Effect of Acute Fatigue of the Hip Abductors on Control of Balance in Young and Older Women

Article Outline

• Abstract
• Methods
  • Subjects
  • Balance Assessment
  • Fatigue Protocol
  • Statistical Analyses
• Results
• Discussion
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Bellew JW, Panwitz BL, Peterson L, Brock MC, Olson KE, Staples WH. Effect of acute fatigue of the hip abductors on control of balance in young and older women.

Objective

To examine the effects of acute fatigue of the hip abductors on the control of balance in young and older women.

Design

Pretest-posttest.

Setting

University research laboratory.

Participants

Healthy young women (n=20; age, 23.0±1.5y; height, 166.52±4.5cm; mass, 65.33±10.5kg) and community-dwelling older women (n=20; age, 71.65±7.2y; height, 162.31±3.8cm; mass, 71.16±11.6kg) without a fall history.

Intervention

Measurements of control of single-limb balance before and after fatiguing the hip abductors of the dominant leg.

Main Outcome Measure

Performance on 3 clinical assessments of control of balance: the modified Functional Reach Test in the forward, left, and right directions; the Lower-Extremity Reach Test in forward and lateral directions; and the Single-Limb Stance Time Test (SLSTT).

Results

Although the younger subjects showed a significantly greater control of balance than the older women in most tests, control of balance after acute fatigue failed to show a significant decline in either age group. The only exception to this was the SLSTT in the younger women in whom a significant 26% decline was noted (P<.05).

Conclusions

Acute fatigue of the hip abductors did not result in a decreased control of balance in healthy young or older women without fall history. Despite considerable changes in movement strategies used to complete the postfatigue tests of balance, quantitative measures of balance did not decrease.

Key Words: Aging, Fatigue, Hip, Rehabilitation

List of Abbreviations: ICC, intraclass correlation coefficient, LERT, Lower-Extremity Reach Test, mFRT, modified Functional Reach Test, ROM, range of motion, SLSTT, Single-Limb Stance Time Test

NEUROMUSCULAR FATIGUE IS A physical phenomenon manifesting in the inability to sustain a given level of effort and is held to underlie many of the changes noted in decreased ability during functional and sporting activities.¹, ², ³, ⁴ Fatigue may involve central mechanisms originating proximal to the motor neuron and/or peripheral mechanisms originating at or distal to the neuromuscular junction, including the intramuscular contractile elements.⁵, ⁶ Central fatigue may be defined as a decline in voluntary activation of skeletal muscle after exercise, whereas peripheral fatigue is associated with contractile slowing and/or increased relaxation time after activation.⁶ Much data⁷, ⁸ have shown that fatigue can reduce muscular output and impair proprioception and muscular reaction times. Appropriate muscle activation during dynamic activity provides stability to joints; thus, a deficiency in stabilization during functional activity secondary to fatigue may impair the ability to safely execute functional activity.

The ability to control balance under conditions of acute muscle fatigue or after periods of increased muscular effort should be of great interest to rehabilitation professionals. This is particularly true for those working with patients of advanced age in whom age-related loss of muscle strength is believed to underlie the decline in balance.⁹ The incidence of falls in older adults exceeds that of younger adults, and falls remain a leading cause of accidental death in older adults.¹⁰, ¹¹ Aging is associated with a decline in controlling postural sway and balance, particularly in undisturbed, static stance.¹² This decline may be exacerbated by acute muscular fatigue and may be a primary contributing factor to increased incidence of falls among the elderly. However, tests of function and control of balance are typically executed under nonfatigued conditions. Consequently, an indication of a person's functional capabilities under conditions of fatigue is not evaluated.¹³ Although testing of performance under fatigued conditions has been advocated, most clinical testing of balance continues to reflect nonfatigue conditions.¹⁴

