Changes in the referent body location and configuration may underlie human gait, as confirmed by findings of multi-muscle activity minimizations and phase resetting

Abstract

Locomotion is presumably guided by feed-forward shifts in the referent body location in the desired direction in the environment. We propose that the difference between the actual and the referent body locations is transmitted to neurons that virtually diminish this difference by appropriately changing the referent body configuration, i.e. the body posture at which muscles reach their recruitment thresholds. Muscles are activated depending on the gap between the actual and the referent body configurations resulting in a step being made to minimize this gap. This hypothesis implies that the actual and the referent leg configurations can match each other at certain phases of the gait cycle, resulting in minimization of leg muscle activity. We found several leg configurations at which EMG minima occurred, both during forward and backward gait. It was also found that the set of limb configurations associated with EMG minima can be changed by modifying the pattern of forward and backward gait. Our hypothesis predicts that, in response to perturbations of gait, the rate of shifts in the referent body location can temporarily be changed to avoid falling. The rate influences the phase of rhythmic limb movements during gait. Therefore, following the change in the rate of the referent body location, the whole gait pattern, for all four limbs, will irreversibly be shifted in time (long-lasting and global phase resetting) with only transient changes in the gait speed, swing and stance timing and cycle duration. Aside from transient changes in the duration of the swing and/or stance phase in response to perturbation, few previous studies have documented long-lasting and global phase resetting of human gait in response to perturbation. Such resetting was a robust finding in our study. By confirming the notion that feed-forward changes in the referent body location and configuration underlie human locomotion, this study solves the classical problem in the relationship between stability of posture and gait and advances the understanding of how human locomotion involves the whole body and is accomplished in a spatial frame of reference associated with the environment.

Stability of gait and interlimb coordination in older adults

“…The longer term (residual) effect of the perturbation on whole body stability was quantified by computing long-lasting phase shifts of all four limbs (Feldman et al 2011;Yamasaki et al 2003). This measure is complementary to the COM analysis, because the COM stability analysis quantifies the return of the COM to the preperturbed stable state, whereas phase resetting describes the residual change in the movement pattern, i.e., changes that persist after the perturbation.…”

“…The anteroposterior position of each limb in the three cycles before perturbation was projected forward, and the minimal time between the actual and projected foot contact/direction reversal was divided by the preperturbed cycle period. In the case of more than one event occurring within the duration of one preperturbed cycle, the first event was used (Ford et al 2007). The obtained value was multiplied by 360°, resulting in a range of possible phase shifts from Ϫ180°to ϩ180°.…”

“…In our study, however, trip perturbations led to a majority of lowering or combined responses that involve premature lowering of the perturbed leg. In this type of response, a transition from swing to stance occurs in one leg and from stance to swing in the other (phase shift; Feldman et al 2011). To completely regain the preperturbed whole body coordination pattern and regardless of the initial response to perturbation, the arms have to switch movement direction (from forward to backward or vice versa) such that the antiphase pattern between the arms and legs is restored and a similar phase shift is observed in upper and lower limbs (Fig.…”

“…Gait can be considered as resulting from translation of body equilibrium in the environment (Feldman et al 2011). To prevent falling, subjects can transiently slow down or speed up the translation in response to perturbation, which can be detected as a long-lasting phase shift in gait.…”

“…Other measures quantify the ability to maintain functional locomotion despite larger perturbations, such as path obstacles or trunk displacements. Transient and long-lasting responses to perturbations have been quantified in terms of limb strategies used (Eng et al 1994), parameters quantifying immediate changes in and gradual return to a stable gait pattern (Bruijn et al 2010), and by the phase resetting of gait reflecting the ability to restore body equilibrium in the environment (Feldman et al 2011). Gait stability is a global property of the motor system, implying that all body segments are involved in the short-and longer term responses to perturbation (Marigold and Misiaszek 2009).…”