Side-to-side differences in neuromuscular function and associated joint mechanics in the lower limbs and trunk during walking and running one year after acute unilateral Achilles tendon rupture.

Abstract

Achilles tendon rupture can result in persistent impairments in neuromuscular function and associated joint mechanics, reflecting altered neural-mechanical coupling despite surgical or non-surgical treatment and structured rehabilitation. However, knowledge of how these deficits affect lower limb muscle coordination and joint power distribution across the kinematic chain remains limited. This cross-sectional exploratory study investigated side-to-side differences in muscle activation, range of motion, joint power, joint moment, and support moment during walking and running one year after unilateral Achilles tendon rupture, with emphasis on adaptations within the ankle-knee-hip complex and their effect on the kinematic chain. Twenty-two individuals (19 male, 3 female; mean (SD) age 48.0 ± 10.9 years) with unilateral Achilles tendon rupture were assessed during walking and running at self-selected speeds at mean (SD) 12.0 + 1.3 months post-injury. Kinematic and kinetic data were collected using a 16-camera optical motion capture system. Electromyography (EMG) was recorded bilaterally from eight muscles. Joint power, joint moments, and total support moment were calculated in the sagittal plane. EMG variability across stance was quantified using the coefficient of quartile variation. Statistical Parametric Mapping identified side-to-side differences in continuous time-series data. During walking, the affected limb showed lower ankle power (∼88-90% stance; p = 0.049), reduced knee power in early stance (∼8-10%; p = 0.031), and lower total support moment at initial contact and toe-off (both p = 0.001). Ankle range of motion was also reduced (∼17-20%, p = 0.047). Lateral gastrocnemius activation increased in late stance (∼88-90%; p = 0,049), suggesting greater reliance on biarticular muscle-tendon energy transfer for propulsion. Soleus activation showed lower coefficient of quartile variation, indicating a more stereotyped activation pattern potentially reflecting compensatory control in response to altered tendon compliance. During running, fewer inter-limb differences were observed, although ankle range of motion (∼77-100%, p = 0.012) and support moment at toe-off remained reduced in the affected limb. One year after Achilles tendon rupture, deficits during walking were primarily localized to the distal limb. Findings suggest altered tendon mechanics with compensatory shifts toward biarticular muscle activation, reduced activation variability, and redistribution of joint power along the limb. Rehabilitation should therefore address tendon stiffness and monoarticular plantarflexor strength and muscle strength restoration. III.