Bio Mechanics of Tai Chi - A Review

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Biomechanics of Tai Chi: A review Youlian Honga; Jing Xian Lib a Department of Sports Science and Physical Education, The Chinese University of Hong Kong, Hong Kong b School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada

To cite this Article Hong, Youlian and Li, Jing Xian(2007) 'Biomechanics of Tai Chi: A review', Sports Biomechanics, 6: 3,

453 — 464

To link to this Article: DOI: 10.1080/14763140701491674 URL: http://dx.doi.org/10.1080/14763140701491674

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Sports Biomechanics September 2007; 6(3): 453–464

REVIEWS

Biomechanics of Tai Chi: A review

YOULIAN HONG1 & JING XIAN LI2 1 1 0 2 y a M

1

Department of Sports Science and Physical Education, The Chinese University of Hong Kong, Hong Kong, and 2School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada

5 5 1 : 9 1 : t A ] s e t r A e D a l o c s E [ : y B d e d a o l n w o D

Abstract Tai Chi Chuan is a favourite form of exercise throughout the world and has drawn increasing research interest from international scientists. Biomechanical research into Tai Chi has grown substantially and has provided evidence of the beneficial effects of Tai Chi exercise on health, fitness, and prevention of falls. This paper reviews studies that have explored the biomechanical aspects of Tai Chi, such as balance, kinematics, kinetics, strength, and neuromuscular activities.

Keywords: Balance, biomechanics, kinaesthesia, muscle latency, strength, Tai Chi

Introduction

In most of the English literature, Tai Chi Chuan is described as a traditional Chinese form of exercise derived from martial arts folk traditions. The word “Chuan” means “boxing”, but in some of the literature it has been omitted and thus it is often called Tai Chi. Tai Chi is a timehonoured and lifetime exercise form derived from martial arts folk traditions, handed down from generation to generation for more than 1200 years (Li, Hong, and Chan, 2001). The slow, supple movement of Tai Chi is rooted in Taoism (a branch of Chinese philosophy). The central doctrine of Taoism is focused on mind tranquillity, and its goal is to achieve longevity by medication and lifestyle modification (Li et al., 2001). Tai Chi differentiated into many schools in the process of development, such as Chen, Yang, Sun, and Wu. Each school has its own distinctive features, but the basic concepts on the Tai Chi exercise are the same (Koh, 1981). Today, millions of people both in the West and the East practise Tai Chi. Tai Chi has become one of the most popular forms of exercise, especially among the elderly. Exercise has positive effects on the health of older people, but appropriate forms are few, as ageing is usually accompanied by a significant decline in organ function. Joint degeneration, poor eyesight, poor balance, and loss of stamina are universal in the older population. Therefore, the appropriate intensity and forms of exercises should be emphasized. Exercise with low speed, low impact and high interest, which are also most beneficial to retain or regain proper balance, may be most suitable for older persons (Lai, Lan, Wong, and Teng, 1995). Of the variety of exercise forms, Tai Chi seemingly provides all the demands that elderly people require from exercise. Correspondence: Y. Hong, Department of Sports Science and Physical Education, Faculty of Education, The Chinese University of Hong Kong, Hong Kong, China. E-mail: [email protected] ISSN 1476-3141 print/ISSN 1752-6116 online q 2007 Taylor & Francis DOI: 10.1080/14763140701491674

454 Y. Hong & J. X. Li

1 1 0 2 y a M 5 5 1 : 9 1 : t A ] s e t r A e D a l o c s E [ : y B d e d a o l n w o D

