Gait of lower limb amputees: Unusual time-course of vertical ground reaction forces of the intact limb.

Background: The impact of an amputation on the intact limb during walking is still unclear. By using vertical ground reaction forces (vGRF), recent reports suggested that not all amputees exhibit the classical M-shape. However, there is no study exploring these "Non-M" strides on both feet. The purpose of present study was to explore the vGRF time-course during gait and to quantify "Non-M" strides, their reproducibility and factors that could influence their apparition.

Methods: vGRF of 29 controls and 23 unilateral lower-limb amputees were recorded during a 5-min walk at comfortable speed on an instrumented treadmill. Each stride was classified as "M" or "Non-M" to determine the percentage of "Non-M" strides per limb that were then compared between groups and limbs. The influence of age, time since amputation, speed, stance phase ASI, and type of prosthetic foot on the percentage of "Non-M" strides was tested. Comparisons between intact and prosthetic sides were made for duration of the stance phase and variability of vGRF patterns.

Results: the percentage of "Non-M" strides was higher in the amputee group (p < 0.001) and higher in the intact limb compared to the prosthetic limb (p < 0.001). The analysis of the variables that could influence vGRF did not identify a cause. The amputees who also produced "Non-M" strides had higher variability in their vGRF patterns on the intact limb compared to those producing only "M" strides. This was not observed on the prosthetic side.

Significance: The results suggest that the intact limb adapts in real-time to manage the body's movements, compensating for limits of prosthetic footwear. The global time-course of the vGRF (rather than the first peak) is a useful tool for studying the impact of the prosthesis on the intact limb.