1364
Improvement of Isokinetic Knee Extensor Strength and Reduction of Postural Sway in the Elderly From Long-Term Tai Chi Exercise Ge Wu, PhD, Fang Zhao, BS, Xinglong Zhou, BS, Liu Wei, BS ABSTRACT. Wu G, Zhao F, Zhou X, Wei L. Improvement of isokinetic knee extensor strength and reduction of postural sway in the elderly from long-term Tai Chi exercise. Arch Phys Med Rehabil 2002;83:1364-9. Objectives: To compare isokinetic strength of leg muscles and foot center of pressure (COP) as a measure of sway between long-term Tai Chi practitioners and controls. Design: Cross-sectional study. Setting: Community setting. Participants: Twenty subjects in the Tai Chi group and 19 subjects in the control group (age, ⬎55y). Intervention: Subjects in Tai Chi group had practiced Tai Chi for a minimum of 3 years. Main Outcome Measures: Concentric and eccentric strength of knee extensors and flexors at 60°/s and 120°/s, and foot COP displacement during quiet stance with eyes open or closed. Results: People in the Tai Chi group had significantly higher knee extensor strength at all speeds tested (P⬍.013), and smaller foot COP excursions for both eyes open and eyes closed conditions (P⬍.05) than people in control group. No significant difference existed in knee flexors between the 2 groups (P⬍.713). The COP excursions correlated significantly with the eccentric strength of knee extensors (P⬍.07) but not with the concentric strength of knee extensors (P⬍.14) or with the isokinetic strength of knee flexors at most of the speeds (P⬍.27). Conclusion: These findings support the hypothesis that the maintenance of eccentric strength of postural muscles in the lower extremities, which is beneficial for maintaining good postural stability, is helped through the long-term practice of Tai Chi. Key Words: Elderly; Knee; Muscle fibers, type II; Posture; Rehabilitation; Tai Chi. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation ALLING AND FALL-RELATED injuries in the elderly are a major health problem for the United States. Not only F does frequency of falls increase with advancing age, but also 1,2
the immobility resulting from falls has more deleterious effects
From the Department of Physical Therapy, University of Vermont, Burlington, VT (Wu, Wei); and Biomechanical Teacher Section, Beijing University of Physical Education, Beijing, China (Zhao, Zhou). Supported by the National Institute of Aging, National Institute of Health (grant no. 1R29AG11602). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Ge Wu, PhD, Dept of Physical Therapy, University of Vermont, Burlington, VT 05401, e-mail:
[email protected]. 0003-9993/02/8310-6956$35.00/0 doi:10.1053/apmr.2002.34596
Arch Phys Med Rehabil Vol 83, October 2002
on structures and functions in the elderly than in the young.3 The scope of this problem will continue to expand in the immediate future because the elderly population is projected to increase dramatically over the next decade.4 Tai Chi has been shown to be beneficial in reducing the incidence of falls in the elderly.5,6 Tai Chi is an ancient Chinese martial art. It consists of a series of slow but continuous movements of every body part. There are many working hypotheses to explain this positive outcome of Tai Chi. They include (1) Tai Chi practice may lead to an optimal use of different components of postural control,7 (2) the slow movement may promote sensory awareness,8-10 (3) precise control of body movement may promote muscle coordination, and (4) shift of body position may promote dynamic balance.10 However, these hypotheses have not been well tested. It is well known that muscle atrophy occurs with aging. In particular, type II fiber distribution and size are more affected than type I fibers by aging. When biopsies taken from the vastus lateralis muscle in a group of sedentary elderly were compared with those from a group of young subjects, the old subjects had twice as many type I muscle fibers as the young subjects, suggesting that old persons have fewer type II fibers than young persons.11 A recent study by Huang et al12 examined the power spectrum of vibromyography of the soleus muscle in the aged population and found a significant decline in the high-frequency range, suggesting a decreased use or loss of type II fibers.13 The age-related changes in type II fiber size and distribution in skeletal muscles are related to the decline in muscle power output,14 eccentric force,15 and level of physical function16,17 in the elderly. It may be that the age-related changes in type II fiber size and distribution are also related to decreased postural stability and increased incidence of falls in the elderly. Recent studies have suggested that eccentric exercises increase type II fibers, muscle strength, and power. Hortobagyi et al15,18 showed a 10-fold increase in type II fiber area and a 77% increase in strength after an eccentric exercise program. Observations of Tai Chi movements show that they involve continuous knee flexion and extension motion during the weightbearing phase of the movement (fig 1 depicts 1 Tai Chi movement). Thus, a movement may require a relatively long duration of eccentric activation of leg muscles over a large range of joint motion. We hypothesized that long-term Tai Chi practice may improve eccentric strength of leg muscles, enhancing an individual’s postural stability. To test this hypothesis, we measured isokinetic strength of leg muscles and postural stability in 2 groups of subjects: those who had practiced Tai Chi regularly for at least 3 years and those who had never practiced Tai Chi before. Comparisons between the 2 groups in the leg muscle isokinetic strength and postural stability measures were made. Correlation between leg muscle strength and postural stability was also examined.