The practice of Tai Chi was originally related to concerns about physical fitness and the capacity for self-defence. However, over the centuries it has become more focused on the homoeostasis of the body’s internal environment. The interaction between mind and body has long been seen as more important than the development of any martial arts skills. In Tai Chi, the body is naturally extended and relaxed, the mind is tranquil but alert, and body movements are slow, smooth, and well coordinated as the various forms are undertaken. Its beneficial effects on health, in terms of flexibility, strength, cardiorespiratory function, mental control, and the immune system, have been observed, and the maintenance of balance control and prevention of falls in older people in particular has drawn the attention of scientific researchers (Li et al., 2001). Studies focusing on the effect of Tai Chi exercise on the reduction of falls are limited. A longitudinal study by Wolf and colleagues (1996) examined the occurrence of falls in three groups of older participants: a group that participated in a 15-week Tai Chi programme, a group that underwent a 15-week computerized balance-training programme, and a group that undertook no training (or control). In total, 200 participants (162 women and 38 men), with a mean age of 76.2 years, participated in the study. All participants were independent and ambulatory. The number of falls was recorded in each group over a period of 164–171 days. Fifty-six, 76, and 77 falls were reported in the Tai Chi, balance training, and control groups, respectively. Those who participated in Tai Chi saw a 47.5% reduction in the risk of falls, whereas the others did not experience a significant reduction. It is clear that the positive impacts of Tai Chi exercise on balance, muscle strength, and flexibility could help to reduce the risk falling. The aim of this paper is to review the studies that have investigated the biomechanical aspects of Tai Chi and the effects of Tai Chi exercise on strength and neuromuscular responses in the elderly. Such a review would enhance our understanding of why Tai Chi exercise improves balance and posture control and subsequently reduces falls. Papers published from 1981 to 2005 were searched for in Medline using the keywords “Tai Chi”, “Tai Chi Chuan”, and “Taijiquan”. The results showed that 132 papers were published between 2001 and 2005, which is significantly more than the total number of papers (n 76) published in the 20 years prior to 2001 (1981–2000). Of these papers, only the relevant ones are reviewed and summarized in this paper. ¼

Kinematic aspects of Tai Chi

Performing Tai Chi depends on either double-stance weight-bearing or single-stance weightbearing manoeuvres, which further require the pivoting of the whole body, the twisting of the trunk, and coordination of head and upper and lower extremities. Many studies on the impact of Tai Chi exercise on balance, muscle strength, and flexibility have demonstrated its beneficial effects, which are closely and directly related to its kinematics characteristics. Most research on the movement kinematics of Tai Chi has been based on qualitative observations. Tse and Bailey (1992) investigated the potential value of Tai Chi in promoting postural control of nine elderly who were well. They described Tai Chi as a series of individual dance-like movements linked together in a continuous, smooth-flowing sequence. The even, slow tempo facilitates a sensory awareness of the speed, force, trajectory, and execution of movement throughout the exercise. Lai et al. (1995) studied the effects of two years’ Tai Chi training on the maintenance of cardiorespiratory function in older individuals. Forty-five individuals formed the Tai Chi group while another 39 people served as control. The authors described Tai Chi movements as being performed in a semi-squatting posture,



