Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 1 , Pages 34-42, January 2009

Prevention of Slip-Related Backward Balance Loss: The Effect of Session Intensity and Frequency on Long-Term Retention

  • Tanvi Bhatt, PT, PhD
    • ,
  • Yi-Chung Pai, PT, PhD

      Affiliations

    • Corresponding Author InformationReprint requests to (Clive) Yi-Chung Pai, PT, PhD, Dept of Physical Therapy, University of Illinois at Chicago, 1919 West Taylor St, Room 426 (M/C 898), Chicago, IL 60612

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL

Article Outline

Abstract 

Bhatt T, Pai Y-C. Prevention of slip-related backward balance loss: the effect of session intensity and frequency on long-term retention.

Objective

To examine the effects of session intensity (number of slip exposures) and frequency on the retention of acquired adaptation for prevention of backward balance loss after repeated-slip training.

Design

A 4-group, randomized, and controlled study.

Setting

Biomechanics research laboratory.

Participants

Healthy young subjects (N=46; 21 men).

Interventions

Twenty-four subjects experienced a high-intensity session of 24 repeated right-side slips; 12 received additional single-slip sessions at a frequency of 1 week, 2 weeks, and 1 month, whereas the rest got no ancillary training. Another 24 subjects received a low-intensity initial session of a single slip; 12 received the same high-frequency ancillary training, whereas the rest got none. All groups were retested with a single slip 4 months after the first session.

Main Outcome Measures

The incidence of backward balance loss, gait stability, and limb support.

Results

The high-intensity groups, irrespective of ancillary training, displayed similar improvements in all 3 outcome measures. Remarkably, the low-intensity group receiving ancillary training also significantly improved in all measures, with retention comparable to that observed in the other 2 groups. A single-slip exposure without ancillary sessions was insufficient to yield a longer-term effect.

Conclusions

Frequent ancillary sessions may be unnecessary for slip-related fall prevention up to 4 months if the initial session intensity is sufficient. Furthermore, the minimum of a single slip may be as effective if the subject is exposed to frequent ancillary sessions.

Key Words: Accidental falls, Gait, Learning, Rehabiliation

Abbreviations: ANOVA, analysis of variance, BOS, base of support, COM, center of mass, CNS, central nervous system

 

THE RISING SUSCEPTIBILITY to falling with increasing age1, 2, 3 poses a significant health threat to the older adult population. Falls are the leading cause of injury-related hospitalization, leading to decreased mobility and quality of life. Approximately 81% to 98% of hip fractures annually are caused by falls,4, 5 with slips comprising 40% of outdoor falls among community-living adults 70 years of age or older.6 These fractures require surgical intervention and extensive postsurgical management at great cost both to patients and to the health care system.2, 4, 7, 8, 9 Thus, reducing the incidence of slip-related falls, which is frequently preceded by a backward loss of balance, is essential.

General exercise-based, combined training paradigms (which could include either 1 or multiple components such as flexibility, strength, endurance, posture control, and balance training) are proposed as one method of improving older adults' functional capacity and preventing falls and fall-related fractures in older adults.10, 11, 12, 13, 14, 15, 16, 17, 18 The fall reduction, although evident, has not always proved to be significant in the studies conducted.16, 19, 20, 21, 22, 23, 24 Similarly, although improvements in postural stability,26, 27 functional balance tests,28 or the fall risk ratio23, 29 with computerized balance training platforms have been linked to the reduction of fall likelihood, evidence is lacking on the direct causal effect of such association.

In contrast, repeated slips induced during activities of daily living, such as sit to stand and walking, can directly reduce falls and balance loss incidence resulting from the improvements in a person's feed-forward and feedback control of dynamic stability displayed during the recovery from a slip.30, 31, 32, 33 Dynamic stability is defined as the ability to restore one's COM state (ie, its position and velocity) without altering his/her original or intended BOS after an externally imposed perturbation or volitional movement.34 Recent findings, in adaptive response to repeated-slip training, have revealed improvements in limb support, the second determinant of fall-recovery outcome in response to slips.35, 36 Although the dynamic stability can be quantified by comparing one's COM position and velocity (ie, its motion state) with the boundary against backward balance loss,37, 38, 39 limb support can be assessed based on the amount of descent in hip height after a slip.35, 36 Limb support is defined as the amount of vertical support provided by the limbs to prevent unintended hip descent. The unintended hip descent is defined when the hip is located 3 SDs below this person's average hip height in an unperturbed gait and when the hip velocity prematurely reverses in the downward direction.34

Such repeated-slip training paradigms, although beneficial, may nevertheless be cumbersome and expensive to implement (eg, require slip-inducing device and protective harness equipment, longer testing time, personnel skills, and liability for injury from simulated falls). Furthermore, this method also involves a greater risk of injury than general exercise or non–perturbation-based training protocols (strength training, Tai-Chi, etc). It is unclear as to the extent to which this new training paradigm can be further improved in terms of efficiency and risk reduction.

