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Applied Sciences: Biodynamics

Comparison of Muscle Torque, Balance, and Confidence in Older Tai Chi and Healthy Adults

TSANG, WILLIAM W.N.; HUI-CHAN, CHRISTINA W.Y.

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Medicine & Science in Sports & Exercise: February 2005 - Volume 37 - Issue 2 - p 280-289
doi: 10.1249/01.MSS.0000152735.06282.58
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Abstract

According to the World Health Organization, there will be more than one billion people aged 60 and older in 2020. Falls have been identified as a major cause of injury and death in older adults. Decrease in muscle strength is one of the intrinsic factors that cause these falls (27). In fact, muscle size and related neuromuscular functions are known to decrease with aging. The decline in muscle strength goes unnoticed until about the sixth decade of life (27). However, this trend can be reversed, as investigations conducted on older adults reveal that resistance training can improve their leg muscle strength (11). Resistance training studies generally use machines or elastic bands to create resistance. Few studies have used body weight as a form of resistance. Tai Chi, a 300-yr-old Chinese mind–body exercise is practiced by millions of Chinese older adults. Because this form of exercise is so widely practiced, we asked the question: Do experienced older adult Tai Chi practitioners have enhanced leg muscle strength compared with older adults who do not practice Tai Chi?

Recent studies have demonstrated that the practice of Tai Chi can reduce anxiety, improve mood and self-esteem, and be beneficial to the cardiorespiratory system and balance control. In 2001, we compared the sway amplitude of older Tai Chi practitioners with that of an older healthy control group, with them standing under six combinations of visual (eyes open, eyes closed, sway-referenced) and support surface (fixed, sway-referenced) conditions in the sensory organization test (21). Our findings demonstrated that older Tai Chi practitioners swayed significantly less when they stood under conditions that demanded an increased reliance on the visual and vestibular systems than the healthy control group similar in age, sex, and physical activity level. Of particular interest is that older Tai Chi practitioners attained the same level of balance control performance in the sensory organization test as young, healthy subjects under conditions that challenged their somatosensory, visual, and vestibular systems (22). Using the limits of stability test, we (20) further showed that Tai Chi practitioners initiated voluntary shifting of their weight to different spatial positions within their base of support more quickly, leaned further without losing their stability, and showed better control of their leaning trajectory than those of the control subjects.

Good balance control during functional activities requires sufficient strength of the agonist and antagonist muscles across the joints (9). Tai Chi is performed with the knees bent most of the time, and it requires both concentric and eccentric contractions of the leg muscles, especially those of the knee. For instance, in one of the Tai Chi forms termed “part the wild horse”s mane,” the practitioner semisquats both her knees (Fig. 1). She then transfers her whole body weight to her right leg and lifts her left leg forward. This form is completed when she extends her right knee and at the same time transfers her weight to the left leg, which is bent about 90° at the knee (23). Tai Chi is performed in a closed kinetic chain position, in which the knee extensors and flexors co-contract to stabilize and control knee movements (9). Long-term practice of Tai Chi may increase strength in the knee muscles, and the resultant muscle strength may augment functional stability in certain daily activities (27).

FIGURE 1— A Tai Chi master illustrating the “part the wild horse’s mane” maneuver.
FIGURE 1— A Tai Chi master illustrating the “part the wild horse’s mane” maneuver.

Wolfson et al. (29) conducted a 3-month intervention study on the effects of a balance and strength training program in healthy community dwellers (mean age = 80 yr). All the subjects were then required to practice Tai Chi over the subsequent 6 months to maintain the gain in their balance and muscle strength. Significant gains were found to have persisted after the study period. Regrettably, the study did not include a control group. Lan et al. (10) investigated the effectiveness of a 12-month Tai Chi training program on knee muscle strength in older adults aged 58–70 yr. They demonstrated that subjects who voluntarily participated in the Tai Chi group had significantly improved concentric isokinetic strength of the knee extensors and flexors (by 19.2 and 15.7%, respectively), whereas the control group showed no significant change in these variables. However, eccentric knee muscle strength was not investigated in this study.

Wu et al. (30) compared the concentric and eccentric strength of the knee muscles between Tai Chi practitioners with the average experience of 21 yr and control subjects, both groups aged > 55 yr. They found that the Tai Chi practitioners had higher concentric and eccentric strength in their knee extensors at 60 and 120°·s−1, but observed no difference in their knee flexor strength from that of the control group. These findings pose questions about possible muscle imbalance, as knee extensor strength, but not knee flexor strength, was higher in the practitioners. However, their finding of insignificant changes in concentric and eccentric knee flexor strength may be due to the higher isokinetic testing speeds (60 and 120°·s−1) used (12,30). Slow isokinetic testing speed may be better at detecting the effect of Tai Chi practice on knee muscle strength, because Tai Chi practice involves slow rather than fast movements (5,12).

