Psychological Reports: Mental & Physical Health 2014, 114, 3, 854-865. © Psychological Reports 2014
EXERTIONAL RESPONSES TO SPRINT INTERVAL TRAINING: A COMPARISON OF 30-SEC. AND 60-SEC. CONDITIONS1 MARCUS W. KILPATRICK AND SAMUEL J. GREELEY School of Physical Education and Exercise Science, University of South Florida Summary.—The purpose of this study was to assess the effect of sprint interval training on rating of perceived exertion. 20 healthy participants (11 men, 9 women; M age = 23 yr.) completed a maximal cycle ergometer test and two high-intensity interval training cycling sessions. Each session utilized the same work-to-rest ratio (1:1), work intensity (90% max), recovery intensity (10% work intensity), and session duration (16 min.). Trials differed on duration of the interval segment, with a 30-sec. trial and a 60-sec. trial. Sessions required the same amount of total work over the duration of the trial. Rating of perceived exertion assessed before, during, and after exercise were higher for the 60-sec. trial than the 30-sec. trial despite no difference in total work. High intensity interval training trials utilizing the same total external work but differing in interval length produced different ratings of perceived exertion. Perceived exertion is significantly higher for sessions of exercise that utilize longer work intervals. These findings suggest that shorter intervals may produce more favorable exertional responses that could positively affect future behavior.
Physical activity guidelines recommend the accumulation of 450–750 MET·min or more of activity per week via multiple combinations of intensities and durations that encourage a lifestyle approach to exercise and primary utilization of moderate intensities (Haskell, Lee, Pate, Powell, Blair, Franklin, et al., 2007). Current epidemiological evidence indicates that most individuals fall short of these stated minimum recommendations (World Health Organization, 2012). High-intensity interval training provides an alternative approach to physical activity programming and may be of great value to public health. This approach to exercise represents a form of activity that is notably different from lower-intensity lifestyle physical activity. However, while prolonged continuous intensities ranging from very low to moderate have been studied extensively, investigations of interval training from a public health perspective have only recently emerged. While many variations of interval training have been developed in recent years, the prescription of interval training receiving most of the attention within research literature is based around high-intensity, low-volume protocols, often referred to as high-intensity interval training (Gibala, 2007). Early work with these protocols utilized up to six 30-sec. supramaximal sprints interspersed with 4 min. of unloaded pedaling on a cycle erAddress correspondence to Marcus Kilpatrick, School of Physical Education and Exercise Science, University of South Florida, 4202 East Fowler Avenue, PED 214, Tampa, FL 33620 or e-mail ([email protected]). 1
DOI 10.2466/06.15.PR0.114k27w8
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gometer (Gibala, Little, van Essen, Wilkin, Burgomaster, Safdar, et al., 2006; Burgomaster, Howarth, Phillips, Rakobowchuk, MacDonald, McGee, et al., 2008). These protocols included three total minutes of exercise stimulus and a total session time of 20–30 min. inclusive of warm-up and cooldown. (Gibala, et al., 2006; Burgomaster, et al., 2008). This type of protocol was compared to continuous endurance training sessions over the course of two weeks on a variety of physiological variables. Despite dramatically lower training volume compared to an endurance training group, research participants performing high intensity interval training showed similar improvements in muscle oxidative capacity, muscle buffering capacity, glucose and lipid metabolism, and endothelial function (Gibala, et al., 2006; Burgomaster, et al., 2008; Little, Gillen, Percival, Safdar, Tarnopolsky, Punthakee, et al., 2011). Due to questions of both acute tolerability and concerns related to long-term adherence (Coyle, 2005), modified protocols utilizing lower intensities and longer exercise intervals were developed (Little, Safdar, Wilkin, Tarnopolsky, & Gibala, 2010). The format of this more contemporary approach to interval training involves 10 near-maximal sprints lasting 1 min. and a very low intensity recovery of the same length, adding up to a 20-min. exercise session. This protocol is proposed to be more practical for implementation and perhaps less intimidating as a physical activity prescription for individuals who undertake exercise for health and fitness (Little, et al., 2011). Recent studies have shown the efficacy of these modified protocols in improving various health markers in sedentary adults (Hood, Little, Tarnopolsky, Myslik, & Gibala, 2011). This research suggests that high-intensity interval training may be an excellent option for promotion of fitness and performance within a variety of populations, including sedentary individuals and type 2 diabetics. However, research investigating perceptual responses to this type of exercise is lacking and it is not yet clear how such training fits into existing physical activity guidelines. One highly relevant perceptual response that is worthy of consideration within the context of high-intensity interval training is perceived exertion, which relates to the sensations and perceptions associated with physical exercise. Though differing viewpoints exist, it is generally understood that perceived effort during an exercise task is determined and regulated by numerous physiological and psychological variables including but not limited to parameters such as metabolism, ventilation, blood flow, muscular fatigue, motivation, mood state, and exercise experience (Robertson & Noble, 1997). Research to date involving perceived exertion is extensive but primarily limited to assessment of momentary perceptions of effort. The earliest research focused on graded exercise testing (Borg, 1970) and was later utilized to monitor and prescribe intensity of effort during training sessions (Foster, 1998). Contemporary guidelines for exer-
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cise typically include a recommendation to exercise at a moderate intensity, with some of these guidelines specifying a particular exertional rating range that are linked to established metabolic and cardiovascular loads and thresholds (DeMello, Cureton, Boineau, & Singh, 1987; Mahon, Gay, & Stolen, 1998). One aspect of the rationale to prescribe exercise using ratings of perceived exertion is to create an exercise load that is metabolically significant in terms of caloric expenditure and physiologically demanding in terms of the cardiovascular response, but tolerable to the individual exerciser. Such a recommendation represents the need to encourage participation in exercise that might foster a positive experience and behavioral maintenance. This type of approach points toward the possibility that consideration of perceived exertion before, during, and after exercise may prove valuable in efforts to both understand and positively impact exercise behavior. Despite a relatively limited body of literature, research on post-exercise exertion has grown in recent years. This ‘session RPE’ was originally used to monitor the training loads of athletes to maximize performance and manage overtraining risk (Foster, 1998). More contemporary research considering the viability of session RPE in conjunction with recreational