Biomechanical Considerations of Foot-Ground Contact in T'ai Chi Chuan

Abstract

Although numerous studies have linked t'ai chi chuan (TCC) practice with benefits for balance, reduction in the number of falls, and in the fear of falling, most of them did not address the causes of these benefits in depth. Some studies, however, sought to determine the causes from the biomechanical point of view. This article aims to thoroughly describe and critically review recent papers on foot-ground contact in TCC practice, one of the parameters involved in balance biomechanics in TCC performance. No previous review on this subject has been found. Nine electronic databases were searched for publications between 1996 and 2013. Studies were excluded if they were not published in English or were abstracts, posters, or summaries from conferences. From a total of 195 articles identified, 4 randomized controlled trials and 3 non–randomized controlled trials were eligible for the analysis. The number of studies that assessed foot-ground contact in TCC and effects on normal gait, postural control improvement, and fall prevention is still quite small. These studies were based on intervention protocols and used populations that were too heterogeneous to allow reliable comparisons. According to the studies analyzed, TCC practice clearly improved parameters associated with foot-ground contact. Nevertheless, the manner in which these benefits are transferred to daily displacement habits still remains unclear.

Introduction

Among the physical activity and sports programs used to improve balance and reduce falls in the elderly, t'ai chi chuan (TCC) has become a common research topic for many authors at the international level. TCC is a physical activity that originated in China, and different schools or styles have been created over time;, the most important are Chen, Yang, Wu, Hao, and Sun. This activity consists of a choreography performed with slow but continuous, circular and fluid movements, postural alignment, and relaxed body, with trunk rotations around the hips and body weight changes from one leg to the other in different directions.¹

Some studies connect TCC practice with improvements in balance whatever the dimension considered: static,^2
–4 dynamic,^5,6 proprioceptive,^7,8 and vestibular,^9,10 as well as with a reduction in the number of falls^11
–13 and the fear of falling.^14,15 Most of these studies did not address the causes for this effect, implying that it is simply the result of improvements in the proprioceptive, vestibular, and visual systems. Nevertheless, with regard to introducing people to TCC practice, it is important to have practical clues for arranging the exercises and the execution of the TCC form in order to obtain the greatest benefits relative to balance. The biomechanics research field can provide some answers for this purpose.¹⁶

In elderly people, gait is conditioned not just by age-related physiologic changes but also by diseases common in this age group, such as osteoarthritis and Parkinson's disease. In general, among healthy elderly people, gait is characterized by a decrease in speed, decreases in step length, increases in step width, a slowed rate of step, lower heel elevation at the toe-off phase, and foot attitude closer to horizontal at heel strike. Furthermore, as far as the musculoskeletal system is concerned, a shorter range of hip flexion-extension and knee flexion at the oscillating phase and a reduction of ankle plantar flexion at the toe off phase are common.¹⁷

Several key aspects of the human gait process should be considered for analysis of the foot-ground contact phase and the involved action-reaction forces: (1) the three basic phases that make up human gait (heel strike, plantar full contact, and forefoot contact until final toe-off); (2) morphologic aspects related to the different types of foot (e.g., normal, flat, high arched, varus, valgus) and foot size (typically, the foot is supported not by the whole sole but by three areas located, respectively, under the heel and under the heads of the first and fifth metatarsal; these areas are linked to each other by three arches: the medial longitudinal, the lateral longitudinal, and the transverse metatarsal arches¹⁸); (3) the way people step on the ground (normal or with some degree of pronation or supination); and (4) the nature of the contact surface with the ground (individual weight, foot strike strength, and impact duration) because the effect of a force depends not only on its magnitude but also on the time it is acting.^18
–20 The process of information transmission from the sole of the foot to the musculoskeletal system by means of the nervous system, which is necessary for people to move in the most suitable mode, should also be considered.²¹

Footwear is another crucial aspect in foot-sole contact. The activity can be done barefoot, but it can also be done using shoes with orthopedic modifications, which can help to overcome both foot defects and malformations and bad gait habits.²² They can also provide improvements or adjustments to optimize activity performance: for instance, very thin soles aimed at maximizing plantar sensation, soles with damping elements to reduce foot-ground impact, antislip elements to prevent falling, and heel cups to improve foot stability.^23,24

The main goal of this article was to describe and critically review the published papers on foot-ground contact, one of the parameters involved in balance biomechanics, when practicing TCC. No previous review on this subject was identified. Normal gait is a complex process involving cyclic movement kinematics of the feet, feet-ground action-reaction forces, joint forces and torques, and the activation of multiple muscle groups according to different sequences and intensities.

