Dynamic Balance Control (DBC) in lower leg amputee subjects; contribution of the regulatory activity of the prosthesis side.

BACKGROUND

Regaining effective postural control after lower limb amputation requires complex adaptation strategies in both the prosthesis side and the non-amputated side. The objective in this study is to determine the individual contribution of the ankle torques generated by both legs in balance control during dynamic conditions.

METHODS

Subjects (6 transfemoral and 8 transtibial amputees) stood on a force platform mounted on a motion platform and were instructed to stand quietly. The experiment consisted of 1 static and 3 perturbation trials of 90 s duration each. The perturbation trials consisted of continuous randomized sinusoidal platform movements of different amplitude in the sagittal plane. Weight distribution during the static and dynamic perturbation trials was calculated by dividing the average vertical force below the prosthesis foot by the sum of forces below both feet. The Dynamic Balance Control represents the ratio between the stabilizing mechanism of the prosthetic leg and the stabilizing mechanism of the non-amputated leg. The stabilizing mechanism is calculated from the corrective ankle torque in response to sway. The relationship between the prosthetic ankle stiffness and the performance during the platform perturbations was calculated.

FINDINGS

All patients showed a (non-significant) weight bearing asymmetry in favor of the non-amputated leg. The Dynamic Balance Control ratio showed that the contribution of both legs to balance control was even more asymmetrical. Moreover, the actual balance contribution of each leg was not tightly coupled to weight bearing in each leg, as was the case in healthy controls. There was a significant positive correlation between the prosthetic ankle stiffness and the Dynamic Balance Control.

INTERPRETATION

The Dynamic Balance Control provides, in addition to weight distribution, information to what extent the stabilizing mechanism of the corrective ankle torque of both legs contributes to balance control. Knowledge of the stiffness properties may optimize the prescription process of prosthetic foot in lower leg amputee subjects in relation to standing stability.