If increased leg muscle strength were found in older Tai Chi practitioners, could Tai Chi practice also help them to achieve better balance control during single-leg stance? The latter question was raised because falls seldom occur during double-leg stance, when the center of mass is well within the base of support. Thus, evaluation of balance control confined to double-leg stance may not reflect the functional capability required of older people in certain activities of daily living. Often the support surface is not stationary, such as stepping into a moving escalator or walking in the aisle of a moving bus. This will demand greater balance control and may pose particular problems. Tai Chi practice requires constant weight shifting between double-leg and single-leg stance (23). This requirement could improve balance control in single-leg stance under less stable conditions, such as when stepping onto a moving support platform.

Better knee muscle strength and balance control during perturbed single-leg stance may enhance older adults” balance confidence, which has been shown to be an important indicator of functional mobility and independence in older adults (13). However, whether those who are experienced practitioners of Tai Chi have better balance confidence remains unknown.

To answer the questions raised, the objectives of the present study were:

  • to examine whether, at low isokinetic movement speed, older Tai Chi practitioners had better concentric and eccentric knee extensor and flexor strength than matched control subjects;
  • to investigate whether muscle imbalance existed in older Tai Chi practitioners, by comparing their agonist/antagonist muscle strength ratio with that of control subjects;
  • to compare the control of body sway between older Tai Chi practitioners and control subjects during both static double-leg stance and single-leg stance subjected to anteroposterior platform perturbations; and
  • to investigate whether older Tai Chi practitioners had better subjective report of balance confidence than control subjects.

If the answers from the above objectives were positive, our final objective was to determine whether any relationship existed: 1) between knee muscle strength and control of body sway in static double-leg stance and during perturbed single-leg stance, or between knee muscle strength and balance confidence; and 2) between body sway during perturbed single-leg stance and balance confidence.

METHODS

Subjects and Study Design

Forty-eight community-dwelling older adults, aged 60 or above, participated in this cross-sectional study. Twenty-four Tai Chi practitioners (12 males and 12 females, mean age = 69.3 ± 5.0 yr) were recruited from local Tai Chi clubs. All of them had practiced Tai Chi for a minimum of 1.5 h·wk−1 for at least 3 yr (mean Tai Chi experience = 8.5 ± 7.6 yr). Twenty-four control subjects (12 males and 12 females, mean age = 71.6 ± 6.1 yr) were recruited from several community older adult centers. They had no previous experience in practicing Tai Chi, though some took morning walks or did stretching exercises. All Tai Chi subjects were independent in their activities of daily living and none required walking aids. To match their physical activity level, the same inclusion criteria for control subjects were adopted. Both groups were also able to communicate and follow the testing procedures. Exclusion criteria include those that could affect the performance of muscle strength and balance control (28). Candidates with poorly controlled hypertension (defined as systolic/diastolic blood pressure ≥ 160/95) (1), those showing severe cognitive impairment (Mini-Mental Status Examination score < 24), or being diagnosed with metastatic cancer, Parkinson”s disease, stroke, or any other neurologic disorder were excluded. Also excluded were people diagnosed with cardiovascular diseases, symptomatic orthostatic hypotension, peripheral neuropathy of the lower extremities, or disabling arthritis that prevented them from completing the muscle strength and balance control tests in the present study. In addition, subjects who reported a history of falls, either injurious or noninjurious, in the past 12 months were excluded. Screening of subjects using a general health questionnaire and blood pressure measurement was conducted by a physical therapist.

Candidates were first interviewed using a general health questionnaire and a physical activity questionnaire. The validated Chinese version of the Mini-Mental Status Examination (4) of Folstein et al. was then administered, with scale ranges from 0 to 30. A score below 24 was considered indicative of cognitive dysfunction, and such subjects were excluded from this study. A modified version of the Minnesota Leisure Time Physical Activity Questionnaire (26) was used to compare the physical levels of the Tai Chi practitioners with those of the control subjects. This instrument evaluated the energy expended in leisure-time physical activities and household tasks. The activities were categorized according to their metabolic equivalent (MET) status as either: light (intensity ≤ 4.0 METs), moderate (4.0 METs < intensity ≤ 5.5 METs), or heavy activities (intensity > 5.5 METs). The project was approved by the Ethics Committee of The Hong Kong Polytechnic University, and written informed consent was obtained from all subjects.

