Differences in knee axis behaviour and muscle activity between open- and closed-chain knee exercises

Understanding knee mechanics and joint loading is vital for optimizing training and rehabilitation. This study analysed knee axis behaviour and lower limb muscle activity during a closed-chain exercise (Bilateral Squat, BLS), compared with an open-chain exercise (Seated Leg Extension, SLE) using time-series analysis of electromyographic (EMG) data and motion capture to assess knee axis inclination and translations through 90 degrees of flexion-extension. EMG data were filtered and normalized to maximum activity, and kinematic curves were analysed across the full movement cycle (0-100%) employing the Instantaneous Helical Axis (IHA) method and Statistical Parametric Mapping (SPM). IHA inclination and position were calculated across eccentric and concentric phases. Results demonstrated significant differences in IHA position and inclination between exercises, with exercise- and phase-dependent associations between IHA parameters and lower-limb muscle activity. IHA position was more anterior during BLS compared to SLE in both eccentric and concentric phases (>80%), whereas IHA inclination was greater during SLE, particularly during latter portions of the eccentric (60-100%) and concentric (85-100%) phases. Associations between IHA parameters, knee kinematics, and muscle activity also differed between exercises and movement phases. These findings indicate exercise- and phase-dependent differences in knee axis behaviour and muscle coordination despite comparable knee ranges of motion.