Methods

Search strategy and exclusion process

Nine electronic databases were used: BIOSIS, Cochrane, Ingenio, MEDLINE-PubMed, Pascal, ScienceDirect, Scirus, SportDiscuss, and Science Citation Index. The following were used as key words: “tai chi,” “tai chi chuan,” “t'ai chi,” “taiji,” “tai ji quan,” “biomechanics,” “plantar pressure,” “center of pressure,” “postural control,” and “ground contact.” The selected publications were published between 1996 and 2013. Studies were excluded if they were not published in English or were abstracts, posters, or summaries from conferences. From a total of 195 articles found, 4 randomized controlled trials and 3 non–randomized controlled trials that met the above-mentioned criteria were selected.

Results

Four out of the seven articles found were reports of randomized controlled trials; the other three did not have control groups (Table 1).

Table 1.

Studies on Foot-Ground Contact in T' ai Chi Chuan

Study	Country	Design	TCC type	Participants (n)	Men/women (n/n)	Age (yr)	Duration (wk)	Frequency
Chen et al.³¹	Australia	RCT	Yang 13	20	11/9	72.9±4.4	12	3 times/wk 60 min/s
Gatts and Woollacott²⁹	U.S.	RCT	Yang 20	11	1/10	77.6±7.0	3	5 times/wk 90 min/s
Hass et al.²⁵	U.S.	RCT	Yang 8	14	NA	79.5±6.0	48	2 times/wk 50 min/s
Manor et al.³²	U.S.	NRCT	Yang 24	25	8/17	71.1±12.8	24	3 times/wk 60 min/s
Mao et al.^27–28	China	NRCT	Yang 42	16	8/8	23.0±5.5	NA	NA
Wu and Hitt²⁶	U.S.	NRCT	Yang (NA)	10	5/5	27.0±4.0	2	7 times/wk 15 min/s
Zhang et al.³⁰	China	RCT	(NA)	15	0/15	65.6±4.2	NA	NA

Age expressed as mean±standard deviation.

TCC, t'ai chi chuan; RCT, randomized controlled trial; NA=not available; NRCT, non–randomized controlled trial.

These articles devoted their attention to foot-ground contact in TCC practice, its effects, the way it influences postural control and balance improvement, and its contribution to the reduction in the number of falls. The studies focused on four variables: displacement of the center of pressure (COP), ground reaction forces, plantar pressure, and contact areas. The first study found²⁵ dealt with the influence of TCC training on the COP trajectory during gait initiation in older adults transitioning to frailty (those who had experienced at least one fall in the previous year). The purpose was to determine how TCC practice contributes to a reduction in the number of falls. The control group followed a theoretical educational program. The 48-week intervention consisted of two weekly 50-minute group sessions. Eight of the 24 simplified TCC forms (modern yang-style TCC) were chosen for training. Data were collected by using a Kistler force platform (Kistler Instrument Corp, Amherst, NY).

Research results showed that TCC improved COP displacement in two aspects: (1) The average magnitude (3.1 cm) of COP backward displacement during the first phase was confirmed, and (2) the COP displacement in the anterior-posterior direction increased in the second phase, indicating greater postural control and improved smoothness of center of mass (COM) displacement. No meaningful differences were observed in the medial-lateral direction. The authors concluded that TCC can contribute to fall reduction in frail older adults.

In 2005, the research of Wu and Hitt²⁶ broadened the scope by including ground reaction forces, plantar pressure, and detection of foot-ground contact areas. Ten young participants were tested, and the experimental design did not include a control group. The participants were asked to learn and practice t'ai chi gait (TCG) following a yang-style form 15 minutes daily for 2 weeks. To have some comparison term, the same variables were measured for the same participants in the slow walk. The experimental setup consisted of two biomechanical force plates: OR6-6-1000 (Advanced Mechanical Technology Inc., Watertown, MA) and biomechanical pressure plate F-Mat (Tekscan Inc., Boston, MA).