resistance and aerobic exercise suggests that this measure has potential value in the development of a greater understanding of the overall exercise experience (Sweet, Foster, McGuigan, & Brice, 2004; Egan, Winchester, Foster, & McGuigan, 2006; Green, Yang, Laurent, Davis, Kerr, Pritchett, et al., 2007; Kilpatrick, Robertson, Powers, Mears, & Ferrer, 2009; Kilpatrick, Bortzfield, & Giblin, 2012). Collectively, this research suggests that perceptions of effort obtained post-exercise may be an important consideration in exercise behavior. In contrast to a notable body of literature investigating post-exercise exertion responses, relatively little research has been conducted on the utility of predicted exertion and how that links to momentary exertion assessed during exercise. Existing research indicates that anticipated effort is not well-matched to the exertion measured during exercise, i.e., momentary RPE. Specifically, prediction errors differ by sex in younger populations whereby boys tend to underestimate momentary effort and girls tend to overestimate momentary effort (Kane, Robertson, Fertman, McConnaha, Nagle, Rabin, et al., 2010). In contrast, two studies in adults suggest that predicted exertion tends to overestimate impending exertion, especially the earlier and middle portions of an aerobic exercise session (Kilpatrick, et al., 2009; Kilpatrick, et al., 2012). This limited body of research suggests that exertion that is anticipated or recalled after exercise may be an important consideration. Given the trend for greater participation in training programs that are intervalbased, it is important to note that only one study has compared the exer-
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tional responses to continuous and discontinuous exercise (Green, et al., 2007). This study compared continuous cycle exercise at a light-to-moderate intensity against 1-min. intervals alternating between high intensity resistance and unloaded pedaling. Results indicated that the interval session was perceived as more effortful during the post-exercise period (ES range ∼ 0.6–1.0) despite both trials being equal on total work. This finding suggests that interval based-exercise may act on exertion in a way that is fundamentally different than continuous exercise. It is possible that intervals in general, and intervals of varied length in particular, exert differential influence on physiological and metabolic strain in a manner that affects perceptions of effort. The present study examined two protocols of low-volume, high-intensity interval training identical in intensity, total work, work-to-rest ratio, and duration, but utilizing differing interval lengths. Therefore, the purpose of the current study was to assess the acute effect of interval length on perceived exertion. Hypothesis. Interval sessions composed of shorter interval segments will be perceived as less effortful despite constant total work across trials. METHOD Participants and Screening Participants were 20 students (11 men, 9 women; M age = 23.4 yr., SD = 3.6) at a large metropolitan university in the southeastern United States. The sample size for this pilot research project is a reflection of existing research (Green, et al., 2007) and is based on an anticipated large effect size (i.e., ES ∼ 0.8), a power level of 0.80, and an alpha criterion of .05. Participants were primarily recruited by announcements in classes. Each participant completed an informed consent document and a health history questionnaire. All participants were screened by a physician's assistant affiliated with the research team for orthopedic, cardiovascular, or pulmonary conditions that would preclude participation. Participants were given instructions to avoid alcohol, caffeine, and tobacco for 3 hr. prior to testing, consistent with American College of Sports Medicine guidelines (2010). Participants provided informed consent in accordance with institutional guidelines prior to the first exercise session. All documents and procedures were approved by the university Institutional Review Board. Research Design A single graded exercise test was conducted prior to two counterbalanced trials of interval exercise on a stationary cycle ergometer. The maximal exercise test was used to measure peak aerobic power. The interval trials were each 20 min. in length. Both trials were matched for duration,
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total work, average intensity, and work-to-rest ratio. The major difference between trials was interval duration; one trial consisted of 60-sec. intervals and the other comprised 30-sec. intervals, to investigate the effect of interval length on perception of effort. A progressive, multi-stage protocol was performed on an electronically braked cycle ergometer (Lode, Netherlands) designed to allow the rider to vary pedal cadence without affecting workload. The primary function of the protocol was to measure peak workload capacity. The exercise protocol began at 50 watts and increased 10 watts every 20 sec. The test was terminated when the participant could not maintain a cycle cadence of 30 rpm. Heart rate and blood pressure were monitored before, during, and after the exercise test. Heart rate was measured using a heart rate monitor (Polar, USA) and blood pressure was determined by auscultation. Upon completion of the test, participants completed 3–5 min. of active recovery. Finally, participants were provided with a brief reminder and description of the two upcoming experimental trials with respect to duration and intensity. The two experimental trials were designed to create low-volume, highintensity interval training sessions. The two sessions differed only in regard to interval length (60 sec. vs 30 sec.). The intensity utilized for all exercise intervals was 90% of the peak power achieved on their initial maximal exercise test. Recovery intervals were set at 10% of the work intensity. Prior to baseline measures for each trial, participants were told which trial they would be performing. Total exercise time was 20 min. inclusive of 2 min. of warm-up, 16 min. of intervals, and 2 min. of cool-down. All workload changes were made by a member of the research staff. All trials were counterbalanced to avoid order effects. Measures The variable of interest for this study was overall or whole body perceived exertion (RPE), which was assessed before, during, and after each trial and input into a tablet computer. In each assessment, perceived exertion was assessed using the Borg Category-Ratio (CR-10) Scale (Borg, 1998). This scale utilizes a single question that includes a response range from 0 to 10. The verbal anchor for 0 is “nothing at all” and the verbal anchor for 10 is “very, very hard (maximal),” with 3 representing “moderate” and 5 representing “hard.” Standardized instructions for utilization of the CR-10 scale were employed (Borg, 1998). Reliability and validity of the CR-10 scale have been extensively demonstrated and reported elsewhere (i.e., Noble, Borg, Ceci, Jacobs, & Kaiser, 1983; Ljunggren & Johansson, 1988; Borg, 1998; Capodaglio, 2001). Perceived exertion was recorded during each phase of the exercise trial: (i) immediately prior to exercise, (ii) every 4-min. period during ex-
