Impact of High-Intensity Interval Duration on Perceived Exertion MARCUS W. KILPATRICK1, NIC M ARTINEZ1, JONATHAN P. LITTLE2, MARY E. JUNG2, ANDREW M. JONES3, NICK W. PRICE4, and DANIEL H. LENDE5 School o f Physical Education and Exercise Science, University o f South Florida, Tampa, FL; 2School o f Health and Exercise Sciences, University o f British Columbia, Kelowna, BC, CANADA; 3School o f Sport and Health Sciences, University o f Exeter, Exeter, UNITED KINGDOM; 4Bayfront Medical Center, St. Petersburg, FL; and 3Department o f Anthropology, University o f South Florida, Tampa, FL ABSTRACT KILPATRICK, M. W., N. MARTINEZ, J. P. LITTLE, M. E. JUNG, A. M. JONES, N. W, PRICE, and D. H. LENDE. Impact of HighIntensity Interval Duration on Perceived Exertion. Med. Sci. Sports Exerc., Vol. 47, No. 5, pp. 1038-1045, 2015. Purpose: RPE is increasingly being considered as a viable tool beyond its original use for monitoring in-task exercise intensity. Research indicates that anticipated, in-task, and postexercise RPE values are often notably different from one another. An important new consideration is how perceptions are impacted by high-intensity interval training (HIT). This study aims to compare RPE responses before, during, and after continuous and HIT exercise trials. Methods: Twenty (11 females and 9 males; mean ± SD age, 22 + 4 yr) overweight (mean ± SD body mass index, 29 + 3 kg m -2) and unfit (mean ± SD V 0 2pcak, 28 + 5 mL-kg-min-1) participants completed a 20-min heavy continuous (HC) trial and three 24-min severe-intensity interval trials that utilized 1:1 work-to-recovery ratios: 30 s (Severe Interval-30), 60 s (Severe Interval-60), and 120 s (Severe Interval-120). Exertion was assessed using the Borg CR10 Scale. Data were analyzed using repeatedmeasures ANOVA and pairwise comparisons. Results: Anticipated exertion was highest in the Severe Interval-120 trial (5.8 ± 2.0; P < 0.05) compared with other trials. Exertion increased from beginning to end in all trials (P < 0.05), with the greatest increases observed within the HC trial. Session RPE for the Severe Interval-120 trial (6.4 ± 2.3) was higher than those for all other trials (P < 0.05), and session RPE for the Severe Interval-30 trial (3.7 ± 1.8) was lower than that for the HC trial (4.9 ± 1.6; P < 0.05). Conclusions: These findings suggest that 30-s HIT protocols limit the perceptual drift that occurs during exercise, in comparison to HC exercise. Moreover, performing more intervals of shorter durations appears to produce lower postexercise RPE values than performing fewer intervals of longer duration and equal intensity. Because effort perception may influence behavior, these results could have implications for the prescription of interval training in overweight sedentary adults. Key Words: EXERTION, INTERVAL, EXERCISE, PERCEPTION

Given the increased attention that HIT has received in recent years, the need for full consideration o f this form o f training is warranted. The modem approach to HIT-style exercise is traced to the training o f track athletes in the middle o f the 20th century (32). HIT takes many forms, but all versions incorporate re­ peated bursts o f highly vigorous exercise interspersed with periods o f recovery. Popular HIT protocols are often charac­ terized as low-volume with respect to total work. A promi­ nent type o f HIT protocol developed more than a decade ago utilizes repeated Wingate-style exercise tests, resulting in a series of supramaximal sprints with a relatively low work-to-rest ratio (14). Repeated Wingate HIT is highly effective in pro­ moting physiological adaptations (13), but concerns related to the tolerability o f this fonnat in general and clinical populations have been raised (9). As such, research has extended the original low-volume HIT protocol to a more practical model utilizing 10 intervals o f 60 s conducted at near-maximal aerobic capacity interspersed with 60-s recov­ eries at very low intensities (23). Research investigating this version o f HIT suggests that these protocols are both feasible and beneficial in healthy young males (24), inactive overweight adults (17), and inactive patients with type 2 diabetes (23). This body o f evidence is encouraging and suggests that HIT exercise

he links between a physically active lifestyle and reduced risk o f chronic diseases are well-established and make clear that regular exercise positively im­ pacts markers o f cardiometabolic health (8,22,34,39). These links are the basis for global physical activity recommen­ dations suggesting participation in 150 min o f moderate activity or 75 min o f vigorous activity per week, with bouts at least 10 min in duration (43). These recommendations are based on a considerable volume o f epidemiological and experimental evidence from continuous aerobic exercise and set the tone for public health messaging. It is notable, how­ ever, that contemporary guidelines for physical activity do not directly consider the potential utility o f intermittent ex­ ercise sessions such as high-intensity interval training (HIT).