It is evident that practice schedules are an important consideration in motor learning. In particular, incorporating random practice (contextual interference)40, 41, 42 and overlearning (ie, continued practice of a task after having reached some success criterion)43, 44, 45 can lead to improved retention effects. For example, a single low-intensity acquisition session consisting of a brief exposure to a block of slips (5 slips only) without any additional ancillary sessions was insufficient to yield retention in stability for the prevention of backward balance loss 12 months later.46 Yet, overlearning by using such principles as combined, blocked, and alternating practice (with an alternating slip, no-slip, and reslip paradigm) yielded significant retention of acquired improvements in the stability and incidence of balance loss up to 4 months.33 Our unpublished findings indicated that a similar retention may exist in limb-support improvements. Such retention may result from 3 additional ancillary sessions, consisting of a single-slip exposure at 1-week, 2-week, and 1-month intervals. It is unclear the extent to which the intensity of the initial session can be reduced but yet achieve the same level of retention. It is equally unclear whether ancillary training is needed to achieve such retention after a low-intensity initial training session or whether it becomes unnecessary in a high-intensity one. Answers to questions such as these would improve the efficiency of fall-prevention training paradigms suitable for a variety of older adult populations ranging from the independent-living community to the older adults who may be frail or institutionalized. If the very minimum of a single-slip exposure used in the initial and subsequent ancillary sessions could yield similar benefit as a high-intensity session, the frail adults who could not tolerate high repetitions may still be able to benefit from this emerging paradigm for fall prevention.34 Furthermore, less slip exposure could, in itself, reduce the risk of training-related injuries.

The purpose of this study was to determine the effect of the practice schedule (session intensity and training frequency) on longer-term retention (ie, up to 4 months) for the prevention of balance loss and falls from a slip. Subjects were randomly assigned to 4 groups each consisting of a combination of intensity and frequency practice schedule. First, we hypothesized that a high session intensity (high intensity, low frequency) would prove to be efficient, such that there would be no measurable difference in performance improvements at the 4-month retest after their initial session compared with improvements induced by the additional training frequency in the high-intensity, high-frequency group. Second, we hypothesized that low session intensity could prove as effective as high-intensity, however, only when combined with high training frequency. Thus, we expected that the measured performance improvements induced by the low-intensity, high-frequency training would be comparable to the high-intensity, low-frequency training, whereas the performance of the low-intensity, low-frequency training would be lower than the low-intensity, high-frequency or the high-intensity, low-frequency groups.

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Methods 

Subjects 

Forty-nine young subjects (26±5y, 21 men) participated in the study after being screened for exclusionary factors including neurologic, musculoskeletal, cardiopulmonary, other systemic disorders, and selected drug usage. The subjects were randomly divided into 4 groups: (1) high intensity, high frequency (24 slips on initial session and 3 ancillary single slip training sessions, n=13); (2) high intensity, low frequency (24 slips on initial session and no ancillary sessions, n=13); (3) low intensity, high frequency (single slip on initial session and 3 ancillary single slip training sessions, n=11); and (4) low intensity, low frequency (single slip on initial session and no ancillary sessions, n=9). Before their participation, all subjects gave informed consent as approved by the institutional review board. One subject from the high-frequency, low-intensity group; 1 from the low-intensity, high-frequency group; and 2 subjects from the low-intensity, low-frequency group did not complete the 4-month retest and were excluded from the analysis.

Experimental Setup 

Slips were induced by means of a device consisting of a movable component attached to a stationary metal beam structure (2.50×0.30×0.21m) anchored to the top of 2 rectangular force platesa bolted to the ground. The movable component comprised a metal plate (0.65×0.30×0.006m, 2.6kg) and its support system.47 This plate can be either locked for regular walking trials or released to simulate a slip. The friction coefficient of the device was consistently below 0.05, within its preset maximum travel distance. A second similar device was placed adjacent to the first for recording the ground reaction forces acting on the nonslipping limb. The top plate was locked and served as a decoy plate to camouflage the sliding plate. These devices were embedded in a 7-m walkway and hidden by the stationary platforms surrounding them.

Slips were induced by a computer-controlled release mechanism that unlocked the moveable platform at the beginning of each trial without the knowledge of the subject. Once released, the moveable platform was free to slide on the linear bearings for up to a maximum travel distance of 1.5m on the right. A computer program written in LabViewb was used for online monitoring of the ground reaction force and generation of the lock-release signal. The subjects wore their own athletic shoes and a full-body safety harness attached to a manually driven trolley on a ceiling-mounted I-beam via an intervening load cell (fig 1).

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  • Fig 1. 

    Schematic diagram of the experimental setup with the approximate position of the subject at touchdown of the training (right limb). The unfilled circles indicate positions of passive-reflective markers on the body segments and movable platform. The solid (right side) and dotted (left side) gray lines joining the markers represent the body-segment links used to calculate the whole body center of mass. The I-beam and safety harness system are much higher than shown (9m above the ground). The I-beam extends the length of the 7-m walkway 49. The low-friction, nonmotorized moveable top plates (right and left) are mounted on a frame with linear bearings. These devices were locked and embedded in a 7-m walkway and made less apparent by the stationary decoy platforms. Once released, the right moveable platform was free to slide on along the sliding track on the linear bearings. The left plate remained locked and served as a decoy.

Protocol 

All subjects, regardless of group, performed 10 regular walking trials at a self-selected speed; in these trials, they were instructed to walk in their ordinary manner and informed that they “may or may not be slipped” on any trial. Subjects were also told that on the occurrence of a slip, they should try to recover their balance and continue walking. Without revealing the purpose, the experimenter adjusted each subject's starting position after each trial so that his/her slipping (right) foot would land entirely on the moveable plate at touchdown. All subjects were able to take at least 3 steps before stepping on the moveable plate. The first slip was induced on the eleventh trial, without explicit warning or practice.

