To the Editor:
It is well established that heart transplantation improves patients’ quality of life, exercise capacity, and survival. However, exercise capacity after a heart transplant still remains subnormal .
Exercise training is a very important intervention for heart transplant recipients. It improves exercise tolerance, quality of life, and cardiac reinnervation. High-intensity interval exercise training has appeared in the literature as a safe and efficient method to improve exercise capacity in patients with coronary artery disease and in patients with heart failure [2, 3].
The study by Nytrøen et al.  is very interesting and adds important information to what we know about exercise training in heart transplant recipients. This randomized clinical trial investigated the effects of high-intensity interval exercise training in peak VO2 and peripheral muscle strength. The main finding of this study  was that high-interval training was safe, applicable and effective in improving peak VO2 and muscular exercise capacity.
It is important to point out that this study  used an unusual and ambitious interval exercise training protocol. The sessions consisted of a 10-minute warm-up, followed by 4-minute exercise bouts at 85–95% of maximum heart rate, interposed by a 3-minute active recovery period. Most of the studies on interval exercise training in heart failure assume an overall recovery time greater than or equal to the period of high-intensity exercise. The protocol used in the study by Nytrøen et al.  opens a new discussion about which method of high-intensity interval exercise training is the most appropriate for cardiac patients, especially for heart transplant recipients. Moreover, it would be interesting to test the interval training in different phases of cardiac reinnervation (and cardiovascular control), once the studied patients had 4 years of transplantation and probably some important degree of reinnervation.
Another important point to be discussed is the peak VO2 improvement. The study by Nytrøen et al.  showed an 11.5% improvement after high-intensity interval training. However, a previous study showed a 25.9% improvement after continuous training . The lack of a control group performing continuous exercise training limits the interpretation of Nytrøen et al.'s study . So, the superiority of high-intensity interval training for heart transplant recipients is not established.
Thus, new trials are needed to elucidate if high-intensity interval training is superior to continuous training for VO2 improvement. Moreover, it is important to investigate the best protocol of high-intensity interval training regarding recovery and exercise bouts in different years of heart transplant and cardiac reinnervation.