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Sensory-Specific Balance Training in Older Adults: Effect on Position, Movement, and Velocity Sense at the Ankle

KP Westlake, PT, PhD, MSc, is Post Doctoral Fellow, Rehabilitation Research and Development Center, VA Palo Alto HCC, 3801 Miranda Ave, Palo Alto, CA 94304 (USA)
Y Wu, MSc, was an MSc degree candidate, School of Rehabilitation Therapy, Queen's University, Kingston, Ontario, Canada, at the time this research was conducted
EG Culham, PT, PhD, is Professor and Director, School of Rehabilitation Therapy, Queen's University

Address all correspondence to Dr Westlake at: westlake{at}rrd.stanford.edu

Submitted September 7, 2006; Accepted January 8, 2007

	Abstract

 
Background and Purpose: Age-related changes in proprioception contribute to impairments in postural control and increased fall risk in older adults. The purpose of this randomized controlled trial was to examine the effects of balance exercises on proprioception.

Subjects: The participants were 36 older people and 24 younger people who were healthy.

Methods: Older participants were randomly assigned to a balance exercise group (n=17) or a falls prevention education group (n=19). Baseline, postintervention, and 8-week follow-up measurements of 3 proprioceptive measures (threshold to perception of passive movement, passive joint position sense, and velocity discrimination) were obtained at the ankle. For comparative purposes, younger participants underwent a one-time assessment of the 3 proprioceptive measures.

Results: Postintervention improvements in velocity discrimination were found in the balance exercise group when compared with values at baseline and in the falls prevention education group. Age-related differences found at baseline were reduced in the balance exercise group after intervention. Improvements were not maintained at the 8-week follow-up. Threshold to perception of passive movement and passive joint position sense did not change as a function of the exercise intervention.

Discussion and Conclusion: The results suggest that short-term improvements in velocity sense, but not movement and position sense, may be achieved following a balance exercise intervention.

	Introduction

Top Abstract Introduction Method Results Discussion Conclusion References

 
Falls occur in approximately one third of adults over the age of 65 years and account for 65% of all injuries in this group.¹ Research aimed at recognizing and reducing risk factors leading to postural instability and falls among this group is crucial. Proprioception represents an essential component of postural control, providing orientation information about movement and position of the joints and muscles.² Age-related changes in this sensation constitutes an important risk factor for falls,³ postural instability,⁴ and impaired function.^5,6 Consequently, much interest and debate have been generated about whether the functioning of this system can be improved with training with the possibility of enhancing the postural responses to orientation cues.

From a physiological perspective, controversy regarding whether proprioception can be trained stems from evidence concerning an ability to selectively enhance activation of proprioceptive receptors⁷ versus a lack of evidence supporting an ability to increase the density of these receptors in humans.⁸ It is well known that proprioception is mediated by receptors located within the skin, joints, and muscles. However, of these receptors, only muscle spindles demonstrate an ability to modulate sensitivity to muscle stretch,^9–11 thereby representing the most promising avenue for training-related improvements to occur.

Apart from investigations at the level of the receptor, discrepancies in the effectiveness of proprioceptive training may be explained by the lack of correlation between different testing measures.^12,13 Traditionally, tests of threshold to perception of passive movement (TPM) and joint position sense (JPS) have been used. A recently developed measure is passive velocity discrimination.¹⁴ Although it is likely that the relative contributions of proprioceptive receptors vary between these tests, muscle spindles are generally considered the primary contributor to movement and velocity sense,¹⁵ with additional velocity information derived from the central nervous system.¹⁶ However, the primary receptor contribution to JPS, measured by passive joint position reproduction, remains unclear, with arguments in favor of both joint¹⁷ and muscle receptors.¹⁸ Nevertheless, it is imperative that these sensations be assessed and reported in complementary terms to expand the understanding of the proprioceptive system. Many investigators continue to assess and report outcomes of only one measure when making judgments about the contributions of the proprioceptive system to motor control.^19–21