Of considerable relevance to older adults is the effect of neuromuscular fatigue on balance. Age-related changes in the neuromuscular system, including motor unit remodeling, and the associated loss of strength and slowed contractile properties of skeletal muscle seen with aging increase fatigability in older adults and likely underlie the increase in falls noted in advanced age.¹⁵, ¹⁶ Although many studies¹⁷, ¹⁸, ¹⁹, ²⁰ have addressed the effects of aging and decreased muscular strength on control of balance, little to no data have been provided regarding the effects of acute muscular fatigue on control of balance in older women. Bellew and Click Fenter²¹ were the first to report the effect of acute fatigue of the ankle and knee on control of static and dynamic balance in older women. Their findings showed that control of balance is specific to the test used and the joint fatigued (ie, tests requiring greater contribution from the ankle, such as single-limb stance, showed a decline after fatigue of the ankle). In contrast, tests of balance using more knee effort, such as the LERT, did not show a decline after fatigue of the ankle but rather the knee.

There are very few studies in the literature that have examined the effect of acute fatigue on control of balance in older adults. This lack of data obscures the ability to understand the effect of fatigue and its relationship to the control of balance in a population with increased fall risk. To date, no data have been reported that describe the effect of acute fatigue of the hip on control of balance in older women. Therein lies the purpose of this investigation. The study hypotheses were that acute fatigue would impair balance in both age groups and that the magnitude of difference would be greater in the older subjects.

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Methods 

Subjects 

Twenty women 18 to 30 years of age (23.0±1.5y, 166.52±4.5cm, 65.33±10.5kg) and 20 community-dwelling and independently living women 60 to 90 years of age (71.65±7.2y, 162.31±3.8cm, 71.16±11.6kg) without a history of falling or acute impairment were examined. Sample-size estimates were computed by using effect sizes from previously published data and our own clinical observations.²¹ Exclusion criteria included the use of an assistive ambulation device, uncontrolled hypertension, diagnosed osteoporosis, transient ischemic attacks, stroke, congestive heart failure, diagnosed vestibular dysfunction, or visual or auditory compromise sufficient to prevent driving. Additionally, subjects with acute arthritis or those reporting a limitation of functional ability secondary to arthritis were excluded. None of the subjects were actively involved in a strength or conditioning program. After an explanation of the study protocol, subjects agreed to participate by signing the consent form approved by the university institutional review board.

Subjects were examined before and immediately after inducing fatigue in the hip abductor muscle group of the dominant leg. Dominance was determined by asking which leg the subject would use to kick a ball. Control of balance was assessed by using a battery of clinical-based tests including the SLSTT, mFRT, and the LERT. The order of balance testing was randomized between and within tests so that tests with multiple directions would be randomized as well. All tests of balance were measured by the same 2 testers, and inter- and intratester reliability was determined before data collection.

Balance Assessment 

Dynamic control of balance was assessed by using a modified version of the Functional and Lateral Reach Tests and the LERT, each of which have been previously described by Bellew and Click Fenter²¹ and Bellew et al.²² In their original form, the Functional Reach and Lateral Reach tests assess the distance reached in the forward and lateral directions while maintaining a 2-legged stance.²³, ²⁴, ²⁵ The mFRT used in our study requires subjects to perform the test while standing on the self-reported dominant leg only while reaching in the forward and lateral directions. To differentiate reach in the lateral directions, the direction was defined relative to whether it was toward the dominant side or away from the dominant side.

For the mFRT forward and to the mFRT dominant side, subjects stood without shoes with their dominant arm raised to shoulder height (parallel to the floor) with finger tips extended and were asked to protract the arm as far as possible in the direction of intended reach to eliminate the effect of scapular protraction on functional reach. Subjects then reached forward or toward the dominant side. Reach to the dominant side was thus over the stance limb. For the mFRT to the nondominant side, subjects remained standing on the dominant leg but were asked to reach with the nondominant arm in the lateral direction away from the stance leg. Subjects were instructed to reach as far as possible in any manner they wished and to return to the starting position (ie, a trial was not counted if the subject took a step in the direction of reach or was unable to resume the start position). The tip of the third (middle) digit was used as a landmark for measuring. Reach was recorded to the nearest 1.27cm (1.5in) as the difference between the starting position and the reach position using a standard yardstick suspended parallel to the floor by a telescoping tripod at shoulder height. Each subject performed 2 successive trials in each direction with a trial repeated if the subject lost balance, took a step, or raised the heel. The average of the 2 trials was used for analysis.

