Its practice consists of fluid, circular, smooth, and slow movements, with the practitioner in a semi-flexed position of the knees. The practice of this exercise requires precise and slow movements of the joints, the maintenance of postural stability, and balance , but also meditative aspects that are combined with breathing exercises .
There are five main styles of Tai Chi: Yang; Wu; Chen; Hao, and Sun. Each has its different focus with regard to the movements, but all share the same essential principles of the practice .
In investigation studies, the 108-movement Yang-style Tai Chi has been utilized, as well as simpler and shorter forms of Tai Chi of between 8 and 108 movements or other Western variants .
Various published studies have calculated that the expenditure of energy during the practice of Tai Chi is between 4.1 and 6.7. Metabolic equivalents (MET) according to the style of the practice , . Lan and collaborators quantified Cardiac frequency (CF) and Oxygen consumption (VO2) with the practice of Tai Chi in adults; mean CF was 58% of the Cardiac frequency reserve (CFR) and a maximal VO2 55%. Thus, according to these parameters, Tai Chi can be considered an aerobic exercise of moderate intensity (CFR, 40–59%) .
There is recent evidence that supports the therapeutic effects of Tai Chi for musculoskeletal, rheumatological, neurological, and geriatric conditions , . Significant improvements have been reported in balance, muscular strength, cardiorespiratory function, joint and muscle flexibility, pain, and even symptoms of depression and anxiety .
In a study that reported the neuromuscular effects of Tai Chi in elderly subjects with alterations in equilibrium, significant progresses were demonstrated in neuromuscular control of the knee and in the organization of muscular response during walking, as well as significant improvement with respect to functional equilibrium appraised with different scales .
There is evidence that Tai Chi reduced the incidence of falls in older adults. Possible mechanisms include improvement in equilibrium, caregiving, postural control, propioceptive sensitivity, and reduction of the fear of falling , . Subjects trained in Tai Chi additionally present improved vestibular reactions in sensorial tests during postural changes and greater propioceptive accuracy when compared with the controls , .
A systematic review and meta-analysis published in 2011 on the effects of Tai Chi in Osteoarthritis (OA) included six Randomized Clinical Trials (Randomized Clinical Trials) that compared Tai Chi with controls and that concluded that Tai Chi was superior with respect to pain, functionality, and joint rigidity on comparing the former with conventional exercise programs or those carried out at home .
Another systematic review, this one published in 2014, on the effects of Tai Chi on Osteoarthritis of the knee included six good-quality Randomized Clinical Trials. The authors concluded that Tai Chi exhibited effectiveness in diminution of pain and improvement in physical function, although the review emphasized the need for conducting Randomized Clinical Trials with larger sample sizes and longer follow-ups to confirm the efficacy of this intervention .
In a recently published systematic review, the effectiveness was evaluated of physical therapy on improvement of equilibrium and reduction in the risk of falls in patients with Osteoarthritis of the knee. Fifteen Randomized Clinical Trials and 1,482 patients were included in the meta-analysis. The interventions taken into account included Tai Chi, strength training, aerobic exercise, and aquatic exercise. The study concluded that Tai Chi, strength training exercises, and aerobic training improved balance and diminished the risk of falls in older adults with Osteoarthritis of the knee .
In the guidelines from the European League Against Rheumatism (EULAR, 2013) for non-pharmacological management knee and hip Osteoarthritis, it is included the recommendation to practice Tai Chi in patients with Osteoarthritis, based on a systematic review of 10 good quality Randomized Clinical Trials, where evidence was found of the usefulness of Tai Chi in pain reduction, with an effect size of 0.28 up to 1.67 , .
In 2012, the American College of Rheumatology established a conditioned recommendation on the practice of Tai Chi as non-pharmacological treatment of Osteoarthritis of the knee . Another international association, the Osteoarthritis Research Society International, in its guidelines of conservative management of Osteoarthritis, does not establish recommendations with respect to the prescription or use of Tai Chi . Recommendations have not been established for these agencies on the usefulness of Tai Chi for other pathologies.
With regard to Quality of life (QOL), there is a systematic review on the effectiveness of Tai Chi in chronic pathologies that includes 21 Randomized Clinical Trials and 1,200 patients in a meta-analysis showing positive effects on QOL in these types of pathologies, specifically on cardiorespiratory, neurological, and musculoskeletal pathologies .
The properties of water promote active movement, muscle relaxation, provide greater bodily and postural support, and stimulate strengthening by means of resistance of water, permitting the training of motor and sensory skills in subjects in water . This rehabilitation technique is based on the potential benefits of hydrodynamic principles such as buoyancy, resistance, relative density, viscosity, turbulence, and hydrostatic pressure and flow, in addition to providing sensory and physiological effects by means of the water temperature .
Hydrotherapy is a technique that is applied as a complementary treatment in multiple pathologies. Evidence exists on the efficacy of this intervention with respect to pain diminution of pain and musculoskeletal function improvement in affectations such as low back pain, rheumatoid arthritis, Osteoarthritis, and fibromyalgia, and in neurological pathologies such as multiple sclerosis . In a meta-analysis published by Barker and colleagues that included 20 studies on the effectiveness of Hydrotherapy on musculoskeletal conditions, the authors found moderate effects on pain diminution and functionality .