According to their results, TCG entails (1) a lower-impact force than in slow walk, which was attributed to the slower execution velocity in TCG, and the coordinated activation of the ankle dorsiflexor muscles, hip flexors/knee extensors, and hip abductor; (2) an even body weight distribution between the fore-foot and rear-foot regions, similar to that of slow walk but with a better location of COP (in TCG, it is centered in the midfoot region, which requires a conscious action and precise neuromuscular control; (3) contact areas that are similar in TCG and slow walk but with some differences in peak pressure positions (peak pressure during TCG tends to occur, initially, under the heel, at the first contact moment, under the first and fifth metatarsal heads during the one leg stance, and under the big toe at the toe off phase; in contrast, in slow walk the peak pressure occurred mostly under the third metatarsal head and only during the one leg stance); and (4) a larger COP both in medial-lateral and anterior-posterior displacements than in slow walk.

A year later, Mao et al.^27,28 aimed at quantifying duration and plantar pressure during the one-leg stance in TCG. These authors compared five basic TCG patterns with those of normal gait in order to explain why TCG helps to prevent falls and improve balance in practitioners, considering that the one-leg stance is the phase where stability is most affected and linked to a greater risk of falls involving hip fracture. Sixteen experienced TCC practitioners participated in this study. They were matched for sex, young, and well trained (3–8 years of experience). Participants were asked to perform 5 typical movements of the 42 movement simplified form of yang style TCC for competition. To collect plantar pressure during the one-leg stance phase, the Pedar X insole system (Novel GmbH, Munich, Germany) was used. Loading duration was also determined. Each insole had 99 force sensors.

Results confirmed several findings. First, no significant difference was found in any of the dependent variables between the male and female groups. Second, the total one-leg stance duration (1.95 seconds) was significantly greater than in normal walking (0.40 seconds). Third, the medial-lateral displacement of the COP during the one-leg stance in TCC movements (25.2%) was significantly greater than in normal walking (21.8%). Displacements were wider in forward, backward, and lateral TCC movements than in the slow walk. Fourth, the COP location was significantly more medial and posterior at initial contact, and more medial and anterior at end contact with the ground during TCG than in normal gait. Fifth, the peak pressure and pressure-time integral of the first metatarsal head and the big toe were significantly greater than those of the other regions during the one-leg stance in TCC exercise. On the contrary, the second, third, fourth, and fifth metatarsal heads showed significantly greater values than other plantar regions.

In 2007, Gatts and Woollacott²⁹ analyzed the COP and COM dynamic response to large and fast walking perturbations in balance-impaired seniors. These authors considered that balance recovery during walking involves the ability to make rapid, coordinated, and accurate adjustments of the body to avoid falling. The participants were 22 older adults, including people with arthritis and a history of back, knee, or hip surgery divided into two groups (TCC and control). The TCC group received a 3-week training period in 12 yang style TCC postures. The experimental set up consisted of two force platforms (Institute of Neuroscience Technical Service Group, University of Oregon, Eugene, OR), one of which (on the right side) was triggered, at heel strike, to move 15 cm forward at 40 cm/s, and six cameras (3D Motion Analysis System, version 6.1; PEAK Performance Technologies, Englewood, CO).

Results showed that with just 14 practice sessions, 5 days per week, with each session lasting 1.5 hours, TCC produced significant changes, such as an increase in COP-COM distance during heel-ground contact. The study confirmed a greater tolerance to imbalance caused by slipping; better strategies for balance recovery from a slip (reduced elevation of arms during recovery, more vertical angle of the trunk, and better foot-ground contact to continue gait). TCC allowed a smooth forward progression, improved control of step width, increased separation of sagittal COM-COP both at slip foot heel strike and foot-off phases, and a significant reduction in anterior-posterior COM speed.

Zhang et al.³⁰ measured plantar pressure when stepping over obstacles. The purpose was to determine whether differences in plantar pressure can provide information about postural strategies used to keep the participant's balance and posture. Participants were a sample of 30 healthy older women. Fifteen had years of experience in TCC and made up the TCC group, and the remaining 15 with experience in walking made up the control group. Data were recorded by digital cameras (JVC 9800; JVC Co., Wayne, NJ) placed around a plantar pressure platform RSscan mat (RSscan International, Olen, Belgium). The obstacles were two 2 cm×100 cm (width×length) wood dowels with heights of 15 cm and 23 cm, which easily fall with slight contact.