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with various amounts of concentric and eccentric contractions that demand moderate work of the musculature of the lower extremities. Quantitative studies of Tai Chi are limited. Chan and colleagues (Chan, Luk, and Hong, 2003) studied the kinematics of a typical Tai Chi movement, “ward off, roll back, press, and push”, performed by an experienced Tai Chi master. The movements were videotaped and digitized. Results showed that the forward and backward shifts in the centre of mass were mainly accomplished by increasing and decreasing, respectively, the joint angles of the bilateral lower limbs, rather than by adopting a forward or backward postural lean. The path of the centre of mass in the antero-posterior and mediolateral component was unique, and the sway or deviation from the path was small. The master maintained an upright posture and maintained a low centre of mass (hips, knees, and ankles bent) while travelling slowly and steadily from one position to another. The pattern of lower limbs movement may help strengthen the muscles. The kinematics of another typical Tai Chi movement, “brush knee and twist steps”, was reported by Xu and colleagues (Xu, Li, and Hong, 2003). Six Tai Chi masters participated in the study. Three movements of each master were video-recorded and analysed. Throughout the whole movement, there was continuous shifting of the centre of mass and a wide range of motionof thejoints. Themovementwas slow andsmooth. Therelatively frequent shifting of the centre of mass in three dimensions may contribute to improving one’s ability to maintain balance. A wide range of motion of the joints may activate more proprioceptors, which are helpful in developing muscle strength and endurance. The slow and smooth action may also require well-controlled muscle coordination. Wu and colleagues (Wu, Liu, Hitt, and Millon, 2004) compared the “Tai Chi Gait”, the stepping-forward gait used in the typical Tai Chi movement “brush knee and twist steps”, with normal walking of 10 healthy young participants. They selected walking as a control because it is the basic form of ambulation in daily activities. Several studies have demonstrated the positive effects of walking training programmes on postural stability (Melzer, Benjuya, and Kaplanski, 2003) and muscle strength (Carmeli, Kessel, Coleman, and Ayalon, 2002) in elderly people. Ten healthy young participants were tested. Compared with normal gait, Tai Chi Gait had longer cycle duration (11.9 ^ 2.4 vs. 1.3 ^ 0.2 s) and longer duration of single-leg stance time (1.8 ^ 0.6 vs. 0.4 ^ 0.05 s). Both joint motion in ankle dorsi-plantar flexion, knee flexion, hip flexion, and hip abduction and the lateral body shift were larger. It was suggested that Tai Chi may help improve joint range of motion in the lower extremity and people who practise Tai Chi regularly may be able to stand on one leg longer. A set of Tai Chi consists of several individual Tai Chi movements and is performed as a specific form of locomotion. In performing a set of Tai Chi, the foot provides seven support patterns (full double-limb support, single-limb left support, single-limb right support, left support with right toe touch, left support with right heel touch, right support with left toe touch, and right support with left heel touch) and six step directions (step forward, step backward, step sideways, up and down step, step turning, and step fixing or fixed step). Thus, analysing foot movement in a set of Tai Chi might provide unique information as why Tai Chi can improve muscle strength and reduce the risk of falls. Mao and colleagues (Chau and Mao, 2006; Mao, Hong, and Li 2006) compared the kinematics of foot movement of the whole set of 42-form Tai Chi and the normal walking performed by 16 experienced Tai Chi practitioners using video analysis. The results showed that compared with normal walking, Tai Chi had a higher percentage of double support and a lower percentage of single support duration; the absolute duration of each support pattern was longer. In Tai Chi, the transition of movement from one pattern to another was slow, the duration of

456 Y. Hong & J. X. Li each step direction was short, and changes of direction were frequent. The challenges of maintaining balance during daily life are often likely to demand changes in the base of support to enhance stability. Successful balance recovery by means of stepping requires accurate control of the foot movement as well as controlling the motion of the centre of mass to arrest it within the boundaries of the new base of support established by the step. Compensatory stepping in all directions is an important strategy for preserving stability (Maki and McIlroy, 1999). It was suggested that Tai Chi may be better than walking at simulating the gait challenges that may be encountered during daily activities. Kinetic aspects of Tai Chi 1 1 0 2 y a M 5 5 1 : 9 1 : t A ] s e t r A e D a l o c s E [ : y B d e d a o l n w o D

The foot is the direct contact between the body and the external environment. The central nervous system relies on sensory input from the muscles and cutaneous receptors in the lower extremities to generate effective motor patterns for human posture and locomotion. Feedback that originated from these receptors provide a constant source of information on loading, joint kinematics, and plantar pressure distribution. Therefore, the plantar pressure distribution during Tai Chi exercise would reflect the function of Tai Chi on neuromuscular control. Four papers have been published on plantar pressure in Tai Chi movements. Lin and colleagues (Lin, Lee, Tang, Huang, and Chen, 1999) studied the vertical reaction force and the pressure profile of the standing foot in the right and left kicking movement of Tai Chi performed by a Tai Chi athlete of national standard using a mat pressure measurement system. The pressure distribution patterns of the stable kick and the unstable kick were compared. The pressure–time diagram of the phalanges, metatarsals, and tarsals showed that the pressure profiles or sequences tended to be stable in the left kick, and the phalanges produced “fragmented” and large pressure points acting on the ground. These observations supported a key point of practising Tai Chi: “five toes grasping the ground to make us stable as a mountain”. Wu and Hitt (2005) studied the ground reaction force profiles, centre of pressure and plantar pressure patterns in the “Tai Chi Gait”, and slow walking performed by ten healthy young participants using a force plate and a pressure plate. The vertical force reached and maintained a peak value of 109 ^ 2% body weight during single stance, and shifted within a range of 10–70% body weight during double-stance phases. There was a uniformly small rate of loading in all three directions throughout stance. The peak plantar pressure was fairly constant throughout stance in the rear-foot region (maximum value of 0.27 ^ 0.07 kPa/kg), but changed from 0 to 0.16 ^ 0.04 kPa/kg in the fore-foot region. The peak pressure difference between the fore-foot and rear-foot regions was less than 0.06 ^ 0.01 kPa/kg during single stance and the second double stance. The maximum plantar contact area during Tai Chi Gait was 60 ^ 9% of the foot area. The foot centre of pressure was displaced largely during the early and late part of the stance and maintained fairly stationary during single stance. The maximum centre of pressure displacement in the medio-lateral direction was 64 ^ 8% of foot width. Together these data indicated that Tai Chi Gait had a low-impact force, a fairly evenly distributed body weight between the fore-foot and rear-foot regions, and a large medio-lateral displacement of the foot centre of pressure. Mao et al. (2006a) studied the plantar pressure in five typical Tai Chi movements, each representing the step forward, backward, sideways, up–down, and fixing, respectively, and normal walking using pressure insoles. Sixteen experienced Tai Chi practitioners were tested. The results showed that during Tai Chi movements the peak pressure and pressure– time integral of the first metatarsal head and the great toe were significantly greater than in other regions, which presents a strong challenge to the exertion of the great toe, and subsequently has a training