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ercise, (iii) immediately after exercise cool-down, and (iv) 10 min. after the exercise session. The pre-exercise assessment was, “How much exertion do you anticipate experiencing during this trial of exercise?” The intask assessment was, “How much exertion are you feeling right now?” Finally, the post-exercise assessment was, “How much exertion did you actually experience during this trial of exercise?” Efforts to make certain participants understood the RPE instructions included a description and review of the assessment procedures on the day of medical screening and the maximal exercise test. Statistical Analysis Analyses of the data proceeded in two phases. The first phase included descriptive analysis of sample weight status and aerobic fitness. The second phase utilized three repeated-measures analyses of variance (ANOVA) including: a 2 (Trial: 30-sec., 60-sec.) × 3 (Time: Pre, Post-0, Post10) ANOVA to consider exertion before and after exercise, a 2 (Trial: 30sec., 60-sec.) × 4 (Time: 25%, 50%, 75%, 100%) ANOVA to consider exertion during exercise, and a one-way ANOVA comparing the grand mean of the four in-task interval RPEs for the 30-sec. and 60-sec. trials. This latter analysis allowed comparisons using a measure of exertion averaged across the duration of the exercise. Significant findings were followed by planned contrasts. Because these comparisons increased the risk for Type I error, the p value for post hoc analyses of means was adjusted to a more conservative .01. Mean difference and standard deviation values were used to estimate Cohen's d, which is reported as the effect size (ES) value (Cohen, 1992). Tests of normality and homogeneity of variance provided assurances that parametric analyses were appropriate. RESULTS Weight status as measured by body mass index (BMI) (M = 24.2 kg·m−2, SD = 3.2) suggested that most participants were normal weight (BMI = 18– 24.9; n = 12), while some were overweight (BMI = 25–29.9; n = 7) or mildly obese (BMI = 30–34.9; n = 1)(ACSM, 2010). Fitness level as measured by the graded exercise test suggested participants were moderately fit (maximal workload capacity, M = 264 W, SD = 58; estimated aerobic capacity based on peak power output, M = 45.4 mL·kg−1·min−1, SD = 6.8) and generally within the 60th and 80th percentile ranks (ACSM, 2010). Data collected during the maximal test for HR (M = 177 beats·min−1, SD = 10) and RPE (M = 9.5, SD = 0.7) suggested maximal effort was achieved. Furthermore, results indicated that 85% of participants (n = 17) reached the criterion for maximal effort (RPE > 9) and 95% of participants (n = 19) reached the criterion for maximal heart rate (HR > 85% age-predicted maximum). Descriptive statistics for the RPE measurements by trial are shown in Table 1.
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M. W. KILPATRICK & S. J. GREELEY TABLE 1 DESCRIPTIVE STATISTICS FOR MEASURED VARIABLES Measurement Time
30-sec. Trial M
SD
Pre-exercise
5.4
1.7
25%
4.2
1.5
50%
5.4
75%
6.0
100%
60-sec. Trial
Grand M
M
SD
6.4
2.2
4.8
1.6
1.6
6.0
1.7
1.7
6.8
1.5
6.2
1.9
7.3
1.7
0 min. Post-exercise
5.2
1.9
6.6
1.8
10 min. Post-exercise
5.0
1.9
6.4
2.1
5.4
Grand M 6.2
The dependent measure of this study was perceived exertion, and the hypothesis that intervals of shorter duration would be perceived as less effortful was supported. Analyses of exertional responses taken before, during, and after trials of cycle exercise revealed several significant findings (Table 2; Fig. 1). First, analysis of RPE before and after exercise indicated a significant main effect for Trial (but the main effect for Time and the interaction between Time and Trial were not significant). Follow-up comparisons indicated significant differences between trials before and after exercise whereby the 60-sec. trial was considered more effortful than the 30-sec. trial (ES = 0.5–0.7), but no differences within trials were observed. Second, analysis of RPE during exercise indicated significant main effects for Trial and Time (but the interaction of Time and Trial was not significant). Follow-up comparisons indicated significant differences between trials whereby the 60sec. trial was considered more effortful than the 30-sec. trial at the midpoint of the sessions through completion (ES = 0.4–0.6). Likewise, both trials demonstrated a significant increase in exertion from the first to final assessment point (ES = 1.2 for 30-sec. trial and 1.5 for 60-sec. trial). Finally, analysis of the grand means for in-task RPE indicated that the 60-sec. trial was perceived as significantly more effortful than the 30-sec. trial (F1, 19 = 14.86, p < .01, ES = 0.5). DISCUSSION This experiment investigated the exertional responses before, during, and after high-intensity interval cycle ergometry exercise. The research design required participants to perform 20 min. of exercise that included 16 min. of interval exercise alternating between 90% and 10% segments of peak work capacity and also included graded warm-up and cool-down of 2 min. each. As such, the trials were similar to the so-called “practical model” linked to recent high-intensity interval training research (Little, et al., 2011). One trial included alternating 30-sec. work and 30-sec. recovery
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TABLE 2 ANALYSES OF VARIANCE FOR RPE BY TRIAL AND RPE MEASUREMENT TIME, WITH PLANNED POST HOC COMPARISONS Source
df
MS
F
η2
Post Hoc
ANOVA: RPE Pre- and Posttest Trial (Tr) 1 Time (Ti) 2 Tr × Ti
2
44.41 35.02‡ 0.65 Between trials: 60-sec. greater at Pre*, Post-0†, & Post-10‡ .61 .63 0.03 Within 30-sec. trial: no significant differences Within 60-sec. trial: no significant differences .76 .75 0.04
ANOVA: RPE during exercise Trial (Tr) 1 Time (Ti) 3 Tr × Ti
3
24.02 14.86† 0.44 Between trials: 60-sec. greater at 50*, 75†, & 100‡ 37.74 37.40‡ 0.66 Within 30-sec. trial: 25 < 50, 75, 100‡; 50 < 100* Within 60-sec. trial: 25 < 50, 75, 100‡ 50 < 75†, 100‡; 75 < 100* .58 1.46 0.07
Note.—Trials are denoted as 30-sec. trial and 60-sec trial in the planned post hoc comparisons; Times are denoted as 25, 50, 75, and 100% trial completion in the planned post hoc comparisons. *p < .05. †p < .01. ‡p < .001.
intervals, and the other used 60-sec. work intervals with 60-sec. recovery intervals. The experimental manipulation produced two trials of exercise of the same length and total external work but different exercise intervals lengths, which led to different perceptions of effort. The present study suggested that equal total work does not necessarily produce equal perceptions of exertion when participating in high-intensity interval exercise sessions that differ on duration of work intervals. While intensity may be primary for exertion responses in continuous exercise, the duration of the work and recovery intervals may drive perceived exertion within interval-based exercise programs when intensity is held constant. This finding is novel, but existing research has demonstrated that interval and continuous sessions equated for total work can produce different post-exercise ratings of perceived exertion. Specifically, Green and colleagues compared moderately intense continuous exercise to interval exercise using repeated 60-sec. segments of work and recovery (Green, et al., 2007). Findings indicated that session ratings of perceived exertion were greater for the interval session compared to the continuous session. Such findings are in general agreement with the current research indicating that equal work distributed in different formats yield different perceptions of overall effort. Importantly, the exercise treatment in the work by Green and colleagues was quite similar to that of the current study with respect to the composition of the interval sessions; i.e., the work and recovery loads were matched. The important extension provided by the current study relates specifically to the comparison of different interval sessions equated for total work.
