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Exercise and Heart Rate Recovery

38

Journal of Exercise Physiologyonline (JEPonline)

Volume 11 February 1 June 2008

Fitness and Training Managing Editor

Tommy Boone, Ph.D. Editor-in-Chief Jon K. Linderman, Ph.D. Review Board Todd Astorino, Ph.D. Julien Baker, Ph.D. Tommy Boone, Ph.D. Lance Dalleck, Ph.D. Dan Drury, DPE. Hermann Engals, Ph.D. Eric Goulet, Ph.D. Robert Gotshall, Ph.D.

Melissa Knight-Maloney, Ph.D.

Len Kravitz, Ph.D. James Laskin, Ph.D. Derek Marks, Ph.D. Cristine Mermier, Ph.D. Daryl Parker, Ph.D. Robert Robergs, Ph.D. Brent Ruby, Ph.D. Jason Siegler, Ph.D. Greg Tardie, Ph.D. Chantal Vella, Ph.D. Lesley White, Ph.D. Ben Zhou, Ph.D. Official Research Journal

of The American Society of Exercise Physiologists

(ASEP)

ISSN 1097-9751

EFFECT OF EXERCISE PROTOCOL (“WARM-UP”) ON POST-EXERCISE HEART RATE RECOVERY GORDON PIERPONT1 1Minneapolis VA Medical Center and University of Minnesota/ Minneapolis, MN, USA

ABSTRACT Pierpont GL. Effect of Exercise Protocol (“warm-up”) on Post-Exercise Heart Rate Recovery JEPonline 2008 11(1):38-44. Delayed heart rate recovery (HRR) from exercise indicates poor prognosis, but effects of variations in exercise protocol on HRR are unclear. This study was therefore designed to test the hypothesis that HRR could vary depending on the protocol used to achieve maximum exercise. Sixteen healthy volunteers exercised to exhaustion using 2 widely disparate protocols. Protocol WmUp used a gradually progressive increase in treadmill speed and elevation, while protocol Sdn was a sudden onset high level exercise. WmUp resulted in a higher peak heart rate (178 + 6 bpm) than Sdn (170 + 6, p<.001). HRR at 1 min was not significantly different from the post WmUp (- 41+14 vs. - 44+15 bpm, p = .07). However, when converted to percent change from maximum (36+10 vs. 42+12%, p = .002), it was significant. By 2 minutes post-exercise there was no difference in HRR (absolute or percent change). These findings demonstrate that exercise protocol can affect maximum achievable heart rate, but the subsequent effect on HRR is too small to be clinically relevant. Key Words: Cardiovascular Physiology, Exercise Prescription, Intensity, Maximal Heart Rate

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INTRODUCTION

It is now fairly well established that a delay in heart rate recovery (HRR) following exercise predicts poor prognosis. This includes risk of sudden death as well as all cause mortality (1). The risk can be demonstrated in asymptomatic individuals (1-5), the elderly, (6) patients referred for exercise screening (7-10), and patients with known coronary artery disease (11-13), diabetes mellitus (14), congestive heart failure (15), and post myocardial infarction (16). It has also been used to predict subsequent ischemic heart disease (17), ischemic cardiac or cerebrovascular events (18), and can improve with physical conditioning (19, 20). However, there is no universally accepted exercise protocol for measuring HRR, and data on the effects of variations in exercise protocol on HRR are lacking. This study was therefore designed to determine if the delay in HRR is influenced by the type of protocol used in the exercise test. This was accomplished by comparing HRR following two very disparate exercise protocols; one of which included a warm-up period while the other did not. METHODS Subjects This study was approved by the Human Studies Subcommittee of the Minneapolis VA Medical Center, conforms to HIPPA guidelines, and to the guidelines of the Declaration of Helsinki. Informed consent was obtained from 16 healthy volunteers (5 women and 11 men) ranging in age from 21 to 59 yrs (mean = 39). Procedures All subjects exercised using two distinct protocols. The first included a warm-up (WmUp) by starting at a very low exercise level. The second protocol increased gradually until exhaustion. The treadmill started at 0% grade and 1 mph, and then increased each min by 1% grade and 0.3 mph alternating with 0.2 mph. (resulting in 1 mph increase every 4 min). The exercise was repeated at least 3 days later using a protocol requiring sudden high level exercise (Sdn). The treadmill was set at the maximum work level achieved on the first effort, and continued at that level to exhaustion. Because the WmUp protocol was always preformed first, training effects were minimized by limiting volunteers to those who exercised regularly and were experienced treadmill users. Subjects sat down immediately at the end of exercise (no “cool-down” period). The ECG was recorded continually. Heart rate (HR) was calculated from successive R-R intervals excluding premature beats. Maximum HR was taken as the average of 6 bts just prior to cessation of exercise. Heart rate during recovery was taken as the HR corresponding to the time in question (1 or 2 min post-exercise) averaged with the 2 previous and two subsequent beats. HRR was measured at both 1 and 2 min post-exercise as the decrease in HR from maximum, and the percent change in HR was calculated using the pre-exercise resting HR as baseline. Statistical Analyses Pre- and post-exercise values were compared using Student’s paired t-test. Results are presented as mean + SD, and differences considered statistically significant for p < 0.05. Assuming a difference of 5 bpm to be physiologically significant, then n = 16 would provide >95% power to detect that difference when estimating the standard deviation of the differences to be 5 bpm, with alpha p = 0.05.