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effect on the muscles controlling the great toe. The ground reaction forces of Tai Chi movements differed from each other and were different from normal walking. The locations of the centre of pressure in the Tai Chi movements were more medial and posterior at initial contact, and were more medial and anterior at the end of contact with the ground than in normal walking, which would induce significant changes in torque in the ankle, knee, and hip joints of the leg (McCaw and DeVita, 1995). Compared with normal walking, the forward, backward, and sideways Tai Chi movements had significantly wider centre of pressure displacements in the medio-lateral direction, and the forward Tai Chi movement had significantly larger centre of pressure displacement in the antero-posterior direction, which would increase not only the intensity of electromyography (Szturm and Fallang, 1998) but also the number of muscles that participated in the movement of the lower extremities when disturbances occur (Nakamura, Tsuchida, and Mano, 2001). It was suggested that regular Tai Chi practice may not only enhance lower extremity muscle strength, but also improve the precise control of activation of these muscles. Stance on a single leg is more critical than stance on two legs. Mao et al. (2006b) studied the spatial and plantar pressure characteristics of one-leg stance between the 42-form Tai Chi and the normal walking performed by 16 experienced Tai Chi practitioners. In Tai Chi, the duration of each one-leg stance was longer and the medio-lateral displacement of the centre of pressure was greater compared with normal walking. The peak pressure and pressure–time integral of the second and third metatarsal heads and the fourth and fifth metatarsal heads were significantly greater than those of other plantar regions during one-leg stance than in normal walking, whereas the peak pressure and pressure–time integral of the first metatarsal head and the great toe were significantly greater than those of other plantar regions during one-leg stance in Tai Chi exercise. Because the one-leg stance is a particularly challenging part of human locomotion and the ability to maintain a one-leg stance is an important predictor of injurious falls for elderly people (Richardson, Ashton-Miller, Lee, and Jacobson et al., 1996), and the great toe plays an important role in both cutaneous feedback and muscle activity to correct the postural disturbances (Meyer, Oddsson, and De Luca, 2004), it is possible that long-term Tai Chi exercise will: (1) not only enhance muscle strength, but also improve the somatosensory input in the great toe region to assist in balance control to a greater extent than would be achieved with normal walking; and (2) improve the ability to balance on one leg, which would lead to a reduced risk of falls and fall-related injuries. Muscle strength and Tai Chi

During Tai Chi exercise, the roles of the muscles continually change between those of stabilizers and movers, weight-bearers and non-weight-bearers, and between contraction and relaxation. Bellew (2002) found that traditional strength training did not improve force control of the lower extremity. However, using an isokinetic dynamometer test, Christou and colleagues (Christou, Yang, and Rosengren, 2003) found that force control was improved following Tai Chi exercise in 16 elderly participants compared with 10 elderly participants in a control group. They suggested that motor learning and muscle coordination may be responsible for the observed decreases in force fluctuations after Tai Chi exercise. Rutherford and Jones (1986) suggested that the performance of Tai Chi is minimally constrained and involves the coordination of various muscle contractions at various intensities. The individuals following Tai Chi may have learned to improve co-activation of the agonist and antagonist muscles and thus improve both strength and force control (Christou et al., 2003). Lai et al. (1995) suggested that Tai Chi is performed in a semi-squatting posture that can

