Effect of muscle fatigue on cycling asymmetry during a constant-power test

Asymmetry in cycling may reduce mechanical efficiency and increase the risk of overuse injuries, particularly as fatigue develops. While asymmetry is often assessed using global power output, analyzing directional force components may reveal biomechanical patterns more relevant to performance and injury. In this study, we investigated the effect of progressive fatigue on bilateral asymmetry in power and pedal force components during a submaximal constant-power cycling test. Twenty healthy recreational participants performed prolonged cycling at 70% of their individual peak power until exhaustion while three-dimensional pedal forces were recorded using an instrumented ergometer. Asymmetry was quantified using the Normalized Symmetry Index (NSI) for power and for tangential, radial, and mediolateral force components, and evaluated at five time points representing progressive fatigue. Fatigue did not significantly alter mean relative interlimb asymmetry in power output or pedal force components as quantified by NSI, nor did it significantly change mean absolute interlimb force or power differences. However, cycle-to-cycle variability of the asymmetry measures and absolute interlimb differences changed across time, suggesting that fatigue influenced variability in limb coordination rather than producing a systematic shift in mean left-right force sharing. Asymmetry also differed significantly between force directions, with mediolateral forces showing the largest NSI range (-10% to 8%) compared to radial (-3% to 6%) and tangential (-7% to 3%) forces. These findings suggest that, in healthy individuals performing seated constant-power cycling, fatigue-related changes may be expressed more strongly through variability and direction-specific force behavior than through systematic changes in mean relative or absolute interlimb asymmetry.