The properties of adaptation within the locomotor and balance control systems directed towards improving one's recovery strategy for fall prevention are not well understood. The purpose of this study was to examine adaptive control of gait stability to repeated slip exposure leading to a reduction in backward loss of balance (and hence in protective stepping). Fourteen young subjects experienced a block of slips during walking. Pre- and post-slip onset stability for all slip trials was obtained as the shortest distance at touchdown (slipping limb) and lift-off (contralateral limb), respectively, between the measured center of mass (COM) state, that is, position and velocity relative to base of support (BOS) and the mathematically predicted threshold for backward loss of balance. An improvement in pre- and post-slip onset stability correlated with a decrease in the incidence of balance loss from 100% (first slip) to 0% (fifth slip). While improvements in pre-slip stability were affected by a proactive anterior shift in COM position, the significantly greater post-slip onset improvements resulted from reductions in BOS perturbation intensity. Such reactive changes in BOS perturbation intensity resulted from a reduction in the demand on post-slip onset braking impulse, which was nonetheless influenced by the proactive adjustments in posture and gait pattern (e.g., the COM position, step length, flat foot landing and increased knee flexion) prior to slip onset. These findings were indicative of the maturing process of the adaptive control. This was characterized by a shift from a reliance on feedback control for postural correction to being influenced by feedforward control, which improved pre-slip stability and altered perturbation intensity, leading to skateover or walkover (>0.05 m or <0.05 m displacement, respectively) adaptive strategies. Finally, the stability at contralateral limb lift-off was highly predictive of balance loss occurrence and its subsequent rapid reduction, supporting the notion of the internal representations of stability limits that could be modified and updated, as a key component in the adaptive control.

A single session of repeated-slip exposure could improve community-dwelling older adults' resilience to postural disturbances and, hence, significantly reduce their annual risk of falls.

With aging, individuals' gaits become slower and their steps shorter; both are thought to improve stability against balance threats. Recent studies have shown that shorter step lengths, which bring the center of mass (COM) closer to the leading foot, improve stability against slip-related falls. However, a slower gait, hence lower COM velocity, does the opposite. Due to the inherent coupling of step length and speed in spontaneous gait, the extent to which the benefit of shorter steps can offset the slower speed is unknown. The purpose of this study was to investigate, through decoupling, the independent effects of gait speed and step length on gait stability and the likelihood of slip-induced falls. Fifty-seven young adults walked at one of three target gait patterns, two of equal speed and two of equal step length; at a later trial, they encountered an unannounced slip. The results supported our hypotheses that faster gait as well as shorter steps each ameliorates fall risk when a slip is encountered. This appeared to be attributable to the maintenance of stability from slip initiation to liftoff of the recovery foot during the slip. Successful decoupling of gait speed from step length reveals for the first time that, although slow gait in itself leads to instability and falls (a one-standard-deviation decrease in gait speed increases the odds of fall by 4 fold), this effect is offset by the related decrease in step length (the same one-standard-deviation decrease in step length lowers fall risk by 6 times).

Objectives To determine whether the fall-resisting skills acquired from a single perturbation training session can be retained for 6-months or enhanced by an intermediate ancillary session. Design A randomized controlled trial. Setting Biomechanics research laboratory. Participants Forty-eight community-dwelling elderly (>65 years). Intervention Initial perturbation training applied to all subjects using low-friction platforms to induce, unannounced blocks of repeated right-side slips, interspersed with non-slips. The single-session group retested with only one slip 6-months later. The dual-session group received an additional slip at 3-month, post initial session, followed by a retest slips at 6-months. Main Outcome Measures Slip outcome (incidence of falls and balance loss), dynamic stability (based on the center-of-mass position and velocity) and vertical limb support (based on hip height). Results Subjects in both groups significantly reduced fall and balance loss incidence from first to last training slips, which resulted from improved stability and limb support control. Both groups demonstrated significant retention in all outcome measures at 6-months compared to the first novel slip; although performance decay was evident in comparison to the last training slip. The ancillary slip at 3-months led to significantly better control of stability, and hence reduced balance loss outcome in the dual-session group at 6-months, than the single-session group. Conclusions Motor memory could be retained for 6-months or longer following a single-session of fall-resistance training, although a single “booster” slip could further impede its decay. Through the experience of slipping and falling, it may be possible to “inoculate” older adults against potentially life threatening falls.

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 sit-to-stands. All (N=174) were healthy and community-dwelling. Intervention Low-friction platforms induced unannounced blocks of 2–8 repeated slips, interspersed with blocks of 3–5 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 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.

. Retention of adaptive control over varying intervals: prevention of slip-induced backward balance loss during gait. J Neurophysiol 95: 2913-2922, 2006. First published January 11, 2006 doi:10.1152/jn.01211.2005. Stability improvements made in a single acquisition session with merely five slips in walking are sufficient to prevent backward balance loss (BLOB) at the end of session, but not after 12 mo. The purpose of this study was to determine whether the effect of an enhanced single acquisition session would be retainable if tested sooner, at intervals of Յ4 mo. Twentyfour young subjects were exposed to blocks of slip, nonslip, and both types of trials during walking at their preferred speed in the acquisition session. In each of the four follow-up sessions around 1 wk, 2 wk, 1 mo, and 4 mo later, these same subjects experienced only a single slip after eight to 13 unperturbed walking trials in an otherwise identical setup. Gait stability was obtained as the shortest distance between the measured center of mass (COM) state (position and velocity) and the mathematically predicted threshold for BLOB at preand postslip, corresponding to the instants of touchdown of the slipping limb and liftoff of the contralateral limb, respectively. During the acquisition session, pre-and postslip stability improved significantly, resulting in a reduction of BLOB from 100% in the first slip (S1) to 0% in the last slip (S24), with improvements converging to a steady state, that enabled all of the subjects to avoid BLOB, regardless of whether a slip occurred. During retest sessions, subjects' preslip stability was not different from that in S24, but was greater than that in S1. Their postslip stability was also greater than that in S1 but less than that in S24, resulting in BLOB at a 40% level. No difference was found in any of these aspects between each follow-up session. These adaptive changes were associated with a range of individual differences, varying from no detectable deterioration in all aspects (n ϭ 8) to a consistent BLOB in all follow-ups (n ϭ 3). Our findings demonstrated the extent of plasticity of the CNS, characterized by rapid acquisition of a stable COM state under unpredictable slip conditions and retention of such improvements for months, resulting in a reduced occurrence of unintended backward falling.

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