APPLIED SCIENCES

T

Address for correspondence: Marcus W. Kilpatrick, Ph.D., School of Physical Education and Exercise Science, University of South Florida, 4202 East Fowler Avenue, PED 214, Tampa, FL 33620; E-mail: [email protected]. Submitted for publication March 2014. Accepted for publication August 2014. 0195-9131/15/4705-1038/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2014 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000000495

1038

PERCEIVED EXERTION

RPE values than 24-s interval sessions when equated for total work and work-to-rest ratio (35). Likewise, exertional responses tend to increase over time during interval exercise trials regardless of interval intensity or rest duration despite interval segments remaining unchanged throughout the ses­ sion (21,35,38,44). These findings are in agreement with research on continuous aerobic exercise indicating that an­ ticipated length of exercise moderates RPE (36) and with related findings that experienced runners modulate attention, often using dissociative techniques, to manage exertion over longer runs (3). However, only a single study has focused on interval training protocols similar to contemporary and practical HIT (16). This study compared moderate continu­ ous exercise and near-maximal 1-min intervals where trials were 16 min long and matched for total work. Findings generally suggested that intense intervals are perceived as more effortful near the end of the sessions and during the postexercise period than continuous trials of equal total work. Despite many important findings from the aforemen­ tioned studies, absent within each of the designs is consid­ eration of how individuals perceive interval sessions of different lengths across the full spectrum of measurement opportunities. Evaluation of the anticipated difficulty of impending exercise and how this prediction compares against in-task and postexercise reflections on the exercise experience could prove instructive because anticipated and actual expe­ rience of HIT exercise may be very different. Such a possi­ bility is supported by research utilizing protocols that are more continuous in nature (19,20), which show differences in per­ ceived exertion over exercise trials. Other exercise research has found that cognitive factors, such as attentional strategies, enjoyment, pain, and anticipated running distance, can shape perceived exertion (3,4,25). Specific research examining how length of exercise impacts perceived exertion, including po­ tential mismatches between anticipated, actual, and reflective exertion, may help better understand potential barriers to physical activity among recreational exercisers and those with very limited exercise experience. Given the effectiveness of interval training in producing physiological benefits and the need to learn more about the likelihood that such forms of training lead to differential engagement in exercise, research investigating the percep­ tual aspects of interval exercise is needed. A plausible hy­ pothesis based on theory and a limited amount of published research is that RPE will differ within and across trials of interval and continuous exercise. Therefore, the purpose of the present study is to investigate exertional responses before, during, and after exercise between one trial of heavyintensity continuous exercise and three trials of severeintensity interval exercise with varied interval durations in a mixed-gender sample of inactive, overweight, unfit adults. M ETHO D Participants and research design. Participants were 20 adults (11 females and 9 males; mean ± SD age, 22 + 4 yr;

Medicine & Science in Sports & Exercise®

1039

APPLIED SCIENCES

is effective in boosting health and fitness and may be equally effective as continuous moderate exercise despite having a lower total work volume. Importantly, research to date has not adequately considered the perceptual aspects of such training. One perceptual variable that requires greater evaluation within the context of HIT is perceived exertion. RPE was pioneered by Borg (6), who defined RPE as the degree of heaviness and strain experienced in physical work. RPE is correlated with a variety of psychophysiological variables. Physiological variables that relate to perceptions of effort include metabolic rate, ventilation, blood flow, and muscular fatigue (6,37). Select psychological considerations that are linked to exertion include motivation, mood state, arousal, mental stress, pacing, and exercise experience (6,37,40). Importantly, RPE is considered the outcome of complex and dynamic interactions among various stimuli and inputs—perhaps best considered as a gestalt of many physiological and psychological sensations, rather than a simple summation of varied parts (6). Related conceptions of perceived exertion argue that physiological responses serve as initial mediators of intensity, which the sensory cortex then interprets as perceptual signals of exertion (37). This cogni­ tive processing is modulated by past experiences, current context, and psychological characteristics. Primary and original uses of RPE include measuring effort sense during graded exercise testing, as well as regulating and prescribing exercise intensity (6,37). More recent uses con­ sider the utility of RPE assessment in the preexercise period before the onset of known exercise (19,20) and as a way of reflecting on the entire exercise session postexercise (10,11). These kinds of assessments have not yet been fully evaluated but do provide an opportunity to consider how perceptions of forthcoming and just completed effort are contextualized and linked to in-task experiences. Moreover, these assessments allow for a broader evaluation of the entire exercise experi­ ence, comparing actual in-task effort against preexercise and postexercise perceptions. Existing research in the exercise domain suggests that anticipated duration of an exercise ses­ sion impacts perceived effort independent of exercise inten­ sity (3), suggesting that effort sense is a dynamic construct that is likely to be sensitive to cognitions outside the exer­ cise session itself. Support for this possibility is provided by research on pain expectations and pain experience, which demonstrates that expectations about painful stimuli can in­ fluence behavior (2). As such, the prospects of important in­ terrelationships along the exercise experience time course are plausible, and potential linkages to future exercise behavior require well-designed experimental studies. Although research has considered exertional responses to intense interval exercise sessions, studies examining intervals similar to those utilized within contemporary HIT are largely absent. Existing research on intervals of varied lengths and work-to-rest ratios provides equivocal findings related to ex­ ertional responses but do suggest that RPE responses vary by the characteristics of the interval training session. Specifically, research indicates that 6-s interval sessions produce lower

