Biomechanical Evidence for the Proactive Balance Strategy as a Functional Analogue of the Ankle Strategy During Stationary Manual Wheelchair Wheelies

Background/Objectives: Maintaining a stationary manual wheelchair wheelie is a highly demanding dynamic postural control task that can be biomechanically modeled as an inverted pendulum. Although skilled wheelchair users employ a Proactive Balance Strategy (PBS) characterized by small rhythmic wheel movements, its underlying biomechanical control mechanism remains incompletely understood. This study investigated whether the PBS serves as a functional analogue of the human standing ankle strategy by examining the dynamic relationships among wheelchair orientation, center of pressure, and center of mass dynamics. Methods: Forty healthy adults with prior wheelchair wheelie training performed stationary wheelie trials on a force platform while three-dimensional kinematic and kinetic data were collected simultaneously. Results: Cross-correlation analysis demonstrated a strong inverse relationship between wheelchair pitch angle and rear-wheel displacement (r = −0.73, phase lag = 0.032 s). A moderate inverse relationship was also observed between the horizontal center of pressure (COPx) and horizontal center-of-mass acceleration (COMaccx) (r = −0.61) with a near-zero mean phase lag (−0.003 s). Conclusions: These findings support a functional biomechanical analogy between PBS and the ankle strategy during standing balance. The coordinated kinematic and kinetic relationships indicate that stationary wheelie balance is maintained through continuous regulation of rear-wheel position, which changes the wheel–ground contact point and thereby modulates the COP and whole-body dynamics. The observed biomechanical coupling is compatible with the proactive characteristics proposed for PBS. Although this study involved healthy adults with prior wheelie experience rather than daily wheelchair users, the findings provide a quantitative biomechanical framework for understanding stationary wheelchair wheelie balance and may inform future wheelchair skills training and rehabilitation strategies.