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The finding from the current study that participants anticipated the exertion to be higher for the 60-sec. interval condition is also noteworthy, since participants were informed that the sessions would be equivalent in terms of total work completed. This difference on exertion that was anticipated prior to exercise may be linked to the relative lack of experience in performing interval exercise, but may also represent a tendency toward an expectation that longer durations at a high intensity will be more difficult despite longer recovery periods and equal total work. Importantly, differences in RPEs between trials were observed throughout exercise and during recovery. One possibility is that the longer intervals produced physiological changes related to blood lactate and ventilation that were not elicited with shorter exposures to high intensity. Such a possibility is supported by research indicating that intervals 5 min. in length at 70% peak capacity produced greater lactate levels than intervals 30 sec. in length at 90% peak capacity (Alkahtani, King, Hills, & Byrne, 2013). This possibility is further supported by research indicating that RPE is highly correlated (r = 0.7) with blood lactate (Scherr, Wolfarth, Christle, Pressler, Wagenpfeil, & Halle, 2013). Therefore, it is plausible that the differences in perceived exertion noted during the current study were related to the discomfort linked to increased lactate production or related ventilation demands associated with the trial comprised of longer intervals. However, strong correlation between RPE and physiological variables does not indicate a causal effect, and further research is required to better understand the mechanistic basis for the perceptual factors that conjoin the physiological experience to the exertion response. Finally, it is also possible that intervals of different durations create different mindsets with respect to motivation, self-efficacy, arousal, etc., that could affect appraisals of effort, but these possibilities have not yet been evaluated within the context of interval exercise. Limitations and Conclusion A primary limitation of the current study is the sample, which was relatively young and somewhat fit. Thus, generalization of the present findings to populations targeted by physical activity initiatives, namely older, overweight, and sedentary individuals, is not possible. However, this study was designed to investigate exertional responses to intervalbased exercise and is therefore an appropriate initial study with potential applications in fitness and clinical settings. Additionally, the current design did not include the measurement of pulmonary gas exchange in the measurement of aerobic fitness and instead predicted fitness based on peak work capacity on a cycle ergometer. Despite the use of a validated prediction equation (American College of Sports Medicine, 2010) and test-
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FIG. 1. Mean exertion responses to interval exercise with standard errors. *Significant difference between 30-sec. and 60-sec. trials (p < .01).
ing data suggesting maximal effort, estimation equations leave open the possibility that prediction values do not accurately reflect participants' fitness. Likewise, blood lactate was not measured, which prevents more definitive explanations of the observed differences. One measurement consideration is that pre-exercise assessment of perceived exertion may have primed all subsequent measurements. A final limitation noted here is that the exercise modality was limited to the cycle ergometer in a laboratory environment, although this modality is consistent with the vast majority of high-intensity interval training research that has been conducted to date. Future research on perceived exertion within the context of highintensity intervals is warranted and might consider the effects of participants' preference for wide-ranging types of interval sessions and how exercisers might go about preparing themselves mentally to perform them. In summary, the current study aimed to assess the effect of interval length on perceived exertion within two 20-min. sessions of high-intensity interval training that were equal on total work, but different on work interval duration. Findings from this experiment indicated that anticipated, momentary, and session ratings of perceived exertion were higher for the 60-sec. interval session than for the 30-sec. interval session, despite equivalent total external work. This project endeavored to provide a provisional answer to a common question raised regarding the perceptual tolerability of interval-based exercise training. Findings clearly indicate that, when controlled for total external work, a session with shorter intervals was perceived as less effortful than a session with longer intervals, which may reflect differential effects on underlying metabolism being driven by
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duration of exposure. Future research should consider utilization of similar designs that incorporate work intervals that are longer or shorter than those utilized in the present study in an effort to better understand the unique influence of duration as an important part of the exercise stimulus. While this study did not investigate the basis for the different perceptual responses or effects on future exercise behavior, it remains plausible that exercise sessions perceived as less effortful may be more desirable and thus may be more likely to facilitate long-term behavioral maintenance. REFERENCES
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HASKELL, W. L., LEE, I. M., PATE, R. R., POWELL, K. E., BLAIR, S. N., FRANKLIN, B. A., MACERA, C. A., HEATH, G. W., THOMPSON, P. D., & BAUMAN, A. (2007) Physical activity and public health: updated recommendation for adults from the American College of Sports Medicine and the American Heart Association. Medicine & Science in Sports & Exercise, 39, 1423-1434. HOOD, M. S., LITTLE, J. P., TARNOPOLSKY, M. A., MYSLIK, F., & GIBALA, M. J. (2011) Lowvolume interval training improves muscle oxidative capacity in sedentary adults. Medicine & Science in Sports & Exercise, 39, 1849-1856. KANE, I., ROBERTSON, R. J., FERTMAN, C. I., MCCONNAHA, W. R., NAGLE, E. F., RABIN, B. S., & RUBINSTEIN, E. N. (2010) Predicted and actual exercise discomfort in middle school children. Medicine & Science in Sports & Exercise, 42, 1013-1021. KILPATRICK, M. W., BORTZFIELD, A. L., & GIBLIN, L. M. (2012) Impact of exercise trials with varied intensity patterns on perceptions of effort: an evaluation of predicted, intask, and session exertion. Journal of Sports Sciences, 30, 825-832. KILPATRICK, M. W., ROBERTSON, R. J., POWERS, J. M., MEARS, J. L., & FERRER, N. F. (2009) Comparisons of RPE before, during, and after self-regulated aerobic exercise. Medicine & Science in Sports & Exercise, 41, 681-686. LITTLE, J. P., GILLEN, J. B., PERCIVAL, M. E., SAFDAR, A., TARNOPOLSKY, M. A., PUNTHAKEE, Z., JUNG, M. E., & GIBALA, M. J. (2011) Low-volume high-intensity interval training reduces hyperglycemia and increases muscle mitochondrial capacity in patients with type 2 diabetes. Journal of Applied Physiology, 111, 1554-1560. LITTLE, J. P., SAFDAR, A., WILKIN, G. P., TARNOPOLSKY, M. A., & GIBALA, M. J. (2010) A practical model of low-volume high-intensity interval training induces mitochondrial biogenesis in human skeletal muscle: potential mechanisms. Journal of Physiology, 