Surprisingly few studies have tested the effect of sensory-specific balance training, intended to alter postural orientation inputs, or general activity programs, such as tennis or swimming, on position and movement sense in older adults. Only 2 studies were found to follow traditional proprioceptive training programs, focusing on wobble-board or mini-trampoline training, in older adults demonstrating improvements in actively replicated JPS at the ankle²² and knee.²³ Other studies used generalized exercise, such as strength (muscle force–generating capacity) training, bicycling, and functional exercises, or balance training programs, such as retro-walking, walking on toes, and standing on one leg, with mixed results.^23–25 However, the majority of research supporting the possibility of enhancing proprioception in older adults used a cross-sectional design,^14,26–29 which does not support a true causal relationship. Moreover, the use of an active measurement of JPS^22,24–27 brings the validity of many of these results into question, as improvements may be due to an enhanced motor response to proprioceptive input rather than an increased ability to extract proprioceptive information.⁸

The hypotheses of this randomized controlled study were that older adults, having completed an 8-week balance exercise program, would demonstrate improvements in TPM and velocity discrimination, but not passive JPS. Postexercise intervention values for TPM and velocity discrimination in older subjects were not expected to differ from values obtained from young adults who were healthy.

	Method

Top Abstract Introduction Method Results Discussion Conclusion References

 
Study Participants and Design

Older participants who were healthy and over 65 years of age were recruited through advertisements and flyers in the community. Younger participants were recruited by an e-mail sent to all students in the in the academic department of the last author (EGC). Exclusion criteria included: (1) preexisting major lower-extremity pathology (ie, chronic ankle instability or severe osteoarthritis), (2) neurological disorders or balance difficulties that would prevent standing for the duration of the testing procedures without the aid of an assistive device (ie, vertigo, poor vision, dizziness, stroke, or epilepsy), and (3) health conditions (ie, heart disease, uncontrolled hypertension, chronic obstructive pulmonary disease, or osteoporosis) that would preclude participation in a balance exercise program.

A brief clinical examination was used to screen for symptoms of peripheral neuropathy that are considered a risk factor for falls.³⁰ This examination identified the presence, diminution, or absence of light touch sensation to the dorsal and plantar aspects of the foot, the Achilles tendon reflex, and position sense of the big toe. Participants demonstrating the absence or diminution of one or more of these symptoms were excluded from participation. Older participants provided physician approval, and all participants gave written informed consent prior to study participation.

A sample size calculation for this study was based on a related study by Teasdale and Simoneau.³¹ Differences in proprioceptive reintegration were determined between older and younger participants using center-of-pressure sway velocity with an effect size of 0.28 and an F value of 7.2. An effect size of this magnitude was desired for the current study to determine differences between trained and untrained older adults. Using Pearson-Hartley charts,³² we determined that 16 participants were needed in each group to achieve 80% power at a 5% level of significance. Considering a 25% attrition rate, a total of at least 40 subjects were targeted for the present study.

This study was a single-blind, randomized controlled trial in which older participants were assigned to either a balance exercise group or a falls prevention education group that received no exercise. Block randomization occurred in 2 groups of 20 and 24 participants with the help of a research assistant following baseline testing using randomly distributed sealed envelopes. Both groups were reassessed at the end of the 8-week intervention. The balance exercise group underwent follow-up testing 8 weeks after intervention. Younger participants were assessed once and did not participate in either intervention.

Intervention

Balance exercise classes were led by a registered physical therapist and one student volunteer for 1 hour, 3 times per week, over 8 weeks. The duration and frequency of this study was comparable to those of previous work demonstrating improvements in balance following an exercise program.^33–35 The exercise protocol followed the Fallproof Program,³⁶ which emphasized static and dynamic balance exercises, including transitions between differing sensory conditions and functional everyday movements. Balance activities were designed to optimize and force use of the somatosensory system.

In addition to standing on a firm support surface with a wide-based stance, challenges to the proprioceptive system were introduced to increase conscious and subconscious awareness of altered proprioceptive cues while simultaneously altering visual and vestibular inputs. Tasks included standing or walking on various support surfaces (eg, a rocker board, foam, narrow beam) and standing in a tandem or semitandem position, standing on one leg, or standing with feet together. To alter visual cues, participants were instructed to close their eyes or to engage vision with a reading or tracking secondary task or by performing balance tasks with a distracting background such as a checked pattern or moving people. To modify vestibular cues, participants were instructed to tilt their head backward or quickly move the head side to side and up and down to focus on a target. Each lesson incorporated a similar general plan: 10-minute warm-up, four 10-minute sensory-specific activities, and 10 minutes of cool-down. To rule out an attention placebo effect, the no-exercise control group was offered a falls prevention education class led by a physical therapist for 1 hour, once per week, over 8 weeks.