After the first slip, subjects having a high-intensity component in their practice schedule (ie, high-intensity, high-frequency and high-intensity, low-frequency groups) were exposed to a repeated-slip training paradigm. The subjects were told that they may or may not be exposed to slipping on the subsequent trials. The entire training paradigm, including the first trial, consisted of a block of 8 repeated slips (S1–S8), a block of 3 nonslip trials (NS1–NS3), another block of 8 slips (S9–S16), a second block of 3 nonslip trials (NS4–NS6), and a final block of 15 mixed trials. The sequence of the mixed block of trials was consistent for each subject: S17 and S18, NS7 and NS8, S19, NS9, S20, NS10, S21 and S22, NS11 and NS12, S23, NS13, and S24.33

After the initial session, subjects having a high-frequency component in their practice schedule (ie, high-intensity, high-frequency and low-intensity, high-frequency) were followed by 3 single-slip ancillary sessions conducted at intervals of 1 week, 2 weeks, and 1 month. For each ancillary session, the setup and instructions were identical to those of the initial session. The protocol consisted of only 1 unannounced slip on the training (right) side induced after 8 through 13 regular walking trials. The instructions given to the subjects throughout the retests were also consistent with those given for the initial session.

All subjects returned for a retest session at 4 months after their initial session. The protocol followed was exactly similar to that used for the single-slip ancillary sessions (eg, 1 slip exposure after 8 through 13 regular walking trials). The exact return date for all the follow-up sessions was within 1 or 2 days of the scheduled date for the 1-week and 2-week sessions and 3 to 4 days beyond or short of a month for the 1-month and 4-month sessions.

Data Collection and Reduction 

A set of 24 full-body light-reflective markers was attached to the bilateral upper and lower extremities and the torso, whereas 1 marker was attached to each moveable platform. Marker coordinates were recorded at 120Hz by using an 8-camera motion capture system.c Marker displacement data were low-pass filtered at marker-specific optimal cutoff frequencies (range, 4.5–9Hz) by using a recursive second-order Butterworth Filter.48 Force plate, harness load cell data, and trigger-release onset signal were collected at 600Hz by using a 64-channel, 16-bit A/D converter.b The ground reaction force data were time synchronized with the motion data externally at the time of data collection.

Backward Loss of Balance 

The outcome of a slip trial was classified either as a backward loss of balance or no backward loss of balance. When the contralateral limb landed posterior to the sliding heel, with negative values in postslip step length during the slip, the trial was classified as a loss of balance trial with recovery stepping. Conversely, trials in which the contralateral limb landed anterior to the sliding heel (having positive postslip step length) were classified as “no loss of balance” trials in which recovery stepping was not needed and forward progression was not disrupted.49 Analysis was restricted to the anteroposterior direction. The instances of step liftoff and touchdown were identified from the vertical ground reaction forces.

Gait Stability 

The COM position and its velocity were computed from the kinematic data by using known sex-dependent segmental parameter information in a 13-segment representation of the body.50 The position of the COM in the anteroposterior direction was expressed relative to the rear of the BOS of the foot most recent to touchdown (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 and normalized as a dimensionless fraction of 51 where g is the acceleration caused by gravity and h is height of the subject.

Stability was assessed through comparison of the COM state relative to the BOS (ie, its position and velocity) with the previously published threshold values for backward balance loss under slip conditions.38 Stability was defined as the shortest distance between this predicted boundary for backward balance loss and that of the instantaneous COM state.30, 49 If the COM state is below the threshold (or to the left), the stability value will be negative, and if the COM state is above the threshold (or to the right) (fig 2), the stability value will be positive. If the COM state lies on the threshold, it will have a stability value of 0. More positive values indicate greater stability against backward balance loss.30, 49 Preslip stability was measured at the touchdown of the slipping limb. Postslip stability was recorded at the liftoff of the contralateral limb.

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  • Fig 2. 

    The instantaneous gait stability for an instantaneous COM state (diamond) is the shortest distance (double-headed arrow) between the boundary and the COM state. The COM state consists of the anterioposterior COM position and its velocity relative to the BOS (XCOM/BOS, ẊCOM/BOS, respectively). The thick black line represents the boundary for backward loss of balance. The XCOM/BOS, ẊCOM/BOS are normalized to foot length and , respectively, where g is acceleration because of gravity, and h is the body height.

Limb support 

The hip vertical motion has been previously associated with limb support.35, 36 Therefore, we investigated the hip height. The hip height was defined as the height of the bilateral hip midpoint (obtained from the hip joint centers) to the surface of the platform. A lower hip height is indicative of poorer limb support, leading to a greater likelihood of falls after the slip.36

Statistics 

To test for measurable retention effects for the incidence of backward balance loss, the Wilcoxon signed-rank test was performed between the initial session slip and the 4-month retest across the groups. Significance, if obtained, was followed up with similar tests within each group. The main effect of group at each session was compared by using the Kruskal-Wallis H test. The main effects of were resolved by using the Mann-Whitney U test for between-group comparisons at each session. Each recovery trial was categorized as a balance loss (value=0) or no balance loss (value=1) and analyzed as described previously.

To analyze the parametric outcome measures (pre- and postslip stability and hip height), an ANOVA for repeated measures was performed on each variable with session as the within-group repeated factor and these 4 predetermined groups (high intensity, high frequency; high intensity, low frequency; low intensity, high frequency; and low intensity, low frequency) as the between-group factor. Significant main effects and interactions were resolved by performing planned within-group and between-group comparisons with paired and independent t tests, respectively. To test the effect of frequency, the high-intensity, high-frequency group was compared with the high-intensity, low-frequency group and the low-intensity, high-frequency group with the low-intensity, low-frequency group. To test the effect of intensity, the high-intensity, high-frequency group was compared with the low-intensity, high-frequency group and the high-intensity, low-frequency group with the low-intensity, low-frequency group. Lastly, to examine the effects of frequency versus intensity the high-intensity, low-frequency group was compared to the low-intensity, high-frequency group. The statistical power and the partial η2 (the proportion of total variability attributable to a factor) are reported for the ANOVAs. Absolute P values between .05 and .01 for the planned comparisons will be reported. A significance level of .05 was used for all the analyses. Analyses were performed by using SPSS.d