Results showed that TCC practitioners exhibited a higher plantar pressure in heel medial region when stepping over the obstacle, while, for experienced walkers, this occurred under the forefoot. The authors attributed this difference to greater ankle flexibility in the TCC practitioners, allowing for greater foot dorsiflexion when stepping over the obstacle. They also confirmed that the normalized toe distance was significantly longer for TCC practitioners than for walkers during both obstacle-stepping conditions. TCG's normalized heel distance was higher only when participants stepped over the 23-cm obstacle. These results indicated that TCC practitioners had better postural control than walkers.

Chen et al.³¹ focused their attention on the soleus Hoffman reflex together with mean displacement of COP both in medial-lateral and anterior-posterior directions for assessment of postural control. The target group was a sample of 20 healthy older adults properly trained in the reduced yang style TCC form of 13 movements. The control group consisted of 14 healthy older adults leading normal daily lives. The study used a force plate (type 9287; Kistler, Winterthur, Switzer-land); electrical stimulator (DS7AH; Digitimer, Hertfordshire, United Kingdom); three self-adhesive electrodes: Uni-Patch EP 84177 (Covidien LP, Wabasha, MN 55981), Axelgaardmodel 895220 (Axelgaard Manufacturing Co, Ltd, Fallbrook, CA), and a Delsys electrode (Boston, MA); and Bagnoli-8 electromyograph system (Delsys). Participants attended a 1-hour session of yang style TCC, 3 sessions per week, while the control group members maintained their regular daily activities during the same period.

After 12 weeks of training, no significant improvements in COP displacements were found. This was attributed to the short training period as suggested in other studies. Nevertheless, in the previously mentioned paper, 14 weeks was considered a sufficiently long period to be able to perceive clear differences, which suggests that there must be a suitable selection of measuring instruments so that results can be confirmed.

Finally, Manor et al.³² adopted a completely different approach and tried to analyze the complexity of COP dynamics. A COP metrics derived from complex systems theory might better capture the multicomponent stimulus that TCC has on the postural control system, balance, and the reduction in the number of falls. The target group consisted of 25 older people with peripheral neuropathy. Participants were asked to complete three 1-hour group TCC training sessions per week for 24 weeks in the 24-form yang style TCC.

Measuring instruments included a stationary force platform (AMTI, Watertown, MA) to measure COP dynamics, a 5.07-gauge Semmes-Weinstein monofilament (North Coast Medical Inc., Gilroy, CA) measured the presence or absence of sensation at the heel, midsole, base of the first and fifth metatarsals, and hallux. Increased complexity of standing COP dynamics in TCC practitioners was reported, associated with improved plantar sensation and physical function. Nevertheless, no changes were found in COP area or speed. Significant correlations between increases in COP complexity, and improvements in sole sensation and physical function were reported.

Table 2 summarizes the results discussed here.

Table 2.

Summary of Results of Reviewed Studies on Foot-Ground Contact in T' ai Chi Practice

Study	Reported positive results
Wu and Hitt²⁶	Lesser force impact on the heel at initial contact
Gatts and Woollacott²⁹ Hass et al.²⁵	Greater COP posterior displacement during the first phase of heel-ground initial contact
Mao et al.^27,28 Zhang et al.³⁰	More medial and posterior location of the heel contact area at the heel-ground initial contact
Hass et al.²⁵ Mao et al.^27,28 Wu and Hitt²⁶	Greater anterior-posterior and medial-lateral displacement of COP during the one-leg stance
Mao et al.^27,28 Wu and Hitt²⁶	Greater peak plantar pressure under the big toe and under the first and fifth metatarsal heads during the one-leg stance.
Zhang et al.³⁰	Greater distance of big toe from the heel when stopping over an obstacle

COP, center of pressure.

Discussion

No systematic review on plantar pressure or foot-ground contact during TCC practice has been found. Only one review of TCC biomechanics has been published, by Hong and Li³³ in 2007, with a specific section on plantar pressure. Nevertheless, it only generally describes results, with no in-depth discussion.

Despite the valuable results reported, the included studies lack information on parameters that should have been considered in the intervention protocols. These parameters would improve understanding of the reported results: Wayne and Kaptchuk³⁴ affirm that TCC practice has a teaching context, and a form of execution that should be considered in designing such studies. Jiménez-Martín et al.¹⁶ list different parameters (TCC style and forms, height of adopted postures, execution speed, movement patterns, session content, teacher experience, and methodology) that should be taken into account in interpreting the benefits that TCC provide, as far as body balance is concerned. These aspects could be easily extrapolated to a biomechanical analysis of plantar pressure. Step width should be added as well because it affects the COP medial-lateral displacement and the distribution of foot-ground contact areas.