Knee Joint Muscle Strength Test

Concentric and eccentric isokinetic strength of the knee extensors and flexors of the subject”s dominant leg were tested using the Cybex Norm dynamometer (Cybex International Inc., Ronkonkoma, NY). The leg that the subject used to kick a ball was considered the dominant leg. Each subject sat on the chair of the dynamometer, with the hips kept at 70° of flexion. The dominant leg was attached to the knee adaptor of the dynamometer with the rotation axis in line with that of the knee joint, defined using the lateral femoral epicondyle. The subject”s trunk and the thigh of the dominant leg were stabilized with straps such that the starting position was 90° of knee flexion, and the endpoint was full knee extension. Knee muscle strength was measured at an angular velocity of 30°·s−1. Before familiarization trials and the actual muscle strength testing, subjects performed a 10-min warm-up, including stretching of the knee muscle groups and 3 min of static bicycling at the subject”s comfortable speed. Familiarization trials were performed with three submaximal and three maximal repetitions for both concentric and eccentric contractions, to ensure reliable data in the isokinetic muscle testing (3). After correction for the gravitational effect on the knee torque, five maximal contractions of the concentric knee extensors and flexors were recorded as a test ensemble. This was followed after a 1-min rest by an eccentric test of both muscle groups (3).

The average of the three highest peak torques from the five repetitions was normalized to subject”s body weight, termed “peak torque-to-body weight” ratio, and was used as a measure to compare the muscle strength of the two groups (25). The hamstrings-to-quadriceps (H/Q) strength ratio was employed to evaluate possible agonist/antagonist muscle imbalance between the two groups (3).

Static Standing Balance Test

Subjects underwent a static standing balance test by standing quietly with the feet together on a force platform (Kistler, model 9286AA, Switzerland) for 30 s, with their eyes open. The maximum body sway angles in the anteroposterior and mediolateral directions during the 30-s stance was calculated, using a method that will be described in the section on data recording and analysis.

Perturbed Single-Leg Stance Test

After the static standing balance test, subjects underwent a dynamic single-leg standing balance test. Wearing a security harness and standing without shoes with their feet at shoulder width, the subjects were on the computerized dynamic posturography machine with a movable platform (NeuroCom International Inc., type Smart EquiTest®, Portland). They were instructed to stand on their dominant leg as still as possible, with their arms by their sides, their eyes looking forward, and their nondominant leg off the ground and flexed 90° at the knee with their hip in neutral position. They were notified that the perturbation began at any time once they flexed the dominant knee. Subjects were then perturbed with either forward or backward platform translation in a random order. To minimize their anticipation, perturbations were initiated after a random delay of 2–7 s. The computerized dynamic posturography equipment scaled the platform translation amplitudes according to the subject”s height, to give an anteroposterior body sway angle of 3.2° (15). Translation lasted for 400 ms. The center of pressure (COP) measured by four sensors mounted on the support surface was used to estimate the anteroposterior body sway angles of subjects undergoing the perturbations. The average of the maximum anteroposterior body sway angles recorded over three trials was used to compare the balance control of the two groups for each perturbation direction.

Activities-Specific Balance Confidence (ABC) Scale

The Activities-specific Balance Confidence (ABC) scale was used to investigate the older adults” perception of their balance confidence in daily activities (16). The scale consisted of 16 items describing daily living tasks. Subjects were asked to rate the 16 items using a 0–100 response continuum, with 0 representing no confidence and 100 complete confidence. Items included: walking around the house, climbing up and down stairs, picking up a slipper from the floor, reaching an object at eye level, reaching an object on tiptoes, standing on a chair to reach, sweeping the floor, walking outside to a nearby bus stop, getting in and out of a bus, walking across a parking lot, walking up and down a ramp, walking in a crowded mall, being bumped while walking in a crowd, using an escalator while holding a rail, using an escalator without holding the rail, and walking in a wet market. Any reference to a “car” in the original scale was replaced by “bus” in this study, as it is not common for older adults in Hong Kong to use cars. In addition, the last item of the original scale was changed from “walking on icy sidewalks” to “walking in a wet market” so as to suit the local situation. A wet market is an indoor market with meat, vegetables, and other goods laid out in the style of a bazaar, where the floors are often wet and slightly slippery. The ABC score ratio was presented as a percentage of the total score of 1600 (a maximum of 100 for each of the 16 items) for comparison between the two groups (16).

Data Recording and Analysis

Static standing balance.

