DISCUSSION
death, MWS, Lower‐extremity
HEART FAILURE, PARKINSON'S DISEASE, SECONDARY
Findings based on the unique hybrid design employed in this study suggest that long‐term, but not short‐term, Tai Chi training is associated with better outcomes in physical function in healthy and already active adults. In cross‐sectional comparisons, for all outcomes except MWS, older adults with long‐term Tai Chi training (i.e., experts) performed better than age‐matched Tai Chi‐naïve adults. Moreover, Tai Chi experts' functional levels were consistently intermediate between Tai Chi‐naïve older and younger adults. For three of the six outcomes (SLST‐EC, Timed Up and Go, and vertical jump), higher levels of function observed in Tai Chi experts compared to Tai Chi‐naïve older adults were statistically significant and, as described in the discussion below, clinically meaningful. By contrast, findings from the randomized trial component of our hybrid design suggest that short‐term practice of Tai Chi (i.e., 6 months of training) did not result in significant improvement in any measures of physical function in our healthy, active population.The effectiveness of Tai Chi for fall prevention in older ambulatory adults and better performance on SLST, FR, and TUG have been associated with reduced fall risk, including in prior cross‐sectional studies evaluating Tai Chi. For SLST‐EC, Tai Chi experts were intermediate to Tai Chi‐naïve younger controls and older adults, supporting the idea that long‐term training may be associated with attenuated age‐related decline in this domain of balance. Prior studies have reported that a reduction of 3–4 s of SLST is associated with 10 years of aging (Jonsson et al., The TUG has been widely used to characterize physical function and mobility. Longer TUG durations are associated with mortality, risk of falls, frailty, and death. As with SLST‐EC, TUG scores in Tai Chi experts were intermediate between Tai Chi‐naïve older and younger adults and statistically better than Tai Chi‐naïve older adults. In healthy older adults aged over 50 years, normative values of mean TUG were reported to be 6–11.8 s (Bohannon, The functional reach test is a proxy of fall risk in older or disabled subjects. Normative functional reach data from community‐dwelling older adults over 60 years old (The routine use of grip strength is recommended for identifying older adults at risk of poor health status. Based on normative data from healthy adults in North America, mean grip strength was 22–55 kg in young adults (20–40 years), with an approximately 15–20 kg reduction in older adults (50–80 years) (Dodds et al., Lower‐extremity strength is correlated with gait speed, gait endurance, and functional balance. In one published study, the mean reported jump height in older adults (age 50–70 years) was 21.4–29.4 cm in females and 29.4–44.3 cm in males (“Vertical Jump Test Normative Data,” Interestingly, although the difference on MWS was not statistically significant between older Tai Chi experts and young healthy controls, the Tai Chi experts tended to walk at a slower speed. Tai Chi is a gentle exercise and consists of slow‐motion workouts with strongly focused attention, which may lead to both highly developed control of body motions and more “mindful walking”. Prior studies have reported slower walking following Tai Chi training, including studies in which balance and fall risk were markedly improved. Data collected in this study and reported elsewhere, add context to these findings (Gow et al., In contrast to the positive impact of long‐term Tai Chi training on physical function, exposure to 6 months of training among the healthy adults recruited for this study had few apparent benefits. This finding was somewhat unexpected, as prior studies of older adults exposed to similar or even shorter periods of training have reported a positive impact on nearly all domains of physical function, including SLST‐EC, FR, TUG, grip strength, lower‐extremity muscle strength, and sometimes MWS. In some studies, such effects were observed even when Tai Chi was compared to an active, alternative exercise control, or standard treatment. One possible explanation for our null finding is the relatively healthy inclusion criteria we employed. When compared to published norms and reference values, our Tai Chi‐naïve participants performed better in several physical function outcomes as mentioned above. This is different from other randomized trials in adults, where specific medical conditions were studied (e.g., high fall risk, Parkinson's disease, and heart failure) and average levels of daily exercise at baseline were more modest. Consequently, it is possible that for this already active population, a more intensive or extended dose of training is needed to impact physical function. Indeed, the exploratory dose–response analyses supported this notion, especially for functional reach and TUG (Figure In our study, we found significant sex differences on some physical function measures (e.g., grip strength, max walk speed, and vertical jump, Figure There are a number of strengths to this study. One strength is our hybrid design and our ability to evaluate long‐ and short‐term training of Tai Chi. Our focus on healthy older adults who were already physically active extends our knowledge beyond Tai Chi's effects on clinical populations. Inclusion of younger healthy controls also allows novel comparisons that inform future hypotheses about Tai Chi's potential to attenuate age‐related functional decline. Second, our hybrid included both longitudinal evaluations of short‐term Tai Chi training in an RCT and cross‐sectional comparisons among Tai Chi experts, Tai Chi naïve older adults, and younger healthy controls. Importantly, our hybrid design utilized the same outcomes assessment protocol in the randomized and observational substudies, eliminating heterogeneity in outcomes, laboratory equipment and testing environment, assessment protocol, and research staff. In addition, our pragmatic RCT enabled us to understand the effects of Tai Chi for older adults learning in real‐world community schools/centers (instead of following a designated research instructor at a makeshift hospital exercise environment), with results generalizable across many styles of Tai Chi. Therefore, the results of our study substantially extend new knowledge to the existing publications.Our study also has important limitations. As an exploratory study of secondary outcomes, sample sizes in both the cross‐sectional and randomized studies were small, and these analyses may have been underpowered to detect differences among the groups. As with any cross‐sectional study, group comparisons may have been confounded by differences between groups other than Tai Chi exposure. Of note, the majority of Tai Chi experts reported cross‐training in other martial arts, thus associations between Tai Chi training and functional abilities should be interpreted cautiously. Future studies might limit inclusion to Tai Chi experts with more narrow training histories. Similarly, Tai Chi‐naïve participants also reported high levels of physical activity (e.g., walking and fitness training) in addition to their 6‐month exposure to Tai Chi. Future, more adequately powered studies should attempt to include data on the types and amounts of extracurricular physical activity as covariates in statistical models. In our cross‐sectional analyses, although we considered multiple potential confounders, observed between‐group differences may have been impacted by additional factors which we did not assess. In our randomized trial, both the short duration of exposure (6 months) and the very high levels of health of participants at baseline may have contributed to the null effect of Tai Chi's short‐term effect on the improvement of physical function, especially given that our older population was already physically active. The short‐term effects of Tai Chi on physical function in active healthy adults will need to be confirmed in larger, adequately powered studies. Finally, as noted above, our pragmatic approach included studying the effects of Tai Chi across many styles of Tai Chi. While this affords generalizability, our study was not designed to systematically assess potential benefits unique to different Tai Chi styles or methods of instruction. Future studies should explore potential differences in effectiveness between styles in well‐designed randomized and cross‐sectional trials.