For the interval protocols, the peak V[Combining Dot Above]O2 was determined by averaging the highest V[Combining Dot Above]O2 at the end of the last 3 work protocols, whereas for the steady-state protocol, the peak V[Combining Dot Above]O2 represents the mean V[Combining Dot Above]O2 of the last 3 minutes of exercise. As seen in Figure 2B, the peak V[Combining Dot Above]O2 was significantly higher in the 90/30 protocol, whereas the 30/30 protocol elicited the lowest value. The peak V[Combining Dot Above]O2 in the 60/30 protocol was significantly lower than in the 90/30 protocol but significantly higher than the 60/60 or steady-state protocol. In all interval protocols, the peak V[Combining Dot Above]O2 fell below the targeted V[Combining Dot Above]O2 of 90% V[Combining Dot Above]O2max.
The mean HR for each protocol, encompassing both work and rest periods, is illustrated in Figure 3A. The mean HR was significantly lowest in the 30/30 and 60/60 protocols, and these 2 protocols did not differ from one another. The mean HR response was similar in the 60/30, 90/30 and steady-state protocols. For the interval protocols, the peak HR was determined by averaging the highest HR at the end of the last 3 work protocols, whereas for the steady-state protocol, the peak HR represents the mean HR of the last 3 minutes of exercise (Figure 3B). The peak HR was the highest in the 90/30 protocol and lowest in the 30/30 protocol. The peak HR did not differ between the 60/30 and 90/30 protocols. Compared with the 90/30 protocol, the peak HR was significantly lower in the 60/60 and steady-state protocols, whereas these latter 2 protocols did not differ from the 60/30 protocol.
The mean resting blood pressure was 116.5/68 mm Hg (Table 1). At the end of 20 minutes steady-state exercise performed at 70% V[Combining Dot Above]O2max, the mean blood pressure was 148/68.7 mm Hg. Blood pressure at the end of the interval protocols conducted at 90% V[Combining Dot Above]O2max did not significantly differ from that of steady-state exercise.
The mean blood lactate concentration, taken at rest and at the end of each exercise protocol, is shown in Figure 4. Blood lactate concentration was significantly higher after all exercise protocols compared with that for rest. There was no significant difference in blood lactate between the 30/30, 60/60, and steady-state protocols. In addition, there was no significant difference in blood lactate concentration between the 60/30 and 90/30 protocols. However, the 90/30 protocol elicited a significantly higher blood lactate than did the 30/30, 60/60, and steady-state protocols. Additionally, blood lactate was significantly higher in the 60/30 protocol compared with that in the 30/30 and steady-state protocols.
Mean total caloric energy expenditure for the 30/30, 60/60, and steady-state protocols averaged approximately 195 kcal, and the total caloric energy expenditure in these 3 protocols was significantly higher than in the 60/30 or 90/30 protocol (∼150 kcal). Lastly, there was no difference in energy expenditure between the 60/30 and 90/30 protocols.
For all exercise protocols, the RPE gradually increased with time. The mean peak RPE taken at the end of the exercise protocol appears in Figure 5. The final RPE was significantly higher in the 90/30 protocol compared with all others. The final RPE did not differ between the 30/30, 60/60, and steady-state protocols. The final RPE in the 60/30 protocol was significantly greater than the 30/30 but not significantly different from the 60/60 or steady-state protocol.
Our present study demonstrates that altering the work-to-rest ratio significantly alters the metabolic and HR response to interval exercise, despite the subjects performing similar amounts of work. Our findings highlight that when using this exercise paradigm, the subjects can achieve several minutes of high-intensity exercise without undue discomfort or fatigue.
The mean and peak V[Combining Dot Above]O2 achieved during the 30/30 protocol averaged 64.1 and 65.6% V[Combining Dot Above]O2max, respectively, whereas during the 60/30 protocol, the mean and peak V[Combining Dot Above]O2 averaged 70.9 and 77.3% V[Combining Dot Above]O2max, respectively. During the 90/30 protocol, the mean and peak V[Combining Dot Above]O2 averaged 75.7 and 83.1% V[Combining Dot Above]O2max, respectively. Thus, the percent of V[Combining Dot Above]O2max reached during interval exercise performed at a workload corresponding to 90% V[Combining Dot Above]O2max was influenced by the work-to-rest ratio, that is, the greater the work-to-rest ratio, the greater the V[Combining Dot Above]O2. Interestingly, however, the V[Combining Dot Above]O2 (mean or peak) never reached the value observed during the continuous graded exercise test in any of the interval protocols.
At the beginning of exercise, energy demand is met through a number of different pathways, including the immediate breakdown of high energy phosphate stores (e.g., adenosine triphosphate, creatine phosphate), glycolysis, and aerobic metabolism (12), and the relative contribution of these pathways depends on a number of variables. The contribution of aerobic metabolism is reflected by oxygen consumption during work, and the extent of rise is dependent not only upon the duration of exercise or the level of fitness within an individual (10) but also on the work intensity. Whipp and Wasserman (16) demonstrated that at low workloads, steady-state V[Combining Dot Above]O2 could be reached within 3 minutes from the start of exercise. With high work intensities, however, the V[Combining Dot Above]O2 kinetics was characterized by an initial rapid phase followed by a slower secondary phase such that steady-state V[Combining Dot Above]O2 was not achieved even in the sixth minute of exercise. Given that our subjects exercised at a workload corresponding to 90% V[Combining Dot Above]O2max with regular intermittent rest periods, it seems likely that the work-to-rest dynamics of the exercise protocol did not allow our subjects sufficient time to achieve the expected V[Combining Dot Above]O2. At present, it is unknown if increasing the number of work intervals will result in a higher mean or peak V[Combining Dot Above]O2, or if the V[Combining Dot Above]O2 response will vary significantly in untrained individuals as compared with trained individuals.