The COP, as measured by four sensors attached to the force platform, was recorded and used to estimate the amount of body sway during the 30 s of quiet stance with the eyes open. This was expressed as the body sway angle, a term commonly used to measure the control of body sway (17), and was used in this study to quantify the body sway. The body sway angle is defined as the angle between a line extending vertically from the center of foot support and a line extending from the center of foot support through the center of mass (COM), when a person moves as a rigid mass about the ankles (Fig. 2) (15). The center of foot support is estimated to be the projection of the COM when the person stands erect. This point is located 2.3° in front of the ankle joint, midway between the lateral borders of the feet (14). In the static standing balance test, the maximum distance swayed was recorded and denoted as PCOP. The height of the COM, expressed as HCOM in the equation below, was estimated to be 55% of the subject”s body height (7). The normalized COP, denoted as PCOP/HCOM, was employed to estimate the maximum body sway angle (θ) as follows (Fig. 2):

FIGURE 2— Definition and calculation of the body sway angle, θ. Modified from the Smart EquiTest® System operators manual
FIGURE 2— Definition and calculation of the body sway angle, θ. Modified from the Smart EquiTest® System operators manual:
(15). PCOP denotes the maximum distance traveled by the subject”s center of mass (equal to “a” in the right diagram). HCOM denotes 55% of the height of the subject (equal to “c”). θ denotes the maximum body sway angle.

This formula takes into consideration the 2.3° “forward shift” of the COM from the vertical when calculating θ from the ankles (14). The COP measured from the force platform was used to determine the PCOP. The maximum body sway angles (θ) in the anteroposterior and mediolateral directions during the 30 s of quiet standing were estimated.

Perturbed single-leg stance.

All the posturography data were smoothed using a second order Butterworth low-pass filter with a cutoff frequency of 0.85 Hz. They were then used to estimate the body sway angles in the perturbed single-leg stance tests. The body sway was first recorded for 2 s before the platform translation. The average value of the body sway angles during the 2 s served as the baseline. After the platform perturbation, the maximum body sway angle was estimated and the difference from the baseline value, termed the perturbed body sway angle, was calculated. A total of three trials for each perturbation direction were performed, and the average value was used to compare the two groups. An amount of 12.5°, the theoretical anteroposterior sway stability limit, was assigned as the perturbed body sway angle if any subject fell during the platform perturbation (15). A “fall” in the perturbed single-leg stance test was recorded when the subject began to fall and touched the visual surround for support, or gained support by using the nondominant leg (15).

Statistical Analysis

Age, weight, and height were compared between the two groups using independent t-tests. Because of the categorical nature of the variables, a chi-square test was considered more appropriate for between-group comparison of the gender distribution and physical activity levels. An intraclass correlation coefficient was applied to assess the test-retest reliability of the knee muscle strength, the body sway angles during static standing and perturbed single-leg stance, and the ABC score ratio. The ICC model 3, denoted by ICC(3,k) was used for assessing intrarater reliability, with “k” denoting the number of trials used in the different tests. Multivariate analysis of variance was used to compare the measures recorded during muscle strength test, as well as static standing and perturbed single-leg stance tests between the Tai Chi and control subjects. If statistically significant differences were found in the multivariate tests, univariate tests were conducted for each of the measures. For between-group comparison of the hamstrings-to-quadriceps (H/Q) muscle strength and ABC score ratios, an independent t-test was employed. A Pearson product-moment coefficient of correlation was used to correlate the measures obtained in the knee muscle strength test, with 1) maximum anteroposterior and mediolateral body sway angles in the static standing balance test, 2) anteroposterior body sway angles in perturbed single-leg stance in forward and backward platform translations, and 3) the ABC score ratios. A significance level (α) of 0.05 was chosen for the statistical comparisons.

RESULTS

Subjects.

Fifty-five community-dwelling subjects volunteered in this project. Two Tai Chi subjects were excluded because of regular tennis playing (N = 1) and history of heart surgery (N = 1). Five control subjects were excluded because of history of falls in the previous 1 yr (N = 2), minor stroke (N = 1), regular badminton playing (N = 1), and knee pain being elicited during isokinetic muscle testing (N = 1). Twenty-four Tai Chi practitioners and 24 controls, all aged 60 or older, underwent all the testing procedures. Independent t-tests showed that there was no statistically significant difference in age, height, or weight between the Tai Chi and control subjects (P > 0.05; Table 1). All the subjects had scored at least 24 on the Mini-Mental Status Examination (mean = 28.5 ± 1.7, ranging from 24 to 30), which indicated no cognitive dysfunction (4). Chi-square tests found no statistically significant difference between the groups in either gender distribution or physical activity level (P > 0.05; Table 1). The Tai Chi and control subjects were thus similar with respect to age, height, weight, gender, and physical activity levels.

