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Logo of bumcprocBaylor University Medical Center ProceedingsAbout the JournalBaylor Health Care SystemSubmit a Manuscript
 
Proc (Bayl Univ Med Cent). 2010 April; 23(2): 126–129.
PMCID: PMC2848087

Myocardial work during endurance training and resistance training: a daily comparison, from workout session 1 through completion of cardiac rehabilitation

Abstract

Patients in cardiac rehabilitation are typically advised to complete a period of supervised endurance training before beginning resistance training. In this study, however, we compared the peak rate-pressure product (RPP, a calculated indicator of myocardial work) of patients during two types of exercise—treadmill walking and chest press—from workout session 1 through completion of cardiac rehabilitation. Twenty-one patients (4 women and 17 men, aged 35 to 70 years) were enrolled in the study; they were referred for cardiac rehabilitation after myocardial infarction, percutaneous coronary intervention, or both. The participants did treadmill walking and chest press exercises during each workout session. Peak values for heart rate (HR) and systolic blood pressure (SBP) were recorded, and the peak RPP was calculated (peak HR [multiply sign in box] peak SBP). Paired t tests were used to compare the data collected during the two types of exercise across 19 workout sessions. The mean peak values for HR, SBP, and RPP were lower during resistance training than during endurance training; the differences were statistically significant (P < 0.05), with only one exception (the SBP for session 1). Across all 19 workout sessions, the participants performed more myocardial work, as indicated by the peak RPP, during treadmill walking than during the chest press.

According to current guidelines from the American College of Sports Medicine, patients can perform resistance training (lifting weights ≥50% of one repetition maximum [1-RM]) “a minimum of 5 weeks after date of myocardial infarction (MI) or cardiac surgery, including 4 weeks of consistent participation in a supervised cardiac rehabilitation endurance training program.” After percutaneous coronary intervention, the wait is 2 to 3 weeks, including 2 weeks of supervised endurance training (1). Thus, the guidelines clearly advocate endurance training over resistance training in the early stage of cardiac rehabilitation.

Like other investigators (some of whom studied this subject 25 years ago), we believe that resistance training should address the realistic goals that most patients want to achieve as soon as possible after hospital discharge, such as being able to shop for groceries, do laundry, mow the lawn, lift grandchildren, or return to manual labor (25). The restrictions on resistance training in cardiac rehabilitation may be especially problematic for younger, more active patients and for those who want to return to physically demanding jobs like firefighting or police work. Concerns about these restrictions prompted us to design the current study.

During workout sessions in cardiac rehabilitation, exertion levels are often estimated from the Borg Rating of Perceived Exertion Scale¯ (6, 7), which ranges from 6 to 20 and has corresponding descriptions of exertion levels that patients can identify at every odd number (8). Rating of perceived exertion and heart rate (HR) are linearly related to each other and to work intensity. By adding a zero to each point on the Borg Rating of Perceived Exertion Scale, one can estimate the HR value of various levels of work intensity: 6 becomes 60 and represents HR at rest, and 20 becomes 200, which might represent a patient's maximal HR (6, 7). The American Association of Cardiovascular and Pulmonary Rehabilitation recommends a rating of perceived exertion of 11 to 15 for patients in cardiac rehabilitation programs (9), which translates to a peak permissible HR of 150 beats/min. In addition, an absolute maximum threshold systolic blood pressure (SBP) of 240 mm Hg is allowed during exercise (9).

Rate-pressure product (RPP), which can be used to estimate the increased metabolic demand that exercise places on the heart, is calculated by multiplying the HR and the SBP. RPP changes during incremental exercise as increased exercise intensity raises both the HR and SBP; the RPP at maximal exercise is five times greater than it is at rest. In practical terms, RPP can be used as an exercise prescription guideline for cardiac patients (10). In our center, for example, we multiply the peak permissible HR of 150 beats/min (from the rating of perceived exertion of 15) by the absolute maximum SBP of 240 mm Hg to yield an RPP of 36,000, which we use as the safe and allowable peak guideline for both endurance and resistance training during cardiac rehabilitation (3).

The purpose of this study was to compare the peak RPP values of patients during two types of exercise—treadmill walking and chest press—from workout session 1 through completion of cardiac rehabilitation.

METHODS

Participants and study design

The institutional review board approved the study, and informed consent was obtained from all participants after their respective physicians gave assent. The study period was March 2006 to July 2008.

