Neuromuscular adaptations and sensorimotor integration following a unilateral transfemoral amputation

Abstract

Background: Following an amputation, the human postural control system develops neuromuscular adaptations to regain an effective postural control. We investigated the compensatory mechanisms behind these adaptations and how sensorimotor integration is affected after a lower-limb transfemoral amputation.

Methods: Center of pressure (CoP) data of 12 unilateral transfemoral amputees and 12 age-matched able-bodied subjects were recorded during quiet standing with eyes open (EO) and closed (EC). CoP adjustments under each leg were recorded to study their contribution to posture control. The spatial structure of the CoP displacements was characterized by measuring the mean distance, the mean velocity of the CoP adjustments, and the sway area. The Entropic Half-Life (EnHL) quantifies the temporal structure of the CoP adjustments and was used to infer disrupted sensory feedback loops in amputees. We expanded the analysis with measures of weight-bearing imbalance and asymmetry, and with two standardized balance assessments, the Berg Balance Scale (BBS) and Timed Up-and-Go (TUG).

Results: There was no difference in the EnHL values of amputees and controls when combining the contributions of both limbs (p = 0.754). However, amputees presented significant differences between the EnHL values of the intact and prosthetic limb (p < 0.001). Suppressing vision reduced the EnHL values of the intact (p = 0.001) and both legs (p = 0.028), but not in controls. Vision feedback in amputees also had a significant effect (increase) on the mean CoP distance (p < 0.001), CoP velocity (p < 0.001) and sway area (p = 0.007). Amputees presented an asymmetrical stance. The EnHL values of the intact limb in amputees were positively correlated to the BBS scores (EO: ρ = 0.43, EC: ρ = 0.44) and negatively correlated to the TUG times (EO: ρ = - 0.59, EC: ρ = - 0.69).

Conclusion: These results suggest that besides the asymmetry in load distribution, there exist neuromuscular adaptations after an amputation, possibly related to the loss of sensory feedback and an altered sensorimotor integration. The EnHL values suggest that the somatosensory system predominates in the control of the intact leg. Further, suppressing the visual system caused instability in amputees, but had a minimal impact on the CoP dynamics of controls. These findings points toward the importance of providing somatosensory feedback in lower-limb prosthesis to reestablish a normal postural control.

Trial registration: DRKS00015254 , registered on September 20th, 2018.

Keywords: Amputees; Center of pressure; Postural control; Prosthesis; Sensory feedback.

Fig. 1

Group x Leg interaction of the EnHL values (left), $\overline{\text{DIST}}$ (middle) and $\overline{\mathrm{VEL}}$ (right). The significant differences between groups are marked with an asterisk (*: p < 0.05, **: p < 0.001). (I: intact leg, in case of controls dominant leg, A: amputated leg, in case of controls non-dominant leg)

Fig. 2

CoP path of the intact (left) and amputated (right) leg of an amputee in the AP and ML direction

Fig. 3

Box plot of the Condition x Group interaction of the EnHL values considering the contributions of both legs (left), and interaction plot of the EnHL values of each limb independently and each group during EO and EC condition. Box plots describe the median (line inside the box) and the 1st and 3rd quartiles (box hinges). The box whiskers represent the largest (or smallest) data value but no larger (or smaller) than 1.5 times the inter quartile range. Values larger (or smaller) than the whiskers are represented by dots. The significant differences are marked with an asterisk (*: p < 0.05). (I: intact leg, in case of controls dominant leg, A: amputated leg, in case of controls non-dominant leg)

Fig. 4

Scatter plot of the WBI values and the TUG times. The correlation between the TUG times and WBI factor is positive in amputees and negative in controls. The lines represent the regression lines. The corresponding Pearson coefficient ρ and p-values are presented in each plot