Repeated-slip training: an emerging paradigm for prevention of slip-related falls among older adults

Abstract

Falls frequently cause injury-related hospitalization or death among older adults. This article reviews a new conceptual framework on dynamic stability and weight support in reducing the risk for falls resulting from a forward slip, based on the principles of motor control and learning, in the context of adaptation and longer-term retention induced by repeated-slip training. Although an unexpected slip is severely destabilizing, a recovery step often is adequate for regaining stability, regardless of age. Consequently, poor weight support (quantified by reduction in hip height), rather than instability, is the major determinant of slip-related fall risk. Promisingly, a single session of repeated-slip training can enhance neuromechanical control of dynamic stability and weight support to prevent falls, which can be retained for several months or longer. These principles provide the theoretical basis for establishing task-specific adaptive training that facilitates the development of protective strategies to reduce falls among older adults.

Figure 1

The predicted feasible stability region (FSR) for both slip and nonslip conditions in the center of mass (COM) state-space (ie, its position and velocity). The position of the COM in the anteroposterior direction was expressed relative to the rear of the base of support (BOS) (X_COM/BOS) of the most recent foot to touch-down (ie, the heel of the sliding foot for slip onset) and normalized to foot length. The COM velocity in the anteroposterior direction was expressed relative to the velocity of the BOS (Ẋ _COM/BOS) and normalized to body height. During regular sit-to-stands prior to their exposure to slipping, 41 older adults' COM state at seat-off was around area 1, which was very near the boundary for backward balance loss for a slip. Subsequently, they all experienced backward balance loss during the first, novel slip. Following repeated exposures to slipping, their COM state at seat-off was shifted from area 1 to area 2, which was inside the stability region for slipping but near the forward balance loss region for the nonslip condition. Nearly all of them experienced forward balance loss when the slip stopped. After the nonslip block, they readjusted their COM state from area 2 to area 3, which is in the middle of the shaded area where stability can be maintained regardless whether a slip occurs or does not occur.

Figure 2

Onset of the unintended hip descent (thin solid vertical line) after the slipping (right) limb touch-down (RTD) from 2 typical young subjects: (A) one who recovered and (B) one who fell. This instant is identified as the point when the hip height (ie, the vertical distance from the ground to the midpoint between 2 hip centers) trajectory (thick broken line) traversed below 3 standard deviations (shaded area) from its mean trajectory (thick solid line) taken from the subjects' regular (unperturbed) gait. This onset timing is nearly identical to the onset of negative vertical (downward) hip velocity (thin broken vertical line). Hip height is normalized as percentage of body height (%bh).

Figure 3

During preparation for chair-rise from movement initiation to losing contact with chair (seat-off) when a slip was unexpectedly induced (∼slip onset), 22 older adults (O) did less net concentric (positive) work, on average, than that generated by the hip extensors (2 half circles: counterclockwise-clockwise) of 43 young adults (Y). This age-related difference (Y>O) caused less increase in upward momentum and in potential energy required for ascending (upward arrow and triangle) and might have predisposed a greater proportion of older adults to falls. All of them took a step for recovery, but from seat-off to step lift-off, fallers (F) did less concentric work generated by the hip extensors (counterclockwise-clockwise circles) and knee extensors (clockwise-counterclockwise circles) than those who recovered (R) (R>F). From step lift-off to touch-down, fallers' knee extensors (clockwise-counterclockwise circles) failed to resist gravitational effect, resulting in more negative (eccentric) work than in those who recovered (R>F), causing knee buckling. Although those who fell and those who recovered both experienced unintended hip descent (limb collapse), hip height for those who fell was significantly lower than that for those who recovered at the step touch-down (downward arrow and triangle for descent, F>R). This mechanism of fall applied to all fallers regardless of their age.

Figure 4

(A) Fall incidence decreased exponentially with repeated exposure to a slip during a sit-to-stand task and at a similar rate in young and older adults. Shown are the number of falls by each age group in each of the 5 trials of the first slip block and the corresponding best-fit exponential relationship identified through nonlinear regression. The parameter for initial fall incidence (C) equaled 14.9 and 30.0 in young and old adults, respectively. The exponential relationship explained 99.7% of the variance in the number of falls, with P<.05 for all parameters. Fall incidence among both groups decreased by the same factor of 3 following each slip exposure. (From: Pavol MJ, Runtz EF, Edwards BJ, Pai YC. Age influences the outcome of a slipping perturbation during initial but not repeated exposures. J Gerontol A Biol Sci Med Sci. 2002;57:M496–M503. Copyright © The Gerontological Society of America. Reproduced by permission of the publisher.) (B) Although instability is neither a sufficient nor a necessary condition to cause falls that result from vertical hip descent and limb collapse, improved stability directly resulted in the reduction in fall incidence among older adults (i=0 for balance loss and 1 for fall in the exponential equation). The rate of incidence decline was almost twice as fast for falls as for balance loss (exponential rate constants of −1.07/trial and −0.48/trial, respectively).

Figure 5

(A) Changes in incidence of balance loss (percentage) and (B) in pre- and post-slip stability (means ± SD), on the first and last slip trials of the acquisition session (S1 and S24, respectively) and the slip trial in each of the follow-up sessions conducted at 1 week, 2 weeks, 1 month, and 4 months after the acquisition session. More positive values indicate greater stability. N=22 for all trials except for the follow-up session at 4 months (n=17). Significant differences with respect to the preceding trial included in the statistical analysis are indicated (*=P<.05, **=P<.001). Data illustrated in Figure 5 are from Bhatt TS, Wang E, Pai YC. Retention of adaptive control over varying intervals: prevention of slip-induced backward balance loss during gait. J Neurophysiol. 2006;95: 2913–2922. Figure 5B, modified and reprinted from the article by Bhatt et al, is used with permission of the American Physiological Society.

Figure 6

(A) Decrease in number of balance losses from first to fourth slip trials for the acquisition (open squares) and the 1-year follow-up (closed circles) sessions. (B) Gait stability (means ± 1 SD) at pre-slip touch-down of slipping limb (open symbols) and post-slip lift-off of contralateral limb (closed symbols) from the acquisition (circles) and the follow-up (squares) sessions for regular walking and first through fourth slip trials (S1 to S4). Significant differences in gait stability (for both pre- and post-slip events) between the 2 sessions for each trial are indicated (*=P<.05). Note that there was no significant difference in post-slip stability and slip outcome, with all subjects experiencing a backward balance loss on the first slip induced 1 year after the acquisition session. Also note the differential increase from pre- to post-slip stability for the follow-up session compared with the acquisition session on the second slip trial (S2), indicating the presence of a priming effect induced by the first follow-up slip. Data illustrated in Figure 6 are from: Bhatt TS, Pai YC. Long-term retention of gait stability improvements. J Neurophysiol. 2005;94:1971–1979. Figure 6B, modified and reprinted from the article by Bhatt and Pai, is used with permission of the American Physiological Society.