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Improvement of isokinetic knee extensor strength and reduction of postural sway in the elderly from long-term Tai Chi exercise

  • Ge Wu
    Affiliations
    Department of Physical Therapy, University of Vermont, Burlington, VT (Wu, Wei); and Biomechanical Teacher Section, Beijing University of Physical Education, Beijing, China (Zhao, Zhou)
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  • Fang Zhao
    Affiliations
    Department of Physical Therapy, University of Vermont, Burlington, VT (Wu, Wei); and Biomechanical Teacher Section, Beijing University of Physical Education, Beijing, China (Zhao, Zhou)
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  • Xinglong Zhou
    Affiliations
    Department of Physical Therapy, University of Vermont, Burlington, VT (Wu, Wei); and Biomechanical Teacher Section, Beijing University of Physical Education, Beijing, China (Zhao, Zhou)
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  • Liu Wei
    Affiliations
    Department of Physical Therapy, University of Vermont, Burlington, VT (Wu, Wei); and Biomechanical Teacher Section, Beijing University of Physical Education, Beijing, China (Zhao, Zhou)
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      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. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

      Keywords

      FALLING AND FALL-RELATED injuries in the elderly are a major health problem for the United States. Not only does frequency of falls increase with advancing age,
      • Hale WA
      • Delaney MJ
      • McGaghie WC.
      Characteristics and predictors of falls in elderly patients.
      • Lord SR
      • Caplan GA
      • Colagiuri R
      • Colagiuri S
      • Ward JA.
      Sensori-motor function in older persons with diabetes.
      but also the immobility resulting from falls has more deleterious effects on structures and functions in the elderly than in the young.
      • Kallinen M
      • Markku A.
      Aging, physical activity and sports injuries. An overview of common sports injuries in the elderly.
      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.
      • Healthy People 2000
      national health promotion and disease prevention objectives.
      Tai Chi has been shown to be beneficial in reducing the incidence of falls in the elderly.
      • Wolf SL
      • Barnhart HX
      • Kutner NG
      • McNeely E
      • Coogler C
      • Xu T.
      Reducing frailty and falls in older persons: an investigation of Tai Chi and computerized balance training. Atlanta FICSIT Group. Frailty and Injuries: Cooperative Studies of Intervention Techniques.
      • Wolfson L
      • Whipple R
      • Derby C
      • et al.
      Balance and strength training in older adults: intervention gains and Tai Chi maintenance.
      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,
      • Forrest WR.
      Anticipatory postural adjustment and Tai Chi Ch'uan.
      (2) the slow movement may promote sensory awareness,
      • Kutner NG
      • Barnhart H
      • Wolf SL
      • McNeely E
      • Xu T.
      Self-report benefits of Tai Chi practice by older adults.
      • Jacobson BH
      • Chen HC
      • Cashel C
      • Guerrero L.
      The effect of Tai Chi Chuan training on balance, kinesthetic sense, and strength.
      • Tse SK
      • Bailey DM.
      T'ai chi and postural control in the well elderly.
      (3) precise control of body movement may promote muscle coordination, and (4) shift of body position may promote dynamic balance.
      • Tse SK
      • Bailey DM.
      T'ai chi and postural control in the well elderly.
      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.
      • Larsson L
      • Sjodin B
      • Karlsson J.
      Histochemical and biochemical changes in human skeletal muscle with age in sedentary males, age 22-65 years.
      A recent study by Huang et al
      • Huang RP
      • Rubin CT
      • McLeod KJ.
      Changes in postural muscle dynamics as a function of age.
      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.
      • Bellemare F
      • Woods JJ
      • Johansson R
      • Bigland-Ritchie B.
      Motor-unit discharge rates in maximal voluntary contractions of three human muscles.
      The age-related changes in type II fiber size and distribution in skeletal muscles are related to the decline in muscle power output,
      • Faulkner JA.
      Power output of fast and slow fibers from skeletal muscles.
      eccentric force,
      • Hortobagyi T
      • Hill JP
      • Houmard JA
      • Fraser DD
      • Lambert NJ
      • Israel RG.
      Adaptive responses to muscle lengthening and shortening in humans.
      and level of physical function
      • Tanaka S
      • Hachisuka K
      • Nara S
      • Ogata H
      • Kobayashi Y
      • Tanaka H.
      Effect of activities of daily living on fiber type atrophy of the vastus lateralis muscle in patients with joint disorders.
      • Bassey EJ
      • Fiatarone MA
      • O'Neill EF
      • Kelly M
      • Evans WJ
      • Lipsitz LA.
      Leg extensor power and functional performance in very old men and women.
      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 al
      • Hortobagyi T
      • Hill JP
      • Houmard JA
      • Fraser DD
      • Lambert NJ
      • Israel RG.
      Adaptive responses to muscle lengthening and shortening in humans.
      • Hortobagyi T
      • Lambert NJ
      • Hill JP.
      Greater cross education following training with muscle lengthening than shortening.
      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).
      Figure thumbnail gr1
      Fig. 1An illustration of 1 Tai Chi movement: the whip.
      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.

      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, ≥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.
      Table 1Subject information
      ControlTai ChiP
      Number1920
      Age (y)63±462±4.79
      Gender (M/F)5/1410/10
      Weight (kg)62±764±9.15
      Height (cm)161±6164±8.19
      Regular exercise (y)1±221±14.00
      Arthritisn33
      Other joint problems43
      Hypertension20
      Heart disease10
      NOTE. Values are mean ± standard deviation or n.
      Abbreviations: M, male; F, female.
      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.