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Results 

Backward Loss of Balance 

There was a significant decrease in the incidence of balance loss from the first slip to the 4-month retest across groups (z=–5.69, P<.001) with no difference in the incidence of balance loss between groups on the first slip of the initial session (χ23,53=0.00, P=1.0); however, a significant difference was found between groups at the 4-month retest (χ23,48=8.05, P=.04). The analysis on the effect of session frequency clearly indicated that high-intensity training alone was sufficient for retention without the need for frequent ancillary sessions. The incidence of balance loss for subjects who had received high-intensity, low-frequency training remained significantly lower 4 months later compared with their initial slip of the first session (100%–50%; P<.001). This training effect was very similar to subjects who received high-intensity and high-frequency training and who, as a group, reduced their incidence of balance loss from 100% at the first slip of the initial session to 39% at the 4-month retest (P>.05). As a result, there was no difference in the incidence of balance loss between the high-intensity, high-frequency and the high-intensity, low-frequency groups (P>.05).

Similarly, low-intensity training with high-frequency ancillary sessions in the low-intensity, high-frequency group was shown to reduce the incidence of balance loss from 100% at the initial session to 55% at the 4-month retest session (P=.02), with no difference between the low-intensity, high-frequency and the high-intensity, low-frequency and high-intensity, high-frequency groups (P>.05). In contrast, there was no improvement in the group receiving both low-intensity and low-frequency training from the initial to the retest session (100% vs 78%, respectively; P>.05) (fig 3). At the retest, both the high-intensity, low-frequency and low-intensity, high-frequency groups were noticeably better than the low-intensity, low-frequency group in resisting balance loss (P=.05 and P=.07, respectively); however, the difference between the high-intensity, high-frequency and low-intensity, low-frequency groups was the greatest (P=.02).

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  • Fig 3. 

    Incidence of backward balance loss in percentage (%) shown on the first slip of the initial session (Initial) and the 4-month retest slip (4-mo) for the 4 participating groups: High-intensity, high-frequency (HI_HF); high-intensity, low-frequency (HI_LF); low-intensity, high-frequency (LI_HF); and low-intensity, low-frequency (LI_LF). *A significant level of P<.05 for the independent and paired t tests performed. The exact values for P<.10 have been reported as well for these comparisons.

Stability and Limb Support 

The continuous variables, pre- and post-slip stability and hip height also demonstrated changes similar to incidence of balance loss. Preslip stability had a significant main effect for session (F1,41=31.62, P<0.001, partial η2=0.43, power=1) and a significant session × group interaction (F3,41=3.23, P=0.03, partial η2=0.2, power=0.7) but no main effect for group (F3,41=1.008, P>0.10, partial η2=0.07, power=0.3). Postslip stability had a significant main effect for session as well (F1,41=78.78, P<0.001, partial η2=0.67, power=1) and a significant session × group interaction (F3,41=3.08, P=0.038, partial η2=0.18, power=0.68) but no main effect for group (F3,41=1.89, P=0.14, partial η2=0.13, power=0.45). Hip height also changed across sessions (F1,41=21.42, P<0.001, partial η2=0.34, power=0.99) with a significant session × group interaction (F3,41=2.82, P=0.04, partial η2=0.18, power=0.64) and no main effect of group (F3,41=0.709, P>0.1, partial η2=0.05, power=0.2).

The retention in both the high-intensity, low-frequency and the high-intensity, high-frequency groups was accompanied by improvements in stability and in limb support (with higher hip height) when the first slip of the initial session was compared with the retest 4 months later (P<.01 for all variables for both groups) (figs 4A, B, and C). There was no between-group difference in any of these variables (P>.05 for all comparisons). The low-intensity, high-frequency group also showed significant improvements in the pre- and postslip stability and in limb support from the initial to the 4-month retest session (P<.01 for all these measures). There was no difference in pre- and postslip stability and limb support between the high-intensity, low-frequency and low-intensity, high-frequency groups (no significant main effects or interaction, P>.05 for all variables). However, both postslip stability and hip height were greater in the high-intensity, high-frequency group compared with the low-intensity, high-frequency group (P=.04 for both comparisons).

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  • Fig 4. 

    Means and SDs of (A) preslip stability, (B) postslip stability, and (C) hip height on the first slip of the initial session (initial) and the 4-month retest slip (4-month) for the 4 participating groups: high-intensity, high-frequency (HI_HF); high-intensity, low-frequency (HI_LF); low-intensity, high-frequency (LI_HF); and low-intensity, low-frequency (LI_LF). Note that the preslip stability was obtained at instant of preslip touchdown of the slipping (right) limb. Postslip stability and hip height were obtained at the instant of postslip liftoff of the contralateral (left) limb. Hip height was normalized to body height (bh). More positive values of stability indicate greater stability. *A significant level of P<.05 for the independent and paired t tests performed. The exact values for P<.10 have been reported as well for these comparisons.

However, in comparison to the training groups, there was no improvement in any of the variables at the 4-month retest in the group receiving low-intensity and low-frequency training in the form of a single slip (P>.05 for all variables). In comparison with the low-intensity, low-frequency group, both the high-intensity, low-frequency and low-intensity, high-frequency groups also had a significantly greater preslip (P=.04 for the high-intensity, low-frequency group; P=.03 for the low-intensity, high-frequency group) and postslip stability (P=.05 for the high-intensity, low-frequency group; P=.04 for the low-intensity, high-frequency group). Similarly, both groups had significantly better weight support, with a higher hip height than the low-intensity, low-frequency group (P=.04 for the high-intensity, low-frequency group; P=.05 for the low-intensity, high-frequency group) (see figs 4A, B, and C). The low-intensity, high-frequency group had a significantly lower stability and limb support compared with the high-intensity, high-frequency group as well (P<.05 for all variables), with this decrease being most pronounced compared with the high-intensity, low-frequency and low-intensity, high-frequency groups.

