed to quantify the effects of arm swings on lower limb kinetics during single-leg forward, vertical, and backward hopping with a focus on knee assessment. Thirty-eight injury-free participants performed single-leg forward, vertical, and backward hopping with and without arm swings on one leg. Hopping performance, duration of the jumping phase, as well as peak power and work of hip, knee, and ankle joints were calculated while jumping. Two-by-three repeated-measures analyses of variance were applied to determine the effects of arm swings and tasks (α = 0.05). Single-leg backward hopping demonstrated the greatest knee demands by the greatest peak knee power and knee work contribution and smallest peak hip and ankle power, hip and ankle work, compared to forward and vertical hopping, regardless of arm swings. Arm swings resulted in greater hopping performance and smaller knee work for all tasks. The results indicate the possibility of using single-leg backward hopping as a clinical-friendly task to evaluate knee function and quadriceps strength following ACL injuries. Standardized instructions on controlling arms are recommended when using performances from hopping tasks to assess knee function. The findings help us better understand lower limb kinetics during single-leg hopping tasks and compensatory strategies when arm swings are involved.