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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,
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.
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.
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 weight-bearing 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.
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, ≥55y). 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.
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.
The means and standard deviations (SDs) of the peak strengths and the maximum COP excursion in the ML and AP directions were calculated. A 2-way analysis of variance (ANOVA) was used to compare the difference in peak strengths between groups and between genders. A 3-way ANOVA was used to compare the difference in COP excursions between groups, genders, and visual conditions. The correlation between peak isokinetic strength of each muscle and maximum COP excursion in each direction was calculated, respectively, by using the Spearman correlation (r).
No significant interaction existed between group and gender for both muscles at all 3 speeds tested. For both knee flexors and extensors, there was a significant gender main effect at all 3 speeds (P<.014) except for the knee extensor's eccentric strength at 120°/s (P=.27). In particular, the isokinetic strength of men was higher than that of women. Furthermore, there was a significant group main effect for the knee extensors at all 3 speeds (P<.013) but not for the knee flexors (P>.713). The Tai Chi group showed significantly higher knee extensor strength at all 3 speeds tested than the control group (see fig 2).
For the COP excursions in both AP and ML directions, no significant interaction existed among all independent variables (P<.333). Nor was there any significant main effect for gender (P<.62) for both directions. However, there was a significant group main effect for both COP excursions (P<.05). Specifically, the Tai Chi group had significantly smaller excursions of the COP in both directions than the control group. Furthermore, the COP excursion in the AP direction was significantly larger in the eyes closed condition than in the eyes open condition (P=.002). No significant difference existed in ML COP excursion between the 2 visual conditions (P=.71). Figure 3 shows the means and SDs of COP excursions.
It was found that the COP excursions in both AP and ML directions correlated significantly with the eccentric strength of the knee extensors (r=−.21 to −.37, P<.07), but not with the concentric strength of the knee extensors (r=−.12 and r=−.18, respectively, P>.14) (see table 2, fig 4).
Table 2Correlation coefficients
Knee Extensor Strength
Knee Flexor Strength
NOTE. Values are correlation coefficient (P value).
The negative correlation coefficient indicated that the higher the isokinetic strength of the knee extensors, the smaller the COP excursion. Furthermore, it was found that the COP excursions did not significantly correlate with the isokinetic strengths of the knee flexors (r=−.13 to.06, P>.27), with 1 exception.
The main findings in this study were that people with long-term Tai Chi practice had higher knee extensor strength and smaller foot COP excursions during quiet stance than people without Tai Chi practice. The eccentric strength of knee extensors was associated with the COP excursions.
To date, it is not clearly understood why Tai Chi practice has a more positive effect on enhancing postural stability and preventing falls in the elderly than other forms of exercise. We hypothesized that Tai Chi practice might lead to the enhanced postural stability or reduced falls in the elderly by improving or maintaining eccentric strength of leg muscles. The results in this study confirmed this hypothesis.
First, the findings that subjects in the Tai Chi group had significantly higher isokinetic strength in the knee extensors and smaller foot COP excursion during quiet stance than subjects in the control group are consistent with previous findings by others. For example, Jacobson et al
compared 20 community-dwelling elderly persons who practiced Tai Chi for 12 months with 18 others who did not practice Tai Chi. They found that all subjects in the Tai Chi group had a significant increase in muscle strength of knee extensors (as measured by the concentric strength at 60°/s), while the subjects in the control group had no significant change. More recently, Lan et al
reported the isokinetic strength change in elderly people after practicing Tai Chi for 6 months. They measured both concentric and eccentric strengths at various speeds and found significant improvements in all strength measures. Although the Lan study was a cross-section design that did not have subjects' strength and foot COP excursion data before their participating in Tai Chi practice, subjects in both groups were comparable in terms of their age range, education background, lifestyle, and so on. The differences found between the 2 groups could be partially, if not completely, attributed to Tai Chi practice.
Second, few studies, to date, have been performed to examine the effect of Tai Chi practice on eccentric strength of postural muscles. Only the study by Lan
directly reported the eccentric strength of leg extensors after a relatively short period of Tai Chi practice. Unfortunately, no control group was used in that study. The present study is the first cross-sectional study to compare the eccentric strength of postural muscles between subjects who have and have not practiced Tai Chi over a long period of time.
Moreover, the present results showed that the eccentric strength of the knee extensors was associated with the foot COP displacement in both AP and ML directions. Specifically, the higher the eccentric strength, the smaller the foot COP displacement during quiet stance. Foot COP displacement, especially in the ML direction during quiet stance, is among the predicting factors for future falls in the elderly.
The inverse relation between the eccentric strength and COP displacement seems to favor greater eccentric strength as a factor that enhances an elderly individual's postural stability and helps prevent future falls.
The association between eccentric strength of knee extensors and foot COP displacement during quiet stance should not be a surprise. Previous studies have shown the effect that eccentric strength has on quality of life in the elderly population. Specifically, eccentric muscle force not only decreases with age
examined the relationship between level of physical function and fiber type atrophy in the vastus lateralis muscle in patients with knee and hip joint problems. They found that the type II fibers were mostly reduced in size, and those patients with predominantly type II fiber atrophy had the lowest functional levels (as measured by the FIM™ instrument). In a separate study, Bassey et al
showed that decreased leg muscle power in frail elders is among the most important limiting factors for performing daily activities such as stair climbing, rising from a chair, and walking. Moreover, a close association exists between an individual's functional disability and his/her susceptibility to falls.
Therefore, a decline in eccentric strength would inevitably decrease an elderly person's postural stability and increase the risks for falls.
The higher eccentric strength we found in those subjects who had practiced Tai Chi over a long time suggests that Tai Chi exercise requires a considerable amount of eccentric activation of leg muscles. Observing Tai Chi movement, one sees that the knee of the weight-bearing leg changes continuously but slowly from extension to flexion.
This action may require the knee extensors to be eccentrically active to control the speed of knee flexion. To date, no quantitative measurement has been performed to examine the type and duration of muscle activation during Tai Chi practice. Future studies are needed to compare the amount of eccentric activation of leg muscles during Tai Chi and during other activities such as walking or jogging.
People who practiced Tai Chi had higher concentric and eccentric strengths of the quadriceps and lower foot COP displacement during quiet stance. A good correlation existed between the COP displacement and the eccentric strength of knee extensors. 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.
We thank Zhong Guan Cun Martial Art Association for providing subjects and Juvena Hitt for helping with data processing.
Characteristics and predictors of falls in elderly patients.
☆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] .