588, 1011-1022. LJUNGGREN, G., & JOHANSSON, S. E. (1988) Use of submaximal measures of perceived exertion during bicycle ergometery exercise as predictor of maximal work capacity. Journal of Sports Sciences, 6, 189-203. MAHON, A. D., GAY, J. A., & STOLEN, K. Q. (1998) Differentiated ratings of perceived exertion at ventilatory threshold in adults and children. European Journal of Applied Physiology and Occupational Physiology, 78, 115-120. NOBLE, B. G., BORG, G., CECI, R., JACOBS, I., & KAISER, P. (1983) A category-ratio perceived exertion scale: relationship to blood and muscle lactates and heart rate. Medicine & Science in Sports & Exercise, 15, 523-528. ROBERTSON, R. J., & NOBLE, B. J. (1997) Perception of physical exertion: methods, mediators, and applications. Exercise & Sports Sciences Review, 25, 407-452. SCHERR, J., WOLFARTH, B., CHRISTLE, J. W., PRESSLER, A., WAGENPFEIL, S., & HALLE, M. (2013) Associations between Borg's rating of perceived exertion and physiological measures of exercise intensity. European Journal of Applied Physiology, 113, 147-155. SWEET, T. W., FOSTER, C., MCGUIGAN, M. R., & BRICE, G. (2004) Quantitation of resistance training using the session rating of perceived exertion method. Journal of Strength and Conditioning Research, 18, 796-802. WORLD HEALTH ORGANIZATION. (2012) Assessing the capacity for the prevention and control of noncommunicable diseases: report of the 2010 global survey. Geneva, Switzerland: World Health Organization Press. Accepted April 2, 2014.
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EXERTIONAL RESPONSES TO SPRINT INTERVAL TRAINING: A COMPARISON OF 30-SEC. AND 60-SEC. CONDITIONS1 MARCUS W. KILPATRICK AND SAMUEL J. GREELEY School of Physical Education and Exercise Science, University of South Florida Summary.—The purpose of this study was to assess the effect of sprint interval training on rating of perceived exertion. 20 healthy participants (11 men, 9 women; M age = 23 yr.) completed a maximal cycle ergometer test and two high-intensity interval training cycling sessions. Each session utilized the same work-to-rest ratio (1:1), work intensity (90% max), recovery intensity (10% work intensity), and session duration (16 min.). Trials differed on duration of the interval segment, with a 30-sec. trial and a 60-sec. trial. Sessions required the same amount of total work over the duration of the trial. Rating of perceived exertion assessed before, during, and after exercise were higher for the 60-sec. trial than the 30-sec. trial despite no difference in total work. High intensity interval training trials utilizing the same total external work but differing in interval length produced different ratings of perceived exertion. Perceived exertion is significantly higher for sessions of exercise that utilize longer work intervals. These findings suggest that shorter intervals may produce more favorable exertional responses that could positively affect future behavior.
Physical activity guidelines recommend the accumulation of 450–750 MET·min or more of activity per week via multiple combinations of intensities and durations that encourage a lifestyle approach to exercise and primary utilization of moderate intensities (Haskell, Lee, Pate, Powell, Blair, Franklin, et al., 2007). Current epidemiological evidence indicates that most individuals fall short of these stated minimum recommendations (World Health Organization, 2012). High-intensity interval training provides an alternative approach to physical activity programming and may be of great value to public health. This approach to exercise represents a form of activity that is notably different from lower-intensity lifestyle physical activity. However, while prolonged continuous intensities ranging from very low to moderate have been studied extensively, investigations of interval training from a public health perspective have only recently emerged. While many variations of interval training have been developed in recent years, the prescription of interval training receiving most of the attention within research literature is based around high-intensity, low-volume protocols, often referred to as high-intensity interval training (Gibala, 2007). Early work with these protocols utilized up to six 30-sec. supramaximal sprints interspersed with 4 min. of unloaded pedaling on a cycle erAddress correspondence to Marcus Kilpatrick, School of Physical Education and Exercise Science, University of South Florida, 4202 East Fowler Avenue, PED 214, Tampa, FL 33620 or e-mail ([email protected]). 1
DOI 10.2466/06.15.PR0.114k27w8
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gometer (Gibala, Little, van Essen, Wilkin, Burgomaster, Safdar, et al., 2006; Burgomaster, Howarth, Phillips, Rakobowchuk, MacDonald, McGee, et al., 2008). These protocols included three total minutes of exercise stimulus and a total session time of 20–30 min. inclusive of warm-up and cooldown. (Gibala, et al., 2006; Burgomaster, et al., 2008). This type of protocol was compared to continuous endurance training sessions over the course of two weeks on a variety of physiological variables. Despite dramatically lower training volume compared to an endurance training group, research participants performing high intensity interval training showed similar improvements in muscle oxidative capacity, muscle buffering capacity, glucose and lipid metabolism, and endothelial function (Gibala, et al., 2006; Burgomaster, et al., 2008; Little, Gillen, Percival, Safdar, Tarnopolsky, Punthakee, et al., 2011). Due to questions of both acute tolerability and concerns related to long-term adherence (Coyle, 2005), modified protocols utilizing lower intensities and longer exercise intervals were developed (Little, Safdar, Wilkin, Tarnopolsky, & Gibala, 2010). The format of this more contemporary approach to interval training involves 10 near-maximal sprints lasting 1 min. and a very low intensity recovery of the same length, adding up to a 20-min. exercise session. This protocol is proposed to be more practical for implementation and perhaps less intimidating as a physical activity prescription for individuals who undertake exercise for health and fitness (Little, et al., 2011). Recent studies have shown the efficacy of these modified protocols in improving various health markers in sedentary adults (Hood, Little, Tarnopolsky, Myslik, & Gibala, 2011). This research suggests that high-intensity interval training may be an excellent option for promotion of fitness and performance within a variety of populations, including sedentary individuals and type 2 diabetics. However, research investigating perceptual responses to this type of exercise is lacking and it is not yet clear how such training fits into existing physical activity guidelines. One highly relevant perceptual response that is worthy of consideration within the context of high-intensity interval training is perceived exertion, which relates to the sensations and perceptions associated with physical exercise. Though differing viewpoints exist, it is generally understood that perceived effort during an exercise task is determined and regulated by numerous physiological and psychological variables including but not limited to parameters such as metabolism, ventilation, blood flow, muscular fatigue, motivation, mood state, and exercise experience (Robertson & Noble, 1997). Research to date involving perceived exertion is extensive but primarily limited to assessment of momentary perceptions of effort. The earliest research focused on graded exercise testing (Borg, 1970) and was later utilized to monitor and prescribe intensity of effort during training sessions (Foster, 1998). Contemporary guidelines for exer-