APPLIED SCIENCES

mean ± SD body mass index, 29 + 3 kg-m-2) at a large university in the southeastern United States. All partici­ pants were overweight or mildly obese (body mass index, 25-35 kg m -2) and not regularly active (1) but otherwise healthy. The sample size is a reflection of related research (15,19,20) and is based on an anticipated medium to large effect size (ES; i.e., 0.5-0.8), a power level of 0.8, and an a criterion of 0.05. Participants completed six trials, each separated by at least 48 h, and trials were completed over a period of 2^1 wk. The first trial was a protocol to measure peak oxygen uptake (V02peak)- The second trial was used to familiarize participants to the forthcoming experimental trials. The remaining experimental trials included one con­ tinuous session at heavy intensity and three interval sessions of various interval lengths performed at severe intensities. All procedures were approved by the university institutional review board. Screening. The first visit to the laboratory included com­ pletion of a written informed consent form, completion of a health history questionnaire, and measurement of height, body mass, resting HR, and resting blood pressure. Partici­ pants were medically screened to determine the presence of contraindications to exercise, with a specific focus on ortho­ pedic, cardiovascular, and pulmonary conditions that would preclude participation in the research study. Participants were also instructed to avoid alcohol, caffeine, and tobacco for 3 h before testing (1). M etabolic testing. A progressive ramp protocol was performed on an electronically braked cycle ergometer that maintains external workload by dynamically adjusting resis­ tance to accommodate cadence changes (Lode, Groningen, The Netherlands). The protocol ramp rate varied between 15 and 25 W-mftU1 and was based on a standardized formula (42). The test was tenninated when the participant could not maintain a pedal cadence of 30 rpm. HR, blood pressure, RPE, and respiratory gases were monitored in accordance with standard exercise testing guidelines (1). HR was mea­ sured using an HR monitor (Polar, Lake Success, NY), and blood pressure was determined by auscultation. RPE was estimated each minute using the Borg CR10 Scale (5). Ex­ pired gases were collected through an air cushion mask and analyzed continuously using a calibrated metabolic cart (Vacumetrics, Ventura, CA). VC^peak was identified as the largest volume of oxygen consumed per minute during the test. Criteria for verifying maximal exertion were as follows: a peak HR of at least 90% of age-predicted maximal HR, a peak RPE of at least 9 (on a 0-10 scale), and a peak RER of at least 1.15 (30). Ventilatory threshold (VT) was identified through visual inspection of ventilatory equivalents for oxy­ gen and carbon dioxide. Fam iliarization. The second visit to the laboratory fo­ cused on familiarizing the research participants to laboratory procedures and experimental sessions. The objective of the familiarization trial was threefold: 1) to ensure that the workloads generated from the maximal test for the experi­ mental trials were reasonable and appropriate; 2) to provide

1040

Official Journal of the American College of Sports Medicine

the participants an opportunity to experience the intensities of the forthcoming interval and continuous exercise sessions so as to ensure that the experimental trials were not entirely unfamiliar; and 3) to provide the participants an opportunity to become familiar with all experimental data collection procedures. Procedure familiarization included the use of an HR monitor and a tablet computer, which was utilized for data collection at baseline and postexercise. Experim ental exercise trials. The remaining visits to the laboratory included the completion of all experimental trials. The experimental trial design yielded cycle ergometer exercise sessions that fit the contemporary descriptions of exercise intensity, suggesting the presence of three inten­ sity domains: moderate, heavy, and severe (12). Moderate intensity considers intensities up to the lactate threshold, heavy intensity spans between VT and critical power, and severe intensity considers intensities above critical power, whereby critical power is estimated to be the midpoint be­ tween VT and maximal capacity (41). Each participant completed one continuous trial within the heavy domain and three interval trials within the severe domain. Continuous and interval trials differed on total duration but were equal in terms of total external work. The continuous trial was 20 min in duration and was conducted at 10% of the distance between VT and maximal capacity (heavy continuous [HC]). The interval trials were 24 min in duration. The work portion of the three interval sessions was conducted at 60% of the distance between VT and maximal capacity, and the recovery portion was conducted at 10%-20% of maximal capacity, based on calculations designed to ensure that total work was equal for all trials. Each interval utilized a 1:1 work-to-recovery ratio and varied only in interval segment duration: 30, 60, and 120 s. The design yielded three different intervals within the severe-intensity domain: Severe Interval-30 (SI-30), Severe Interval-60 (SI-60), and Severe Interval-120 (SI-120). SI-30 included twenty-four 30-s intervals, SI-60 in­ cluded twelve 60-s intervals, and SI-120 included six 120-s intervals. All work intervals were followed by recovery in­ tervals of the same duration. All trials included a 2-min warm-up and cool-down standardized relative to peak capacity. Procedures. RPE was assessed before, during, and after exercise using the single-item Borg CR10 Scale ranging from 0 to 10 (0, “nothing at all”; 3, “moderate”; 5, “hard”; 10, “very, very hard (maximal)”) (6). Instructions and anchoring procedures related to RPE assessment were standardized and based on established methodologies (37). RPE was assessed before, during, and after exercise. Preexercise RPE was taken immediately after the participant was provided a description of the upcoming trial. In-task RPE was assessed 12 times during the trials and occurred during the last 10 s of the work and recovery intervals approximating 1/6, 1/3, 1/2, 2/3, 5/6, and 6/6 of trial completion. Postexercise RPE was assessed immediately following cool-down and again 10 min later. All RPE assessments taken outside of the exercise trial were performed while the participant was seated comfortably on a reclinable chair in a partitioned area adjacent to the exercise

