Development of a theoretical model for upright postural control in lower limb prosthesis users.

Methods used to assess quiet standing in unilateral prosthesis users often assume validity of an inverted pendulum model despite this being shown as invalid in some instances. The aim of the current study was to evaluate the validity of a proposed unilaterally-constrained pin-controller model in explaining postural control in unilateral prosthesis users. Prosthesis users were contrasted against the theoretical model as were able-bodied controls that stood on a platform which unilaterally constrained movement of the CoP. All participants completed bouts of quiet standing with eyes open, eyes closed and with feedback on inter-limb weight bearing asymmetry. Correlation coefficients were used to infer inverted pendulum behavior in both the anteroposterior and mediolateral directions and were derived from both kinematic (body attached markers) and kinetic (centre of pressure) experimental data. Larger, negative correlation coefficients reflected better model adherence, whilst low or no correlation reflected poorer model adherence. Inverted pendulum behavior derived from kinematic data, indicated coefficients of high magnitude in both mediolateral (all cases range 0.71-0.78) and anteroposterior (0.88-0.91) directions, irrespective of groups. Inverted pendulum behavior derived from kinetic data in the anteroposterior direction indicated validity of the model with large negative coefficients associated with the unconstrained/intact limbs (prosthesis users: - 0.45 to - 0.65, control group: - 0.43 to - 0.72), small coefficients in constrained/prosthetic limbs (prosthesis users: - 0.02 to 0.07, control group: 0.13-0.26) and large negative coefficients in combined conditions (prosthesis users: - 0.36 to - 0.56, control group: - 0.71 to - 0.82). For the mediolateral direction, coefficients were negligible for individual limbs (0.03-0.17) and moderate to large negative correlations, irrespective of group (- 0.31 to - 0.73). Data suggested both prosthesis users' and able-bodied individuals' postural control conforms well to that predicted by a unilaterally-constrained pin-controller model, which has implications for the fundamental control of posture in transtibial prosthesis users.