TABLE 1
TABLE 1:
Comparison of age, height, body weight, gender, and physical activity level between elderly Tai Chi and control subjects.

Test-retest reliability of knee muscle strength, body sway angles during the two standing balance tests, and ABC score.

Another four males and seven females with a mean age of 70.8 yr (± SD 4.0 yr) were recruited for the test-retest reliability study. The knee muscle strength test, the two standing balance tests, and the ABC scale were readministered to these subjects 1 wk afterward.

For the isokinetic knee muscle strength test, the ICC(3,3) values for the concentric knee extensors, concentric knee flexors, eccentric knee extensors, and eccentric knee flexors were 0.97 (confidence intervals abbreviated as CI henceforth 0.89–0.99), 0.86 (CI 0.43–0.97), 0.97 (CI 0.89–0.99), and 0.95 (CI 0.81–0.99), respectively. In the static standing balance test, the ICC(3,3) values for the maximum anteroposterior and mediolateral body sway angles were 0.82 (CI 0.21–0.96) and 0.85 (CI 0.23–0.97), respectively. For the perturbed single-leg stance test, the ICC(3,3) values for the maximum anteroposterior body sway angles in forward translation and backward translation were 0.81 (CI 0.51–0.93) and 0.74 (CI 0.35–0.90), respectively. The ICC(3,1) value for the ABC score was found to be 0.91 (CI 0.71–0.98).

Knee muscle strength.

Multivariate tests of the knee strength results indicated an overall statistically significant effect across the four measures between the Tai Chi and control subjects (P = 0.044). Univariate tests showed that there were statistically significant differences in the muscle strength of concentric knee extensors (P = 0.004; Table 2), concentric knee flexors (P = 0.021), eccentric knee extensors (P = 0.049), and eccentric knee flexors (P = 0.007) between the two groups. Tai Chi practitioners achieved significantly higher peak torque-to-body weight ratios with both their knee extensors and flexors in both concentric and eccentric isokinetic testing at 30°·s−1. An examination of the concentric H/Q strength ratios for the Tai Chi and control subjects yielded 0.49 and 0.50, respectively (P = 0.764 in the independent t-test), whereas the eccentric H/Q strength ratios were 0.67 and 0.61, respectively (P = 0.131). Our findings thus show that Tai Chi practitioners had similar agonist/antagonist strength ratios in concentric and eccentric muscle contractions as those of the healthy control subjects.

TABLE 2
TABLE 2:
Comparison of knee muscle strength, body sway angle in static standing and perturbed single-leg stance, and ABC score between elderly Tai Chi and control subjects.

Static standing and perturbed single-leg stance tests.

Multivariate test results showed no significant difference between the Tai Chi and control subjects (P = 0.498). The mean maximum anteroposterior body sway angles for the Tai Chi and control subjects were 1.6 ± 0.4 and 1.8 ± 0.5°, respectively (P = 0.411; Table 2). The mean maximum mediolateral body sway angles were 1.5 ± 0.3 and 1.6 ± 0.5°, respectively (P = 0.235; Table 2).

Multivariate tests of the perturbed single-leg stance results indicated an overall statistically significant effect across these two measures between the Tai Chi and control subjects (P < 0.001). The univariate tests showed that the Tai Chi practitioners achieved less body sway angles during forward platform translation (mean = 7.2 ± 2.1°) than did the control subjects (mean = 10.0 ± 2.9°; P < 0.001; Table 2), as well as during backward platform translation (mean = 6.2 ± 2.1 and 9.7 ± 3.2°, respectively; P < 0.001).

ABC scale.

Tai Chi practitioners reported having more balance confidence in performing their daily tasks as assessed by the ABC scale than did the control subjects, with score ratios of 98.0 ± 3.0 and 90.7 ± 9.5%, respectively (P = 0.001; Table 2).

Correlation of knee muscle strength with measures of static standing, perturbed single-leg stance, and ABC tests.

Correlations among the four measures from the knee muscle strength test on the one hand, and the measures from the static standing test (maximum anteroposterior and mediolateral body sway angles), the perturbed single-leg stance test (maximum anteroposterior body sway angles in forward and backward platform translations), and the ABC score ratio on the other hand, were analyzed using Pearson”s product-moment coefficient of correlation. Table 3 shows that the four measures from the knee muscle strength tests were not statistically correlated with the maximum body sway angles in the static standing balance test (all P > 0.05). However, they were all inversely correlated with the body sway angles obtained from forward and backward platform perturbations during the single-leg stance test (all P < 0.05; Table 3). Subjects who exhibited higher peak torque-to-body weight ratios swayed less during platform perturbations in either a forward or backward direction. All the measures of knee muscle strength were also correlated with the ABC score ratios (all P < 0.05; Table 3). After performing further correlation analysis, we discovered that the body sway angles during single-leg stance subjected to forward and backward platform perturbations were in fact negatively correlated with the ABC score ratio (−0.383 and −0.432, respectively, all P < 0.05).