http://www.acsm-msse.org

of participants (n = 19) reached the criterion for perceived effort (RPE > 9), 90% of participants (n = 18) reached the criterion for maximal HR (HR > 90% of age-predicted maximum), and 100% of participants (« = 20) reached the criterion for maximal RER (RER > 1.15). Analysis of work performed during experimental trials indicates that the HC trial was performed at 50% + 4% of peak workload, whereas work and recovery segments of the Severe Interval trials were performed at 78% ± 2% and 5% + 4% of peak work­ load, respectively. Further analysis of work within the trials revealed that all trials were not different from each other in terms of estimated total caloric cost (approximately 165 kcal; P > 0.10) and external work (approximately 691 kJ; P > 0.10). HR responses. Analysis of HR revealed significant increases during exercise for all trials (P < 0.001). Approx­ imated rate increases were as follows; 140-150 bpm for the SI-30 trial, 145-170 bpm for the SI-60 and HC trials, and 155-180 bpm for the SI-120 trial. Recovery HR increased throughout all interval trials (P < 0.001), with increases of approximately 15 bpm for all trials. The grand mean HR created from all in-task and recovery HR values provided an estimate of average cardiovascular work but did not consider total cardiovascular demand because continuous and interval trials were of different total durations. These analyses re­ vealed significant differences and a trend toward increased HR response, with increased interval length and the greatest HR response arising from the continuous trial: HC was significantly higher than all Severe Interval trials (P < 0.05), SI-30 was significantly lower than SI-60 (P < 0.05) and SI120 (P < 0.05), but SI-60 was not different from SI-120 (P < 0.10). HR responses are reported in Figure 1. Perceived exertion responses. A two-way ANOVA revealed a significant effect for time (P < 0.001) and trial (P < 0.001) and an interaction between time and trial (P < 0.001). Follow-up one-way ANOVA and planned con­ trasts revealed several differences within and between trials. Predicted RPE was significantly higher in the SI-120 trial (RPE ~ 6) than in the HC and SI-60 trials (RPE ~ 5; P < 0.05;

190 * _- — ”

_ — -J t

fc

.......... - * ................ — , /

-Q qj

' ^ ------------- -

-----

________ _

I :: :: :

m

A



150

........ .................... *

e

RESULTS

-----------

^

I

S



5/6

6/6

GrandMean

ro at X

Descriptive data. Participants had a mean ± SD VCHpeak of 28 ± 5 mL kg-min” 1, a mean ± SD peak work­ load of 199 ± 42 W, and a mean ± SD VT at 44% ± 5% of peak workload. Testing data revealed the participants had a mean ± SD maximal test HR of 188 ± 10 bpm and a mean + SD maximal RPE of 9.8 + 0.5, suggesting that maximal effort was achieved. Furthermore, results indicate that 95%

PERCEIVED EXERTION

m "

1/6

^

'

2/6

- •-S I-3 0 W ork -•-S I-3 0 Recovery - — HC

3/6

4/6

- • -si-60 W o rk -■ -S I-60 Recovery

-♦-S I-1 2 0 W ork -♦-S I-1 2 0 Recovery

FIGURE 1—HR responses. ^Different from beginning to end of trial ( P< 0.001).

Medicine & Science in Sports & Exercise®

1041

APPLIED SCIENCES

equipment and were entered by the participant into a tab­ let computer. All RPE assessments taken during the exer­ cise trial were conducted by asking the participants to report their perceptions. Preexercise assessment asked “How much exertion do you anticipate experiencing during this trial of exercise?” and served as the prediction or anticipated RPE. In-task assess­ ment asked “How much exertion are you feeling right now?” and served as the momentary RPE. Finally, postexercise as­ sessment asked “How much exertion did you actually expe­ rience during this trial of exercise?” and served as the session or reflective RPE. Efforts to ensure that participants under­ stood the RPE instructions included a description and review of the assessment procedures on the day of medical screening and maximal exercise testing. Finally, HR was assessed using an HR monitor and was recorded during collection of RPE data. As such, HR served as the objective measure of exer­ cise intensity. Workload changes were controlled by software linked to the testing system. Interactions between research staff and participants were limited to required data collection, and members of the research staff remained largely out of view of the participants during trials. The order of experi­ mental trials was counterbalanced. S tatistical analysis. Initial analyses provided descrip­ tive characteristics of the sample and exercise trials. Next, HR responses to the experimental trials was analyzed using a series of one-way ANOVA assessing HR change over time during the exercise trials, with change representing the dif­ ference between initial and final HR values. Likewise, a separate one-way ANOVA allowed for the comparison of average (or grand mean) HR values between trials so as to evaluate the average cardiovascular response for the exper­ imental trials. Finally, RPE responses were analyzed using a two-way ANOVA: 4 (HC, SI-30, SI-60, SI-120) x 9 (Pre, 1/6, 1/3, 1/2, 2/3, 5/6, 6/6 completion, Post-0, Post-10). A separate one-way ANOVA assessed changes in RPE during exercise and between sessions. Significant differences were followed by planned contrasts. Planned contrasts within tri­ als compared change over time from the first to the final measurement point for work and recovery segments. Crite­ rion for significance was set at P < 0.05. Significant differ­ ences are reported to the lowest appropriate criterion level. Mean differences were utilized to determine ES differences (0.2, small effect; 0.5, medium effect; 0.8, large effect) (7). All analyses were performed using SPSS 22 (IBM SPSS, Chicago, IL).