Whether participants are barefoot or use footwear is important because that variable could affect plantar pressure distribution and gait pattern and their measurements through the influence of reinforcements, damping elements, heel height, shoelaces, and soles of the shoes.^23,24 Only five of the analyzed studies^26

–29,31 provide information on whether the participants wore footwear or were barefoot; only two^27,28 offer information about the footwear used; and only two of the studies present data on the kind of surface on which TCC was practiced and the type of surface covering/mat which they used. Other aspects—such as what each participant's dominant leg was, assessment of the way participants strike the ground (pronator, supinator, or neutral) when walking,^19,20 the features of the participants' feet (such as size)²⁶, foot deformities, or the functional force and muscular activation of the participant's lower limbs—were not assessed.^35,36

As Manor et al.³² pointed out, all these variables provide key information when dealing with studies on sole-ground contact complexity in TCC and its potential translation to body balance improvement. These authors provided researchers in this area with a new analysis method that entails not only an important break with previous studies but even a new starting point, affirming that body balance is a complex system in which many variables and their mutual interactions can have an impact. The most suitable approach would be one of global scope, such as the one implied in the theory of complex systems. The underlying hypothesis, which has been already proposed in other studies,^37
–39 is that complex system evolution has a multicomponent cause involving mutual interactions. Therefore, any intervention method should affect all of them or, at least, as many as possible. Manor et al.³² present a research limitation in that they paid attention to only four variables when analyzing COP dynamics: postural control, foot sole sensation, leg strength, and functional capacity and mobility.

On the other hand, even though all the reviewed studies concern yang style TCC, they exhibit great heterogeneity, making it difficult to establish clear conclusions. Some of them focused just on assessing variables related to foot-ground contact in TCG;^25,26 others, on selected movements of the TCC form they have chosen;^25

–28 and others, finally, on normal gait,²⁹ stepping over obstacles,³⁰ or standing postural control.^31,32 With regard to the data collection protocol, COP displacement assessment has been performed both in static and dynamic positions. Stepping over obstacles has also been addressed in this context;²⁹ most of the studies focused on elder adults, with the exception of Wu and Hitt²⁶ and Mao et al.,^27,28 who centered their attention on young people. Only two papers provide information about measurement correction in terms of the participant's body weight. There is a lack of homogeneity in the strategies used to identify plantar pressure distribution, which renders making suitable comparisons difficult. Wu and Hitt²⁶ performed a global assessment, while Mao et al.^27,28 divided the sole into nine regions.

Finally, some other points should be clarified: (1) the effect of TCC practice benefits for the body balance and movements of older people in their daily lives, taking into account that TCC is a conscious training activity in contrast with the relative unconsciousness of daily life movements; (2) how stable the above-mentioned improvements are over time and the way they depend on experience; (3) the minimum period needed for TCC training to produce benefits; and (4) TCC benefits as compared with those of other sport activities that are more popular among older people.

Furthermore, it would be interesting to have a unifying study to ensure the coherence of the achieved results both in biomechanical studies focused on TCC and balance and displacement features in kinematic and joint movement aspects.^35,36,40,41 Other relevant items should include the muscular dynamics of the lower limbs, the amount and duration of isolated muscle activation, and the simultaneous activation of agonistic and antagonistic pairs of muscles, bearing in mind the isometric and isotonic muscular actions.^42
–44 Finally, other authors have analyzed improvements in vestibular, proprioceptive, and visual systems.^{7

–10,45

–48}

In comparing the studies carried out in TCC on the process of foot-ground contact with other research on this topic, it is clear that further research is needed in areas such as the following:

1. The way greater plantar pressure in TCG, in determined foot areas, affects people with foot pain and how could it be overcome, with adapted insoles, to increase damping in those regions. Mickle et al.⁴⁹ confirmed that the greater the plantar pressure, the greater the foot pain. This factor is associated with an increased risk of falls and balance disruption. Along this same line of thought, it would be interesting to investigate how TCG affects people with plantar fasciitis and Achilles tendonitis.