The HR response in most respects mirrored the V[Combining Dot Above]O2 response during the exercise protocols. Perhaps most striking was that the mean HR was significantly lower in the 30/30 and 60/60 protocols as compared with 20 minutes of steady-state exercise conducted at 70% V[Combining Dot Above]O2max. Heart rate averaged approximately 153 b·min−1 during both the 30/30 and 60/60 protocols, and approximately 167 b·min−1 during the steady-state protocol. This occurred even though the total caloric energy expenditure was similar across the 3 protocols. By the end of the steady-state protocol, the HR(peak) averaged 171 b·min−1. Additionally, the mean HR during either the 60/30 or 90/30 protocol was not significantly different than the steady-state protocol. With respect to the peak HR achieved during the interval exercise, only the 90/30 protocol was significantly higher than the steady-state protocol. Moreover, the blood pressure response did not differ across all protocols. These findings indicate that high-intensity interval exercise (at workloads corresponding to 90% V[Combining Dot Above]O2max) employing a protocol described in this study can likely be carried out as safely as traditional steady-state exercise performed at 70% V[Combining Dot Above]O2max.
We determined whether the interval exercise protocols used in this study resulted in excessively high concentrations of blood lactate, which could ultimately influence work tolerance. Blood lactate concentration was highest after the 90/30 protocol but only averaged 4.5 mmol. Lowering the work-to-rest ratio from 3:1 (90/30) to 1:1 (60/60 or 30/30) resulted in a significantly lower blood lactate concentration, even though the subjects performed similar amounts of work. Ballor and Volovsek (2) previously noted that increasing the work-to-rest ratio significantly increased V[Combining Dot Above]O2, HR, and blood lactate when the subjects performed interval work at the same intensity. Our findings indicated a similar but modest relationship between blood lactate and work-to-rest ratios. Compared with the steady-state protocol, only the 60/30 and 90/30 protocols resulted in a higher lactate concentration, but even these concentrations were modest at best.
Perceived effort during exercise is an important (but not sole) determinant in a subject's ability or willingness to complete an exercise session. Consistent with the work by Green et al. (9), the RPE gradually increased with time in all 5 exercise protocols. Of the 4 interval protocols performed in this study, only the 90/30 protocol was rated higher than steady-state exercise conducted at 70% V[Combining Dot Above]O2max, whereas the RPE during the steady-state protocol did not differ from the 30/30, 60/30, or the 60/60 protocol. Thus, the high interval training (HIT) protocols with a work-to-rest duration of 60/30, 60/60, or 30/30 can be performed by young subjects without undue discomfort. Whether or not older sedentary individuals respond in a similar manner is yet to be determined.
Sprint interval training using the Wingate protocol has been studied extensively during the last several years. Although short-term studies have documented significant improvements in work endurance and peak power output (5,7), oxidative enzyme potential and lipid oxidation (6), and flow-mediated dilation (13), little to no changes in aerobic capacity have been observed (5,7). Moreover, because this exercise is conducted at supramaximal intensity, this type of training is likely not feasible for sedentary older individuals who may be at risk for cardiovascular or metabolic disease. However, other types of high-intensity interval training paradigms have been used for normal and patient populations, which lead to significant improvements in aerobic capacity. For example, Wisloff et al. (17) had heart patients train at approximately 95% of peak HR 3 times per week for 12 weeks using an interval protocol consisting of four 4-minute work intervals interspersed with 12 3-minute rest intervals conducted at 50–70% of peak HR. After interval training, peak V[Combining Dot Above]O2 increased 46%. Using a similar training paradigm, Slordahl et al. (14) reported that V[Combining Dot Above]O2max increased 18% in young healthy subjects. However, in both these studies, the total training duration exceeded 30 minutes per workout and therefore may not be construed as time effective by a majority of Americans.
Recently, Little et al. (11) employed a similar HIT protocol as described in this study in healthy young men. Each subject trained at approximately 100% of peak power output and performed 8- to 12 60-second intervals interspersed with 75-second recovery periods (3 d·wk−1 × 2 weeks). The total time commitment, including warm-up and cool-down was approximately 20–29 minutes depending upon the number of intervals performed. The results indicate that cycling endurance, muscle oxidative capacity, resting muscle glycogen and total GLUT 4 content significantly increased. Their findings indicate that a lower volume HIT paradigm is effective in improving physiological function. We propose that the total duration of exercise may be reduced to <20 minutes by employing the 60/30 protocol described in this study. Whether or not this protocol will lead to significant training adaptations in cardiorespiratory endurance or muscle metabolism remains to be established, but recent data by Little et al. (11) are encouraging.
Because exercise intensity is typically titrated according to the HR response, exercise specialists should consider the work-to-rest ratio, and work and rest duration when prescribing high-intensity aerobic interval exercise. For example, if using the protocol described in this study, a lower work-to-rest ratio could be used for sedentary middle-aged subjects yet still obtain the same amount of mechanical work. As aerobic capacity and exercise tolerance improves, the subjects could be progressed to a higher work-to-rest ratio but with fewer intervals, thereby shortening the total workout duration. Whether or not similar training adaptations will be observed between the different protocols (e.g., 90/30, 60/30, etc.) remains to be determined.
The authors are grateful for the technical assistance of Pedro Sotelo-Peryea. This work was supported in part by funds from the United University Professions.
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Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
lactate; heart rate; blood pressure; training; RPE