Instrumentation and Data Collection

The principal investigator, who was masked to group assignments, conducted all testing. A torque motor (Compumotor, model 605)^* with an attached footplate and potentiometer for measuring angular displacement was used for testing. Participants sat on a plinth with the right knee slightly flexed to 5 degrees and the ankle in a neutral position (Fig. 1). Recent work in our laboratory has shown results obtained with this testing position to be comparable to those obtained with other commonly used positions (ie, seated with knee flexion and standing) for testing passive proprioception at the ankle in older adults.³⁷ The right foot rested lightly against the footplate, with the dorsal and lateral aspects of the foot free from contact.

View larger version (36K): [in this window] [in a new window]   Figure 1. Experimental set-up of the torque motor and foot position for tests of proprioception.

The TPM was determined by passively rotating the ankle joint in the direction of dorsiflexion or plantar flexion at a velocity of 0.25°/s starting from a neutral position. Participants pressed a switch to stop joint rotation once movement was perceived and the direction of the displacement could accurately be stated. The incidence of guessed responses was reduced with constant verbal reminders and the occasional sham trial in which the torque motor was turned on but no movement occurred. Differences between the start and stop positions (in degrees) were calculated, and a mean of 6 trials was obtained. The test demonstrated excellent test-retest reliability with an intraclass correlation coefficient (2,1) of .95 tested at the ankle within a 2-week period.³⁸

Testing passive JPS involved passive joint movement at a velocity of 5°/s to 1 of 3 random test positions: 10, 12, and 15 degrees of plantar flexion. Each position was tested twice for a total of 6 trials. Following a 5-second hold, the ankle was passively moved to 5 degrees of dorsiflexion and then passively returned toward the test angle. Participants pressed a stop switch once they perceived that the test angle had been reproduced. Previous work in our laboratory demonstrated no differences in passive JPS based on the selected test positions of 10, 12, and 15 degrees of plantar flexion.³⁷ Therefore, absolute differences between test and reproduced positions were calculated, and a mean of the 6 trials was determined. Excellent test-retest reliability of JPS at the subtalar joint was previously identified (Pearson correlation coefficient of .99).³⁹

Tests for velocity discrimination required participants to choose the faster of 2 randomized passive movements. Displacement was from 0 to 20 degrees of plantar flexion for the first velocity and from 20 degrees of plantar flexion to 5 degrees of dorsiflexion for the second velocity. Subsequent velocity pairs began at the end angle from the previous trial, moving within a 20- to 25-degree range. The reference velocity was always set at 5°/s, and the initial test velocity was 10°/s with a random order of reference and test velocity for each presented pair. Test velocities were reduced by 1°/s until an incorrect response was reported and then increased in increments of 0.25°/s. The threshold value was the smallest difference between the reference and test velocities in which a correct response was obtained and confirmed over 3 trials. Test-retest reliability of this measure was determined in our laboratory for 8 older participants with an intraclass correlation coefficient (2,1) of .86.⁴⁰

Data Analysis

Data were analyzed on an intention-to-treat basis using SPSS, version 11.5. Baseline characteristics were compared using independent t tests for continuous variables and chi-square tests for categorical variables. Effects of the 8-week interventions were determined using a 2x2 analysis of variance (ANOVA) with group as the between-subject factor and visit as the within-subject factor. Outcomes of the 8-week follow-up were analyzed using a repeated-measures ANOVA with visit as the within-subject factor. Comparisons between the younger and older groups were determined using a one-way ANOVA. Statistical significance was set at P<.05, and significant interaction effects were followed with Bonferroni adjusted post hoc tests.

