The importance of the functional base-of-support for clinical biomechanical balance analysis

Abstract

The occurrence of falls and balance problems are common in persons of higher age or with neuromuscular disorders. While clinical balance scales are unable to accurately identify balance, biomechanical balance models (such as the extrapolated center-of-mass) need missing information on the base-of-support formed by the feet [1]. People can balance their body mass above this area formed by the feet without taking a compensatory step. Common impairments such as muscle degeneration likely decrease this support area. Therefore, we evaluated changes in the functional base-of-support (fBOS) resulting from ageing and neuromuscular disorders and the impact on gait balance analysis. We assessed the fBOS in 20 young persons (28±7 yrs), 7 with lower leg muscle weakness due to slowly progressive neuromuscular disorders (63±5 yrs; caption Fig. 1), 7 age-matched middle-aged (62±8 yrs) and 7 old persons (80±3 yrs). Ground forces and foot markers were recorded while participants slowly moved their center-of-pressure in as large circles as possible without moving their feet. The fBOS is modeled was the convex hull enclosing this circled area normalized to marker-based foot dimensions [2]. The effect of ageing of the fBOS on dynamic balance outcomes during walking at heel strike (anterior-posterior direction) was assessed in a dataset of 138 persons across the lifespan [3,4]. The fBOS was only 24% of the foot outline formed by markers for young persons (Fig. 1A) and is 84% smaller in patients with neuromuscular disorders (pttest<0.001). The fBOS decreased with age (pANOVA=0.003), with similar values in mid-age (-24%, pttest=0.11) and a 52% decrease in old age (pttest=0.002) compared to young (Fig. 1A). When taken the fBOS into account, dynamic balance shifts from inside to outside the support area. Extrapolating the age-reduction in fBOS, balance changes from increasing to decreasing with age. Fig. 1: Functional Base of Support (fBOS) for the different participant groups.Download : Download high-res image (333KB)Download : Download full-size image Studies overlook the base-of-support as part of dynamic balance analysis [1]. This study shows the importance of using an accurate model of the fBOS, as a single reference marker does not capture 1) the shape of the effective fBOS; 2) the effects of age and disorder; and 3) changes over the gait cycle. Use of the fBOS revealed reductions in balance in older persons, compared to safer margins without the fBOS. The large group variances indicate that individual fBOS measurements are needed for precise balance assessment. We provide the fBOS model per group and code to apply this to measured markers, so researchers can establish clinical meaningful differences in dynamic balance outcomes. As such, this study strives towards the integration of accurate biomechanical balance analysis in clinical gait analysis.