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Discussion 

In accordance with our hypothesis, the repeated-slip paradigm was efficient in improving performance to slips, such that the motor training induced by a single high-intensity session was retained for at least 4 months. The additional ancillary sessions did not yield significant benefits in stability control to reduce the incidence of backward balance loss shown by the comparable results found between the high-intensity, high-frequency and the high-intensity, low-frequency groups. Alternatively, in accordance with our second hypothesis, low-intensity training could also yield significant retention when combined with the ancillary sessions. This was shown by the comparable performance between the low-intensity, high-frequency and the high-intensity, low-frequency groups. Nonetheless, a single slip alone, without any ancillary training, did not generate sufficient retainable benefits at 4 months.

Our findings from the first hypothesis were appealing, indicating that with sufficient stimuli from the high slip repetitions in a single session, the subject was able to retain the acquired motor skills for prevention of balance loss. Our previous findings have indicated a dominant role of stability and limb support (as measured by hip height) for recovery from slips during both tasks of sit-to-stand35, 36 and gait (unpublished data). Subjects in both the groups receiving the intense session of repeated-slip training showed a significant increase in both pre- and postslip stability, explaining the approximately 60% success rate in the prevention of backward balance loss and the diminished need for recovery stepping.32 The increase in hip height from the first slip of the initial session to the 4-month retest further enhanced the success of recovery from slip by providing adequate limb support even under low-friction conditions, enabling subjects to maintain a natural progression in stepping with their contralateral limb. It must be noted that postslip stability showed the greatest difference between the groups, which has been associated with the most accurate prediction of the incidence of balance loss32 compared with the other 2 variables.

Although the finding that a single high-intensity training session can generate long-term retention is very promising, extrapolation of such a paradigm to a variety of populations may be premature. For example, individuals may not be able to tolerate such a large number of slips (ie, about 40 trials with around 24 slips). However, results from our second hypothesis may prove to be promising in this situation. A single-slip exposure when repeated frequently generated a cumulative effect resulting in a significant increase in performance over a long period of 4 months. The results suggest that there is a coupling between the intensity of the initial session and the frequency of training in which simultaneously increasing one and decreasing the other could yield similar effects, ultimately resulting in long-term retention of adaptive skills of preventing a backward balance loss in slips.

The similar motor retention observed between the group that received a single session of high-intensity training and the group that received the high-intensity training with frequent ancillary sessions is not surprising, and this finding can be explained based on emerging concepts on the mechanisms of acquisition and storage of motor memory. It is proposed that the CNS acquires new sensory-motor relationships or strategies through the process of adaptation to enhance stability to cope with changes in external constraints and prevent the incidence of balance loss and falls. Such a process is associated with building or modifying internal representation of one's stability limits for the prevention of backward balance loss after forward BOS perturbations.30, 32 After repeated slips, adaptive improvements possibly shift from reliance on sensory feedback-based error correction to a feedforward influenced, recalibrated control that regulates BOS perturbation kinematics and limb support.32, 46 Such a shift would also result in developing an enhanced memory from the short-term labile state to a longer-lasting stable state through the process of consolidation.52, 53, 54, 55 However, once in the consolidated stage, it is difficult to disintegrate the memory from interference resulting from competing or disrupting factors. It is proposed that if a single session with sufficient training intensity by itself served sufficient to induce permanent structural changes, additional training sessions, even if provided, might not serve to induce any further changes in performance. The relatively constant performance previously shown on each ancillary session by subjects who received high-intensity training on the initial session further supported the above postulation.33

The poorer results on the retest slip compared with the last (twenty-fourth) slip of the initial session in the high-intensity, high-frequency and high-intensity, low-frequency groups also indicated that the stored motor memory was subject to decay. Apparently, providing additional ancillary sessions was not able to further prevent this decay in the high-intensity, high-frequency group. Our previous findings33 indicated that motor memory decay was associated largely with postslip stability control rather than with feedforward control of preslip stability. Motor memory decay may be affected by subjects' inability to retain the minimal feedback-based reactive control, still needed postslip, to control the BOS perturbation intensity once perturbation onset occurs.32 Such evidence suggests that adaptation in feedback control mediated by reflex responses may not be transformed into permanent representations similar to that of triggered responses or motor programs within the CNS. Regular gait pattern is re-enforced on a daily basis and may be the interference in the consolidation processes, leading to decay in motor memory of the training-acquired gait pattern. The performance of the high-intensity, low-frequency and high-intensity, high-frequency groups was still significantly better than that of the first slip in the initial session, indicating retention still overweighed decay at that instant.

Our results further suggest that increments in motor memory could continue to occur over spaced time intervals if the initial experience or training did not result in sufficient memory saturation, as shown by the improvements in the low-intensity, high-frequency group. The session-to-session performance improved in the low-intensity, high-frequency group and appeared to resemble the learning process observed in the trial-to-trial improvements of the high-intensity training in the initial session for the high-intensity, low-frequency group. Our previous results32, 33 have indicated a trial-to-trial reduction in balance loss from 100% to less than 10% within 3 to 5 trials during the initial training with repeated slip exposure. We observed a similar trend of session-to-session improvement with the single-slip exposures in the low-intensity, high-frequency group spaced at least 1 week apart where backward balance loss reduced from 100% on the first slip to 67% at the 1-week session, to 58% at the 2-week session, and about 40% at the 1-month training session (fourth slip exposure). With each slip exposure, the CNS can update its internal representation of stability limits to better control its COM state and prevent balance loss when experienced with a similar perturbation in future. The modification of the stability thresholds with each session would simultaneously allow the memory to be consolidated, resulting in permanent changes that are harder to fade away and explaining the additive effect of training seen in this group. It is postulated that the consolidation of memory could be as fast as a few minutes to a few hours to even as long as months.54, 56 Thus, the consolidation could have occurred rapidly by using high-intensity, large repetition training but could have also been achieved as a gradual process with low session intensity but high session frequency.