The LERT, a lower-extremity analog of the mFRT test previously described by Bellew et al,²² is an assessment tool that incorporates dynamic control of single-limb balance with lower-extremity neuromuscular control. To quantify lower-extremity reach, subjects stood without shoes on their self-reported dominant leg and reached as far as possible with the opposite limb in the forward direction and away from the stance leg. Subjects were required to maintain the reach position for at least 1 second while the tester recorded the distance reached. Subjects were instructed to position their foot pointing in the forward direction for both forward and lateral reach and were free to use various reaching strategies. The maximal distance reached was measured to the nearest 1.27cm (1.5in) for 2 consecutive trials, with the average of the 2 trials used for data analysis.

Static balance was assessed by using the SLSTT, which has been previously described by several authors.¹², ²¹, ²² In summary, the SLSTT quantifies the duration a subject remains standing on the dominant limb with the arms by her side and eyes opened. The SLSTT has been reported to be the strongest predictor of injurious falls in healthy community-dwelling adults over the age of 60 years.¹² The average of 2 trials was used for data analysis. For all tests of balance, testing order was randomized between subjects and within subjects from pre- and postfatigue conditions. Additionally, intra- and intertester reliability was determined for all tests of balance before testing.

Fatigue Protocol 

The principle muscle group of interest in this study was the hip abductors of the self-reported dominant leg. To induce acute fatigue, subjects were placed side lying on the nondominant side on a standard padded examination plinth. A Velcro strap was placed around the subject's pelvis and table just distal to the pelvic crest and proximal to the hip joint to prevent rolling forward or backward. Before testing, the subject was asked if the belt was uncomfortable and adjusted if so. The subject's maximal active range of hip abduction motion was determined by standard goniometric measurement. From this measurement, the subjects performed the hip abduction motion through 50% of the available active ROM. A dowel rod was positioned above the ankle at a height representing 50% of the subject's maximal ROM. Subjects were instructed to raise their leg to touch the dowel rod on each repetition. A metronome was used to pace the lift-and-lower phases, yielding a normalized rate of 25 lifts per minute for all subjects. To facilitate the onset of fatigue, weighted resistance equal to 3% of the subject's body weight (rounded to the nearest whole pound) was applied to her ankle. Fatigue was defined as the point at which the subject failed to reach the target ROM or became out of synch with the pacing for 3 consecutive repetitions. To ensure maximal effort from each subject, strong verbal encouragement was given throughout the testing.

Statistical Analyses 

The analyses attempted to answer 2 questions: (1) Did balance performance change after fatigue within each age group and if yes for both age groups? and (2) Was the percent change greater in one group than the other? ICCs_2,1 for the 2 testers were completed. The means of the pre- and postfatigue trials and the calculated mean percent change for balance for each age group were used for analyses (SPSS version 16.0^a). The independent variables were group (young or older) and conditions (prefatigue or postfatigue), and the dependent variables were measures of balance (mFRT forward, mFRT dominant, mFRT nondominant side, SLSTT, LERT forward, LERT lateral) and percent change in each test. Because the mFRT had 3 directions (forward, dominant side, nondominant side), Bonferroni-corrected paired-sample t tests for 3 related measures were used to assess change from pre- to postfatigue conditions. Thus, alpha was set at P less than .017 (.05÷3) for the measures of mFRT. Accordingly, because the LERT had 2 levels of direction, alpha was set at P less than .025 (.05÷2). The level for significance remained P less than .05 for the SLSTT. Paired samples t tests were also used to assess differences in mean percent change.

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Results 

Intratester reliability (ICCs) ranged from .887 to .947 for the mFRT, from .809 to .970 for the LERT, and .820 to .828 for the SLSTT. Intertester reliability ranged from .953 to .980 for the mFRT, from .980 to .987 for the LERT, and .880 to .980 for the SLSTT. Subject demographics are noted in table 1.