TABLE 3
TABLE 3:
Pearson product-moment coefficient of correlation among the measures of the knee muscle strength test and those of the static standing, perturbed single-leg stance, and ABC tests.

DISCUSSION

Test-retest reliability.

The ICC values found in this study ranged from 0.74 to 0.97. Therefore, all the tests used were found to produce reliable measures. However, the confidence intervals for concentric knee flexors and maximum anteroposterior and mediolateral body sway angles in static standing balance test appeared large. This might be a result of the small sample size in the reliability test (N = 11).

Effect of Tai Chi practice on knee muscle strength.

The Tai Chi practitioners demonstrated significantly higher relative knee extensor and flexor muscle strength in both concentric and eccentric contractions when compared with matched control subjects (P = 0.044; Table 2). They were 36% stronger in concentric knee extensor contraction (mean = 1.5 ± 0.4 N·m·kg−1 for Tai Chi practitioners and 1.1 ± 0.4 N·m·kg−1 for control subjects; P = 0.004; Table 2), and 40% stronger in concentric knee flexor contraction (mean = 0.7 ± 0.3 and 0.5 ± 0.3 N·m·kg−1, respectively; P = 0.021; Table 2). These results are in agreement with those of Lan et al. (10), who showed that older participants (age ranged from 58 to 70 yr) had significant strength improvement in both concentric knee extension and flexion after 12 months of Tai Chi training. However, the respective increases of 19.2 and 15.7% found by these investigators were comparatively less than those found in our present cross-sectional study (36 and 40%). There are two possible explanations for the discrepancy between our findings and theirs: 1) we had adopted a slower isokinetic testing speed of 30°·s−1 compared with theirs (60°·s−1), or 2) our Tai Chi subjects had longer practice experience (mean = 8.5 ± 7.6 yr) than their study (mean of 1 yr) (10).

Our results also demonstrate that Tai Chi practitioners had higher eccentric contraction strength in their knee extensors (21% difference, mean = 1.7 ± 0.6 N·m·kg−1 for Tai Chi practitioners and 1.4 ± 0.4 N·m·kg−1 for control subjects; P = 0.049; Table 2) and knee flexors (37.5% difference, mean = 1.1 ± 0.3 and 0.8 ± 0.3 N·m·kg−1, respectively; P = 0.007). Our findings of significantly higher knee flexor muscle strength in both concentric and eccentric contractions among the older Tai Chi practitioners were different from those observed by Wu et al. (30). These investigators found no significant difference in knee flexor strength between Tai Chi practitioners and control subjects. Because the experimental set-up was similar in terms of the functional range of muscle testing, the reason might be the slower isokinetic testing speed used in our study (30°·s−1) versus their study (60 and 120°·s−1). Note that Tai Chi practice involves mainly slow movements (23). Various studies have shown that muscle strength gains are consistently greatest at the training velocity, termed “velocity specificity” (5,12). Therefore, the slower isokinetic testing speed used in the present study may explain the higher muscle strength of our Tai Chi practitioners, especially for the knee flexor muscles, which was not found by Wu et al. at higher isokinetic testing speed (30).

The frequent knee bending involved in performing various Tai Chi forms may explain the greater knee extensor strength of the Tai Chi practitioners (6). Different knee flexor strength may be due to the training in the closed kinetic chain position adopted during Tai Chi practice. Primary knee flexors are the hamstrings and gastrocnemius muscles. Both muscle groups are two-jointed muscles, with the hamstrings crossing the hip and knee joints and the gastrocnemius crossing the knee and ankle joints. Bending the knees in a closed kinetic chain standing position requires hip flexion with simultaneous ankle dorsiflexion (6). Hence, both the hamstrings and gastrocnemius are required to contract with their origin and insertion reversed. EMG studies have shown that the hamstrings and gastrocnemius contracted at 20–60% of their maximum isometric voluntary contractions in different phases during double-leg squatting (6). In a single-leg stance squatting protocol, the maximum hamstrings activation was found to be 81% of the maximum isometric voluntary contraction (2). Tai Chi practice involves a lot of squatting in double-leg and single-leg stance, which may explain why experienced Tai Chi practitioners had higher knee flexor muscle strength. Further investigation of the muscle work during different Tai Chi forms is warranted.