The participants were 21 patients (4 women and 17 men, aged 35 to 70 years) who were referred to the cardiac rehabilitation program at the Baylor Jack and Jane Hamilton Heart and Vascular Hospital in Dallas, Texas, after MI, percutaneous coronary intervention, or both. All participants were able to safely walk on a treadmill and perform the chest press.

Exclusion criteria included permanent pacemaker/defibrillator placement, antiarrhythmic medications, congestive heart failure, coronary artery bypass graft surgery, valvular disease and/or surgery, history of hernia or aneurysm, unstable angina pectoris, physical disability that limits treadmill ambulation, uncontrolled hypertension (systolic >160 mm Hg or diastolic >100 mm Hg), symptomatic dysrhythmias, and/or regular performance of resistance training exercise since the date of MI.

Each participant served as his or her blocking factor. This matched-sample design has the advantage of greater power and economy, especially when each participant is measured multiple times, as in this study.

Determination of exercise baselines

The Borg Rating of Perceived Exertion Scale was thoroughly explained to the study participants, who each performed an exercise tolerance test to determine the speed that represented an 11, or “fairly light” intensity; this level was used to ensure consistency across the group of participants. Treadmill intensity was increased 0.4 mph every 3 minutes. During the last 30 seconds of each 3-minute interval, participants were asked to rate their perceived exertion. The speed at which the rating of perceived exertion was reported to be “fairly light” (rating of 11) was used as the participant's baseline treadmill speed. For participants who reported a rating of perceived exertion >11, an increment of 0.2 mph was subtracted for each unit above 11, and the resulting speed was used as the baseline for the treadmill walking protocol.

A resistance test protocol on the chest press machine was used to assess each participant's estimated 1-RM lifting capability, defined as the maximum amount one can lift one time with proper technique. Before beginning the baseline resistance test protocol, the participants were shown the proper techniques for breathing and lifting. They were asked to perform repetitions of a specified weight on the chest press machine. If the participant performed more than 10 repetitions with proper form, the weight was increased by 10 pounds for men or 5 pounds for women. The participants repeated the sets until they could only perform fewer than 10 repetitions for the selected weight. Using the final weight lifted, the Brzycki equation (1-RM = weight lifted/[1.0278 – (0.0278 [multiply sign in box] number of repetitions)] was used to estimate the participants' 1-RM strength capability (11).

Intervention and data collection

Before each workout session, a resting lead II electrocardiogram and resting HR and blood pressure measurements were recorded for each participant. For each workout session, the participants did a warm-up and then performed both the treadmill walking and chest press exercises. To make these sessions as normal as possible, we allowed the participants to choose which exercise they did first; however, they always cooled down to within 10 units of the baseline HR and SBP (10 beats/min and 10 mm Hg, respectively) before beginning the second exercise.

During both types of training, the peak electrocardiogram and peak HR and blood pressure measurements were recorded. Because older individuals may take up to 5 minutes to reach their steady state (12) (when cardiovascular variables plateau for that level of exercise intensity [13]), the rating of perceived exertion, electrocardiogram, HR, and blood pressure were assessed after 5 minutes of treadmill walking and the peak values recorded.

Participants performed the chest press at 50% of the estimated 1-RM, and their blood pressure was taken immediately after each set of repetitions. Because systolic and diastolic pressures return to normal approximately 10 seconds after the last repetition of a resistance training set (14), we placed the blood pressure cuff on the arm before the exercise was started and inflated it after two thirds of the set was completed. Thus, we measured the blood pressure immediately after the set was finished. Participants completed three to five sets of 10 to 12 repetitions, with 3 minutes of rest between each set. The highest SBP measurement was recorded as the peak value.

The study activities during the 24 cardiac rehabilitation sessions are summarized in the Figure. The first session was used to orient each participant; the second was for baseline testing (determining his or her starting treadmill speed and lifting amount). During the remaining sessions, each participant's treadmill walking intensity was increased incrementally according to a written protocol. Treadmill speed was increased from baseline 0.2 miles per hour every third session. Upon reaching maximal comfortable walking speed, treadmill grade was then increased 0.5% each third session until completion of the cardiac rehabilitation program. To ensure that adequate weight was used for resistance training, participants were retested at every sixth session to determine a new estimated 1-RM weightlifting capability. New weight amounts were adjusted so participants continually performed the resistance training at 50% of 1-RM strength capability. On average, the participants completed at least 15 workout sessions. A physician was present during each workout session, and a crash cart was available in case of emergency.