Table 1. Subject Demographic

To reach fatigue, the young group completed an average of 63±29 repetitions with an average of 4.39±.68lbs through an average range of 33° of abduction. The older group completed an average of 41±22 repetitions with an average of 4.70±.73lb through an average range of 18° of abduction. There was no significant difference in the amount of resistance used (P=.186), but the number of repetitions completed (P= .011) and ROM (P<.001) was significantly greater in the young women.

With the exception of the LERT forward, balance measures were significantly greater in the young group compared with the older group for all tests during both pre- or postfatigue sessions (P<.001–.001). The induction of acute fatigue of the hip abductors had no effect on the control of balance in the young group (table 2). The only exception to this occurred in the SLSTT in which a significant average decrease of 26% (P<.05) was noted. In the group of older women, no significant changes were noted in any of the tests of balance (see table 2). The range of mean percent change across all tests showed no greater than a 4% decline (in mFRT forward) and a 5.5% improvement (in LERT forward) in the young. Accordingly, the range of difference in the older women was from a 2.3% decline (in the mFRT forward) to a 1.5% improvement (in the mFRT nondominant side).

Table 2. Balance Performance

Abbreviations: LERTF, LERT forward; LERTL, LERT lateral; mFRTD, modified Functional Reach Test, dominant side; mFRTND, modified Functional Reach Test, nondominant side.

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Discussion 

This study examined the effect of acute fatigue of the hip abductors on the control of balance in young and older women. Our hypotheses were that acute fatigue would impair balance in both age groups and that the magnitude of difference would be greater in the older group. Although measures of balance were greater in younger than older subjects, these data do not support the hypotheses and warrant further discussion.

The single-joint hip abductor muscle group was selected for 2 reasons; previous data reporting control of balance after localized fatigue at the knee and hip have been reported by Bellew and Click Fenter²¹ with no such data available regarding the hip and because the hip abductor group has a significant role in stability of the pelvis and lower extremity in upright stance and activity.²⁶, ²⁷, ²⁸ Serving as the articulation between the lower extremity and the pelvis, the hip simultaneously provides stability and mobility between the upper and lower body. The collective action of the primary muscles of the hip abductor group (gluteus medius, gluteus minimus, and tensor fascia latae) is to stabilize the pelvis over the femur during single-limb stance.²⁶, ²⁷ Thus, weakness of the hip abductors may result in difficulty with functional activities that require single-limb stance such as walking and stair climbing. Weakness of the hip abductors during single-limb stance often presents as a frontal plane drop of the contralateral pelvis or compensatory lean of the trunk to over the stance leg, both of which may lead to decreased control of or loss of balance.²⁸ Given the significance of the hip abductor muscle group and the recent but limited understanding of the effects of acute fatigue isolated to a single joint, it was of interest to examine the effect of fatigue of this muscle group on control of balance.

Accurate input from the visual, vestibular, and proprioceptive systems is used to guide both volitional and reflexive muscle responses necessary to maintain postural stability and balance. Muscular fatigue is held to alter the afferent kinesthetic and proprioceptive feedback, which may in turn manifest in a decreased ability to sense changes in postural stability.²⁹, ³⁰ Fatigue of both central and peripheral origin likely alters the efferent output, leading to decreased physical capacity.¹ A decline in somatosensory afferent input and central interpretation of, or response to, fatigue may underlie fatigue-induced decline in physical performance. Theoretically then, fatigue of the hip abductors would be expected to alter neuromuscular control of the hip, thus compromising maintained alignment of body segments in the erect posture, the coordination of voluntary movements, or the reaction to external stimuli.³¹ However, although our subjects performed resisted hip abduction to fatigue, performance on our balance tests did not reflect a decline in balance control.