manipulation produced trials of exercise that fit within established exercise intensity domains and were similar to existing research protocols that are known to be highly ef­ fective in producing cardiometabolic benefits (17,23,24). The observation within the present study that perceived exertion drifted upward over time during all trials is con­ sistent with research designs employing HC exercise and intervals near peak work capacity (4,11,16,21). The specific details of perceptual drift are not fully understood, but two possibilities have been posited. It is possible that effort sense may be the response to sensory receptors in the pe­ riphery that detect workloads (26). Alternatively, percep­ tions of effort may be generated centrally by forwarding neural signals from motor areas to sensory areas of the ce­ rebral cortex (27). Both possibilities remain plausible, but considerably more evidence supports a central origin for effort sense and suggests that exertion is significantly the conscious awareness of central motor command to working muscle (27). Importantly, the current findings indicate that perceptual drift is a function of interval segment length, in agreement with established findings within continuous ex­ ercise indicating that exertion rises during constant load work over time (18). Perceived exertion increased by ap­ proximately 1.5 units for the 30-s trial and by 2 units for the 60-s trial. In contrast, the HC and 120-s trials produced increases of about 3 units despite being equated for total work. These findings are insightful given recent research suggesting that exercise tolerance is significantly limited by perception of effort (29). As such, it seems that shorter in­ tervals provide an exposure to highly beneficial exercise intensities while limiting perceptions of effort that are asso­ ciated with muscle fatigue, which may inhibit performance and participation (28). The factors that create differential interpretation of per­ ceived exertion with varied interval lengths are not fully understood. Some portion of the upward drift in perceived

ES ~ 0.5) and was nearly significantly higher than the SI-30 trial (RPE ~ 5; P < 0.10; ES ~ 0.5). Predicted RPE for all other trial comparisons were similar (P > 0.10). In-task RPE increased over time in all trials for the work phase (P < 0.001; ES range, 0.9-1.6) and the recovery phase (P < 0.05; ES ~ 0.4), except over time during recovery within the SI-120 trial (P > 0.10). The magnitude of change from the beginning to the end of the trials was approximately 1.5 units for the SI-30 and SI-60 trials (ES ~ 1.0), 2.5 units for the SI-120 trial (ES = 1.4), and 3 units for the HC trial (ES = 2.4). Session RPE measured at both postexercise time points was significantly higher in the SI-120 trial (RPE ~ 6.5) than in all other trials (RPE = 4-5; P < 0.05; ES range, 0.7-1.4), and the HC trial was significantly higher than the SI-30 trial (.P < 0.05; ES ~ 0.7). Predicted and session RPE values were also compared and revealed no differences for HC and SI-60 trials (P > 0.10) but indicated that session RPE was significantly lower than predicted RPE for the SI-30 trial (P < 0.05; ES ~ 0.6) and predicted RPE was significantly lower than session RPE for the SI-120 trial (P < 0.05; ES = 0.5) immediately postexercise, but not 10 min postexercise (P > 0.05). Perceived exertion re­ sponses are reported in Table 1 and Figure 2.

DISCUSSION The present investigation examined perceived exertion before, during, and after continuous and high-intensity in­ terval exercise matched for total work among a group of inactive overweight adults. The continuous trial was 20 min in duration and was perfonned at 50% of peak work capacity and slightly above VT. The three intervals were 24 min in duration and were performed at about 80% of peak work capacity and slightly above critical power utilizing a 1:1 work-to-rest ratio and interval lengths of 30, 60, and 120 s. All trials were matched for total work. The experimental TABLE 1. Perceived exertion responses.

S I- 3 0

APPLIED SCIENCES

HC

Preexercise In-task 1/6 complete 1/3 complete 1/2 complete 2/3 complete 5/6 complete 6/6 complete Recovery 1/6 complete 1/3 complete 1/2 complete 2/3 complete 5/6 complete 6/6 complete Postexercise Post-0 Post-10

S I- 6 0

S I- 1 2 0

5.8 ± 2.0

5.0 ± 2.0a

4.9 ± 1 .5 “

4.9 ± 1 .7 “

2.9 3.5 4.1 4.8 5.5 5.9

2.9 3.7 3.8 4.3 4.4 4.5

±1 .3 ±1 .3 ± 1 .6 ± 1 .6 ±1 .9 ± 1 .8 *

3.3 3.9 4.5 4.8 5.2 5.1

±1 .6 ± 1 .7 ±1 .5 ± 1 .7 ±1 .8 ± 2.2*

4.1 4.9 5.6 5.9 6.7 6.8

± 1.9 ± 1 .9 ±2 .0 ± 1 .9 ± 2 .0 ± 2.0*

2.3 2.5 2.9 3.2 3.3 3.0

±1 .5 ±1 .7 ±1.8 ±2.1 ±2 .3 ± 2.3*

2.0 2.2 2.7 3.0 3.1 2.8

±1 .0 ±1 .2 ± 1 .6 ± 2 .0 ±2.1 ± 2 .0 *

2.3 2.4 2.5 2.6 2.9 3.0

± 1.1 ±1.5 ±1.5 ±1.8 ± 2.0 ± 2 .5 *

± 0.8 ± 1 .0 ± 1 .0 ± 1 .0 ± 1 .4 ± 1 .7 *

5.1 ± 1 .4 “ 4.9 ± 1 .6 “

3.9 ± 1 .8 “'* * 3.7 ± 1 .8 “'* *

4.6 ± 1 .9 “ 4.5 ± 1 .9 “

6.7 ± 2 .2 * 6.4 ± 2.3

Data are presented as mean + SD RPE. All notations indicate statistically significant differences at P < 0.05. “Significantly different from SI-120 at preexercise. “Significantly different from beginning to end of trial. “Significantly different from preexercise. “Significantly different from HC.