It is proposed that the adaptive effect generated by an environmental threat is proportional to its intensity or the severity of the consequences in case of a failure. Exposure to a single slip is sufficient to induce immediate adaptive changes such that approximately 50% of subjects prevent the incidence of balance loss and 80% of the incidence of falls on the second slip of the same session.33 The adaptation response is conditioned depending on the penalties imposed by the CNS for an inappropriate response and the potential of experiencing an injury.57, 58 Fear-conditioning studies59, 60 in mice have shown that a single acquisition session is sufficient for long-term retention of the acquired stimulus-response behavior within the CNS.59, 60 Thus, it was speculated whether a single slip generated by our sliding device on the first session, having large displacement and velocity profiles, could in itself be sufficient to induce longer-term retention lasting beyond a single session. Our results indicated otherwise; a single slip could not significantly reduce the incidence of backward balance loss on the 4-month retest. However, it must be noted that for 1 in 10 subjects this single slip was sufficient to induce longer-term memory changes such that he/she had an improved outcome to slip at 4 months.

However, the question still arises if the findings revealed in the present study could be reproduced among healthy older adults or could be generalized to everyday life situations. Previous balance retraining studies13, 23, 25, 28, 61, 62 have tested and observed improvements up to several months (4–6mo); however, the training was extensive, consisting of several sessions per week continuing for several months.13, 23, 25, 61, 62 Furthermore, there is a limited correlation of such improvements with a reduction in fall incidence. Evidence from motor learning for skilled voluntary tasks have indicated that older adults can completely retain an acquired skill for 2 years63 and partially up to 5 years64 without any intervening rehearsals. In fact, even after 5 years, they retain the ability to quickly reacquire the skill.64 Although the retention of fine motor skills might be different from that of locomotor skills, there is evidence that older adults can display similar adaptability65 and retention (from our preliminary study) as the young, with a strong likelihood of future fall prevention as well (unpublished data). Further transfer of such acquired motor skills to the opposite limb66 and other environmental contexts has also been shown in the young. Despite the known deterioration in other sensorimotor and cognitive processes with human aging, evidence is emerging revealing the components of motor adaptation and memory that could be preserved and remain intact in the presence of these aging processes and that could be exploited to reduce fall incidence. Hence, such a relatively simple approach of repeated slip exposure can potentially have a lasting practical impact.

Finally, because slip-related falls can cause serious injuries even among healthy, active young adults, the acquisition and retention of fall-resisting skills as proposed in the present study can benefit these people. Certainly, this paradigm to prevent future falls can be particularly attractive to older adults who are especially prone to having bone mass loss and vulnerable to fracture. At the very least, our findings derived from healthy young adults have provided a strong evidence-based platform and sound rationale to extend such investigation to other populations.

Study Limitations 

Because the statistical power could have been compromised by the small sample size in each group, corrections for multiple comparisons were not performed so as not to further increase chances of a type II error. Because of the chance there could be a type I error (rejecting the hypothesis when it is true), results should indeed be considered preliminary and be interpreted and extrapolated with caution. Our analysis indicated that a sample size of 20 subjects in each group would indeed result in significant difference at the corrected significance level of .02 with at least a power of .80 between the low-intensity, low-frequency group, and the high-intensity, low-frequency and the low-intensity, high-frequency groups on each of the outcome variables at the 4-month retest. However, in concurrence with our results, the same larger sample size would still fail to yield significant differences between the high-intensity groups with and without ancillary training and between these 2 groups and the low intensity group with ancillary training.

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Conclusions 

We propose that a practice schedule consisting of a single training session incorporating principles of blocked and random practice and overlearning could be applied to the healthy younger and community dwelling older adults to induce longer-term retainable effects for prevention of slip-related falls. However, a single slip exposure could have greater appeal to those frail individuals who have limited tolerance or inability to withstand intense training consisting of lengthy session duration. For these individuals, we propose that the single-slip training paradigm can still be effective when combined with frequent ancillary sessions. We conclude that further research is required to determine whether this type of training actually leads to fewer slip related falls in older adults and that the final choice of the training protocol in this population would depend on the balance of patients'/clients' tolerance to repeated slips and the costs of conducting frequent ancillary sessions.

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Acknowledgments 

We thank Feng Yang, PhD, and Ting-yun Wang, MSc, for assisting in data collection and processing.