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RESULTS Summary results of the responses to the 2 exercise protocols are presented in Figure 1. WmUp allowed subjects to reach a higher peak HR by an average of 9.0 + 5.4 bpm (range -1 to 21, p < 0.001). Thus, recovery began from a higher HR following WmUp than following Sdn. Individual post-exercise responses are presented in Figure 2 as the change in HR from maximum. HRR was slightly less complete at 1 min following WmUp. This difference was not statistically significant (p = 0.07). However, when the data are expressed as percent recovery at 1 min. The difference between the two protocols (WmUp and Sdn) becomes statistically significant (p = 0.002), but the magnitude of this difference remains small (6.0 + 6.5%). By 2 min post-exercise, there was no difference between the protocols in absolute level of HR recovery (0.6 + 10.8 bpm, p = 0.84) while the difference in percent

H ea

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(b pm

)

0

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maximum 1 min. post 2 min. post

Figure 1. Average heart rate response (+ SD) to the warm-up protocol (WmUp) compared to the sudden protocol (Sdn).

recovery was significance (5.4 + 10.0%; p = 0.05).

2 min. post exercise

WmUp Sdn

p = 0.84

1 min. post exercise

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at e

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Figure 2. Heart rate recovery (as change from maximum heart rate) for each individual at 1 and 2 min. post exercise, comparing the warm-up protocol (WmUp) to the sudden exercise protocol (Sdn).

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DISCUSSION Autonomic Control of Heart Rate During Exercise It is generally accepted that heart rate increases at the onset of exercise primarily due to parasympathetic withdrawal (21, 22). As exercise progresses to higher levels, sympathetic activation predominates, and is primarily responsible for attaining maximum heart rate at peak exercise (23, 24). Thus, autonomic dynamics with exercise can potentially differ significantly depending on how exercise is initiated, and how it progresses. Consequently, maximum exercise ability and subsequent recovery could be affected. Indeed, a “warm-up” period is considered preferable to sudden onset of major physical effort, and warm-up has become a routine part of competitive sports and recreational exercise programs (25, 26). During Recovery The relative role of parasympathetic reactivation versus sympathetic withdrawal in determining HR recovery following exercise is less well understood. While both are clearly involved, some authors suggest that parasympathetic reactivation dominates the early phases of exercise recovery (27, 28). Other authors (29) emphasize the role of sympathetic withdrawal. In either case, data are lacking on potential differences in the dynamics of HRR due to differences in the nature of the exercise effort. Most studies used symptom-limited maximal exercise, while submaximal levels have also been used (ref). Additionally, some studies (ref) include a “cool down” period of low exercise level at the end of the exercise effort while others go immediately to rest, and it is not always clear if the resting state is sitting or lying down. All of the studies referenced in the introduction that used treadmill exercise found delayed HRR to predict poor outcome. Effect of Exercise Protocol (Warm Up) It has long been held that a proper warm-up enhances physical performance and allows for maximal output (ref). This study did not attempt to measure or compare the amount of work achieved or maximum aerobic capacity. However, it is not surprising that the normal subjects were able to achieve a higher HR with the WmUp protocol than with the Sdn protocol. Since the subjects achieved a higher HR with WmUp, it is also not surprising that HRR took slightly longer with this protocol. Catecholamines were not measured post-exercise, but it is reasonable to speculate that a higher norepinephrine level was reached with WmUp. Thus, a longer time would be required to clear the norepinephrine, and HR would remain higher longer. This would be consistent with the fact that HR recovers faster from lower levels of exercise, where there is less activation of the sympathetic system compared to higher exercise levels. The 3 bpm difference in HRR between protocols at 1-min post-exercise was not statistical significance by a narrow margin (p = 0.07), but was statistically significant when analyzed as percent change. However, this small difference is not likely of physiologic significance, even if a beta error should occur in analysis of the absolute changes. This study estimated the standard deviation in the methods section at 5 bpm. With the 16 subjects included in the study, the standard deviation of the differences between the 2 protocols turned out to be 6.1 bpm. This would still provide 87% power to detect a significant difference of 5 bpm or greater. It is therefore unlikely that including more subjects in our sample would alter the conclusions. Limitations of the Study Neither of the protocols in this study is commonly used in most exercise laboratories. However, they were purposely chosen to represent the far ends of the spectrum of what could be done on a treadmill, with the supposition that if there is no difference between such disparate protocols, there would not likely be a difference among protocols with lesser disparity. As such, the results could be

