Physical fitness improved in both groups
In this study, a randomized 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. During the 12 months exercise intervention, HIIT and MICT groups were found significantly improved in VC, grip strength, sit-and-reach, choice reaction time, one-leg stand with eyes closed, push-up. The blood pressure and the craving level in the two groups decreased significantly, but no significant differences were found between the HIIT group and MICT group.
Choice reaction time declined
Choice reaction time (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, 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(29). In this study, The CRT in the experimental group decreased by 0.03s, the CRT in the control group decreased by 0.04s. There was no significant difference between groups, whereas, both groups found a significant difference in CRT compared to the baseline. The results are consistent with that physical activity can improve CRT, which indicates that no matter what form of exercise, it is helpful to improve the reaction time. The improvement of reaction time may be explained by the fact that physical exercise can improve the function of central nervous system (Figure 5).
The one-leg stand 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 of their balance ability. Previous study showed an improvement in balance control after long term practice of Tai Chi (30). In this study, the MICT group was able to stand on one leg with eyes closed for 14s longer than baseline while the HIIT group was 11s longer than baseline. The evidence proved that both HIIT and MICT can improve balance ability. However, although there was no significant difference between groups, MICT group seems to have more potential to improve balance than HIIT group because of Tai Chi content arranged.
Blood pressure decreased
The subjects in HIIT and MICT group had the same effect on diastolic and systolic blood pressure, both types of training effectively reduced blood pressures for individuals of SUD. A reduction in blood pressure can have a positive effect on the health of individuals. At present, systemic blood pressure is controlled by total peripheral resistance, which is determined by the diameter of small arteries and arterioles (30). The decrease in blood pressure 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 blood pressure, which may be due to the Tai Chi and HIIT exercise which improve the cardiac function for subjects, as well as increase stroke output and blood circulation. The relaxation form of exercise may reduce the tension of vascular center and vascular smooth muscle, and accelerate blood flow leading to the reduction of blood pressure. The practice of regular physical exercise can reduce resting blood pressure chronically, as a result, it induce post-exercise hypotension, which is a reduction in blood pressure below resting values after exercise training (31). Therefore, HIIT and MICT have a similar effect to reduce blood pressure. Both HIIT and MICT can be applied as effective exercises for rehabilitation for individuals with SUD in term of reducing blood pressure.
Flexibility and vital capacity improved
HIIT, MICT or Tai Chi exercise for different types of participants were all able to improve the flexibility for subjects (32, 33), which is the same as the results of this study. Result shows that the two kinds of exercises have similar promoting effects on muscle flexibility.
In this study, vital capacity in HIIT group was increased by 496 ml, whereas, MICT group was increased by 514 ml. The results demonstrated improvements in vital capacity were similar. Some preliminary evidence suggests that Tai Chi exercise may increase lung function (34, 35). One of the explanations is that Tai Chi combines breathing with movement closely to improve breathing efficiency. Furthermore, study shows HIIT and MICT are both effective for improving lung function(36). HIIT is at least as effective as MCT for improving functional capacity and quality of life measures in patients with pulmonary disease (36). Therefore, the experimental results are similar to those of some studies, which prove that both HIIT and MICT have similar effects on improving lung vital capacity.
HIIT is better in strength development
In this study, from the data, the grip strength of HIIT group was increased by 5.48kg, while subjects in MICT group was increased by 4.68kg; The number of push-up in HIIT group was increased by 7 repetitions, while subjects in MICT group was increased less than 5 repetitions. The results showed that the improvement of grip strength in the two groups was similar, but the improvement of push up in HIIT group was better than that in MICT group. This demonstrated both groups increased (P < 0.05) muscle strength; whereas, greater muscle strength gains in HIIT group. The possible explanation is the training content of HIIT group includes resistance training (37), which can effectively improve the muscle strength. This result has been supported in the previous study. A study reported HIIT is better at improving upper limb, waist and abdominal strength for individuals with SUD (33).
Interestingly, muscle strength increased was also observed in MICT group. Some preliminary evidence suggests that Tai Chi exercise may increase the body’s muscle strength (38, 39). However, study finds Tai Chi has no advantages in improving muscle strength (30). It is not sure whether Tai Chi intervention improved the strength in MICT group, because subjects in MICT group not only practiced Tai Chi, but also engaged in medium intensity broadcast exercise.
Does HIIT training has better effect than MICT?
After 12 months exercise training, the results showed HIIT and MICT training have the same effect on the physical fitness for individuals with SUD. The results are not consistent with the previous hypothesis. Generally, the greater the intensity of exercise causing the more profound the stimulation to the human body, the more significant the training effect has. However, the differences of 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 control group was even better than those of the experimental group. The advantage of HIIT over MICT is mainly in effect of cardiovascular function (40), but there is no evidence that HIIT training is superior to MICT.
Several studies have shown the same effect of HIIT and MICT. In the exercise intervention with HIIT for type 2 diabetes over 12-week, it was found that there was no significant difference in the test indicators of positively altering body fat, increasing peak power output, glucose control, cardiovascular risk, and microvascular complication markers in the HIIT group and the MICT group (19). In an exercise intervention for obese men, HIIT was verified as an equally effective exercise mode for improving 24-h glycemic control in overweight and obese adults comparing to MICT (41). However, Haykowsky et al. (42) showed that HIIT was 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 HIIT has greater effect on ventilatory threshold and VO2peak compared to MICT, with similar effects on blood pressure (43, 44).
As can be seen from this, the advantages and disadvantages of HIIT and MICT are still uncertain, many studies believe that these two training methods can produce equal benefits. There may be several reasons for this phenomenon. First, patients with different symptoms are suitable for different training methods, and different training methods can produce different 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 ability of self-care and clinical benefits during aging (45-47). Short HIIT is useful in the initial and improved stages of cardiac rehabilitation, while MICT or HIIT regimens seem to be more appropriate for the improvement and maintenance stages, as they have higher physiological stimulation (24). 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. (48) compared the 6-week HIIT based on Wingate with the traditional endurance training, it was found that although the weekly training amount (90% reduction in HIIT group) and time input (67% decreased in HIIT group), 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, there is no exploration of the optimal intensity of HIIT training currently. Different intensity of HIIT training may have different effects on the various subjects.