1042

Official Journal of the American College of Sports Medicine

http://www.acsm-msse.org

SI-30

SI-60

W o rk • • • ■ * • R e c o v e ry

T im e P o in t



Wo r k

•••« ••

R e c o v e ry

T im e P o in t

SI-120

HC



W o rk R e c o v e ry

T im e P o in t

FIGURE 2—Perceived exertion responses. *Different from beginning to end of trial (P < 0.001 for work; P < 0.05 for recovery). #Different from preexercise (P < 0.05).

PERCEIVED EXERTION

important in dealing with the discomfort associated with ex­ ertion (31). A potential influence on discomfort is that the anticipation of a long sustained interval effort can be per­ ceived as more aversive. Research on dread has shown that some individuals prefer receiving painful stimuli sooner rather than waiting in anticipation of a less painful stimuli (5). This limited research therefore points to the possibility that “getting it out of the way” might be an important cognitive advantage of shorter HIT approaches. That is, the opportunity to more quickly work through potentially noxious stimuli, such as muscular fatigue and dyspnea, may be preferable to the anticipation of a much longer exposure to the same stimulus. Second, inserting periods of recovery in between high-intensity interval segments allows for a break from both the physical and the cognitive demands associated with intense exercise. Limiting cognitive demands in this manner may be important given research demonstrating that mental stress increases perceptions of effort and reduces endurance performance (27,33). Likewise, these frequent breaks from severe intensities might contribute to lesser increases in perceived effort during shorter intervals. As such, “catching one’s breath” might make it cognitively easier to maintain interval work over time. The present data provide some support for both mecha­ nisms as contributing to exertional differences. RPE in the 30-s trial starts near a 3 rating and rises to about 4.5, for an overall increase of 1.5 units. In contrast, RPE in the 120-s trial starts near 4 and ends near 7, with an overall increase of almost 3 units. As such, the average first rating is lower in shorter intervals than in longer intervals, pointing to

Medicine & Science in Sports & Exercise®

1043

APPLIED SCIENCES

exertion may be attributable to physiological factors such as blood lactate accumulation and pH disturbance (35,37). However, an equal or greater consideration in exertional drift is likely linked to factors other than physiological changes. The present study observed an increase in per­ ceived exertion for the longest interval that was approxi­ mately double the increase in the shorter interval despite no difference in total work. Prior research suggesting the importance of exercise experience, cognition, and task struc­ ture in the formation of perceived exertion (37) may also be instructive for interval-based exercise among inactive over­ weight participants who were not accustomed to dealing with long intervals of exercise. Although only speculative, it is possible that prior fitness and sport training experiences could impact the cognitive approach to managing the significant exertion that comes with intense exercise. Given findings in active and fit populations that attentional strategies are used to manage exertion over time (3) and that such strategies can be taught to physically active individuals (25), future research should address how attentional focus can be used to attenuate perceived exertion in less active populations. It is also clear from the current findings that shorter in­ tervals are perceived as requiring less effort. The HIT ap­ proach is perceived as requiring less exertion than continuous exercise, except in the longest HIT intervals of 2 min. This effect was observed even though the overall workload was the same, and this finding could be explained through at least two mechanisms. First, prior research indicates that recovery from effortful work is more than just recovery from peripheral muscle fatigue and suggests that cognitive mechanisms are as

anticipatory effects. Separately, the much smaller increase in RPE for the shorter interval highlights that frequent breaks seem to blunt the upward drift of RPE over time, which connects with the idea that shorter intervals provide more opportunities for recovery between periods of extreme work. It is possible that the shorter intervals were perceived as less effortful in part because shorter bursts may have at­ tenuated the disruption of physiological homeostasis. Future research should aim to delineate the respective contributions of these two different mechanisms by which length of in­ tervals can affect ratings of perceived effort. Likewise, the demonstration of intervals no greater than 60 s points to the need for future research to determine how cardiometabolic benefits may differ between HIT protocols of differing in­ terval lengths. Limitations to the current study design include consider­ ations related to the sample, laboratory-based research de­ sign, and absence of measures that link to future exercise behavior. Concerns related to the sample focus primarily on age, but a larger sample would have also allowed for consideration of potential moderators such as gender and weight status. Although the sample targeted a desirable de­ mographic of unfit and overweight adults, the participants were young and mostly college students, limiting generalizability. Likewise, the design allowed for careful laboratory manipulation of important research variables; however, the resulting exercise sessions lack significant ecological validity but are representative of common approaches to exercise. Another laboratory-related limitation is that the findings do not include metabolic markers of exercise in­ tensity or potentially valuable measures of fatigue and af­ fect. A final significant limitation relates to the absence of design features that would provide insights into the likelihood that the trials would be repeated for exercise intention and self-efficacy, both of which are important within the context of maintenance of exercise behavior. Collectively, these limitations suggest that future research in this area should consider these issues when develop­ ing research designs. Likewise, research investigating the implications for differential exertion before, during, and

