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Yoshi Heffernan
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Tripping over obstacles is considered one of the most common causes of falls in older people1,2,3. The older adults were found to increase the foot-clearance4,5,6 and increased lead foot-clearance linearly with increasing obstacle height9, apart from slower crossing speed, and shorter step length and heel-obstacle distance compared with young adults4,5,6,7,8. Increasing foot-clearance to avoid tripping4,9,10 may lead to perturbations to balance and whole-body posture changes, which is, in turn, another major contributing factor to falls in older people1,2,11. Alterations in whole-body posture may include smaller frontal pelvic motion12, greater variabilities in the lower limb inter-joint coordination13, and altered COM-COP control14, leading to a greater risk of imbalance during obstacle-crossing. Once a trip or loss of balance occurs, failure to recover balance would lead to falls. Therefore, understanding the changes in the balance control of older people is essential in developing and evaluating methods for reducing fall risks.
Although many training programmes addressing single factors, such as resistance, endurance, and feedback training, are available to older people to improve their balance capacity and reduce the risk of falling21,22, multi-factor exercises appear to be more effective in improving balance capability23,24. These multi-factor exercises can be structured to emphasize muscular strength/endurance training and other functions such as balance/somatosensory reaction and confidence. Tai-Chi Chuan (TCC), an ancient Chinese martial art, has been shown to improve multiple factors in older people22,25,26,27,28,29,30,31, and is a popular exercise among older people for enhancing their general physical condition. The practice of TCC consists of a series of slow, continuous, gentle motions transitioning from double-limb to single-limb support, thus focusing on dynamic weight shift to a narrower base of support. Participating in long-term TCC exercise can increase muscle strength26,29,30, flexibility31,32, balance22,28,29, sensory organization in postural control27, and reduced COP sway area during challenging tasks such as single-leg stance and tandem stance25. These changes are all critical components for preventing falls in older people33,34. TCC may cause practitioners to modify their gait and movement behaviours, leading them to move more cautiously. These modifications may provide some explanation for the observed reductions in falls35.
Knowledge of the effects of TCC on the COM-COP motions during obstacle-crossing may help better understand the balance control strategy and plan programmes to prevent fall-related injuries in older people. However, previous studies on older people with TCC experience during obstacle-crossing have focused mainly on the gait modifications in terms of temporospatial parameters, ground reaction forces, foot pressure distribution and joint kinematics36,37,38. Older people with TCC experience showed significantly larger forward ground reaction forces to propel the body with greater hip flexion of leading limb36, faster crossing velocity with altered pressure distribution in the trailing foot37, and longer periods of single-limb support38 while crossing obstacles. No study has reported the effects of TCC on foot-clearances and the whole-body balance control in terms of IA and RCIA in older people when crossing obstacles of different heights. To form a complete assessment of the TCC effects on balance control relevant to fall risks, data on the COM-COP motions in terms of IA and RCIA from both TCC and non-TCC practitioners are needed.
The purpose of the current study was to quantify the effects of long-term TCC practice and obstacle height on foot-clearances and the whole-body balance control in terms of IA and RCIA during obstacle-crossing in healthy older people. It was hypothesized that healthy older people with long-term experience in practising TCC would show more conservative COM-COP control with greater toe-obstacle clearance than those who did not practise TCC and that these results were not affected by obstacle heights.
There were no statistical interactions between group and obstacle height factors for any of the tested variables. Therefore, only the main effects are reported here. The TCC group showed significantly greater leading and trailing toe-obstacle clearances but smaller trailing stride lengths when compared to the control group (Table 1). No significant between-group differences were found in crossing speed, toe-obstacle and heel-obstacle distances (Table 1). With increasing obstacle height, the leading toe-obstacle clearances were significantly increased linearly, but the heel-obstacle distances, leading stride lengths and crossing speeds were decreased linearly (Table 1). Since significant obstacle height effects were found on crossing speed, the following results on IA and RCIA are adjusted values for differences in crossing speed.
In the frontal plane, similar patterns in the IA and RCIA curves were found in both Control and the TCC group (Fig. 3). Quantitatively, between-group differences were found mainly in RCIA (Table 3). No significant between-group differences in IA were found. When compared to Control, the TCC group showed significantly smaller RCIA at leading-toe crossing (T2) and peak values during DLS, but significantly greater RCIA at trailing-toe crossing (T5) (Table 3). No significant height effects were found for any IA and RCIA variables in the frontal plane (Table 3).
The current study aimed to determine the effects of long-term TCC practice and obstacle height on foot-clearances and whole-body balance control during obstacle-crossing in older people, in terms of IA and RCIA of the COM relative to COP. The TCC group was found to cross obstacles with significantly greater leading and trailing toe-obstacle clearances and significantly more posterior COM position relative to the COP throughout the crossing cycle, showing specific COM-COP control. The current results suggest that long-term TCC practitioners crossed obstacles with a more conservative COM-COP control strategy for less risk of tripping and better balance control compared to non-practitioners of TCC.
The current study was the first to document the effects of long-term TCC practice on foot-clearances and the whole-body balance control in terms of IA and RCIA in older people when crossing obstacles of different heights during walking. Although previous studies have reported the positive benefits of TCC training, including increased foot-obstacle clearance during obstacle crossing36 and improved standing and walking balance25,33, no studies have provided quantitative descriptions of the dynamic control (COM relative to the COP) during each phase of the crossing cycle to reveal the general control strategy employed by experienced older TCC practitioners, nor has the current literature reported on such COM-COP controls associated with increased leading and trailing toe-obstacle clearances. Nonetheless, the current study was limited to the dynamic balance control when negotiating fixed obstacles in healthy older adults with long-term TCC experience. It is also worth investigating the benefits of TCC in older people when negotiating unexpected obstacles that are commonplace in real-life conditions and are more challenging to cross. Further studies will be needed to determine the efficacy in older adults with compromised neuromusculoskeletal function, such as frailty, neurological diseases, and/or joint degeneration. Future longitudinal studies will be needed for determining direct correlations between muscle strength and the observed COM-COP control during obstacle-crossing in TCC practitioners. Studies on the joint kinematics and kinetics of the locomotor system associated with the observed COM-COP control strategies adopted by the TCC subjects will be needed to reveal the underlying mechanisms.
Long-term TCC practitioners displayed an obstacle-crossing technique with a lesser risk of tripping and better balance control, as indicated respectively by significantly increased toe-obstacle clearances and a more posterior COM position relative to the COP with smaller IA and RCIA during leading-limb crossing, and greater posterior IA and frontal RCIA during trailing-limb crossing compared to non-practitioners of TCC. The observed between-group differences in the COM-COP control appeared to be related to the main features of TCC movements that emphasize maintaining the trunk in the upright position with the COM at a more posterior position when moving forward and keeping the body weight more on the trailing limb during the slow weight-shifting of double-limb support.
C.-C.K., S.-C.C. and, T.-J.H., J.-G.L. and T.-W.L. conceived and designed the experiments. C.-C.K., S.-C.C., T.-Y.C. and T.-J.H. performed the experiments. S.-C.C. and T.-Y.C. analyzed the data. T.-W.L., T.-J.H. and J.-G.L. provided the subjects, materials, and analysis tools. C.-C.K., S.-C.C., T.-J.H. and T.-W.L. wrote the main manuscript text. All authors reviewed the manuscript.
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