Dynamic balancing responses in unilateral transtibial amputees following outward-directed perturbations during slow treadmill walking differ considerably for amputated and non-amputated side

Abstract

Background: Due to disrupted motor and proprioceptive function, lower limb amputation imposes considerable challenges associated with balance and greatly increases risk of falling in presence of perturbations during walking. The aim of this study was to investigate dynamic balancing responses in unilateral transtibial amputees when they were subjected to perturbing pushes to the pelvis in outward direction at the time of foot strike on their non-amputated and amputated side during slow walking.

Methods: Fourteen subjects with unilateral transtibial amputation and nine control subjects participated in the study. They were subjected to perturbations that were delivered to the pelvis at the time of foot strike of either the left or right leg. We recorded trajectories of center of pressure and center of mass, durations of in-stance and stepping periods as well as ground reaction forces. Statistical analysis was performed to determine significant differences in dynamic balancing responses between control subjects and subjects with amputation when subjected to outward-directed perturbation upon entering stance phases on their non-amputated or amputated sides.

Results: When outward-directed perturbations were delivered at the time of foot strike of the non-amputated leg, subjects with amputation were able to modulate center of pressure and ground reaction force similarly as control subjects which indicates application of in-stance balancing strategies. On the other hand, there was a complete lack of in-stance response when perturbations were delivered when the amputated leg entered the stance phase. Subjects with amputations instead used the stepping strategy and adjusted placement of the non-amputated leg in the ensuing stance phase to make a cross-step. Such response resulted in significantly larger displacement of center of mass.

Conclusions: Results of this study suggest that due to the absence of the COP modulation mechanism, which is normally supplied by ankle motor function, people with unilateral transtibial amputation are compelled to choose the stepping strategy over in-stance strategy when they are subjected to outward-directed perturbation on the amputated side. However, the stepping response is less efficient than in-stance response.

Keywords: Center of mass; Center of pressure; Dynamic balancing response; Ground reaction force; In-stance strategy; Perturbed walking; Stepping strategy; Unilateral transtibial amputation.

Fig. 1

Experimental setup for assessing balance responses after perturbations applied to pelvis. Perturbations were applied in forward, backward, inward, outward directions and were triggered at either left or right foot strike

Fig. 2

Kinematics and kinetics following outward-directed perturbation. Mean values and standard deviations are shown for the group representatives

Fig. 3

Group mean values and standard deviations for the duration of in-stance and stepping periods of unperturbed walking for all three groups. P-values indicate statistically significant effect of group factor and statistically significant differences between groups in Bonferroni post-hoc paired comparisons were found

Fig. 4

Temporal parameters of unperturbed and outward-directed perturbed walking. Group mean values and standard deviations are shown for all three groups. P-values indicate statistically significant effect of group factor and statistically significant differences between groups in Bonferroni post-hoc paired comparisons were found

Fig. 5

ΔCOM of outward-directed perturbed walking. Group mean values and standard deviations are shown for all three groups. P-values indicate statistically significant effect of group factor and statistically significant differences between groups in Bonferroni post-hoc paired comparisons were found

Fig. 6

ΔCOP and ΔGRF of outward-directed perturbed walking. Group mean values and standard deviations are shown for all three groups. P-values indicate statistically significant effect of group factor and statistically significant differences between groups in Bonferroni post-hoc paired comparisons were found. Note that positive values of ΔCOP_ML indicate change in the direction of perturbation direction. Also note that positive values of ΔGRF_ML indicate change in the direction opposite to perturbation direction