Inoculation against falls: rapid adaptation by young and older adults to slips during daily activities

Abstract

Objective: To determine whether aging diminishes one's ability to rapidly learn to resist falls on repeated-slip exposure across different activities of daily living.

Design: Quasi-experimental controlled trial.

Setting: Two university-based research laboratories.

Participants: Young (n=35) and older (n=38) adults underwent slips during walking. Young (n=60) and older (n=41) adults underwent slips during a sit-to-stand task. All (N=174) were healthy and community dwelling.

Intervention: Low-friction platforms induced unannounced blocks of 2 to 8 repeated slips interspersed with blocks of 3 to 5 nonslip trials during the designated task.

Main outcome measures: The incidence of falls and balance loss. Dynamic stability (based on center of mass position and velocity) and limb support (based on hip height) 300 ms after slip onset.

Results: Under strictly controlled, identical low-friction conditions, all participants experienced balance loss, but older adults were over twice as likely as young to fall on the first, unannounced, novel slip in both tasks. Independent of age or task, participants adapted to avoid falls and balance loss, with most adaptation occurring in early trials. By the fifth slip, the incidence of falls and balance loss was less than 5% and 15%, respectively, regardless of age or task. Reductions in falls and balance loss for each task were accomplished through improved control of stability and limb support in both age groups. A rapidly reversible age- and task-dependent waning of motor learning occurred after a block of nonslip trials. Adaptation to walk slips reached a steady state in the second slip block regardless of age.

Conclusions: The ability to rapidly acquire fall-resisting skills on repeated-slip exposure remains largely intact at older ages and across functional activities. Thus, repeated-slip exposure might be broadly effective in inoculating older adults against falls.

Fig. 1

Protocol for the a) walking and b) STS tasks. Blocks of slip trials (S) are shown in gray, blocks of nonslip trials (NS) in white, and the block of mixed slip and nonslip trials in vertical lines. For the walking task, the sequence of trials comprised 10 initial nonslipping (NS) trials, followed by a block of 8 slips, a block of 3 nonslipping trials, a second block of 8 slips, a second block of 3 nonslip trials, then a mixed block of 8 slip and 7 nonslip trials (S17–18, NS×2, S19, NS, S20, NS, S21–22, NS×2, S23, NS, S24). For the STS task, the sequence comprised 4 nonslip trials, followed by a block of 5 slips, a block of 3–5 nonslip trials (a 4^th or 5^th trial was performed if stepping occurred in the preceding trial), and a final block of 2 reslips (RS). For both tasks, all trials were unannounced, and participants were only aware that a slip “may or may not occur.”

Fig. 2

Changes in the incidence of a) falls and b) backward balance loss with repeated exposure of young adults (OPEN symbols) and older adults (FILLED symbols) to slips during walking (squares) and during a STS (circles). Shown is the percentage of participants who fell or experienced a backward balance loss as a function of trial across the first slip block (S1 through S8 for walking; S1 through S5 for STS). The curves shown correspond to the models predicted by the generalized estimating equations (Table 2). Also shown are the incidence of falls and backward balance losses on the first and second reslip trials (S9 and S10 for walk-slips, RS1 and RS2 for STS-slips), induced after 3–5 nonslip trials. STS data from Pavol et al. and Pai et al.

Fig. 3

Trial-to-trial changes by young adults (OPEN symbols) and older adults (FILLED symbols) in a) stability during walk-slips, b) stability during STS (STS) slips, c) hip height during walk-slips, and d) hip height during STS-slips. Shown are the group mean ± 1SD results for the first through fifth slips of the first slip block (S1, S2, S3, S4 and S5), as well as for the first and second reslip trials (S9 and S10 for walk-slips; RS1 and RS2 for STS-slips) induced after 3–5 nonslip trials. All values correspond to 300 ms after slip onset. Stability in (a) and (b) was defined as the shortest distance from the center of mass state (i.e. its normalized anteroposterior position and velocity relative to the base of support) and the model-derived limits of dynamic stability under slip conditions. Stability is a dimensionless quantity, with more positive values indicating greater stability against backward balance loss., Hip height was measured from the ground to the midpoint of the two hip joint centers and was expressed as a fraction of body height (bh). * = P < 0.05 between trials. † = P < 0.05 between young and older adults.

Fig. 4

Changes in the incidence of a) falls and b) backward balance loss with repeated-slip exposure during walking in young adults (OPEN squares) and older adults (FILLED circles). Shown is the percentage of participants who fell or experienced a backward balance loss during: the first and last slips of the first slip block (S1 and S8), the first, second, and last slips of the second slip block (S9, S10, and S16), and all the slips of the mixed block (S17 through S24). Also indicated are the occurrences and number of non-slip (NS) trials between blocks and within the mixed block. * = P < 0.05 between trials.