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References 

  1. Tinetti ME, Speechley M, Ginter SF. Risk factors for falls among elderly persons living in the community. N Engl J Med. 1988;319:1701–1707
  2. Rubenstein LZ, Josephson KR, Robbins AS. Falls in the nursing home. Ann Intern Med. 1994;121:442–451
  3. Baker SP, Harvey AH. Fall injuries in the elderly. Clin Geriatr Med. 1985;1:501–512
  4. Hedlund R, Lindgren U. Trauma type, age and gender as determinants of hip fracture. J Orthop Res. 1987;5:242–246
  5. Parkkari J, Kannus P, Palvanen M, et al. Majority of hip fractures occur as a result of a fall and impact on the greater trochanter of the femur: a prospective controlled hip fracture study with 206 consecutive patients. Calcif Tissue Int. 1999;65:183–187
  6. Luukinen H, Herala M, Koski K, Honkanen R, Laippala P, Kivela SL. Fracture risk associated with a fall according to type of fall among the elderly. Osteoporos Int. 2000;11:631–634
  7. Hayes WC, Myers ER, Robinovitch SN, Van Den Kroonenberg A, Courtney AC, McMahon TC. Etiology and prevention of age-related hip fractures. Bone. 1996;18:77S–86S
  8. Phillips S, Fox N, Jacobs J, Wright W. The direct medical costs of osteoporosis for American women aged 45 and older, 1986. Bone. 1988;9(1 Suppl):271–279
  9. Grisso JA, Kelsey JL, Strom BL, et al. Risk factors for falls as a cause of hip fracture in women (The Northeast Hip Fracture Study Group). N Engl J Med. 1991;324:1326–1331
  10. Wolfson L, Whipple R, Derby C, et al. Balance and strength training in older adults: intervention gains and Tai Chi maintenance. J Am Geriatr Soc. 1996;44:498–506
  11. Wolfson L, Whipple R, Judge J, Amerman P, Derby C, King M. Training balance and strength in the elderly to improve function. J Am Geriatr Soc. 1993;41:341–343
  12. Michael A, Hadley EC, Hornbrook MC, et al. The effects of exercise on falls in elderly patients. JAMA. 1995;273:1341–1383
  13. Lord SR, Ward JA, Willams P, Strudwick M. The effect of a 12-month exercise trial on balance, strength, and falls in older women: a randomized controlled trial. J Am Geriatr Soc. 1995;43:1198–1206
  14. Lord SR, Ward JA, Willams P. Exercise effect on dynamic stability in older women: a randomized controlled trial. Arch Phys Med Rehabil. 1996;77:232–236
  15. Judge JO, Lindsey C, Underwood M, Winsemius D. Balance improvements in older women: effects of exercise training. Phys Ther. 1993;73:254–262; discussion 263-5
  16. Campbell AJ, Robertson MC, Gardner MM, Norton RN, Tilyard MW, Buchner DM. Randomised controlled trial of a general practice programme of home based exercise to prevent falls in elderly. BMJ. 1997;315:1065–1069
  17. Chandler JM, Duncan PW, Kochersberger G, Studenski S. Is lower extremity strength gain associated with improvement in physical performance and disability in frail, community-dwelling elders?. Arch Phys Med Rehabil. 1998;79:24–30
  18. Guralnik JM, Ferrucci L, Pieper CF, et al. Lower extremity function and subsequent disability: consistency across studies, predictive models, and value of gait speed alone compared with the short physical performance battery. J Gerontol A Biol Sci Med Sci. 2000;55:M221–M231
  19. Campbell AJ, Robertson MC, Gardner MM, Norton RN, Buchner DM. Falls prevention over 2 years: a randomized controlled trial in women 80 years and older. Age Ageing. 1999;28:513–518
  20. McMurdo ME, Mole PA, Paterson CR. Controlled trial of weight bearing exercise in older women in relation to bone density and falls. BMJ. 1997;314:569
  21. Steinberg M, Cartwright C, Peel N, Williams G. A sustainable programme to prevent falls and near falls in community dwelling older people: results of a randomised trial. J Epidemiol Community Health. 2000;54:227–232
  22. Rubenstein LZ, Josephson KR, Trueblood PR, et al. Effects of a group exercise program on strength, mobility, and falls among fall-prone elderly men. J Gerontol A Biol Sci Med Sci. 2000;55:M317–M321
  23. Wolf SL, Barnhart HX, Kutner NG, McNeely E, Coogler C, Xu T. Reducing frailty and falls in older persons: an investigation of Tai Chi and computerized balance training. Altanta FICSIT Group. Frailty and injuries: cooperative studies of intervention techniques. J Am Geriatr Soc. 1996;44:489–497
  24. Buchner DM, Cress ME, de Lateur BJ, et al. The effect of strength and endurance training on gait, balance, fall risk, and health services use in community-living older adults. J Gerontol A Biol Sci Med Sci. 1997;52:M218–M224
  25. Carter ND, Kannus P, Khan KM. Exercise in the prevention of falls in older people: a systematic literature review examining the rationale and the evidence. Sports Med. 2001;31:427–438
  26. Woollacott MJ, Shumway-Cook A, Nashner LM. Aging and posture control: changes in sensory organization and muscular coordination. Int J Aging Hum Dev. 1986;23:97–114
  27. Wrisley DM, Stephens MJ, Mosley S, Wojnowski A, Duffy J, Burkard R. Learning effects of repetitive administrations of the sensory organization test in healthy young adults. Arch Phys Med Rehabil. 2007;88:1049–1054
  28. Judge JO, Whipple RH, Wolfson LI. Effects of resistive and balance exercises on isokinetic strength in older persons. J Am Geriatr Soc. 1994;42:937–946
  29. Wolf SL, Kutner NG, Green RC, McNeely E. The Atlanta FICSIT study: two exercise interventions to reduce frailty in elders. J Am Geriatr Soc. 1993;41:329–332
  30. Pai YC, Wening JD, Runtz EF, Iqbal K, Pavol MJ. Role of feedforward control of movement stability in reducing slip-related balance loss and falls among older adults. J Neurophysiol. 2003;90:755–762
