The contribution of counter-rotation movements during fall recovery: A validation study

Three mechanisms of maintaining standing stability include M₁ - moving the COP within the base of support, M₂ - segment counter-rotation, and M₃ - applying an external force. To date, the contributions of these mechanisms have not been quantified for the response to an external postural disturbance. The purpose of this study was to evaluate the construct validity of measures that quantify the M₂ contribution to anteroposterior fall recovery. We evaluated the whole-body rotation contribution, as well as a measure specific to arm motion (M_ARMS). With segment counter-rotation as the main focus of this study, we examined standing feet-in-place responses to treadmill-induced falls. The treatment validity of our measures was assessed by comparing unconstrained responses to those with constrained arm motion. The convergent validity of our measures was assessed by correlating peak shoulder flexion and extension velocities with counter-rotation contributions. Eleven unimpaired participants responded to anteroposterior belt accelerations from a treadmill, and the M₂ and M_ARMS contributions were quantified from three-dimensional segment motion. The treatment validity of these measures was partially supported. Constraining the arms reduced M₂ for anterior, but not posterior falls. Conversely, M_ARMS was reduced for posterior, but not anterior falls. Convergent validity was supported for M_ARMS (r = 0.64-0.78), but not M₂ (r = -0.40 to -0.15). These results support the use of M_ARMS over M₂ when interested in the role of arm motion. Given that arm constraints did not change the contribution of M_ARMS during a forward fall, unimpaired participants may not necessarily rely on arm motion as part of their recovery strategy in this context.