Discussion

The main finding of the study was that higher intensities of exercise elicit greater improvements in V?O_2max than lower intensities of exercise over a 4- to 6-wk training period in healthy, young adults. This finding is consistent with the original hypothesis. Unlike V?O_2max, there were no changes observed in resting HR and resting BP after training.

A major strength of this study was the control of total volume of exercise between training groups. Each group performed the same amount of exercise based on V?O₂R or HRR, which was expressed as ATU. This is a better means of matching exercise volume than matching based on energy expenditure per se because each subject in this study performed the same amount of exercise relative to his or her aerobic capacity. On the other hand, if each subject had been asked to expend a set number of kilocalories per week, then this would have been a greater relative challenge for the lesser-fit subjects than the higher-fit subjects. Nonetheless, although exercise volume was matched using ATU, each group performed the same amount of external work per week, given that the mean V?O_2max values at baseline were similar. The average external work in the sixth week of training, ?1000 kJ, translates to approximately 1000 kcal of energy expenditure, given 4.186 kJ·kcal^-1 and assuming a human efficiency of ?24%. The actual caloric cost of the exercise would be somewhat higher than this value, given friction in the drive train of the ergometer and the energy cost of spinning the legs.

Aerobic Capacity

The ACSM currently recommends 20 to 60 min of exercise performed at 40/50-85% HRR or V?O₂R for most adults, where 40% is considered a threshold level for deconditioned individuals and 50% is a threshold for average adults.^[2] There has been previous evidence suggesting that exercise of a higher intensity will result in greater gains in cardiovascular fitness.^[32,33] However, only a few reports included a sufficient number of subjects to confirm that groups training at higher intensities experienced significantly greater increases in V?O_2max than groups training at lower intensities when the total volume of exercise was controlled. In this study, each of the exercising groups experienced a significant absolute increase in V?O_2max versus baseline values, and the absolute increase in the near-maximal-intensity group was significantly greater than that in the moderate-intensity group. Moreover, when the increases in V?O_2max were expressed as percent changes, the response in each intensity group was significantly greater than that in the lower-intensity groups.

This study is unusual in including a group that exercised with intervals at an intensity that approached V?O_2max. Such intervals have been included in training programs as early as 1977, when Hickson et al.^[17] reported a 44% increase in V?O_2max after 10 wk of training that consisted of six 5-min intervals of bicycling at V?O_2max on 3 d·wk^-1 plus 40 min of vigorous running on 3 d·wk^-1. However, Hickson et al.^[17] did not compare this training program with any other. Three recent studies have compared high-intensity interval training with lower-intensity continuous training in cardiac patients with total work controlled.^[27,37,38] Significantly greater benefits were found in the interval group than the continuous group for V?O_2max,^[27,38] ventilatory threshold and treadmill time to exhaustion,^[37] and left ventricular performance.^[38]

A study recently published by Helgerud et al.^[16] examined the effects of 8 wk of aerobic endurance training at various exercise intensities in healthy, young-adult males. Groups performed running at a moderate-intensity (70% HR_max for 45 min each session), vigorous-intensity (85% HR_max for ?24 min per session), and two maximal-intensity interval training regimens that both alternated 90-95% HR_max with 70% HR_max, one using multiple 15-s intervals and one using four 4-min intervals. Both interval training groups significantly increased V?O_2max, whereas neither continuous training group did. Using a previously published formula,^[32] the moderate- and vigorous-intensity groups of Helgerud et al.^[16] were exercising at ?47% and ?72% HRR, respectively, which are comparable to the current study. The failure of the continuous training groups of Helgerud et al.^[16] to increase V?O_2max was probably due to their high baseline fitness, which averaged 58 mL·min^-1·kg^-1. Esfarjani and Laursen^[11] recently compared interval training at V?O_2max with continuous training at 75% HRR in male runners. As in the study of Helgerud et al.,^[16] the subjects' baseline V?O_2max was greater than 50 mL·min^-1·kg^-1, and only the interval group increased V?O_2max. Both of these studies differed from the current study in the population (only males vs both males and females; high vs average fitness) and the mode of exercise (running vs cycling).

