Physical fitness improved in both groups
In this study, a randomised controlled trial of HIIT and MICT was conducted to explore whether HIIT is more conducive to the rehabilitation of physical functions for individuals with SUD than MICT. During 12 months of exercise intervention, the HIIT and MICT groups exhibited a significant improvement in VC, grip strength, push-up, sit-and-reach, one-leg standing with eyes closed and CRT. The BP and craving level in the two groups decreased significantly, but no significant differences were found between the HIIT and MICT groups.
CRT is one of the most important indicators of physical fitness. It reflects the function of the human nervous system to some extent. The smaller the reaction time, the faster the human body responses, and the better the function of the nervous system is. Performance in a CRT task is applied as an indicator of cognitive capacity, including aspects of eye-hand reaction time, attentiveness and processing speed . The results consistently showed that physical activity can improve CRT, which indicates that any form of exercise is helpful to improve the CRT.
The one-leg standing with eyes closed test was performed to measure the balance control of the subjects. The increase in time spent standing on one foot with eyes closed in individuals with SUD indicated the improvement in their balance ability. A previous study showed an improvement in balance control after long-term practice of Tai Chi . In this study, the MICT group was able to stand on one leg with eyes closed for 14 s longer than the baseline, while the HIIT group was only able to do so for 11 s longer than the baseline. These findings proved that both HIIT and MICT could improve balance ability. Although there was no significant difference between groups, the MICT group showed more potential to improve balance than the HIIT group because of the Tai Chi content.
Blood pressure decreased
The subjects in the HIIT and MICT groups had the same effect on DBP and SBP; both types of training effectively reduced the BP for individuals with SUD. A reduction in BP can have a positive effect on the health of individuals. SBP is controlled by total peripheral resistance, which is determined by the diameter of small arteries and arterioles . The decrease in BP in individuals with SUD may be due to increased flexibility in their blood vessels through training and reduced peripheral resistance. In this study, exercise intervention can significantly reduce BP, which may be due to Tai Chi and HIIT, which improved the cardiac function of subjects, as well as increase stroke output and blood circulation. The relaxation form of exercise may reduce the tension of the vascular centre and vascular smooth muscles and accelerate blood flow, leading to the reduction in BP. The practice of regular physical exercise can reduce resting BP and induce post-exercise hypotension, which is a reduction in BP below resting values after exercise training . Therefore, HIIT and MICT have a similar effect of reducing BP. Both HIIT and MICT can be applied as effective exercises for rehabilitation for individuals with SUD in terms of reducing BP.
Flexibility and vital capacity improved
HIIT, MICT or Tai Chi exercise for different types of participants can improve flexibility , which was consistent with the results of this study. The results showed that the two kinds of exercises have similar promoting effects on muscle flexibility.
In this study, VC in the HIIT group increased by 496 ml, whereas that in the MICT group increased by 514 ml. These results demonstrated that improvements in VC were similar. Some preliminary evidence suggests that Tai Chi exercise may increase lung function , because Tai Chi combines breathing with movement closely to improve breathing efficiency. Furthermore, another study showed that HIIT and MICT are both effective for improving lung function . HIIT is at least as effective as MCT for improving functional capacity and quality of life measures in patients with pulmonary disease. Therefore, the experimental results were similar to those of some studies, which proved that both HIIT and MICT have similar effects on improving lung VC.
HIIT is better in strength development
The results of this study showed that the grip strength of the HIIT group increased by 5.48 kg, whilst that of subjects in the MICT group increased by 4.68 kg. The number of push-ups in the HIIT group increased by seven repetitions, whilst that in the MICT group increased by less than five repetitions. These results showed that the improvement in grip strength in the two groups was similar, but the improvement in push-ups in the HIIT group was better than that in the MICT group. Both groups increased (P < 0.05) muscle strength, whereas greater muscle strength was gained in the HIIT group. The possible explanation is the training content of the HIIT group includes resistance training , which can effectively improve muscle strength. This result has been supported in a previous study. A study reported that HIIT is better at improving upper limb, waist and abdominal strength for individuals with SUD compared with MICT .
Interestingly, increased muscle strength was also observed in the MICT group. Some preliminary evidence suggests that Tai Chi exercise may increase the body’s muscle strength . However, another study found that Tai Chi has no advantages in improving muscle strength . Whether Tai Chi intervention improved the strength in the MICT group is unclear, because subjects in the MICT group not only practiced Tai Chi but also engaged in medium-intensity broadcast exercise.
Does HIIT training have a better effect than MICT?
After 12 months of exercise training, the results showed that HIIT and MICT training had the same effect on the physical fitness of individuals with SUD. The results were not consistent with the previous hypothesis. In general, the greater the intensity of exercise, the more profound the stimulation to the human body, and the more significant the training effect is. However, the differences in physical fitness and craving level between the two groups were not significant. In terms of standing on one leg with eyes closed, the benefits of the subjects in the control group were even better than those in the experimental group, thereby indicating that Tai Chi has a better effect on improving balance ability. The advantage of HIIT over MICT is mainly in terms of cardiovascular function , but there is no evidence to support that HIIT training is superior to MICT.
Our results showed the same positive effects on VC, grip strength, sit-and-reach, choice reaction time, one-leg standing with eyes closed and push-up in the two groups. Notably, craving levels decreased in the two groups, which proved the correlation between physical fitness and craving level. Several studies have also demonstrated the same effect of HIIT and MICT. In the exercise intervention with HIIT for type 2 diabetes over 12 weeks, another study found no significant difference in the test indicators of positively altering body fat and increasing peak power output, glucose control, cardiovascular risk and microvascular complication markers in the HIIT and MICT groups . In an exercise intervention for obese men, HIIT was verified as an equally effective exercise mode for improving 24 h glycaemic control in overweight and obese adults compared with MICT . However, Haykowsky et al.  showed that HIIT is superior to MICT in terms of improving VO2 peak for patients with congestive heart failure, with similar effects on left ventricular function and exercise compliance. Similarly, in patients with coronary heart disease, studies demonstrated that HIIT has greater effect on ventilatory threshold and VO2 peak compared with MICT, with similar effects on BP .
The advantages and disadvantages of HIIT and MICT remain uncertain, but many studies believe that these two training methods produce equal benefits. There may be several reasons for this phenomenon. Firstly, patients with different symptoms are suitable for varying training methods, and different training methods can produce varying benefits at different stages of the disease. For example, long-term continuous aerobic exercise training plays an important role in maintaining the health and well-being of patients with cardiovascular disease, including the potential to maintain self-care ability and clinical benefits during aging . Short HIIT is useful in the initial and improved stages of cardiac rehabilitation, whilst MICT or HIIT regimens are more appropriate for the improvement and maintenance stages, as they have higher physiological stimulation . Secondly, the correlation between the physical benefits and the amount of exercise generated by HIIT and MICT is greater than the intensity of exercise. Martin J. Gibala et al.  compared the six-week HIIT based on Wingate with traditional endurance training; they found a reduction in the weekly training amount (90 % reduction in the HIIT group) and time input (67 % decrease in the HIIT group), but the training had similar improvement on various indexes of skeletal muscle and cardiovascular adaptation. Related studies focus more on the difference between these two training methods and less on the comparison of the total amount of exercise in the same or different situations. Thirdly, no study has explored the optimal intensity of HIIT training. Different intensities of HIIT training may have varying effects on various subjects. Although research suggested that the two training methods have similar effects, as individuals with SUD lose interest to MICT, HIIT and MICT can be alternatively arranged to maintain training interest.