458 Y. Hong & J. X. Li place a large load on the muscles of the lower extremities, and various amounts of concentric, eccentric, and isometric contractions are needed in this unique posture. In a cross-sectional study, Tsang and Hui-Chan (2005) examined the concentric and eccentric isokinetic strength of the dominant knee extensors and flexors at an angular velocity of 30 /s using an isokinetic dynamometer. Twenty-four elderly Tai Chi practitioners and 24 elderly sedentary individuals participated in this study. The results showed that Tai Chi practitioners had higher peak torque-to-body weight ratios in concentric and eccentric isokinetic contractions of their knee extensors and flexors. The authors concluded that long-term Tai Chi practitioners had better knee-muscle strength. More recently, Xu and colleagues (Xu, Li, and Hong, 2006) compared the muscle strength and endurance of the lower extremities of 21 long-term older Tai Chi practitioners with those of 18 older regular joggers and 22 older sedentary individuals using isokinetic measurement. Maximum concentric strength of the knee flexors and extensors was tested at angular velocities of 30 and 120 /s. The ankle dorsiflexors and plantar flexors were tested at 30 /s. Moreover, the dynamic endurance of the knee flexors and extensors was assessed at a velocity of 180 /s. The differences in muscle strength of the knee joint among the three groups were significant at the higher speed. The strengths of the knee extensors and flexors in the control group were significantly lower than those in the jogging group and marginally lower than those in the Tai Chi group. For the ankle joint, the participants in both the Tai Chi and the jogging groups generated more torque in their ankle dorsiflexors than their sedentary counterparts. In addition, muscle endurance of the knee extensors was more pronounced in Tai Chi practitioners than the controls. The authors suggested that older regular Tai Chi practitioners and joggers showed better scores than the sedentary controls on most of the muscle strength and endurance measures. However, the magnitude of the effects of exercise on muscles could depend on the characteristics of different types of exercise. It is believed that the increase in lower extremity muscle strength is one of the factors that enhance the balance and posture control of regular Tai Chi practitioners. 8


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Electromyography and neuromuscular activity in Tai Chi

To date, few studies have focused on the characteristics of electromyography and neuromuscular activity in Tai Chi. Zhang and colleagues (Zhang, Huang, Hu, and Gao, 1989) undertook an electromyography (EMG) study of experienced Tai Chi practitioners aged 55 years. Using surface electrodes, the EMG activities of deltoideus, sacrospinalis (erector spinae), rectus femoris, and gastrocnemius on both sides of the body were recorded while performing the whole set of Tai Chi. The integrative EMG pattern showed an alternation of higher and lower amplitude. The higher amplitude was accompanied by higher frequency while the lower amplitude was accompanied by lower frequency. Analysis of the movement “Yan shou hong chui” (one basic form in Chen Tai Chi) showed that there was a time difference of 0.014 s in muscle activity between the upper and lower body extremities. Also, the muscles of the upper extremity on the left side were activated first, followed by those of the lower extremity on that side; subsequently, the muscle on the right side contracted in the sequence from upper to lower extremities. The electromyograms during Tai Chi provided preliminary experimental evidence that Tai Chi exercise demands the involvement and coordination of upper and lower extremities and unilateral and bilateral support. In addition, the power frequency did not show any evidence of muscle fatigue during Tai Chi practice. There are three recent EMG studies in Tai Chi, each focusing on a different Tai Chi movement. Chan et al. (2003) reported the EMG activities of the lumbar erector spinae,