2. The effect that static postures, movements included in TCC choreography, movement slowness, and a longer oscillation phase duration have on the sole tactile sensors. Meyer et al.⁵⁰ and Wang and Lin⁵¹ confirmed that the loss of sensation in the foot contributes to balance disruption. Later, Zhang and Li⁵² concluded that tactile sensation in the sole plays an important role just at balance recovery during the static phase but not during the gait process. TCC, because of its slow execution mode and long oscillating phase, may work as a hybrid between a static and a dynamic process as far as the supporting leg is concerned. It may help to maintain body balance, thus producing a greater plantar sensation as Manor et al.³² suggest. On the other hand, Maurer et al.⁵³ confirmed that in talking about foot sole sensors, two different types must be distinguished: (a) exteroceptors that provide information about plantar pressure and tactile qualities of the supporting surface and (b) somatic receptors that deal with information relative to COP displacement at the limits of the foot support base. It would be interesting to confirm how TCC movements act on both types of receptors.

3. If foot sole sensation were confirmed as a key factor in TCC, it would be interesting to test the influence of using insoles with stimulating inserts. They could enhance balance even more, and determine whether, as Corbin et al.⁵⁴ and Palluel⁵⁵ affirm, they act positively only in static positions and not in dynamic displacement.

4. The influence of shoe features for TCC practice, in programs aiming at the prevention of falls in older adults. Although traditional t'ai chi shoes are flexible, with thin, flat soles, Menant et al.²⁴ recommend low-heel shoes, thin soles, hard-sole shoes, treated soles, treated beveled heels, slip-resistant soles, and supported heel-collars for fall prevention older adults.

5. The way balance improvements achieved by TCC practice develop into compensation strategies for avoiding falls when slipping in any one of the four directions: forward, backward, left lateral, or right lateral. The only work on TCC that addressed this challenge was the one by Gatts and Woollacott,²⁹ but they focused exclusively on the forward direction and did not consider the response of plantar receptors. Perry et al.⁵⁶ showed that compensation strategies and foot sole–sensitive receptor activation are different when the imbalance happens in another direction and that this information is crucial to understanding the mechanisms for preventing falls, whatever the considered direction. It is important to make this point clear in order to understand the way TCC helps prevent falls in older adult, as has been established in different studies.^11
–13

6. How toe deformations influence plantar pressure distribution and its potential contribution to an increase in the number of falls.⁵⁷ The influence of the foot features—normal, flat, and high arch—would also be crucial.^19,20

As far as this current review is concerned, the main limitation is the small number of available studies, which could have been conditioned by the search criteria and the selection process.

Conclusions

The number of studies performed on the features of foot-ground contact in TCC and the way their results affect normal gait, postural control improvement, and fall prevention is still quite small. These studies are founded on intervention protocols and populations that are too heterogeneous to permit reliable comparisons.

According to the studies reviewed here, TCC practice improves parameters associated with foot-ground contact. Nevertheless, the way these benefits are transferred to daily displacement habits still remains unclear.

Balance analysis as a complex system is still being addressed by means of a reduced set of variables that in most cases are considered in isolation. As a consequence, the global vision needed to understand the process and allow the design of suitable intervention programs is still lacking. To date, no published study has gathered all the achieved results related to the influence of TCC on balance improvement, a reduction in the number of falls, and fear of falling from the biomechanical and physiological points of view.

Footnotes

Author Disclosure Statement

No competing financial relationships exist.

References

Chang

, Nien

, Tsai

, et al. Physical activity and cognition in older adults: the potential of Tai Chi Chuan. J Aging Phys Act, 2010; 18:451–172.

Lelard

, Doutrellot

, David

, et al. Effects of a 12-week Tai Chi Chuan program versus a balance training program on postural control and walking ability in older people. Arch Phys Med Rehabil, 2010; 91:9–14.

, Harmer

, Fitzgerald

, et al. Tai Chi and postural stability in patients with Parkinson's disease. N Engl J Med, 2012; 366:511–519.

Qin

, Choy

, Leung

, et al. Beneficial effects of regular Tai Chi exercise on musculoskeletal system. J Bone Miner Metab, 2005; 23:186–190.

Hackney

, Earhart

. Tai Chi improves balance and mobility in people with Parkinson disease. Gait Posture, 2008; 28:456–460.

Thorton

, Sykes

, Tang

. Health benefits of Tai Chi exercise: improved balance and blood pressure in middle-aged women. Health Promot Intern, 2004; 19:33–38.

Fong

, Ng

. The effects on sensoriomotor performance and balance with Tai Chi training. Arch Phys Med Rehabil, 2006; 87:82–87.

Gyllensten

, Hui-Chan

, Tsang

. Stability limits, single-leg jump, and bodyawareness in older Tai Chi practitioners. Arch Phys Med Rehabil, 2010; 91:215–220.