Tai Chi practitioners not only had higher knee muscle strength, our findings demonstrated that the extent of their strength difference was similar in both the agonists and antagonists. Briefly, the concentric H/Q strength ratio was 0.49 and the eccentric H/Q strength ratio was 0.67, which was no different from the respective value of 0.51 and 0.61 found in control subjects (all P > 0.05). This may be due to the closed kinetic chain positioning adopted during Tai Chi practice, as it requires cocontraction of the agonist-antagonist muscles, which controls joint movement in three dimensions (9). Exercises targeting only the agonists may cause muscle imbalance, which will predispose the participants to injury. Muscle imbalance between the agonist and antagonist muscles may cause joint instability, as often seen in sports activities (19). Closed kinetic chain exercise like Tai Chi may have the advantage of improving the muscle strength of agonists and antagonists to a ratio similar to that of healthy control subjects.

Administering eccentric training to older adults may not be easy (27). LaStayo et al. (11) tried to overcome this by designing an eccentric ergometer powered by a motor driving the pedals in a backward direction. However, the applicability of this technique is limited by the fact that the intensity of the ergometer must be individually designed and monitored, and by the need for having such special equipment for training. Tai Chi practice does not require equipment or large space. Using one”s body weight for resistance training is safe and cost-effective, and practitioners can vary the degree of knee bending according to their ability and comfort. There has been no known report of adverse effects from long-term Tai Chi practice. The eccentric training components of Tai Chi can thus become an effective and easy means of eccentric muscle strength training for older adults.

Static standing and perturbed single-leg stance.

The static standing balance test did not reveal any difference in balance performance between Tai Chi practitioners and the control subjects. More specifically, no significant difference in the maximum anteroposterior and mediolateral body sway angles was found between the two groups (P = 0.411 and 0.235, respectively; Table 2). In contrast, during the perturbed single-leg stance test, Tai Chi practitioners demonstrated significantly less body sway angles during both forward (P < 0.001; Table 2) and backward platform translations (P < 0.001).

The results of our static standing tests differ from those of Wu et al. (30) in that their older Tai Chi subjects had significantly less body displacement in both the anteroposterior and mediolateral directions than their control subjects. This may be due to the more stringent inclusion criteria adopted in the present study, such as recruitment of control subjects with similar physical activity level (P = 0.232; Table 1), as well as the absence of a history of falls over the past 12 months.

The practice of Tai Chi requires constant weight shifting between double-leg and single-leg stances, and execution of various arm and leg movements during single-leg stance such as in a form called “golden hen single-leg stance.” Such weight shifting demands a higher degree of balance control from the Tai Chi practitioners. In a cross-sectional study, Tse and Bailey (24) found that older people (ages ranging from 65 to 84 yr) with more than 1 yr of Tai Chi practice could maintain single-leg standing longer than their sedentary counterparts in right and left leg standing with the eyes open. Later, Hong et al. (8) found that practitioners (mean age = 67.5 yr) with more than 10 yr of Tai Chi experience could maintain single-leg standing with their eyes closed for a significantly longer period than non–Tai Chi practitioners. The present study demonstrated that Tai Chi practitioners had better balance control even in single-leg stance against platform perturbations. All these observations may be explained by the fact that in different Tai Chi forms, the practitioners are required to perform different movements with their other limbs during single-leg stance, which clearly demands greater balance control (23). For example, in one Tai Chi maneuver termed ”kick with the heel,” the practitioner lifts the nonweight-bearing leg as high as possible while simultaneously opening and holding their arms horizontally (Fig. 3). In another Tai Chi move, “repulse the monkey,” once the practitioners have lifted the leg, they need to extend it backward while simultaneously pushing forward with their opposite arm (Fig. 4). These two examples illustrate the demand for greater balance control during single-leg stance, when movements of both arms and the other leg are performed simultaneously in different directions.

FIGURE 3—The “kick with the heel” maneuver.
FIGURE 3—The “kick with the heel” maneuver.
FIGURE 4—The “repulse the monkey” maneuver.
FIGURE 4—The “repulse the monkey” maneuver.

Our findings thus suggest that long-term practice of Tai Chi may contribute to better control of body sway during perturbed single-leg stance in the practitioners with mean Tai Chi experience of 8.5 ± 7.6 yr. As falls seldom occur during double-leg stance, the better single-leg stance during platform perturbation could explain the reduction in the risk of multiple falls by 47.5% in the Tai Chi practitioners (mean age = 76.9 yr), as shown by Wolf et al. (28).

ABC scale.