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cise typically include a recommendation to exercise at a moderate intensity, with some of these guidelines specifying a particular exertional rating range that are linked to established metabolic and cardiovascular loads and thresholds (DeMello, Cureton, Boineau, & Singh, 1987; Mahon, Gay, & Stolen, 1998). One aspect of the rationale to prescribe exercise using ratings of perceived exertion is to create an exercise load that is metabolically significant in terms of caloric expenditure and physiologically demanding in terms of the cardiovascular response, but tolerable to the individual exerciser. Such a recommendation represents the need to encourage participation in exercise that might foster a positive experience and behavioral maintenance. This type of approach points toward the possibility that consideration of perceived exertion before, during, and after exercise may prove valuable in efforts to both understand and positively impact exercise behavior. Despite a relatively limited body of literature, research on post-exercise exertion has grown in recent years. This ‘session RPE’ was originally used to monitor the training loads of athletes to maximize performance and manage overtraining risk (Foster, 1998). More contemporary research considering the viability of session RPE in conjunction with recreational resistance and aerobic exercise suggests that this measure has potential value in the development of a greater understanding of the overall exercise experience (Sweet, Foster, McGuigan, & Brice, 2004; Egan, Winchester, Foster, & McGuigan, 2006; Green, Yang, Laurent, Davis, Kerr, Pritchett, et al., 2007; Kilpatrick, Robertson, Powers, Mears, & Ferrer, 2009; Kilpatrick, Bortzfield, & Giblin, 2012). Collectively, this research suggests that perceptions of effort obtained post-exercise may be an important consideration in exercise behavior. In contrast to a notable body of literature investigating post-exercise exertion responses, relatively little research has been conducted on the utility of predicted exertion and how that links to momentary exertion assessed during exercise. Existing research indicates that anticipated effort is not well-matched to the exertion measured during exercise, i.e., momentary RPE. Specifically, prediction errors differ by sex in younger populations whereby boys tend to underestimate momentary effort and girls tend to overestimate momentary effort (Kane, Robertson, Fertman, McConnaha, Nagle, Rabin, et al., 2010). In contrast, two studies in adults suggest that predicted exertion tends to overestimate impending exertion, especially the earlier and middle portions of an aerobic exercise session (Kilpatrick, et al., 2009; Kilpatrick, et al., 2012). This limited body of research suggests that exertion that is anticipated or recalled after exercise may be an important consideration. Given the trend for greater participation in training programs that are intervalbased, it is important to note that only one study has compared the exer-
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tional responses to continuous and discontinuous exercise (Green, et al., 2007). This study compared continuous cycle exercise at a light-to-moderate intensity against 1-min. intervals alternating between high intensity resistance and unloaded pedaling. Results indicated that the interval session was perceived as more effortful during the post-exercise period (ES range ∼ 0.6–1.0) despite both trials being equal on total work. This finding suggests that interval based-exercise may act on exertion in a way that is fundamentally different than continuous exercise. It is possible that intervals in general, and intervals of varied length in particular, exert differential influence on physiological and metabolic strain in a manner that affects perceptions of effort. The present study examined two protocols of low-volume, high-intensity interval training identical in intensity, total work, work-to-rest ratio, and duration, but utilizing differing interval lengths. Therefore, the purpose of the current study was to assess the acute effect of interval length on perceived exertion. Hypothesis. Interval sessions composed of shorter interval segments will be perceived as less effortful despite constant total work across trials. METHOD Participants and Screening Participants were 20 students (11 men, 9 women; M age = 23.4 yr., SD = 3.6) at a large metropolitan university in the southeastern United States. The sample size for this pilot research project is a reflection of existing research (Green, et al., 2007) and is based on an anticipated large effect size (i.e., ES ∼ 0.8), a power level of 0.80, and an alpha criterion of .05. Participants were primarily recruited by announcements in classes. Each participant completed an informed consent document and a health history questionnaire. All participants were screened by a physician's assistant affiliated with the research team for orthopedic, cardiovascular, or pulmonary conditions that would preclude participation. Participants were given instructions to avoid alcohol, caffeine, and tobacco for 3 hr. prior to testing, consistent with American College of Sports Medicine guidelines (2010). Participants provided informed consent in accordance with institutional guidelines prior to the first exercise session. All documents and procedures were approved by the university Institutional Review Board. Research Design A single graded exercise test was conducted prior to two counterbalanced trials of interval exercise on a stationary cycle ergometer. The maximal exercise test was used to measure peak aerobic power. The interval trials were each 20 min. in length. Both trials were matched for duration,
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total work, average intensity, and work-to-rest ratio. The major difference between trials was interval duration; one trial consisted of 60-sec. intervals and the other comprised 30-sec. intervals, to investigate the effect of interval length on perception of effort. A progressive, multi-stage protocol was performed on an electronically braked cycle ergometer (Lode, Netherlands) designed to allow the rider to vary pedal cadence without affecting workload. The primary function of the protocol was to measure peak workload capacity. The exercise protocol began at 50 watts and increased 10 watts every 20 sec. The test was terminated when the participant could not maintain a cycle cadence of 30 rpm. Heart rate and blood pressure were monitored before, during, and after the exercise test. Heart rate was measured using a heart rate monitor (Polar, USA) and blood pressure was determined by auscultation. Upon completion of the test, participants completed 3–5 min. of active recovery. Finally, participants were provided with a brief reminder and description of the two upcoming experimental trials with respect to duration and intensity. The two experimental trials were designed to create low-volume, highintensity interval training sessions. The two sessions differed only in regard to interval length (60 sec. vs 30 sec.). The intensity utilized for all exercise intervals was 90% of the peak power achieved on their initial maximal exercise test. Recovery intervals were set at 10% of the work intensity. Prior to baseline