These findings are not inconsistent with previous evidence examining the effect of acute fatigue on the control of balance. Johnston et al³² assessed both single- and double-limb stance during static and dynamic conditions after simultaneous fatigue of the hip, knee, and ankle using a stair-stepping effort. They reported a significant decrease in static balance under single- and double-limb stance; however, the dynamic test of balance failed to show a significant change. Alderton and Moritz³³ assessed single-limb stance after acute fatigue of the calf muscles and noted no change in the control of balance. Alderton and Moritz³³ suggested that undetected compensatory mechanisms, such as increased reflex activity of the muscle spindles or increased muscle stiffness, may be used for balance control under conditions of fatigue. Johnston et al³² suggested a primary reason for not detecting a decline in balance after fatigue is recovery. Our findings that SLSTT declined in the young are consistent with Johnston,³² whereas the lack of decline in the older subjects is consistent with Alderton and Moritz.³³ We assert that the magnitude of difference in SLSTT between old and young subjects before fatigue must be considered. The fact that the younger subjects had a 12-fold greater duration in SLSTT than the young before fatigue (227s vs 18s) makes it more likely that the younger subjects would show a decline in balance. Compensatory mechanisms or recovery may provide a possible explanation for the lack of change in balance performance measures noted in our study.

Although functional clinical tests of balance such as the mFRT, LERT, and SLSTT provide quantifiable data with regard to distance reached or duration of balance, they fail to account for changes in biomechanics of the task from pre- to postfatigue conditions. To our knowledge, few studies exist that assess the effects of acute fatigue of the hip on control of single-limb balance in older adults. However, changes in the biomechanics of single-leg hopping, sprinting, and running have been well documented after the induction of acute fatigue in younger populations.¹³, ³⁴, ³⁵ As such, observations of neuromuscular control from these studies after fatigue may be used to interpret the findings of this study because these activities and the tests of this study require periods of single-leg stabilization of the lower extremity as the center of mass moves in various directions. That the LERT requires a considerable level of control and that all older subjects were able to complete testing is evidence that the older subjects examined are likely of above-average physical ability compared with age-matched peers. Given the use of muscular effort to fatigue and the necessary exclusion criteria used, the interpretation of above-average ability is supported.

A common finding after fatigue of the thigh muscles is a significant decrease in hip motion¹³ or no change in total hip ROM³⁵ with significant compensatory increases in knee and ankle motion and significant changes in joint moments.¹³, ³⁵ Collectively, these changes have been purported to compensate for more proximal weakness. In a similar manner, the use of compensatory responses at other joints may explain the lack of change in the measures of functional reach assessed in this study (ie, our study focused on fatigue at the hip, but prior data suggest a decrease or no change in hip motion after fatigue). From the data of Augustsson et al¹³ and Orishimo and Kremenic,³⁵ it is possible that a decrease or no change in hip motion compensated for the localized fatigue at the single-joint abductor muscles of the hip used in this study.

It is quite likely that the biomechanical actions observed at the hip, knee, and ankle during our tests were not consistent from the pre- to postfatigue trials of this study. This is consistent with findings of other studies.¹³, ³⁵ It was our observation that both age groups showed noticeable changes in the strategies used to complete the balance testing in the postfatigue conditions. In the young, we observed increased lateral trunk flexion over the fatigued side and increased hip and knee flexion combined with ankle dorsiflexion on the stance limb, whereas in the older women we observed increased hip flexion but with decreased knee flexion and decreased ankle dorsiflexion as the subjects shifted weight posterior to their heels. Common to both groups, we noted that during postfatigue trials the subjects performed the repetitions more quickly and with greater compensatory movement from the upper extremities than observed prefatigue. Again, because balance tests predicated on functional reach do not account for changes in the movement pattern used, these observations were not quantified as part of this study and are likely not documented as part of a clinical balance examination, but herein may lay clinical information of great utility in identifying patients at risk for fall or those for whom training for fall prevention is warranted. Although the control of balance did not show a quantitative change, subjectively, control appears to be altered by acute fatigue. Thus, it would appear that compensatory measures were adequate to maintain balance and thus do not necessarily suggest increased risk for falling in this group. However, the quantification of changes in movement patterns may prove useful in identifying compensatory movement characteristics of subjects with fall history. These recommendations should be addressed in future research.