after exercise would be useful in designing effective exer­ cise interventions. In summary, the current design evaluates how a sample of unfit and overweight adults perceive high-intensity in­ terval exercise and continuous exercise matched for total work. Findings from this study indicate that anticipated ex­ ertion, exertion experienced during exercise, and reflections on just completed effort are not always well-matched. Al­ though perceptions of effort for 60-s intervals in the latter minutes of the session tended to match that which was an­ ticipated and experienced upon recovery, intervals of only 30 s tend to be less effortful than anticipated, and intervals of 120 s are considerably harder than expected. Although each session required the same amount of total work, these results point toward the possibility that intervals of similar relative intensity not exceeding 60 s may be preferable if the goal of exercise is to produce significant cardiometa­ bolic stress while minimizing exposure to perceptions of exertion being less effortful, especially in individuals not accustomed to the physical sensations associated with in­ tense exercise. The novel assessments of anticipated and reflective exertion utilized in the current study provide in­ sights on comfort level associated with the exercise experi­ ence. Fitness professionals and clinicians are often required to make modifications to exercise prescriptions based on physiological responses to exercise, and the current find­ ings indicate that perceptual aspects of the exercise experi­ ence may need to be considered alongside more objective indicators of actual workout intensity. These results, al­ though preliminary, suggest that shorter HIT intervals are more palatable than longer intervals for novice exer­ cisers, and that maintenance of relatively modest levels of perceived exertion throughout the entire exercise ex­ perience may be advisable to encourage sustained exer­ cise behavior. This research project was funded in part through a grant program sponsored by USF World at the University of South Florida. There is no conflict of interest in any aspect of this study. Results of the present study do not constitute endorsement by the American College of Sports Medicine.

APPLIED SCIENCES

REFERENCES 1. American College of Sports Medicine. ACSMS's Guidelines fo r Exercise Testing and Prescription. Philadelphia (PA): Lippincott Williams & Wilkins; 2013. pp. 19-59. 2. Atlas LY, Wager TD. How expectations shape pain. Neurosci Lett. 2012;520:140-8. 3. Baden DA, Warwick-Evans L, Lakomy J. Am I nearly there? The effect of anticipated running distance on perceived exertion and attentional focus. J Sport Exerc Psychol. 2004;26:215-31. 4. Bartlett JD, Close GL, MacLaren DP, Gregson W, Drust B, Morton JP. High-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise: implications for ex­ ercise adherence. J Sports Sci. 2011 ;29:547—53. 5. Bems GS, Chappelow J, Cekic M, Zink CF, Pagnoni G, MartinSkurski ME. Neurobiological substrates o f dread. Science. 2006; 312:754-8.

1044

Official Journal of the American College of Sports Medicine

6. Borg G. Borg’s Perceived Exertion and Pain Scales. Champaign (IL): Human Kinetics; 1998. 7. Cohen J. A power primer. Psychol Bull. 1992; 112:155-9. 8. Comelissen VA, Fagard RH. Effects of dynamic aerobic en­ durance training on blood pressure, blood pressure regulation mechanisms and cardiovascular risk factors. Hypertension. 2005;46:667-75. 9. Coyle EF. Very intense exercise-training is extremely potent and time efficient: a reminder. J Appl Physiol. 2005;98:1983-4. 10. Foster C. Monitoring training in athletes with reference to over­ training syndrome. Med Sci Sports Exerc. 1998;30(7): 1164-8. 11. Foster C, Florhaug JA, Franklin J, et al. A new approach to mon­ itoring exercise testing. J Strength Cond Res. 2001;15:109-15. 12. Gaesser GA, Poole DC. The slow component of oxygen uptake kinetics in humans. Exerc Sport Sci Rev. 1996;24:35-71.

