The use of a descriptor of instability to characterize and compare transfemoral amputees’ gait with healthy subjects

Abstract

Introduction Falls during daily activities are one of the most important causes of morbidity in the elderly population. Several descriptors exist to quantify instability. However, these descriptors are limited, and sometimes impossible to calculate in realistic walking conditions on uneven surfaces or during multi-contact phases (Bailly et al. 2018). One proposed descriptor is the distance between the body center of mass (BCOM) and the minimal moment axis (MMA) of the external contact wrench dCOM-Δ: The benefits of using such descriptor are first its ability to describe the variation of the angular momentum at the BCOM throughout the phases of the gait cycle, and its capacity to be computed in various locomotion conditions. In addition, this descriptor has shown promising results in previous studies for characterizing instability during gait. For example, Bailly et al. (2018) observed that the distance increases as the walking terrain increases in difficulty. Abiad et al. (2020) also studied the relative position of MMA and BCOM through the vector linking BCOM and its projection on the MMA. They showed that the norm and the anterior-posterior component of this vector can be used to highlight some instability features in the gait of people with amputation. The mean norm of distance and the anterior posterior component of distance calculated on amputees was higher than that calculated on healthy subjects. Nevertheless, there remain several intriguing questions to be answered about the role of other components of the vector on the mediolateral and vertical directions in detecting instabilities. In this article, we compare the mediolateral and vertical vector components between transfemoral amputees and healthy subjects. The hypothesis is that the parameters associated with the relative position of MMA with respect to BCOM will be higher in amputees. 2. Methods Under the approval of the ethics committee, the study was composed of five subjects with unilateral transfemoral amputation (44 ± 14 years, 73 ± 17 kg) and fourteen healthy subjects (25.6 ± 5.8 years, 73 ± 8 kg). Each participant was equipped with 47 retro-reflective skin markers tracked at 200 Hz using a Vicon system (Oxford Metrics, UK). Ground reaction forces (GRF) and moments were simultaneously measured at 2 kHz by force plates (AMTI, USA) and filtered with a 4th order Butterworth lowpass with a 15Hz cut-off frequency. Each subject performed at least one trial at self-selected walking velocity. To calculate the MMA, along which moment of the external contact forces is minimal, the GRF and moments were used. Three-dimensional BCOM was determined using anthropometric tables for healthy subjects and sound limbs of amputees (Dumas et al. 2007). The BCOM of the prosthetic limb is obtained by the method described in Abiad et al. (2020). The components of the vector between BCOM and its projection on the MMA dCOM-Δ is computed according to the following equation: 3. Results and discussion Figure 1 shows the mean and the range of variation of dML and dV: Both the means of dML and dV were greater for amputee subjects. Considering dML, this result agrees with the result found in the previous study of (Sheehan et al. 2015) which states thatindividuals with lower limb amputation are mediolaterally more unstable during walking than able-bodied individuals. Moreover, individuals with unilateral transtibial amputation have a greater mean range of angular momentum in the frontal plane or anteriorposterior direction (Silverman and Neptune 2011). Figure 2 shows the shape of the mediolateral vector component for one amputee and on average for healthy subjects. For healthy subjects, this shape is similar to the variation of mediolateral angular momentum derivate (Herr and Popovic 2008). Equation (2) is an approximation of mediolateral component in Equation (1). The product FAP ×Mv is negligible with respect Fv × MAP : It shows a relationship between the mediolateral component dML and the anterior-posterior component of the angular momentum. where H AP = MAP is the angular momentum derivate in the frontal plane. In Herr and Popovic (2008), it was inferred that the anterior-posterior angular momentum was the second contributor in whole-body angular momentum. Similarly, the mediolateral vector component is the second contributor in the norm of the distance. Considering dV, even though the vertical component value is lower than anterior posterior mediolateral distance, a difference of 6mm between amputees and healthy subjects could be found. Finally, the instability in amputees gait appeared through the higher distance along vertical and mediolateral direction. Limitations of this study include the low number of amputees, heterogeneity of levels of amputation, of prosthetic types and walking speeds. 4. Conclusions As a conclusion, the distance vector components computed in the two mentioned directions: mediolateral and vertical can differentiate between the amputee and healthy gait. They may detect instabilities that amputees have along the mediolateral and vertical directions. Ongoing works will involve experimental analysis of different instability conditions of gait using instrumented bi-band treadmill. Furthermore, this distance can be used to assess the ability of the prosthesis to provide a functional gait as compared to healthy subjects.