Both groups in this study reported high ABC score ratios. With the mean scores above 90%, they can be regarded as having good balance confidence (16). The high ABC score ratios could be explained by the fact that our participants in both groups were healthy and had no history of falls in the past 12 months. Falls can lead to fractures, soft tissue injuries, joint dislocations, and mobility impairments. They may also lead to fear of falls, which can result in self-imposed activity restriction (13). This reduced activity can cause further deterioration of balance control in older adults. However, such a vicious cycle can be broken by a suitable intervention program (13). Wolf et al. (28) conducted a 15-wk Tai Chi intervention program with a group of community-dwelling older adults aged 70 or above. They found that the Tai Chi participants had reduced their fear of falling as compared with a control group after intervention. The Tai Chi practitioners in this study had significantly higher ABC scores (98.0%) than those of the control subjects (90.7%; P = 0.001; Table 2). The better balance control in the Tai Chi practitioners as shown in our previous studies (20–22), coupled with the better single-leg stance performance and balance confidence shown in the present study, might help to maintain their physical activity level and balance control and reduce the risks of their falling as they age further.

Correlation of knee muscle strength with measures of the static standing, perturbed single-leg stance, and ABC tests.

Our correlation findings differ from those of Wu et al. (30), who found a significant correlation of knee extensor but not flexor strength with body sway angles in static double-leg standing. The significant negative correlations observed in the present study between the strength of both the knee extensors and flexors, and body sway angles in perturbed single-leg stance, may be due to the need to recruit both agonists and antagonists to maintain balance control during single-leg stance. Even during double-leg stance, platform perturbations demand additional muscle groups to counteract the perturbations. For example, in a forward platform translation, which will induce backward body sway by virtue of inertial force, the ankle dorsiflexors, knee extensors, and abdominal muscles are all recruited to maintain bipedal balance and erect standing (18). In a backward platform translation, which will lead to forward body sway, the ankle plantarflexors, knee flexors, and back extensors are recruited (18). The more demanding balance control in perturbed single-leg stance is suggested by the relatively higher negative correlation coefficient values between knee extensor strength and body sway angles, ranging from −0.529 to −0.441 (all P < 0.05; Table 3), when compared with those of Wu et al. (30) in double-leg stance (−0.37 to −0.28). Negative correlation coefficient values between knee flexor strength and body sway angles during perturbed single-leg stance in our study were all significant, with values ranging from −0.440 to −0.335 (P < 0.05; Table 3). These findings illustrate that better muscle strength can enhance more demanding balance control leading to less body sway during perturbed single-leg stance.

Significant positive correlations between concentric and eccentric knee extensor and flexor strength and the ABC score ratios were found (all P < 0.05; Table 3). This result suggests that greater knee muscle strength leads to more confidence in activities requiring good balance control. On further correlation analysis, we discovered significant negative correlations between body sway angles during perturbed single-leg stance and ABC score ratios (all P < 0.05). This finding suggests that less body sway during perturbed single-leg stance was associated with better subjective report of balance confidence, when older adults perform functional activities involving single-leg stance and moving support surface. A notable example is stepping onto a moving escalator without holding the rail.

Because we used a cross-sectional design in the present study, a causal relation between the Tai Chi practice and the improvement of muscle strength, perturbed single-leg stance, and subjective balance confidence cannot be established. A longitudinal study is required to establish the causal relation. Because only healthy older adults were examined in this study, our findings cannot be inferred to frail older individuals or those who have a history of falls. In our study, more Tai Chi practitioners were engaged in moderate and heavy physical activities (N = 5 and 2, respectively) when compared with control subjects (N = 3 and 0, respectively; Table 1). However, statistical analysis of the physical activity level did not show any significant difference between the two groups. Limitations aside, findings from this cross-sectional study demonstrate that experienced Tai Chi practitioners had stronger knee extensor and flexor muscle strength, less body sway in perturbed single-leg stance, and greater balance confidence than matched control subjects. Significant correlations among these three measures highlight for the first time the importance of knee muscle strength and balance control during perturbed single-leg stance in older adults” balance confidence.

The authors thank The Hong Kong Polytechnic University for financial support of this study through an Area of Strategic Development Grant to C. W. Y. Hui-Chan, W. W. N. Tsang, et al.; the subjects for their participation; and the older adult centers for permission to recruit their subjects. Thanks are also due to Mr. Bill Purves for his English editorial advice.

Part of the study was presented at the Third Pan-Pacific Conference on Rehabilitation, Hong Kong (SAR), China, August 23–25, 2002.

No commercial party having a direct financial interest in the research findings reported here has or will confer a benefit upon the authors or upon any organization with which the authors are associated.

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Keywords:

AGING; FALLS; EXERCISE; ISOKINETIC TESTING

©2005The American College of Sports Medicine