Figure
Study activities during the 24 sessions of cardiac rehabilitation. 1-RM indicates one repetition maximum, or the maximum amount of weight one can lift one time with proper technique.

Data analysis

Peak RPP values were calculated (peak HR [multiply sign in box] peak SBP) for both the treadmill walking and chest press exercises. For each of the 19 workout sessions, mean peak values for HR, SBP, and RPP were calculated for each type of exercise. Paired t tests were used to determine whether differences in these values were statistically significant when the two types of exercise were compared; the one-sided type I error rate was set at 0.05. All statistical analyses were performed using JMP version 8.0 (SAS Institute Inc., Cary, NC).

RESULTS

The study results are summarized in the Table. For all 19 workout sessions, the mean peak values for HR and SBP were lower for the chest press than for treadmill walking; the differences were statistically significant (P < 0.05), with only one exception (SBP for session 1). Likewise, the mean peak values for RPP were lower for the chest press than for treadmill walking; the difference was statistically significant for all workout sessions (P < 0.05). Throughout the study, the participants' peak RPP values during both types of exercise remained far below the clinically safe limit of 36,000 for exercise training.

Table
Paired t test comparison of mean values obtained during chest press (CP) and treadmill walking (TW) exercises in 19 workout sessions of cardiac rehabilitation

Adverse events tracked during the study were arrhythmias, ST depression, and angina. During treadmill walking, one participant reported two episodes of angina, and another had ST depression. One participant had bigeminy 1 day after performing the chest press; the same person had occasional premature ventricular contractions on another day during the chest press. These episodes were transient and did not warrant additional treatment.

Most participants (16/21, or 76%) were taking various medications that reduced HR and blood pressure; one additional participant was taking medication that reduced blood pressure alone.

DISCUSSION

For this study, we designed an exercise plan for patients that featured traditional endurance training and concurrent resistance training, from workout session 1 through completion of cardiac rehabilitation. We reevaluated the participants to ensure comparable and uniform progression during both types of exercise. On any given day, the participants performed more myocardial work, as indicated by the peak RPP, during treadmill walking than during the chest press even though they began the resistance training regimen immediately instead of first completing 2 to 4 weeks of supervised endurance training. This finding remained consistent through the end of the workout sessions, when a heavier amount of weight was lifted.

Resistance training is recommended in the guideline books most commonly used by cardiac rehabilitation professionals (1, 9). However, even though resistance training has been deemed safe for cardiac rehabilitation, it is typically used only to complement endurance training. This practice presents a conundrum for patients who need to return to physically demanding jobs.

The American College of Sports Medicine recognizes the importance of occupation-specific exercise training for those returning to work, stating that “exercise training must be specific to the muscle groups and energy systems used for occupational tasks, particularly for those whose employment involves manual labor” (1). Nevertheless, the prescribed delay of resistance training means that post-MI patients whose jobs require heavy lifting cannot begin the specific training they need until at least week 5 of a typical 6- to 8-week (9) cardiac rehabilitation program.

To ensure safety during endurance training, cardiac rehabilitation professionals use responsible and reliable methods to help patients progress gradually. Increases in intensity are based on the staff's observations and the patients' subjective responses. The same should be true for resistance training, with cardiac rehabilitation professionals choosing the right exercises for each patient—just as they guide patients in their endurance activities, which also carry some risk.

The sample size for this study was relatively small; we recommend further investigation on a larger scale to confirm the findings and enhance their generalizability. In addition, arterial catheterization would have been more accurate than auscultation for detecting blood pressure changes during resistance training, but invasive methods were impractical in the study's clinical setting and would have exposed our participants to undue risk of harm.

Despite these limitations, the study results support our belief that resistance training can be a safe component of cardiac rehabilitation programs and that it should not be delayed by weeks of prerequisite endurance training. Most patients must do some form of lifting, carrying, or pushing activity in their daily routine; some want to return to physically demanding jobs. The inclusion of concurrent resistance training across all cardiac rehabilitation sessions may accelerate patients' return to desired levels of daily activity and facilitate a safe return to work.

Acknowledgment

We thank Beverly Peters, MA, ELS, for her help in formulating the manuscript.

References

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