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generalized to multiple protocols using treadmill exercise. It would be more suspect to generalize to other types of exercise, such as bicycle or arm ergometry. Including patients with heart failure or other conditions with known autonomic dysfunction was beyond the scope of this study. Since patients with heart failure have abnormal kinetics for changes in oxygen consumption during onset of exercise (31), it is possible that their altered exercise physiology or effects of medication use to treat their condition could make them more sensitive to changes in exercise protocol and subsequent HRR. This study did not address the question as to which measure of HRR to use. The HR curve post- exercise was modeled using exponential decay (27, 29), but this approach has been problematic (32). Again, this study used the recovery at 1 and 2 min post-exercise because of its simplicity and proven predictive utility (1, 2, 4, 6-16). CONCLUSIONS In summary, exercise protocol can influence maximum achievable HR and subsequent HRR. However, even with major differences in protocol used in this study, the magnitude of effect on HRR was relatively small. Hence, clinical inferences from HRR analysis should be valid using most protocols that allow subjects to reach their cardiovascular limits. For simplicity and reproducibility, the preferred way to obtain comparable results for HRR may well be to use symptom-limited maximum treadmill exercise (using the preferred protocol of the individual laboratory), followed by sitting down immediately post-exercise. ACKNOWLEDGEMENTS This work was supported by the United States Department of Veterans Affairs. Address for correspondence: Pierpont GL, MD, PhD, Department of Cardiology, Minneapolis VA Medical Center and University of Minnesota, 1 Veterans Drive, Minneapolis, MN, USA, 55113. Phone (612) 467-3662; FAX: (612) 970-5899; Email. [email protected]. REFERENCES 1. Jouven X, Empana JP, Schwartz PJ, Desnos M, Courbon D, Ducimetiere P. Heart-rate profile during exercise as a predictor of sudden death. N Engl J Med 2005;352(19):1951-1958. 2. Cole CR, Foody JM, Blackstone EH, Lauer MS. Heart rate recovery after submaximal exercise testing as a predictor of mortality in a cardiovascularly healthy cohort. Ann Intern Med 2000;132(7):552-555. 3. Mora S, Redberg RF, Cui Y, Whiteman MK, Flaws JA, Sharrett AR, Blumenthal RS. Ability of exercise testing to predict cardiovascular and all-cause death in asymptomatic women: a 20-year follow-up of the lipid research clinics prevalence study. JAMA 2003;290(12):1600-1607. 4. Aktas MK, Ozduran V, Pothier CE, Lang R, Lauer MS. Global risk scores and exercise testing for predicting all-cause mortality in a preventive medicine program. JAMA 2004;292(12):1462-1468. 5. Morshedi-Meibodi A, Larson MG, Levy D, O'Donnell CJ, Vasan RS. Heart rate recovery after treadmill exercise testing and risk of cardiovascular disease events (The Framingham Heart Study). Am J Cardiol 2002;90(8):848-852.

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