1365
TAI CHI ON ECCENTRIC STRENGTH AND POSTURAL SWAY, Wu
Fig 1. An illustration of 1 Tai Chi movement: the whip.
METHODS Participants Subjects were a convenience sample from 2 organized groups in Beijing, China: a Tai Chi exercise group and a leisure activity group. They were recruited on a voluntary basis. Persons in the Tai Chi exercise group were assigned to the Tai Chi group and those in the leisure group to the control group. All subjects were retired faculty members from universities in Beijing, China (age, ⱖ55y). They all lived within the vicinity of a concentrated university campus area. They have similar educational backgrounds and lifestyles such as using bicycles as a primary mode of transportation. The subjects in the Tai Chi group (n⫽20) had practiced Tai Chi in a group setting on a regular basis at least 3 days a week, 1 hour daily, for a minimum of 3 years (mean, 21y). The hour-long practice included a brief warm-up stretching, a complete long-form Tai Chi, and free practice. A Tai Chi master was present to teach and lead the Tai Chi practice. Six subjects in the Tai Chi group had arthritis or previous joint injuries. The subjects in the control group (n⫽19) had never practiced Tai Chi before. But, all of them had been physically active, and some of them did regular exercises. Seven control subjects had arthritis or joint problems, and 3 had hypertension or coronary heart disease. At the time of testing, all subjects were free of pain or centrally active medications that would have affected their physical functioning or postural balance. Table 1 summarizes the subjects’ information in both groups. No significant difference existed between the 2 groups in age (P⫽.79), weight (P⫽.15), or height (P⫽.19). All subjects participated in the study voluntarily and signed the informed consent form approved by the University of Vermont Institutional Review Board. Protocol For each subject, the foot center of pressure (COP) displacement during quiet stance was first measured. Subjects were asked to stand on a biomechanical forceplate with heels 10cm apart and toes 10° out and with eyes either open or closed, respectively. When eyes were open, they were asked to look at a target in front of them. For each visual condition, subjects were instructed to stand as stable as possible for 30 seconds. A
total of 5 trials were repeated for each visual condition, with at least a 1-minute break between each trial. The COP displacement in both anteroposterior (AP) and mediolateral (ML) directions was collected at 20Hz. Each subject was then tested for isokinetic (concentric, eccentric) muscle strength in 2 muscle groups: knee extensors (quadriceps) and knee flexors (hamstrings) of the right limb. The measure of strength was peak torque as indicated by an isokinetic force dynamometer.a Each subject was first stabilized in a testing seat by means of straps at the chest, waist, thigh, and the ankle. The rotating axis of the dynamometer was aligned with the subject’s knee joint. Each subject was then given the instruction on the task and was asked to practice a few times to get familiar with the task. During testing, 3 continuous repetitions of concentric or eccentric strength were taken at each speed through the full range of motion. The concentric strength was measured at 60°/s, and eccentric strength at 60° and 120°/s, respectively. To minimize the effects of muscle fatigue, the sequence of measurements was randomized, and the subjects were given at least a 2-minute rest period between tests or as much time they needed.
Table 1: Subject Information
Number Age (y) Gender (M/F) Weight (kg) Height (cm) Regular exercise (y) Arthritis Other joint problems Hypertension Heart disease
Control
Tai Chi
P
19 63⫾4 5/14 62⫾7 161⫾6 1⫾2 3 4 2 1
20 62⫾4 10/10 64⫾9 164⫾8 21⫾14 3 3 0 0
— .79 — .15 .19 .00 — — — —
NOTE. Values are mean ⫾ standard deviation or n. Abbreviations: M, male; F, female.
Arch Phys Med Rehabil Vol 83, October 2002
1366
TAI CHI ON ECCENTRIC STRENGTH AND POSTURAL SWAY, Wu
Fig 2. The means and SDs of isokinetic strength of knee extensors and knee flexors at various speeds. Abbreviations: con, concentric; ecc, eccentric; TC, Tai Chi. * Significant difference (P
