Timing paradox of stepping and falls in ageing: not so quick and quick(er) on the trigger

Abstract

Physiological and degenerative changes affecting human standing balance are major contributors to falls with ageing. During imbalance, stepping is a powerful protective action for preserving balance that may be voluntarily initiated in recognition of a balance threat, or be induced by an externally imposed mechanical or sensory perturbation. Paradoxically, with ageing and falls, initiation slowing of voluntary stepping is observed together with perturbation‐induced steps that are triggered as fast as or faster than for younger adults. While age‐associated changes in sensorimotor conduction, central neuronal processing and cognitive functions are linked to delayed voluntary stepping, alterations in the coupling of posture and locomotion may also prolong step triggering. It is less clear, however, how these factors may explain the accelerated triggering of induced stepping. We present a conceptual model that addresses this issue. For voluntary stepping, a disruption in the normal coupling between posture and locomotion may underlie step‐triggering delays through suppression of the locomotion network based on an estimation of the evolving mechanical state conditions for stability. During induced stepping, accelerated step initiation may represent an event‐triggering process whereby stepping is released according to the occurrence of a perturbation rather than to the specific sensorimotor information reflecting the evolving instability. In this case, errors in the parametric control of induced stepping and its effectiveness in stabilizing balance would be likely to occur. We further suggest that there is a residual adaptive capacity with ageing that could be exploited to improve paradoxical triggering and other changes in protective stepping to impact fall risk.

Single and multiple step balance recovery responses can be different at first step lift-off following lateral waist-pull perturbations in older adults

“…There were no significant differences in step length between the responses, but smaller stability margins were observed at first step lift-off for the multiple steps for level 3 and 4 pulls, which appears to result in smaller stability margins at landing. An estimation of the mechanical state conditions for stability as early as first step lift-off based on a forward internal model, may have been used to determine the use of single or multiple step recoveries (Rogers and Mille, 2016). It should also be noted that the COM motion state for the multiple stepping responses was closer to the functional boundary, rather than the BOS boundary.…”

“…When balance is disturbed by external means, the central nervous system (CNS) appears to estimate the current and future states of motion of the whole-body center of mass (COM) in relation to the base of support (BOS) on the basis of ongoing sensory information (Maki and Mcilroy, 1999; Rogers and Mille, 2016), which may determine the number of steps used to prevent falling. The CNS appears to monitor the evolving state of balance stability and modify the triggered response in an online manner to safe-guard stability.…”

Perturbation-evoked lateral steps in older adults: Why take two steps when one will do?

“…The CNS appears to alter the response in real time (Fujimoto et al, 2017) based on the evolving balance state, i.e. the central nervous system (CNS) monitors the current and future states of the CoM relative to the BoS (Maki and McIlroy, 1999; Rogers and Mille, 2016). Reduced dynamic stability at first step foot contact, and as early as first step lift off, has been linked with additional steps following lateral perturbations (Fujimoto et al, 2017), although only cross-over step types were analyzed.…”

Editorial

“…). These concepts of disruption in neuromuscular systems and locomotion are major contributors to the increased incidence of falls in the aged and this is reviewed in relation to the process and protection of stepping in reducing the risk of falls by Rogers & Mille (). Ageing is also associated with changes in sensory function, which is highlighted here in relation to mechanosensory and chemosensory function in the gut (Keating et al .…”

“…These age-related neuromuscular changes are characterized by loss of motoneurons, enlargement of motor units and associated loss in synaptic function and muscle mass, which have been carefully studied using electromyography in the vastus lateralis (Piasecki et al 2016). These concepts of disruption in neuromuscular systems and locomotion are major contributors to the increased incidence of falls in the aged and this is reviewed in relation to the process and protection of stepping in reducing the risk of falls by Rogers & Mille (2016). Ageing is also associated with changes in sensory function, which is highlighted here in relation to mechanosensory and chemosensory function in the gut (Keating et al 2016) that contribute to gastrointestinal dysfunction and morbidity.…”