  31. Pavol MJ, Runtz EF, Pai YC. Young and older adults exhibit proactive and reactive adaptations to repeated slip exposure. J Gerontol A Biol Sci Med Sci. 2004;59:494–502
  32. Bhatt T, Wening JD, Pai YC. Adaptive control of gait stability in reducing slip-related backward loss of balance. Exp Brain Res. 2006;170:61–73
  33. Bhatt T, 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
  34. Pai YC, Bhatt T. Repeated-slip training: an emerging paradigm for prevention of slip-related falls among older adults. Phys Ther. 2007;87:1478–1491
  35. Pai YC, Yang F, Wening JD, Pavol MJ. Mechanisms of limb collapse following a slip among young and older adults. J Biomech. 2006;39:2194–2204
  36. Pavol MJ, Pai YC. Deficient limb support is a major contributor to age differences in falling. J Biomech. 2007;40:1318–1325
  37. Pai YC, Patton JL. Center of mass velocity-position predictions for balance control. J Biomech. 1997;30:347–354
  38. Pai YC, Iqbal K. Simulated movement termination for balance recovery: can movement strategies be sought to maintain stability even in the presence of slipping or forced sliding?. J Biomech. 1999;32:779–786
  39. Yang F, Anderson FC, Pai YC. Predicted threshold against backward balance loss in gait. J Biomech. 2007;40:804–811
  40. Dail TK, Christina RW. Distribution of practice and metacognition in learning and long-term retention of a discrete motor task. Res Q Exerc Sport. 2004;75:148–155
  41. Wrisberg CA, Liu Z. The effect of contextual variety on the practice, retention, and transfer of an applied motor skill. Res Q Exerc Sport. 1991;62:406–412
  42. Del Rey P. Training and contextual interference effects on memory and transfer. Res Q Exerc Sport. 1989;60:342–347
  43. Hart M, Poremba A, Gabriel M. The nomadic engram: overtraining eliminates the impairment of discriminative avoidance behavior produced by limbic thalamic lesions. Behav Brain Res. 1997;82:169–177
  44. Markowitsch HJ, Kessler J, Streicher M. Consequences of serial cortical, hippocampal, and thalamic lesions and of different lengths of overtraining on the acquisition and retention of learning tasks. Behav Neurosci. 1985;99:233–256
  45. Lemoine HE, Levy BA, Hutchinson A. Increasing the naming speed of poor readers: representations formed across repetitions. J Exp Child Psychol. 1993;55:297–328
  46. Bhatt T, Pai YC. Long-term retention of gait stability improvements. J Neurophysiol. 2005;94:1971–1979
  47. Yang F, Pai YC. Correction of the inertial effect resulting from a plate moving under low friction conditions. J Biomech. 2007;40:2723–2730
  48. Winter D. Biomechanics and motor control of human movement. 3rd ed.. Hoboken: Wiley; 2005;
  49. Bhatt T, Wening JD, Pai YC. Influence of gait speed on stability: recover from anterior slips and compensatory stepping. Gait Posture. 2005;21:146–156
  50. de Leva P. Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters. J Biomech. 1996;29:1223–1230
  51. McMahon TA. Muscles, reflexes, and locomotion. Princeton: Princeton Univ Pr; 1984;
  52. McGaugh JL. Memory—a century of consolidation. Science. 2000;287:248–251
  53. Kandel ER, Schzwartz JH, Jessell TM. Principles of neural science. 4th ed.. New York: McGraw Hill; 2000;
  54. Shadmehr R, Holcomb HH. Neural correlates of motor memory consolidation. Science. 1997;277:821–825
  55. Shadmehr R, Brashers-Krug T. Functional stages in the formation of human long-term motor memory. J Neurosci. 1997;17:409–419
  56. Krakauer JW, Shadmehr R. Consolidation of motor memory. Trends Neurosci. 2006;29:58–64
  57. Adkin AL, Frank JS, Carpenter MG, Peysar GW. Postural control is scaled to level of postural threat. Gait Posture. 2000;12:87–93
  58. Carpenter MG, Frank JS, Silcher CP, Peysar GW. The influence of postural threat on the control of upright stance. Exp Brain Res. 2001;138:210–218
  59. Kim JJ, Fanselow MS. Modality-specific retrograde amnesia of fear. Science. 1992;256:675–677
  60. Sacchetti B, Scelfo B, Tempia F, Strata P. Long-term synaptic changes induced in the cerebellar cortex by fear conditioning. Neuron. 2004;42:973–982
  61. Shumway-Cook A, Gruber W, Baldwin M, Liao S. The effect of multidimensional exercises on balance, mobility, and fall risk in community-dwelling older adults. Phys Ther. 1997;77:46–57
  62. Hu MH, Woollacott MH. Multisensory training of standing balance in older adults: II. Kinematic and electromyographic postural responses. J Gerontol. 1994;49:M62–M71
  63. Smith CD, Walton A, Loveland AD, Umberger GH, Kryscio RJ, Gash DM. Memories that last in old age: motor skill learning and memory preservation. Neurobiol Aging. 2005;26:883–890
  64. Rodrigue KM, Kennedy KM, Raz N. Aging and longitudinal change in perceptual-motor skill acquisition in healthy adults. J Gerontol B Psychol Sci Soc Sci. 2005;60:P174–P181
  65. 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
  66. Bhatt T, Pai YC. Immediate and latent interlimb transfer of gait stability adaptation following repeated exposure to slips. J Mot Behav. 2008;40:380–390
  • a OR6-5-1000; AMTI, 176 Waltham St, Newton, MA 02472
  • b National Instruments Inc, 11500 N Mopac Expwy, Austin, TX 78759
  • c Motion Analysis Corp, 3617 Westwind Blvd, Santa Rosa, CA 95403
  • d SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606

 Supported by the National Institutes of Health (grant no. 2R01 AG16727).No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

PII: S0003-9993(08)01496-2

doi:10.1016/j.apmr.2008.06.021

Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 1 , Pages 34-42, January 2009

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