measures for each trial, participants were told which trial they would be performing. Total exercise time was 20 min. inclusive of 2 min. of warm-up, 16 min. of intervals, and 2 min. of cool-down. All workload changes were made by a member of the research staff. All trials were counterbalanced to avoid order effects. Measures The variable of interest for this study was overall or whole body perceived exertion (RPE), which was assessed before, during, and after each trial and input into a tablet computer. In each assessment, perceived exertion was assessed using the Borg Category-Ratio (CR-10) Scale (Borg, 1998). This scale utilizes a single question that includes a response range from 0 to 10. The verbal anchor for 0 is “nothing at all” and the verbal anchor for 10 is “very, very hard (maximal),” with 3 representing “moderate” and 5 representing “hard.” Standardized instructions for utilization of the CR-10 scale were employed (Borg, 1998). Reliability and validity of the CR-10 scale have been extensively demonstrated and reported elsewhere (i.e., Noble, Borg, Ceci, Jacobs, & Kaiser, 1983; Ljunggren & Johansson, 1988; Borg, 1998; Capodaglio, 2001). Perceived exertion was recorded during each phase of the exercise trial: (i) immediately prior to exercise, (ii) every 4-min. period during ex-
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ercise, (iii) immediately after exercise cool-down, and (iv) 10 min. after the exercise session. The pre-exercise assessment was, “How much exertion do you anticipate experiencing during this trial of exercise?” The intask assessment was, “How much exertion are you feeling right now?” Finally, the post-exercise assessment was, “How much exertion did you actually experience during this trial of exercise?” Efforts to make certain participants understood the RPE instructions included a description and review of the assessment procedures on the day of medical screening and the maximal exercise test. Statistical Analysis Analyses of the data proceeded in two phases. The first phase included descriptive analysis of sample weight status and aerobic fitness. The second phase utilized three repeated-measures analyses of variance (ANOVA) including: a 2 (Trial: 30-sec., 60-sec.) × 3 (Time: Pre, Post-0, Post10) ANOVA to consider exertion before and after exercise, a 2 (Trial: 30sec., 60-sec.) × 4 (Time: 25%, 50%, 75%, 100%) ANOVA to consider exertion during exercise, and a one-way ANOVA comparing the grand mean of the four in-task interval RPEs for the 30-sec. and 60-sec. trials. This latter analysis allowed comparisons using a measure of exertion averaged across the duration of the exercise. Significant findings were followed by planned contrasts. Because these comparisons increased the risk for Type I error, the p value for post hoc analyses of means was adjusted to a more conservative .01. Mean difference and standard deviation values were used to estimate Cohen's d, which is reported as the effect size (ES) value (Cohen, 1992). Tests of normality and homogeneity of variance provided assurances that parametric analyses were appropriate. RESULTS Weight status as measured by body mass index (BMI) (M = 24.2 kg·m−2, SD = 3.2) suggested that most participants were normal weight (BMI = 18– 24.9; n = 12), while some were overweight (BMI = 25–29.9; n = 7) or mildly obese (BMI = 30–34.9; n = 1)(ACSM, 2010). Fitness level as measured by the graded exercise test suggested participants were moderately fit (maximal workload capacity, M = 264 W, SD = 58; estimated aerobic capacity based on peak power output, M = 45.4 mL·kg−1·min−1, SD = 6.8) and generally within the 60th and 80th percentile ranks (ACSM, 2010). Data collected during the maximal test for HR (M = 177 beats·min−1, SD = 10) and RPE (M = 9.5, SD = 0.7) suggested maximal effort was achieved. Furthermore, results indicated that 85% of participants (n = 17) reached the criterion for maximal effort (RPE > 9) and 95% of participants (n = 19) reached the criterion for maximal heart rate (HR > 85% age-predicted maximum). Descriptive statistics for the RPE measurements by trial are shown in Table 1.
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M. W. KILPATRICK & S. J. GREELEY TABLE 1 DESCRIPTIVE STATISTICS FOR MEASURED VARIABLES Measurement Time
30-sec. Trial M
SD
Pre-exercise
5.4
1.7
25%
4.2
1.5
50%
5.4
75%
6.0
100%
60-sec. Trial
Grand M
M
SD
6.4
2.2
4.8
1.6
1.6
6.0
1.7
1.7
6.8
1.5
6.2
1.9
7.3
1.7
0 min. Post-exercise
5.2
1.9
6.6
1.8
10 min. Post-exercise
5.0
1.9
6.4
2.1
5.4
Grand M 6.2
The dependent measure of this study was perceived exertion, and the hypothesis that intervals of shorter duration would be perceived as less effortful was supported. Analyses of exertional responses taken before, during, and after trials of cycle exercise revealed several significant findings (Table 2; Fig. 1). First, analysis of RPE before and after exercise indicated a significant main effect for Trial (but the main effect for Time and the interaction between Time and Trial were not significant). Follow-up comparisons indicated significant differences between trials before and after exercise whereby the 60-sec. trial was considered more effortful than the 30-sec. trial (ES = 0.5–0.7), but no differences within trials were observed. Second, analysis of RPE during exercise indicated significant main effects for Trial and Time (but the interaction of Time and Trial was not significant). Follow-up comparisons indicated significant differences between trials whereby the 60sec. trial was considered more effortful than the 30-sec. trial at the midpoint of the sessions through completion (ES = 0.4–0.6). Likewise, both trials demonstrated a significant increase in exertion from the first to final assessment point (ES = 1.2 for 30-sec. trial and 1.5 for 60-sec. trial). Finally, analysis of the grand means for in-task RPE indicated that the 60-sec. trial was perceived as significantly more effortful than the 30-sec. trial (F1, 19 = 14.86, p < .01, ES = 0.5). DISCUSSION This experiment investigated the exertional responses before, during, and after high-intensity interval cycle ergometry exercise. The research design required participants to perform 20 min. of exercise that included 16 min. of interval exercise alternating between 90% and 10% segments of peak work capacity and also included graded warm-up and cool-down of 2 min. each. As such, the trials were similar to the so-called “practical model” linked to recent high-intensity interval training research (Little, et al., 2011). One trial included alternating 30-sec. work and 30-sec. recovery
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TABLE 2 ANALYSES OF VARIANCE FOR RPE BY TRIAL AND RPE MEASUREMENT TIME, WITH PLANNED POST HOC COMPARISONS Source
df
MS
F
η2
Post Hoc
ANOVA: RPE Pre- and Posttest Trial (Tr) 1 Time (Ti) 2 Tr × Ti
2
44.41 35.02‡ 0.65 Between trials: 60-sec. greater at Pre*, Post-0†, & Post-10‡ .61 .63 0.03 Within 30-sec. trial: no significant differences Within 60-sec. trial: no significant differences .76 .75 0.04
ANOVA: RPE during exercise Trial (Tr) 1 Time (Ti) 3 Tr × Ti
3
24.02 14.86† 0.44 Between trials: 60-sec. greater at 50*, 75†, & 100‡ 37.74 37.40‡ 0.66 Within 30-sec. trial: 25 < 50, 75, 100‡; 50 < 100* Within 60-sec. trial: 25 < 50, 75, 100‡ 50 < 75†, 100‡; 75 < 100* .58 1.46 0.07
Note.—Trials are denoted as 30-sec. trial and 60-sec trial in the planned post hoc comparisons; Times are denoted as 25, 50, 75, and 100% trial completion in the planned post hoc comparisons. *p < .05. †p < .01. ‡p < .001.