Tsang

, Wong

, Fu

, et al. Tai Chi improves standing balance control underreduced or conflicting sensory conditions. Arch Phys Med Rehabil, 2004; 85:129–137.

10.

Tsang

, Hui-Chan

. Standing balance after vestibular stimulation in Tai Chipracticing and non-practicing Healthy Older Adults. Arch Phys Med Rehabil, 2006; 87:546–553.

11.

Deschamps

, Onifade

, Decamps

, et al. Health-related quality of life in frail institutionalized elderly: effects of a cognition-action intervention and Tai Chi. J Aging Phys Act, 2009; 17:236–248.

12.

Wolf

, Barnhart

, Kutner

, et al. Reducing frailty and falls in older persons: an investigation of Tai Chi and computerized balance training. J Am Geriatr Soc, 1996; 44:489–497.

13.

, Keyes

, Callas

, et al. Comparison of telecommunication, community, and home-based Tai Chi exercise programs on compliance and effectiveness in elders at risk of falls. Arch Phys Med Rehabil, 2010; 91:849–856.

14.

Taggart

. Effects of Tai Chi exercise on balance, functional mobility, and fear of falling among older women. Appl Nurs Res, 2002; 15:235–242.

15.

Zhang

, Takata

, Yamazaki

, et al. The effects of Tai Chi Chuan on physiological function and fear of falling in the less robust elderly: an intervention study for preventing falls. Arch Gerontol Geriatr, 2006; 42:107–116.

16.

Jiménez-Martín

, Meléndez

, Ulrike

, et al. A review of Tai Chi Chuan and parameters related to balance. Eur J Integr Med, 2013; 5:469–475.

17.

Daley

, Spinks

. Exercise, mobility and aging. Sport Med, 2000; 29:1–12.

18.

Dawe

. Anatomy and biomechanics of the foot and ankle. Orthop Trauma, 2011; 25:279–286.

19.

Buldt

, Murley

, Butterworth

, et al. The relationship between foot posture and lower limb kinematics during walking: a systematic review. Gait Posture, 2013; 38:363–372.

20.

Mootanah

, Song

, Lenhoff

, et al. Foot type biomechanics. Part 2: Are structure and anthropometrics related to function?. Gait Posture, 2013; 37:452–456.

21.

Le Vay

. Human Anatomy and Physiology. New York: McGraw-Hill, 2001.

22.

Kurup

, Clark

CIM

, Dega

. Footwear and orthopaedics. J Foot Ankle Surg, 2012; 18:79–83.

23.

Koepsell

, Wolf

, Buchner

, et al. Footwear style and risk of falls in older adults. J Am Geriatr Soc, 2004; 52:1495–1501.

24.

Menant

, Steele

, Menz

, et al. Optimizing footwear for older people at risk of falls. J Rehabil Res Dev, 2008; 45:1167–1182.

25.

Hass

, Gregor

, Waddell

, et al. The Influence of Tai Chi training on the center of pressure trajectory during gait initiation in older adults. Arch Phys Med Rehabil, 2004; 85:1593–1598.

26.

, Hitt

. Ground contact characteristics of Tai Chi Gait. Gait Posture, 2005; 22:32–39.

27.

Mao

, Li

, Hong

. The duration and plantar pressure distribution during oneleg stance in Tai Chi exercise. Clin Biomech, 2006; 21:640–645.

28.

Mao

, Li

, Hong

. Plantar pressure distribution during Tai Chi exercise. Arch PhysMed Rehabil, 2006; 87:814–820.

29.

Gatts

, Woollacott

. How Tai Chi improves balance: Biomechanics of recovery to a walking slip in impaired seniors. Gait Posture, 2007; 25:205–214.

30.

Zhang

, Mao

, Riskowski

, et al. Strategies of stepping over obstacles: the effects of long-term exercise in older adults. Gait Posture, 2011; 34:191–196.

31.

Chen

, Zhou

, Cartwright

. Effect of 12 weeks of Tai Chi training on soleus Hoffmann reflex and control of static posture in older adults. Arch Phys Med Rehabil, 2011; 92:886–891.

32.

Manor

, Lipsitz

, Wayne

, et al. Complexity-based measures 450 inform tai chi's impact on standing postural control in older adults with peripheral neuropathy. BMC Complement Altern Med, 2013; 13:87.