http://www.acsm-msse.org

13. Gibala MJ, Little JP, Macdonald MJ, Hawley JA. Physiological adaptations to low-volume, high-intensity interval training in health and disease. J Physiol. 2012;590:1077-84. 14. Gibala MJ, Little JP, van Essen M, et al. Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance. J Physiol. 2006;575:901-11. 15. Green JM, Pritchett RC, Crews TR, Tucker DC, McLester JR, Wickwire PJ. RPE drift during cycling in 18 degrees C vs 30 degrees C wet bulb globe temperature. J Sports Med Phys Fitness. 2007; 47:18-24. 16. Green JM, Yang Z, Laurent CM, et al. Session RPE following interval and constant-resistance cycling in hot and cool environ­ ments. Med Sci Sports Exerc. 2007;39(11):2051—7. 17. Hood MS, Little JP, Tamopolsky MA, Myslik F, Gibala MJ. Lowvolume interval training improves muscle oxidative capacity in sedentary adults. Med Sci Sports Exerc. 2011;43( 10): 1849-56. 18. Jones NL, Kilian KJ. Exercise limitations in health and disease. N Engl J Med. 2000;343:632-41. 19. Kilpatrick MW, Bortzfield AL, Giblin LM. Impact of exercise trials with varied intensity patterns on perceptions of effort: an evaluation of predicted, in-task, and session exertion. J Sports Sci. 2012;30:825-32. 20. Kilpatrick MW, Robertson RJ, Powers JM, Mears JL, Ferrer NF. Comparisons of RPE before, during, and after self-regulated aer­ obic exercise. Med Sci Sports Exerc. 2009;41(3):682-7. 21. Laurent CM, Vervaecke LS, Kutz MR, Green JM. Sex-specific responses to self-paced, high-intensity interval training with vari­ able recovery periods. J Strength Cond Res. 2014;28:920-7. 22. Leon AS, Sanchez OA. Response o f blood lipids to exercise training alone or combined with dietary intervention. Med Sci Sports Exerc. 2001 ;33(6 Suppl):S502-15. 23. Little JP, Gillen JB, Percival ME, et al. Low-volume high-intensity interval training reduces hyperglycemia and increases muscle mi­ tochondrial capacity in patients with type 2 diabetes. J Appl Phys. 2011;111:1554-60. 24. Little JP, Safdar A, Wilkin GP, Tamopolsky MA, Gibala MJ. A practical model of low-volume high-intensity interval training in­ duces mitochondrial biogenesis in human skeletal muscle: poten­ tial mechanisms. J Physiol. 2010;588:1011-22. 25. Lohse KR, Sherwood DE. Defining the focus of attention: effects of attention on perceived exertion and fatigue. Front Psychol. 2011;2:332. 26. Luu BL, Day BL, Cole JD, Fitzpatrick RC. The fiisimotor and reafferent origin of the sense of force and weight. J Physiol. 2011;589:3135^17. 27. Marcora SM. Perception of effort during exercise is independent of afferent feedback from skeletal muscles, heart, and lungs. J Appl Physiol. 2009;106:2060-2.

28. Marcora SM, Bosio A, de Moree HM. Locomotor muscle fatigue increases cardiorespiratory responses and reduces performance during intense cycling exercise independently from metabolic stress. Am J Physiol. 2008;294:R874-83. 29. Marcora SM, Staiano W. The limit to exercise tolerance in humans: mind over muscle? Eur J Appl Physiol. 2010;109:763-70. 30. Maud PJ, Foster C. Physiological Assessment o f Human Fitness. Champaign (IL): Human Kinetics; 1995. pp. 9-18. 31. Neyroud D, Maffiuletti NA, Kayser B, Place N. Mechanisms of fatigue and task failure induced by sustained submaximal con­ tractions. Med Sci Sports Exerc. 2012;44(7): 1243—51. 32. Noakes T. Lore o f Running. Champaign (IL): Human Kinetics; 2003. pp. 261-361. 33. Pageaux B, Marcora SM, Lepers R. Prolonged mental exertion does not alter neuromuscular function o f the knee extensors. Med Sci Sports Exerc. 2013;45(12):2254—64. 34. Pattyn N, Comelissen VA, Eshghi SRT, Vanhees L. The effect of exercise on the cardiovascular risk factors constituting the meta­ bolic syndrome. Sports Med. 2013;43:121-33. 35. Price M, Moss P. The effects of work:rest duration on physiolog­ ical and perceptual responses during intermittent exercise and performance. J Sports Sci. 2007 ;25:1613-21. 36. Rejeski WJ, Ribisl PM. Expected duration and perceived effort: an attributional analysis. J Sport Psychol. 1980;2:227-36. 37. Robertson RJ, Noble BJ. Perception of physical exertion: methods, mediators, and applications. Exerc Sport Sci Rev. 1997; 25:407-52. 38. Seiler S, Sjursen JE. Effect of work duration on physiological and rating scale of perceived exertion responses during self-paced in­ terval training. Scand J Med Sci Sports. 2004;14:318-25. 39. Thomas D, Elliot EJ, Naughton GA. Exercise for type 2 diabetes mellitus. Cochrane Database Syst Rev. 2006;15:539-53. 40. Tucker R. The anticipatory regulation of performance: the physi­ ological basis for pacing strategies and the development of a perception-based model for exercise performance. Br J Sports Med. 2009;43:392-400. 41. Vanhatalo A, Jones AM, Burnley M. Application of critical power in sport. Int J Sports Physiol Perform. 2011 ;6:128—36. 42. Wasserman K, Hansen JE, Sue DY, Casaburi R, Whipp BJ. Principles o f Exercise Testing and Interpretation. Philadelphia (PA): Lippincott Williams & Wilkins; 1999. pp. 115^12. 43. World Health Organization. (2010). Global Recommendations on Physical Activity fo r Health. Geneva, Switzerland: World Health Organization Press, pp. 15-34. 44. Zuniga JM, Berg K, Noble J, Harder J, Chaffin ME, Hanumanthu VS. Physiological responses during interval training with different intensities and duration of exercise. J Strength Cond Res. 2011 ;25: 1279-84.

APPLIED SCIENCES

PERCEIVED EXERTION

Medicine & Science in Sports & Exercise®

1045

Copyright of Medicine & Science in Sports & Exercise is the property of Lippincott Williams & Wilkins and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.

Impact of high-intensity interval duration on perceived exertion.

RPE is increasingly being considered as a viable tool beyond its original use for monitoring in-task exercise intensity. Research indicates that antic...
6MB Sizes 0 Downloads 8 Views