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rectus femoris, medial hamstrings, and medial head of the gastrocnemius when performing the basic Tai Chi movement “ward off, roll back, press, and push”. Results showed that the medial hamstrings and medial head of the gastrocnemius maintained low activity, with higher electromyographic values in the lumbar erector spinae and substantially higher ones in the rectus femoris during the push movement. Both concentric and eccentric contractions occurred in muscles of the lower limbs, with eccentric contraction occurring mainly in the anti-gravity muscles, such as the rectus femoris and the medial head of the gastrocnemius. It was suggested that the eccentric muscle contraction of the lower limbs in the push movement of Tai Chi may help to strengthen the muscles. Xu et al. (2003) used electromyography to study the Tai Chi movement “brush knee and twist steps”. The EMG activities of the rectus femoris, semitendinosus, gastrocnemius, and anterior tibialis were synchronously recorded and analysed. They suggested that continuous alteration of muscle loading and types of contraction of the flexor and extensor of each joint produced different intensities of muscular activity, which was helpful in developing muscle strength and endurance. The slow and smooth action of the Tai Chi movement also required well-controlled muscle coordination. The vigorous contraction, moderate tension, and relaxation may be helpful in developing muscle strength and endurance. Wu et al. (2004) compared the contractions of the muscles of the lower extremities between the“Tai ChiGait” andnormalgait using EMG. Theauthors found that ankle dorsiflexion, knee extension, and hip abduction are activated more and over a longer duration during Tai Chi Gait than during normal gait. Tai Chi Gait involves a long duration of co-activation of the anterior muscles (e.g. tibialis anterior and rectus femoris) and lateral muscles (e.g. peroneus longus and tensor fasciae latae). These muscles are important for preventing backward falls andlateral falls. Tai Chi Gait requires muscles to change their functions between being movers (concentric contraction) and stabilizers (isometric or eccentric contraction). The findings suggested thatTai Chi may be an effective strengthening and endurance exercise for lower extremity muscles. Long-term Tai Chi practice may enforce motor control strategies that are important for postural control, which may help reduce the risk of falls. Research has shown that postural control stability is significantly affected by a decline in proprioception in the lower limb (Lord, Clark, and Webster, 1991). Clinical research has demonstrated that individuals with proprioception and neuromuscular reaction deficits as a result of injury, lesions, and joint degeneration are less capable of maintaining postural stability and equilibrium (Pintsaar, Brynhildsen, and Tropp, 1996). The classic methods of testing proprioception involve the determination of the lowest threshold for the detection of joint rotation, or kinaesthesia, and the determination of joint position sense from the accuracy with which contralateral joint angles can be matched or a limb segment repositioned in three-dimensional space without the aid of vision. Measurements of muscle latency in the ankle (the time between the beginning of an ankle perturbation stimulus and the onset of EMG activity) are used to study neuromuscular reaction in both basic research and clinical studies (Eils and Rosenbaum, 2001; Konradsen and Ravn, 1990) and have shown good repeatability (Lynch, Eklund, Gottlieb, Renstrom, and Beynnon, 1996). To the best of my knowledge, four studies have been published on proprioception and neuromuscular reaction during Tai Chi. Tsang and Hui-Chan (2003) measured the passive knee joint repositioning of 21 elderly Tai Chi and control participants using an isokinetic test to evaluate joint proprioceptive acuity. The results showed that Tai Chi practitioners had better knee-joint proprioceptive acuity, in that they made fewer absolute-angle errors than controls in passive kneejoint repositioning. These results suggest that long-term Tai Chi practitioners had improved knee-joint proprioception.