33.

Hong

, Li

. Biomechanics of Tai Chi: a review. Sport Biomech, 2007; 6:453–464.

34.

Wayne

, Kaptchuk

. Challenges inherent to T'ai Chi research: part II “Defining the intervention and optimal study design.”. J Altern Complement Med, 2008; 14:191–197.

35.

. Age-related differences in Tai Chi gait kinematics and leg muscle electromyography: a pilot study. Arch Phys Med Rehabil, 2008; 89:351–357.

36.

, Millon

. Joint kinetics during Tai Chi gait and normal walking gait in young and elderly Tai Chi Chuan practitioners. Clin Biomech, 2008; 23:787–795.

37.

Costa

, Goldberger

, Peng

. Multiscale entropy analysis of complex physiologic time series. Phys Rev Lett, 2002; 89:1–4.

38.

Litaker

, Tomolo

, Liberatore

, et al. Using complexity theory to build interventions that improve health care delivery in primary care. J Gen Intern Med, 2006; 21:30–34.

39.

Thuraisingham

, Gottwald

. On multiscale entropy analysis for physiological data. Physica A, 2003; 366:323–332.

40.

, Liu

, Hitt

, et al. Spatial, temporal and muscle action patterns of Tai Chi gait. J Electromyogr Kinesiol, 2004; 14:343–354.

41.

, Li

, Hong

. Tai Chi movement and proprioceptive training: a kinematics and EMG analysis. Res Sport Med, 2003; 11:129–143.

42.

Chan

, Luk

, Hong

. Kinematic and electromiographic analysis of the push movement in tai chi. Br J Sports Med, 2003; 37:339–344.

43.

Tseng

, Liu

, Finley

, et al. Muscle activation profiles about the knee during Tai-Chi stepping movement compared to the normal gait step. J Electromyogr Kinesiol, 2007; 17:372–380.

44.

, Ren

. Speed effect of selected Tai Chi Chuan movement on leg muscle activity in young and old practitioners. Clin Biomech, 2009; 24:415–421.

45.

, Xu

, Hong

. Changes in muscle strength, endurance, and reaction of the lower extremities with Tai Chi intervention. J Biomech, 2009; 42:967–971.

46.

McGibbon

, Krebs

, Parker

, et al. Tai Chi and vestibular rehabilitation improve vestibulopathic gait via different neuromuscular mechanisms: preliminary report. BMC Neurol, 2005; 5:1–12.

47.

Wong

, Lin

, Chou

, et al. Coordination exercise and postural stability in elderly people: effect of Tai Chi Chuan. Arch Phys Med Rehabil, 2001; 82:608–612.

48.

, Hong

, Chan

. Effect of tai chi exercise on propioception of ankle and knee joints in old people. Br J Sports Med, 2004; 38:50–54.

49.

Mickle

, Munro

, Lord

, et al. Foot pain, plantar pressure, and falls in older people: a prospective study. J Am Geriatr Soc, 2001; 58:1936–1940.

50.

Meyer

, Oddsson

LIE

, De Luca

. The role of plantar cutaneous sensation in unperturbed stance. Exp Brain Res, 2004; 156:505–512.

51.

Wang

, Lin

. Sensivity of plantar cutaneous sensation and postural stability. Clin Biomech, 2008; 23:493–499.

52.

Zhang

, Li

. The differential effects of foot sole sensory on plantar pressure distribution between balance and gait. Gait Posture, 2013; 37:532–535.

53.

Maurer

, Mergner

, Bolha

, et al. Human balance control during cutaneous stimulation of the plantar soles. Neurosci Lett, 2001; 302:45–48.

54.

Corbin

, Hart

, Palmieri-Smith

, et al. The effect of textural insoles on postural control in double and single limb-stance. J Sport Rehabil, 2007; 16:363–372.

55.

Palluel

, Nougier

, Olivier

. Do spike insoles enhance postural stability and plantar surface cutaneous sensivity in the elderly?. Age (Dordr), 2008; 30:53–61.

56.

Perry

, McIlroy

, Maki

. The role of plantar cutaneous mechanoreceptors in the control of compensatory stepping reactions evoked by unpredictable, multidirectional perturbation. Brain Res, 2000; 877:401–406.

57.

Mickle

, Munro

, Lord

, et al. Gait, balance and plantar pressures in older elderly people with toe deformities. Gait Posture, 2011; 34:347–351.