As a global public health problem , substance use disorder (SUD) has elicited increasing concern. Complex reasons may explain why individuals with SUD are difficult to cure. Numerous studies have demonstrated that SUD or drug addiction is a chronic and relapsing brain disease in which addicted patients continually exhibit drug-seeking behavior despite its adverse consequences for their psychology and physiology [2–4]. Neuropsychological studies have verified that the cognitive function of individuals exposed to certain substances (e.g., drugs, alcohol, cigarettes) for extended periods of time show significant defects that can be associated with abnormal brain function and metabolism caused by substance addiction [5, 6]. The cognitive impairment of drug addicts is mainly manifested by their higher level of impulsive behavior and decision-making compared with the normal population [7, 8]. Compared with assessments obtained after recent methamphetamine (Meth) use, the impulsivity self-reported by Meth-dependent subjects was significantly higher following approximately 1 week of abstinence from drugs . Elevated impulsivity and impaired decision-making cognition have been reported in heavy drug users; both conditions are strongly state-dependent in a SUD population and may be suitable for monitoring treatment success [9, 10]. A number of studies have confirmed the presence of learning and memory disorders in drug addicts, and these disorders have been proposed to increase the vulnerability of the latter to drug addiction [11, 12]. That is to say, learning and memory defects render individuals likely to become addicted to drugs and increase the difficulty of abstinence. Therefore, improving the cognitive function of drug addicts can be assumed to play a vital role in drug rehabilitation.
Physical exercise has been strongly recommended as a rehabilitation program for SUD [6, 13] and has been proven to be a positively effective means of drug rehabilitation . Numerous studies have assessed the effects of short exercise sessions and long-term exercise protocols as an adjunct therapy in SUD. The majority of these studies focus on physical and mental health, craving, duration of abstinence, and withdrawal symptoms [15–20]. The impacts of exercise on cognitive psychology and brain neuroscience have received growing interest in efforts to explore the mechanism of drug addiction. Studies have shown that exercise can improve age-related declines in cerebral blood flow, which is associated with improved cognitive function . The beneficial effects of long-term exercise on cognition has been demonstrated [14, 22], and even a single session of aerobic exercise (i.e., acute exercise) has been consistently shown to exert a positive influence on cognitive function . Acute exercise effectively improves inhibition control in individuals with SUD . However, despite the numerous positive effects of exercise on the human body, systematic research on the application of this therapy to individuals with SUD remains at an early stage. Studies in this area are assigned the strictest criteria of scientific rigor, and the related mechanisms and modalities have yet to be thoroughly investigated . Most available studies solely explore the effectiveness of aerobic or moderate-intensity exercise on the physical attributes, mental recovery, and brain function of patients with SUD. While previous studies have shown benefits in motoric and cognitive domains, how these effects are functionally related remains unclear. The effectiveness of different doses of specific exercise parameters (e.g., intensity) must also be investigated , as this topic seldom discussed in research on exercise rehabilitation for drug addicts . Some studies have explored the effects of moderate- and high-intensity exercise, as well as aerobic and anaerobic forms of exercise, on drug rehabilitation [25, 27]. However, the single-feature (e.g., moderate intensity) exercise protocols used therein  do not allow precise exercise prescriptions to be developed for individuals with SUD.
Tai chi (TC), a traditional Chinese sport, has been increasingly applied to the field of drug rehabilitation. Previous studies reported that TC can be beneficial to the physical and mental health of individuals with SUD [17, 18], but how TC affects the cognition of SUD is not completely understood. A large number of studies have proven that TC can improve cognition in the elderly and cognitive-impairment groups . The cognitive impairment of drug addicts has been confirmed , and the results provide indications for the improvement of cognitive function, especially improvement of inhibition control, are related to drug abstinence [11, 12]. A greater understanding of the cognitive impact of TC on patients with SUD is warranted.
Depending on its style, TC is described as a form of physical exercise with low to moderate intensity [18, 29, 30]. In general, a TC style with moderate intensity is believed to assist detoxification, relieve withdrawal syndrome, and inhibit relapse impulses and behaviors; it is also considered relatively safe [31–33]. Moderate-intensity continuous training (MICT) is widely used in drug rehabilitation programs . The effects of high-intensity interval training (HIIT) on drug addicts have received great attention  as HIIT is considered a time-saving, highly efficient, and economical form of exercise . HIIT is derived from the Tabata protocol  and characterized by a short series of high-intensity repetitions interspersed with low-intensity recovery phases. However, increased exercise intensity is widely believed to result in greater risk of adverse events, such as muscle injury and systemic inflammation [36–38]. The application of high-intensity exercise to drug dependents is considered a critical endeavor because patients with SUD are not normally in excellent physical condition, [17, 18] and participation in high-intensity sport features an inherent risk of sports-related injuries . Nevertheless, several long- and short-term studies argue that HIIT has the same value as, if not is better than, MICT in terms of improving body composition, cardiovascular function, metabolic health tolerance, safety, and exercise enjoyment and adherence [40–45]. More importantly, some authors have emphasized that HIIT is beneficial in reducing cardiovascular risk and indirectly improving mortality risk [27, 46]. Unlike TC or other moderate exercise, HIIT may require more exercise monitoring and specific patterns to reduce potential risks because high-intensity exercise may be challenging for the SUD population. Different exercise intensities or forms could incite different expectations of cognitive benefits. Sport-physiological research supports the premise that higher-intensity exercise treatments lead to greater cognitive benefits compared with moderate-intensity exercise . HIIT has been reported to have a positive effect on improving cognition in the elderly and cognitive-impairment groups [48–50]. Currently, the evidence in favor of high-intensity training for SUD is weak. Few studies have discussed the cognitive rehabilitation of patients with SUD through high-intensity training [26, 27, 51]. Consequently, the impact of HIIT on the cognition of patients with SUD is unclear and requires further research.
Whether different forms or intensities of exercise exert different effects on substance use-related outcomes must be determined. The theoretical basis of TC and HIIT beside the cognitive treatment of SUD has not been fully confirmed in the scientific literature. Given the benefits of TC and HIIT in physical and cognitive rehabilitation in different groups, this study seeks to explore which form of exercise is more suitable for the cognitive rehabilitation of SUD. The purpose of this study is to compare the effects of TC and HIIT on the recovery of cognitive inhibition control in patients with SUD by using specific cognitive tests that are typically associated with executive functions.