A study on the validation of techniques that are used to measure knee proprioception found that joint kinaesthesia is more repeatable and precise than the joint position sense techniques (Beynnon et al., 2000). Based on this finding, Xu and colleagues (Xu, Li, Hong, and Chan, 2004) examined the kinaesthesia of the knee and ankle joints and muscle latency of the ankle joints of 21 elderly regular Tai Chi practitioners, 20 elderly long-term swimming– running exercisers, and 27 elderly sedentary controls (control group). The results showed that ankle joint kinaesthesia differed significantly among the three groups. The Tai Chi practitioners could detect a significantly smaller amount of motion than the swimming– running exercisers and sedentary individuals. No significant difference was found between the swimming– running group and the sedentary controls. For the knee joint, the threshold for detection of passive motion was significantly different in knee extension and flexion. For knee flexion, the Tai Chi group showed a significantly smaller mean threshold for detection of passive motion than did the participants in the control group, while there were no significant differences between the swimming–running group and control group and between the Tai Chi group and swimming– running group. For knee extension, no significant difference was noted among the three groups. It was suggested that the elderly people who regularly practised Tai Chi not only showed better proprioception in the ankle and knee joints than sedentary controls, but they also had better ankle kinaesthesia than the elderly regular swimming–running group. The prominent benefits of Tai Chi exercise on proprioception may be one of the most important factors in that Tai Chi practices maintain balance control in older people. Xu and colleagues (Xu, li, and Hong, 2005) measured the latency of the muscles, defined as the time from the moment that unexpected ankle inversion perturbation began to the moment of onset of the EMG response, of 21 long-term elderly Tai Chi practitioners, 18 regular elderly joggers, and 22 sedentary individuals. The results showed that there were significant differences in the latency of the rectus femoris and tibialis anterior muscles among the three groups, but that there were no differences in the latency of the semitendinosus and the gastrocnemius muscles. Further tests indicated that the rectus femoris and tibialis anterior in the Tai Chi and jogging groups were activated faster than those in the control group. No significant difference was found for the muscle onset latencies between the Tai Chi and jogging groups. Maintaining information-processing speed during ageing is important because of the role that it plays in many everyday events. The rectus femoris and tibialis anterior in the regular Tai Chi and jogging groups showed faster responses to unexpected ankle inversion perturbations, which is helpful for the timely correction of postural disturbances, than those in the sedentary control group. Studies have found that proprioception can be improved though exercise, especially proprioceptive exercise that requires three actions: the proprioception of the joints, balance capacity, and neuromuscular control (Eils and Rosenbaum, 2001; Irrgang and Neri, 2000). Tai Chi seemingly provides ideal proprioceptive exercise to the older population. Balance control and Tai Chi

Several cross-sectional and longitudinal studies have provided evidence of the benefits of Tai Chi for balance. Tse and Bailey (1992) reported that Tai Chi practitioners performed significantly better on right and left single-leg stances with eyes open and heel-to-toe walking than non-practitioners. But this was not the case with single-leg stances with eyes closed. Lan and colleagues (Lan, Lai, Wong, and Yu, 1996) found that long-term Tai Chi practitioners recorded better scores in a stand-and-reach test. Hong and colleagues (Hong, Li, and Robinson, 2000) reported that long-term (13.2 years) Tai Chi practitioners



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performed better in tests of single-leg stance with eyes closed (right and left) and in tests of sit-and-reach than sedentary individuals. Tsang et al. (2003) examined whether elderly Tai Chi practitioners have developed better standing-balance control than elderly healthy controls. Body sway was measured using a force plate. Tai Chi practitioners initiated voluntary weight shifting in the limits of the stability test more quickly than controls. Moreover, they could lean further without losing stability and showed better control of their leaning trajectory. It was suggested that long-term Tai Chi practitioners had expanded their limits of stability during weight shifting in a stance. Tsang and colleagues (Tsang, Wong, Fu,and Hui-Chan,2004)investigatedthe effect of long-term Tai Chi practice on balance control among 20 healthy elderly Tai Chi practitioners, 20 healthy elderly non-Tai Chi practitioners, and 20 young participants when standing under reduced or conflicting somatosensory, visual, and vestibular conditions. Computerized dynamic posturography was used. It was shown that the Tai Chi practitioners had significantly better balance control than the non-Tai Chi participants based on the visual and vestibular ratios, but not the somatosensory ratio. Furthermore, there were no significant differences in any of these three sensory ratiosbetween theTai Chipractitioners andthe young, healthy participants.It was suggested that long-term Tai Chi practice improves balance control in the elderly population when there is an increased reliance on the visual and vestibular systems during stance. Of particular interest is that the elderly Tai Chi practitioners attained the same balance-control performance as the young, healthy participants when standing under reduced or conflicting somatosensory, visual, and vestibular conditions. Tsang and Hui-Chan (2005) examined whether older Tai Chipractitioners hadless body sway in perturbed single-leg stanceand greater balance confidence than healthy older adults. Control of body sway was assessed in static double-leg stance and in single-leg stance perturbed by forward or backward platform perturbations. The results showed thatTai Chi practitioners hadless antero-posteriorbodysway angles in perturbed single-leg but not static double-leg stance than did controls. The authors concludedthatlong-term Tai Chi practitioners hadless body sway in perturbed single-leg stance and greater balance confidence. Besides the cross-sectional studies, a number of Tai Chi intervention studies on balance capacity in elderly people have been conducted. Three main findings have been reported. First, Tai Chi improves movement capacity, including balance, strength, and flexibility in healthy older people. In a study of 24 Tai Chi practitioners and 22 sedentary participants, Schaller (1996) found that 10 weeks of an easy-to-learn westernized form of Tai Chi resulted in a significant improvement in the scores for single-leg stance with eyes open, but not in the single-leg stance with eyes closed. Jacobson and colleagues (Jacobson, Chen, Cashel, and Guerrero, 1997) reported significantly better balance control (tilting board test), strength of knee extension (maximal voluntary extension test), and kinaesthetic sense (glenohumeral media rotation at 60 ) in 12 adult Tai Chi participants than in 12 sedentary participants after a 12-week Tai Chi intervention. Shih (1997) reported a 16-week Tai Chi intervention on the average speed of sway using a force plate in 11 elderly participants. Tai Chi exercise was associated with substantial changes in the sway velocities in the anterior and posterior directions between the pre- and post-tests. Second, Tai Chi contributes to the maintenance effects on gains in balance and strength in healthy older adults. Low-intensity Tai Chi has resulted in maintenance of gains in balance (single stance test) and strength (isokinetic torque test) (Wolfson et al., 1996). Third, Tai Chi exercise has the therapeutic effects of improving the upper extremity range of motion, preventing further deterioration, and helping alleviate joint pain and increase strength, flexibility, and balance in older patients with osteoarthritis (Lumsden, Baccala, and Martire, 1998) and rheumatoid arthritis (Kirsteins, Dietz, and Hwang, 1991). 8



