Effects of Long Versus Short of Exercise in Healthy Subjects
Robert F. DeBusk, MD, Ulf Stenestrand, MD, Mary Sheehan, MS, and William L. Haskell, PhD
To evaluate the ‘threshold” duration of exercise required to produce training effects, 18 healthy men aged 51 f 6 years comple6ng 30 mimrtes of exercise training/day were compared with 18 men aged 52 f 6 years completing three lo-minute bouts of exercise/day, each separated by at least 4 hours. Exercise training intensity was moderate (6s to 75% of peak treadmill heart rate). During the &week study period VO2 max imased significantly in both groups from 33.3 f 3.2 to 37.9 f 3.5 ml/kg/min in men performing long exercise bouts and from 32.1 f 4.6 to 34.5 f 4.5 ml/kg/ min in men performing short exercise bouts (p 6S% of the prescribed duration. Thus, multiple shorl bouts of moderate-intensity exercise training slgnifkantly increcue peak oxygen uptake. For many individuals short bouts of exercise training may fit better into a busy schedule than a single long bout. (Am J Cardiol1990;65:1010-1013)
From the Stanford Cardiac Rehabilitation Program, Division of Cardiology, Department of Medicine, Stanford University School of Medicine, Palo Alto, California. This study was supported in part by grant HL-36272 from the National Heart, Lung and Blood Institute, Bethesda, Maryland. Manuscript received August 31, 1989; revised manuscript received and accepted December 14, 1989. Address for reprints: Robert F. DeBusk, MD, 780 Welch Road, Suite 106, Palo Alto, California 94304. 1010
rends toward greater participation in physical exercise conditioning by the American public during the past 2 decades appear to be stabilizing or reversing.1,2 This may reflect in part a perceived or actual reduction in leisure time as Americans work longer hours or undertake multiple jobs. Lack of time is often stated as the reason for nonparticipation in physical exercisem3Many individuals believe that physical benefits from exercise accrue only with exercise bouts of 120 minutes, consistent with exercise guidelines developed by the American Heart Association.4 Shorter bouts are perceived by many to be of little value in enhancing physical fitness. A regimen 120 minutes of relatively intense exercise performed L3 times/week is commonly used by researchers to increase functional capacity.5 However, short and long bouts of physical exercise of comparable total duration might enhance physical fitness to a similar extent. To evaluate the “threshold” of exercise duration required to produce training effects we compared the effects on functional capacity of a single “long” (30 minutes) bout of moderate-intensity exercise with 3 “short” (10 minutes) bouts of moderate-intensity physical exercise.
METHODS Study population: Forty sedentary healthy men aged
40 to 60 years employed at the Lockheed Missiles and Space Corporation in Sunnyvale, California, were recruited with a notice in the company newsletter. Medical exclusions included heart disease, hypertension, disorders limiting participants’ ability to perform exercise, and smoking. Subjects >25% overweight, based on 1959 Metropolitan Life Insurance weight tables, were excluded.6 All subjects had sedentary jobs. In the preceding 6 months they had not walked or jogged for >20 minutes continuously 23 times/week, walked a total of 60 minutes/day 23 times/week or participated in any active sport >l time/week. A pretraining medical examination included recording of a 1Zlead electrocardiogram, body height and weight and triplicate measurement of resting blood pressure using a Hawksley random zero device. Symptom-limited treadmill exercise testing was performed using a multistage protocol with continuous monitoring of the electrocardiogram and oxygen uptake. Workload was increased by 2 METS or 2 multiples of resting energy consumption every 2 minutes. Oxygen uptake was measured using an on-line, computer-based open-circuit method. Blood pressure was recorded during the last 30 seconds of each treadmill workload. This evaluation was repeated after 8 weeks of exercise training.
After the pretraining treadmill test, subjects were randomly assigned to 1 of 2 exercise training groups. Twenty subjects were assigned to a single “long” exercise bout of 30 minutes daily and the other 20 to 3 daily “short” exercise bouts of 10 minutes, each separated by at least 4 hours. Exercise was performed at home or at work by subjects training alone. There was no medical supervision. Subjects were instructed in monitoring their exercise pulse rate and were provided with an electronic heart rate monitor (ExerSentry Model ExIIIA, Computer Instruments Corp.) to help them to exercise within their training heart rate range. They were requested to make no major changes in their diet. Both training regimens consisted of 30 minutes of jogging/day at an intensity producing a heart rate 65 to 75% of the peak heart rate attained during the pretraining treadmill exercise test. This corresponded to a range from 116 to 133 beats/min. This moderate-intensity regimen was chosen beause it minimizes sweating and was well received by participants of our previous training studies.‘J Training occurred 5 days/week over a period of 8 weeks. Subjects recorded the adherence to exercise on logs which were completed daily and mailed to a project staff every 2 weeks. These logs also inquired about the following aspects of each exercise bout: heart rate during the last minute; rating of perceived exertion during the last minute on a scale from 6 to 209; overall level of enjoyment; and level of convenience and whether sweating occurred. Responses to continuous variables were
pooled across bouts for each individual. Responses to the question regarding sweating were pooled for each individual. A staff member called each participant every 2 weeks to check on progress, to review the previous log and to answer questions. For 2 days on 2 occasions during the 8-week training period, subjects wore a portable solid state microcomputer (Vitalog Inc.) connected to the chest by electrocardiographic electrodes and connected to a motion sensor attached to the thigh.‘O This provided a continuous recording of heart rate and body movements throughout the day (Figures 1A and 1B). These recordings were analyzed to verify self-reported exercise intensity, duration and frequency by methods previously described. lo Differences in body weight, resting blood pressure and treadmill test variables between the 2 groups before and after training were compared by 2 sample t tests. A p value of