	Results

Top Abstract Introduction Method Results Discussion Conclusion References

 
Of the 64 older volunteers who responded to study advertisements, 45 subjects met the study criteria and were randomly assigned to either the balance exercise group or the falls prevention education group (Fig. 2). By the end of the 8-week interventions, 36 subjects remained, with 17 and 19 participants in the balance exercise and falls prevention education programs, respectively. The mean number and percentage of all visits attended by participants in the balance exercise group and the falls prevention education group was 21.5 (89.9%) and 5.4 (66.3%), respectively. Fifteen participants in the balance exercise group were available for 8-week follow-up testing. Twenty-four young subjects responded to the e-mail request, met eligibility criteria, and were tested.

View larger version (30K): [in this window] [in a new window]   Figure 2. Participant flow chart. Progress of participants from time of initial contact to analysis.

The ratio of incorrect responses for TPM in terms of movement direction to the total number of trials was 3:102 for the balance exercise group and 6:114 for the falls prevention education group at baseline and 2:102 for the balance exercise group and 6:114 for the falls prevention education group after intervention. This low number of incorrect responses indicated that the participants understood the instructions, with minimal "guessed" responses. In terms of TPM outcomes, no effects of group (F=0.02; df=1,34; P=.88), visit (F=0.16; df=1,34; P=.69), or groupxvisit interaction (F=0.001; df=1,34; P=.98) were identified. Comparisons between younger and older participants for TPM revealed a main effect of group (F=8.99; df=5,110; P<.001), with differences between the younger group and the balance exercise group (P<.001) and the falls prevention education groups (P<.001) at baseline and between the younger group and the balance exercise group (P<.001) and the falls prevention education group (P<.001) after intervention.

Measures of JPS demonstrated no main effect of group (F=0.003; df=1,34; P=.96), visit (F=0.02; df=1,34; P=.90), or groupxvisit interaction (F=0.02; df=1,34; P=.90). No group differences were identified between the younger participants and the older participants at baseline or after intervention.

No within-group differences were identified between outcomes at the 8-week follow-up and either baseline or postintervention measurements in terms of velocity discrimination, TPM, or JPS in the balance exercise group (P>.14). Group differences, however, were found between the younger group and the balance exercise group at the 8-week follow-up in terms of velocity discrimination (P=.018) and TPM (P=.001). No differences in JPS were found between the younger group and the balance exercise group at follow-up (P=1.00).

	Discussion

Top Abstract Introduction Method Results Discussion Conclusion References

 
This study represents the first investigation of the effects of a sensory-specific balance-training program on 3 passive proprioceptive measures at the ankle in older adults. As predicted, tests of velocity discrimination responded to the training protocol, demonstrating within-group differences between baseline and postintervention measurements in the balance exercise group and postintervention differences between the balance exercise and falls prevention education groups. Values of velocity discrimination in the balance exercise group were lowered after intervention to values that no longer differed from those of the younger subjects. The other 2 measures—TPM and JPS—showed no significant change as a function of the exercise intervention. Contrary to the lack of improvement in TPM in the present study, Thompson et al²⁵ noted improvements in knee joint TPM in both a general strength-training group and a range-of-motion exercise group. However, a hamstring muscle curl with and without weights was included, which replicated the motion used to test TPM and may have contributed to an enhanced familiarity with the proprioceptive feedback from this particular movement pattern. Indeed, repetition of testing motion has been shown to lead to improved proprioceptive performance,⁴¹ making it difficult to ascribe improvements in TPM unrelated to task familiarity in this research design.

Although underlying physiological mechanisms influencing the results of the present study are speculative at this point, observations that only velocity discrimination, and not TPM, improved with exercise merit some discussion. The original hypothesis that both TPM and velocity discrimination would demonstrate improvements was based on previous research indicating a dependent relationship between movement and velocity sense with fast, low displacement amplitudes being detected similarly to the slow, large amplitudes.⁴² In the present study, when movement sense was tested with constant velocity of 0.25°/s and velocity sense was tested with a relatively constant amplitude of 20 to 25 degrees, the independent nature of these proprioceptive submodalities became apparent.

