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

Abstract

An inability to recover lateral balance with a single step is predictive of future falls in older adults. This study investigated if balance stability at first step lift-off (FSLO) would be different between multiple and single stepping responses to lateral perturbations. 54 healthy older adults received left and right waist-pulls at 5 different intensities (levels 1–5). Crossover stepping responses at and above intensity level 3 that induced both single and multiple steps were analyzed. Whole-body center of mass (COM) and center of pressure (COP) positions in the medio-lateral direction with respect to the base of support were calculated. An inverted pendulum model was used to define the lateral stability boundary, which was also adjusted using the COP position at FSLO (functional boundary). No significant differences were detected in the COP positions between the responses at FSLO (p ≥ 0.075), indicating no difference in the functional boundaries between the responses. Significantly smaller stability margins were observed at first step landing for multiple steps at all levels (p ≤ 0.024), while stability margins were also significantly smaller at FSLO for level 3 and 4 (p ≤ 0.048). These findings indicate that although reduced stability at first foot contact would be associated with taking additional steps, stepping responses could also be attributable to the COM motion state as early as first step lift-off, preceding foot contact. Perturbation-based training interventions aimed at improving the reactive control of stability would reduce initial balance instability at first step lift-off and possibly the consequent need for multiple steps in response to balance perturbations.

Introduction

An impaired ability to control lateral balance is one of the important balance problems relevant to falls (Maki and McIlroy, 1996, Maki and McIlroy, 2006, O'Neill et al., 1994, Robinovitch et al., 2013, Rogers and Mille, 2003). In response to lateral perturbations of standing balance, older adults are much more likely than younger adults to take multiple balance recovery steps with a crossover or medial step strategy, which increases the potential for inter-limb collisions and subsequent falls (Bair et al., 2016, Maki et al., 2000, Mille et al., 2013, Mille et al., 2005). Use of multiple recovery steps has been a consistent finding that distinguishes older adults from younger adults and older fallers from non-fallers (Hilliard et al., 2008, Luchies et al., 1994, Maki and McIlroy, 2006, Mille et al., 2005, Patton et al., 2006). An inability to recover lateral balance with a single step is predictive of future falls (Hilliard et al., 2008). As protective responses with multiple steps have been shown to be a sensitive predictor of fall risk, it is important to identify underlying factors that lead to multiple steps to recover lateral balance.

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 safeguard stability. Such an estimation of dynamic stability may underlie the choices for recovery step strategies (i.e., type of step and single versus multiple steps).

Dynamic balance stability has been quantified based on the position-velocity relationship between the COM and BOS (Carty et al., 2011, Hof et al., 2005, Pai and Patton, 1997, Pai et al., 1998). The tendency for older adults to step more often than younger subjects has been well predicted by a reduced margin of stability for forward or backward steps, where the level of instability of the initial step, indicated by the reduced margin of stability at first foot contact, has been reported to be strongly predictive of the recovery strategies employed (Carty et al., 2011, Hsiao and Robinovitch, 2001, Maki and Mcilroy, 1999). These studies indicate that instability at foot contact would be a determinant of the use of multiple stepping.

Although reduced stability at foot contact has been linked with the use of additional steps, stepping strategies for balance recovery may also be determined by factors preceding foot contact. For example, older adults may preplan to take a series of small steps to gain a greater opportunity to make corrective adjustments in the response (Luchies et al., 1994, Maki et al., 2000). Since more adjustments can be made to correct for any ill-chosen responses, multiple stepping by older adults may be a more conservative response to ensure stability to disturbances that fully stress their balance capacity (Luchies et al., 1994). Thus, even earlier than the first step ground contact, the CNS might estimate the evolving level of instability and select to make different protective stepping responses. In doing so, stepping strategies can be enacted in a pre-determined manner and the use of multiple steps may be attributable to the level of stability as early as the instant of first step lift-off (FSLO).

To further examine these issues, the objective of this study was to compare balance stability at FSLO between single and multiple step recoveries in response to lateral perturbations of standing balance in older adults. An inverted pendulum model was used to define the BOS lateral stability boundary at FSLO, which was also adjusted using the COP position at FSLO (functional boundary). It was hypothesized that reduced balance stability would be observed for multiple step recovery responses at FSLO.