A potential explanation for the lack of decline in the control of balance after fatigue may be early recovery. After the fatigue portion of the study, most subjects reported the sensation of having given a significant amount of effort, but, shortly thereafter, this perception had abated. The majority of subjects, and more so in the older women, reported feeling little to no consequences of the fatigue portion of testing by the completion of the balance tests. Furthermore, subjects reported no residual discomforts associated with the fatigue testing during follow-up telephone calls the day after participation. The fact that the older subjects performed an average of one-third less repetitions than the young may help explain an early recovery in the older women.

Although subjective in nature, it appears that the duration of our balance testing protocol may have allowed a recovery from fatigue before completion of the tests. Recovery time after fatigue of the knee extensors and ankle dorsiflexors has been reported by Milner-Brown et al³⁶ as 5.9 and 1.5 minutes. Such data from the hip abductors have not been found. Nevertheless, although we did not time each subject during balance testing, it certainly required greater than 1.5 minutes and likely more than 5.9 minutes to complete the postfatigue portion of our balance tests. The evidence provided by Milner-Brown³⁶ supports the possibility that recovery from fatigue may have occurred before the completion of balance testing.

Study Limitations 

The possibility of recovery from fatigue before completion of the balance assessment is a considerable limitation to this study as is the limited number of subjects examined. Although efforts were made to maintain consistency in the amount of effort exerted by each subject including the use of a pacing metronome, standardizing the hip movement to 50% of the maximal active range and using 3% of body weight as a resistance, variability among subjects existed nonetheless. As an example, our determination of fatigue was predicated on the inability to reach the target range or failure to maintain pace with the metronome. Although some subjects clearly showed efforts consistent with a maximal attempt, others (most notably in the older group) volitionally terminated their effort stating that they had done enough to suit them or that they felt they had worked hard enough. Perhaps a more quantitative and controlled method of monitoring fatigue would have yielded different results. Observation over a larger sample size is necessary to better assess this variability.

A further limitation of our study involves the use of resistance in addition to the weight of the leg. We chose to use 3% of gross body weight to increase resistance during the abduction movement. This procedure was selected to facilitate the onset of acute muscular fatigue. It is reasonable to assume that the number of repetitions completed and the time to reach fatigue was less than could have been completed without additional resistance. The effect this had on the onset of fatigue and subsequent ability on the balance portion of the testing is unclear. The average number of repetitions completed by each subject (63 in the young and 41 in the older) and time to reach fatigue would indicate that our fatigue stress was more anaerobic than aerobic in nature. Although some discussion could take place as to the potential difference of using anaerobic or aerobic activity to induce fatigue, previous literature², ³, ⁴ examining various fatigue protocols suggests that changes in performance after fatigue appear to be more similar than dissimilar, thus indicating that type of fatigue used may be irrelevant.

In light of previous research on the effect of fatigue on biomechanical performance, it is reasonable to suggest that the lack of change in control of balance as reported here is the result of complex compensatory changes in motor control at, but not limited to, the stance extremity. Visual observation of changes in movement strategy used by subjects to complete the balance tests provides subjective support of this assertion. To reiterate, clinical tests of balance that are predicated on measures of distance reached or time while balanced do not and cannot objectively account for the alteration in movement strategy used to execute the balance task. Perhaps greater exploration and delineation of movement strategies used before and after fatigue may elucidate common patterns or expose biomechanical alterations that can be trained, developed, or potentiated in attempts to improve control of balance. Although this remains a matter of intellectual discourse, it nevertheless represents an area of potential investigative work with great potential.

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Conclusions 

This study has shown that acute fatigue of the hip abductor muscle group did not result in a decrease in control of balance. This finding was consistent in both the young and older women. Although the tests of balance used in this study provided quantitative measures used to document control of balance, changes in the movement strategies used to complete the tests of balance after fatigue are not accounted for. Clinical examination of balance should be performed both before and after physical effort in order to account for the potential use of compensatory movement strategies after physical effort.

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