One reason for these differences may be the decision criterion necessary for tests of velocity discrimination, reflecting a higher degree of mental concentration compared with the time-limited paradigm used for TPM. Selective activation of fusimotor neurons through enhanced central processing and drive during cognitively challenging events has previously been demonstrated using tasks involving mental computation.⁴³ Although relevant studies directly linking an increase in fusimotor activity to enhanced proprioception have yet to be conducted, the theoretical possibility is supported by the results of our study. Because a primary focus during the exercise sessions was to enhance the attention paid to proprioceptive cues, the test used to determine velocity discrimination could have been reflected in this strategy more than the test used for TPM. One possible functional scenario extending from this assumption is that, if an individual can learn to pay more attention to these cues, an increased ability to use relevant proprioceptive information during a challenging balance task may follow.

Although conclusive evidence regarding the relationship between proprioception and balance cannot be drawn at this point, the finding that velocity sense was sensitive to targeted interventions may represent an additional important consequence in terms of functional tasks. Considering that velocity information is crucial and more accurate than position and acceleration information for the small postural corrections required during quiet stance,^44,45 the possibility to reverse age-related changes in velocity sense is encouraging. Moreover, impairments in postural control and fear of falling in older adults that remain unexplained by muscle weakness⁴⁶ may potentially be alleviated by improvements in velocity discrimination following balance interventions. Nevertheless, it is important to note that despite the postintervention improvements in velocity discrimination, these changes were not retained at the 8-week follow-up session. Thus, to maintain improvements in velocity discrimination, a regular targeted exercise regimen must be continued.

Potential limitations of this study should be mentioned. First, despite participant instructions and observational monitoring by the investigator, the light pressure on the sole of the foot was not recorded. Because foot pressure is a known contributor to proprioception, the variability in pressure among subjects may have skewed the results. However, previous findings that TPM or JPS at the ankle did not differ when tested in a standing, weight-bearing position and a seated, knee-extended position^37,47 suggest that cutaneous receptors are unlikely to account for results of our study.

A second limitation of this study is the lack of sensitivity of passive JPS to age-related changes. Compared with other studies, the mean passive JPS error of the younger group in the present study (3.27°) fell within the wide range of previously reported values, ranging from below 2 degrees^17,28,48,49 to above 4.5 degrees.^50,51 However, the large discrepancies in values between studies and controversies regarding age-related changes indicate poor consistency and high variability of this measure. Thus, further research is warranted to enhance the validity of data obtained with this measure.

A third possible limitation is the lower total contact time as well as the attendance of participants in the falls prevention education group compared with the exercise group. Although there was greater therapist-client interaction in the balance exercise group, a postintervention survey indicated that the majority of participants in the education group were satisfied with the one-visit-per-week interaction and would not have preferred to increase the number of visits. Consequently, the motivation and attention required for proprioceptive testing were not likely to have been influenced by differences in the number of intervention sessions.

	Conclusion

Top Abstract Introduction Method Results Discussion Conclusion References

 
The results of this study showed that velocity sense improved after a sensory-specific training intervention. Further research is needed to determine the contributing physiological mechanisms and to confirm the functional relevancy of this finding.

	Footnotes

 
Dr Westlake and Dr Culham provided concept/idea/research design, writing, project management, and consultation (including review of manuscript before submission). Dr Westlake and Ms Wu provided data collection and subjects. All authors provided data analysis. Ms Westlake provided fund procurement. Dr Culham provided facilities/equipment. The authors acknowledge Karen Parsons, PT, and Joanna Sarnecka, PT, for help in leading the group intervention sessions and Dr Mui Lam for comments on statistics.

This study was approved by the Queen's University and Affiliated Teaching Hospitals Health Science Human Research Ethics Board.

Ms Westlake received financial support from a Canadian Institutes of Health Research Fellowship.

^* Parker Hannifin Corp, Compumotor Div, 5500 Business Park Dr, Rohnert Park, CA 94928.

SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

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Top Abstract Introduction Method Results Discussion Conclusion References

 

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				K. P Westlake and E. G Culham Sensory-Specific Balance Training in Older Adults: Effect on Proprioceptive Reintegration and Cognitive Demands Physical Therapy, October 1, 2007; 87(10): 1274 - 1283. [Abstract] [Full Text] [PDF]

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