It should be noted that although interval training groups spend some of their training time at a very high intensity, a similar amount of time is spent at a lower intensity, and therefore the mean intensity of training may not be any higher than that of a continuous training program. In the current study, the interval training group used 5 min each for the work and the recovery phases of the intervals and had an average intensity of 72% HRR, which is slightly less than the 75% HRR of the vigorous group. The work-recovery periods of Helgerud et al.^[16] were 4 min at ?93% HR_max and 3 min at 70% HR_max, for a mean intensity of 83% HR_max in the interval group, whereas one of the continuous groups used 85% HR_max. Warburton et al.^[37] used 2 min at 90% HRR and 2 min at 40% HRR for the work and the recovery phases, yielding a mean intensity of 65% HRR in the interval group, and had the continuous training group use 65% HRR. Wisloff et al.^[38] used 4-min work phases at ?93% HR_max and 3-min recovery phases at 60% HR_max, for a mean intensity of 79% HR_max in the interval group, and used ?73% HR_max in the continuous training group. Despite the similarity of mean intensity between the interval and the continuous training groups, the interval groups in all of these studies experienced greater improvements in aerobic fitness after training. Therefore, although intensity is a key variable in cardiorespiratory training (as shown by comparing the two continuous training groups in this study), the mean intensity may not be as important as the highest intensity that is used for a significant portion of the training. A topic for future research is to determine what portion of training should be done at high intensities and using what work-recovery periods to obtain the greatest results.

Interval training has been used previously with elderly female cardiac patients,^[27,38] but the current study appears to be the first to compare the effectiveness of interval training and continuous training among healthy females. Accordingly, a post hoc analysis was performed to determine the results in the female subjects alone. V?O_2max increased 15.5% among females in the near-maximal-intensity interval group, 13.6% in the vigorous-intensity group, and 8.0% in the moderate-intensity group. All increases were significant, and the interval group's increase was significantly greater than the moderate-intensity group's increase. A similar trend for greater increases in V?O_2max among men in the higher-intensity groups was observed, but it did not reach significance (P = 0.13 for one-way ANOVA on percent changes) due to the relatively low number of men (4-5 per group vs 8-10 women per group).

Resting HR and BP

Endurance-trained subjects are known to have a significant resting bradycardia.^[29] However, only a few studies have examined the role of training intensity in lowering resting HR. In studies that have controlled total exercise volume in two groups training at different intensities, two found no change in resting HR in either group,^[13,35] two found similar decreases in both groups,^[5,22] and one found that women, but not men, in the higher-intensity group decreased resting HR, whereas neither women nor men did in the lower-intensity group.^[24] In the current study, there was no significant change for any of the exercising groups. This is likely attributed to the fact that the subjects were young, healthy, and had a low resting HR at baseline (?66 bpm). A greater duration of training may be needed to elicit the magnitude of bradycardia exhibited by athletes.

The ACSM's position stand on exercise and hypertension concluded that aerobic training reduces resting BP and that there is no intensity effect.^[26] However, clinical trials that have compared more than one intensity of training while controlling total volume generally support a greater decrease with higher intensities. Specifically, four of five such studies found a decrease in diastolic BP only in the higher-intensity group;^[3,19,24,35] one found a greater decrease in systolic BP in the higher-intensity group,^[24] and one found similar decreases in both systolic and diastolic BP in both groups.^[5] The current study found that none of the training groups, regardless of intensity, experienced a significant decrease in either systolic or diastolic BP at rest. This lack of effect was likely dependent on the fact that the subjects were young, healthy, and had a low resting BP at baseline (?108/65 mm Hg).

Conclusions

These results indicate that when exercise volume is controlled, vigorous-intensity exercise is more effective for improving V?O_2max than moderate-intensity exercise in a healthy adult population at low risk for cardiovascular disease. Furthermore, the most effective training was interval exercise performed at near-maximal intensity. This study contributes to a growing body of evidence concerning the beneficial effects of higher-intensity exercise for improvements in V?O_2max, as well as potential benefits for cardiovascular health.^[33]

One concern with higher-intensity exercise is the possibility of poor adherence or "burnout." In the current study, vigorous-intensity training was performed for 5 wk, and interval training was performed for only 4 wk, with excellent compliance. Of course, subjects were supervised and received incentives for completing the study. Nonetheless, no problems were observed over the duration of the training program. Interval training is not traditionally used for extended periods of time, but further research should consider its benefits and potential deleterious effects. Additional clinical trials should investigate possible long-term health benefits of vigorous and higher-intensity training. This study illustrated short-term improvements in aerobic fitness for all groups, but further experimentation is warranted. A longer duration of training may result in a greater training effect on other variables, such as resting HR and BP.

The results of the present study do not constitute endorsement by ACSM.

What else is published in Medicine & Science in Sports & Exercise_®? Visit www.acsm-msse.org.