The technique and advantages of Tai Chi can theoretically be complemented with the benefits of Hydrotherapy. This concept has already been explored and although it is presently found in an experimentation phase, various studies have reported the efficacy of water-based postural exercise programs, suggesting positive effects on coordination and equilibrium on combining these . When equilibrium training has been conducted on the floor, an individual’s performance can be affected by lack of confidence, fear of falling, or joint pain. In an aquatic environment, the viscosity inherent in water serves as postural support, promoting confidence and reduction in fear of falling. The water temperature can be beneficial in pathologies that produce chronic pain, muscle contractions, or spasticity .
Novel techniques have been developed that combine the principles of Tai Chi and those of other disciplines similar to Hydrotherapy, or even techniques such as Ai Chi, aquatic exercise, which include Tai Chi, Qui Gong, Shiatsu or Watsu movements within an aquatic and musicotherapeutic environment. These programs have been tested in Randomized Clinical Trials as measures of the outcomes of pain, functionality, and disability, with significantly better results with respect to the group only engaging in breathing and relaxation exercises outside of the water .
Although Ai Chi is not synonymous with underwater Tai Chi, it does include movements of the latter with the potential benefits of Hydrotherapy, specifically in a pool, and variants of the technique have been developed that take advantage of the good aspects of both. Teixeira et al (2011)  conducted a Randomized Clinical Trials on the effects of Ai Chi on equilibrium and fear of falling in older adults. Thirty participants at the daycare center of a home for the fragile elderly were randomly assigned to an Ai Chi class or to a control group where they received customary caregiving. The intervention group received 16 sessions of Ai Chi during 6-week period. The results suggested that an Ai Chi program produces improvement in static and dynamic equilibrium in the elderly in comparison with conventional caregiving. The Ai Chi group maintained the same level of fear of falls, but in the control group, this increased .
In another Randomized Clinical Trials, with the objective to analyze the effects of an Ai Chi program on equilibrium in older adults, 54 subjects aged between 60 and 85 yr were included who presented a high risk of falls. These subjects were distributed into two groups: the experimental group that participated in 12 sessions of Ai Chi, and the control group, the members of which carried out a conventional Hydrotherapy treatment in a pool. Equilibrium, joint mobility and pain were evaluated at the beginning and at the end of the intervention. Significant improvement in equilibrium in both groups was reported but, on performing the comparison between the groups, the experimental group was significantly better with respect to balance .
Calandre et al. (2009)  published a Randomized Clinical Trials in which the authors compared an Ai Chi technique with another involving floor exercises in persons with fibromyalgia; the authors concluded that there were no global differences between both groups, although there was significant improvement in symptoms and sleep quality in members of the Ai Chi group .
In a pilot study published by Pérez-de la Cruz and colleagues (2015) , the authors analyzed the results of an Ai Chi program conducted in the water on QOL, depression, and pain in patients with fibromyalgia. Twenty patients were included, in whom functionality, pain, and QOL were measured; the intervention consisted of 20 sessions of the water-based program. The authors reported significant differences in the outcome variables in favor of the experimental group .
Noh et al. (2008)  reported another Randomized Clinical Trials in which they included 25 patients with chronic cerebrovascular-event sequelae who were assigned to two treatment groups: the experimental group underwent aquatic training, which included a combination of Ai Chi and Halliwick (another aquatic training technique) or a control group, with gymnastic floor exercises. The authors concluded that the aquatic therapy significantly improved equilibrium .
Tai Chi has demonstrated, in multiple studies, its usefulness on musculoskeletal system pathologies, due to its slow movements that involve the four limbs together with the regulation of breathing, postural control, and balance, and Tai Chi is now even recommended in some evidence-based clinical guidelines.
Hydrotherapy has been utilized with proven benefits derived from the mechanical properties of water that combine with the effects of the exercise, improving the muscle-skeletal conditions of the subjects.
Both treatment types involve the physical work of equilibrium, mobility, strength, coordination, and sensory input such as proprioception, with complementary advantages, which theoretically render them as possessing potential synergy in their benefits. In addition to this relaxation and breathing can provide a complete exercise, with low joint impact and strength work at the same time.
Combined Tai Chi and Hydrotherapy have been tested principally in older adults with risk of falls or fragility, and in some neurological or rheumatological pathologies. It is important to know whether Tai Chi practiced in water is useful in other chronic pathologies that cause pain and alterations in walking, such as knee or hip Osteoarthritis. For this, it is necessary to develop good-quality Randomized Clinical Trials in those included in conventional Tai Chi control groups, conventional Hydrotherapy groups, and in groups with both Tai Chi and Hydrotherapy combined, or conventional floor exercises (stretching, strengthening, equilibrium), in order to acquire general evidence that establishes whether there is an advantage on combining both techniques. In addition, it is necessary to carry out Randomized Clinical Trials with adequate samples sizes and greater follow-up to observe potential medium- and long-term benefits and to observe distinct techniques and modalities of Tai Chi as well as of Hydrotherapy.
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