Control of ankle and hip joint stiffness for arm-free standing in paraplegia

Objective. To investigate whether a simple static stiffness model adequately relates the angles and net joint torques (NJT) developed in the ankles (sagittal plane) and in the hips (frontal plane) following perturbations delivered in multiple directions to partially constrained subjects standing quietly. Materials and Methods. Six subjects were standing on two force platforms while an apparatus randomly delivered controlled perturbations at the level of pelvis in eight directions in the transverse plane. Results. Ankle sum stiffness was found to be invariant of the perturbation directions for the group of forward directed perturbations (17 ± 5 Nm/deg, p= 0.93) as well as for the group of backward directed perturbations (13 ± 6 Nm/deg, p= 0.92). The correlation coefficients were confined between 0.95 and 0.98 across perturbation directions. Hip sum stiffness was found to be invariant of the perturbation directions (15 ± 4 Nm/deg, p= 0.98). The correlation coefficients were confined between 0.96 and 0.98 across perturbation directions. Conclusions. The relationship between the ankle angles and NJT and hip angles and NJT following perturbed stance can be described as static stiffness. The implications for arm-free paraplegic standing are discussed and a new control scheme is proposed.