intervals, and the other used 60-sec. work intervals with 60-sec. recovery intervals. The experimental manipulation produced two trials of exercise of the same length and total external work but different exercise intervals lengths, which led to different perceptions of effort. The present study suggested that equal total work does not necessarily produce equal perceptions of exertion when participating in high-intensity interval exercise sessions that differ on duration of work intervals. While intensity may be primary for exertion responses in continuous exercise, the duration of the work and recovery intervals may drive perceived exertion within interval-based exercise programs when intensity is held constant. This finding is novel, but existing research has demonstrated that interval and continuous sessions equated for total work can produce different post-exercise ratings of perceived exertion. Specifically, Green and colleagues compared moderately intense continuous exercise to interval exercise using repeated 60-sec. segments of work and recovery (Green, et al., 2007). Findings indicated that session ratings of perceived exertion were greater for the interval session compared to the continuous session. Such findings are in general agreement with the current research indicating that equal work distributed in different formats yield different perceptions of overall effort. Importantly, the exercise treatment in the work by Green and colleagues was quite similar to that of the current study with respect to the composition of the interval sessions; i.e., the work and recovery loads were matched. The important extension provided by the current study relates specifically to the comparison of different interval sessions equated for total work.
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The finding from the current study that participants anticipated the exertion to be higher for the 60-sec. interval condition is also noteworthy, since participants were informed that the sessions would be equivalent in terms of total work completed. This difference on exertion that was anticipated prior to exercise may be linked to the relative lack of experience in performing interval exercise, but may also represent a tendency toward an expectation that longer durations at a high intensity will be more difficult despite longer recovery periods and equal total work. Importantly, differences in RPEs between trials were observed throughout exercise and during recovery. One possibility is that the longer intervals produced physiological changes related to blood lactate and ventilation that were not elicited with shorter exposures to high intensity. Such a possibility is supported by research indicating that intervals 5 min. in length at 70% peak capacity produced greater lactate levels than intervals 30 sec. in length at 90% peak capacity (Alkahtani, King, Hills, & Byrne, 2013). This possibility is further supported by research indicating that RPE is highly correlated (r = 0.7) with blood lactate (Scherr, Wolfarth, Christle, Pressler, Wagenpfeil, & Halle, 2013). Therefore, it is plausible that the differences in perceived exertion noted during the current study were related to the discomfort linked to increased lactate production or related ventilation demands associated with the trial comprised of longer intervals. However, strong correlation between RPE and physiological variables does not indicate a causal effect, and further research is required to better understand the mechanistic basis for the perceptual factors that conjoin the physiological experience to the exertion response. Finally, it is also possible that intervals of different durations create different mindsets with respect to motivation, self-efficacy, arousal, etc., that could affect appraisals of effort, but these possibilities have not yet been evaluated within the context of interval exercise. Limitations and Conclusion A primary limitation of the current study is the sample, which was relatively young and somewhat fit. Thus, generalization of the present findings to populations targeted by physical activity initiatives, namely older, overweight, and sedentary individuals, is not possible. However, this study was designed to investigate exertional responses to intervalbased exercise and is therefore an appropriate initial study with potential applications in fitness and clinical settings. Additionally, the current design did not include the measurement of pulmonary gas exchange in the measurement of aerobic fitness and instead predicted fitness based on peak work capacity on a cycle ergometer. Despite the use of a validated prediction equation (American College of Sports Medicine, 2010) and test-
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FIG. 1. Mean exertion responses to interval exercise with standard errors. *Significant difference between 30-sec. and 60-sec. trials (p < .01).
ing data suggesting maximal effort, estimation equations leave open the possibility that prediction values do not accurately reflect participants' fitness. Likewise, blood lactate was not measured, which prevents more definitive explanations of the observed differences. One measurement consideration is that pre-exercise assessment of perceived exertion may have primed all subsequent measurements. A final limitation noted here is that the exercise modality was limited to the cycle ergometer in a laboratory environment, although this modality is consistent with the vast majority of high-intensity interval training research that has been conducted to date. Future research on perceived exertion within the context of highintensity intervals is warranted and might consider the effects of participants' preference for wide-ranging types of interval sessions and how exercisers might go about preparing themselves mentally to perform them. In summary, the current study aimed to assess the effect of interval length on perceived exertion within two 20-min. sessions of high-intensity interval training that were equal on total work, but different on work interval duration. Findings from this experiment indicated that anticipated, momentary, and session ratings of perceived exertion were higher for the 60-sec. interval session than for the 30-sec. interval session, despite equivalent total external work. This project endeavored to provide a provisional answer to a common question raised regarding the perceptual tolerability of interval-based exercise training. Findings clearly indicate that, when controlled for total external work, a session with shorter intervals was perceived as less effortful than a session with longer intervals, which may reflect differential effects on underlying metabolism being driven by
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duration of exposure. Future research should consider utilization of similar designs that incorporate work intervals that are longer or shorter than those utilized in the present study in an effort to better understand the unique influence of duration as an important part of the exercise stimulus. While this study did not investigate the basis for the different perceptual responses or effects on future exercise behavior, it remains plausible that exercise sessions perceived as less effortful may be more desirable and thus may be more likely to facilitate long-term behavioral maintenance. REFERENCES
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