The single-leg standing test has been used to test the balance of the elderly in various studies (Hong and Robinson, 2000; Tse and Bailey, 1992). The time dependency of effects was found. In the study by Tse and Bailey (1992), a group of Tai Chi practitionerswith a practice history of 1– 20 years anda group of age-matched non-practitioners were compared. All participants were aged 65 or older. Single-leg stance time with eyes open and closed was measured. Tai Chi practitioners were able to stand significantly longer only with eyes open. However, this finding is challenged by the study by Hong and Robinson (2000), who found a significant difference between Tai Chi practitioners with a minimum of 10 years’ experience and non-practitioners in single-leg stance time with eyes closed. The discrepancy in the results of these two studies could be due to the difference in the practice history of the Tai Chi practitioners. It was reasoned that maintenance of balance with vision excluded is not a usual life experience. This time dependency was also seen in the longitudinal studies. Schaller (1996) also measured single-leg stance time with eyes open and closed, and found that Tai Chi participants (10 weeks Tai Chi practice, once a week in class and three times a week of selfpractice) improved more than 50%, whereas an age-matched control group deteriorated by 2%. This significant change was only with eyes open. Ross and colleagues (Ross, Bohannon, Davis, and Gurchiek, 1999) conducted a similar study in 11 elderly females with only 8 weeks of training (three times a week). They found no significant improvement, although there was a positive trend, in single-leg stance time. Similarly, Hartman et al. (2000) found no significant changes after 12 weeks of training (two times a week) in 18 Tai Chi participants and 15 controls. The total number of practice sessions in the latter two studies is much smaller than that in Schaller’s study (24 vs. 40). It should be noted that Hartman et al. (2000) studied older people with osteoarthritis, whereas Schaller (1996) and Ross et al. (1999) examined healthy individuals. The physical condition and pain in the osteoarthritis population might have prevented them from performing better in the physical function tasks. In summary, single-leg stance time showed improvement with Tai Chi practice only in some studies. The improvement is most evident with eyes open and after sufficient practice time. Single-leg stance time with eyes closed has shown significant improvement only with very long-term practice. Conclusion

Although the studies reported here provide useful scientific data about the biomechanical aspects of Tai Chi, most of the data were based on single movements. Moreover, information about movement coordination in Tai Chi exercise is still lacking. The quantitative basis for determining the adequacy of Tai Chi for individuals is limited. Differences in Tai Chi performance between beginners and masters, the young and old, the healthy and physically impaired require examination. It needs to be asked whether Tai Chi really is a safe exercise for patients with joint problems in the lower extremities. More importantly, there is no comprehensive and quantitative basis for understanding the mechanisms of Tai Chi. To address why Tai Chi is a better exercise for improving balance and posture control than other traditional balance exercises, and what makes it uniquely effective, further investigation is required.

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