      Data analysis

      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).

      Results

      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).
      Figure thumbnail gr2
      Fig. 2The 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<.05) between the Tai Chi and control groups.
      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.
      Figure thumbnail gr3
      Fig. 3The means and SDs of foot COP displacement in both AP and ML directions, with eyes open (EO) and eyes closed (EC). Abbreviation: disp, displacement. * Significant difference (P<.05) between the Tai Chi and control groups.
      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 StrengthKnee Flexor Strength
      Concentric 60°/sEccentric 60°/sEccentric 120°/sConcentric 60°/sEccentric 60°/sEccentric 120°/s
      AP COP−.12 (.34)−.37 (.00)−.21 (.07)−.13 (.27).01 (.92).06 (.62)
      ML COP−.18 (.138)−.35 (.00)−.28 (.02).21 (.08)−.04 (.77).00 (.98)
      NOTE. Values are correlation coefficient (P value).
      Figure thumbnail gr4
      Fig. 4The scatterplots of isokinetic strength of (A–C) knee extensors versus foot COP displacement in AP direction at various speeds, (D–F) knee extensors versus foot COP in ML direction at various speeds, (G–I) knee flexors versus foot COP displacement in AP direction at various speeds, and (J–L) knee flexors versus foot COP in ML direction at various speeds. The linear regression line is also shown. * Significant correlation (P<.10) between the Tai Chi and control groups.
      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.

      Discussion

      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
      • Jacobson BH
      • Chen HC
      • Cashel C
      • Guerrero L.
      The effect of Tai Chi Chuan training on balance, kinesthetic sense, and strength.
      tested 24 young volunteers before and after a 12-week Tai Chi exercise and found a significant group difference in the isometric strength of the dominant knee extensor and body sway. Lan et al
      • Lan C
      • Lai JS
      • Chen SY
      • Wong MK.
      12-month Tai Chi training in the elderly: its effect on health fitness.
      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
      • Lan C
      • Lai JS
      • Chen SY
      • Wong MK.
      Tai Chi Chuan to improve muscular strength and endurance in elderly individuals: a pilot study.
      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
      • Lan C
      • Lai JS
      • Chen SY
      • Wong MK.
      Tai Chi Chuan to improve muscular strength and endurance in elderly individuals: a pilot study.
      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.
      • Maki BE
      • Holliday PJ
      • Topper AK.
      A prospective study of postural balance and risk of falling in an ambulatory and independent elderly population.
      • Means KM
      • Rodell DE
      • O'Sullivan PS.
      Obstacle course performance and risk of falling in community-dwelling elderly persons.
      • Luukinen H
      • Koski K
      • Laippala P
      • Kivela SL.
      Risk factors for recurrent falls in the elderly in long-term institutional care.
      • Lord SR
      • Clark RD
      • Webster IW.
      Postural stability and associated physiological factors in a population of aged persons.
      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
      • Hortobagyi T
      • Zheng D
      • Weidner M
      • Lambert NJ
      • Westbrook S
      • Houmard JA.
      The influence of aging on muscle strength and muscle fiber characteristics with special reference to eccentric strength.
      but also is closely related to the atrophy of type II muscle fibers
      • Martin JC
      • Farrar RP
      • Wagner BM
      • Spirduso WW.
      Maximal power across the life span.
      and to overall physical performance.
      • Bassey EJ
      • Fiatarone MA
      • O'Neill EF
      • Kelly M
      • Evans WJ
      • Lipsitz LA.
      Leg extensor power and functional performance in very old men and women.
      For example, Tanaka et al
      • Tanaka S
      • Hachisuka K
      • Nara S
      • Ogata H
      • Kobayashi Y
      • Tanaka H.
      Effect of activities of daily living on fiber type atrophy of the vastus lateralis muscle in patients with joint disorders.
      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
      • Bassey EJ
      • Fiatarone MA
      • O'Neill EF
      • Kelly M
      • Evans WJ
      • Lipsitz LA.
      Leg extensor power and functional performance in very old men and women.
      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.
      • Gialloreti LE
      • Marazzi MC.
      Risk for falls in the elderly. [Role of activities of daily living and of the subjective assessment of health status. A case-control study].
      • Stevens JA
      • Powell KE
      • Smith SM
      • Wingo PA
      • Sattin RW.
      Physical activity, functional limitations, and the risk of fall-related fractures in community-dwelling elderly.
      For example, fallers have significantly more disabilities in performing activities of daily living than nonfallers,
      • Chan KM
      • Pang WS
      • Ee CH
      • Ding YY
      • Choo P.
      Epidemiology of falls among the elderly community dwellers in Singapore.
      and persons with limited physical function are more likely to sustain injuries from their falls than those without any functional limitations.
      • Stevens JA
      • Powell KE
      • Smith SM
      • Wingo PA
      • Sattin RW.
      Physical activity, functional limitations, and the risk of fall-related fractures in community-dwelling elderly.
      • Asada T
      • Kariya T
      • Kinoshita T
      • et al.
      Predictors of fall-related injuries among community-dwelling elderly people with dementia.
      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.
      • Jacobson BH
      • Chen HC
      • Cashel C
      • Guerrero L.
      The effect of Tai Chi Chuan training on balance, kinesthetic sense, and strength.
      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.

      Conclusion

      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.

      Acknowledgements

      We thank Zhong Guan Cun Martial Art Association for providing subjects and Juvena Hitt for helping with data processing.

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