PURPOSE: The convalescent period after myocardial infarction (MI) has been associated with a "spontaneous" improvement in functional aerobic capacity that may be because of normal recovery processes unrelated to formal exercise training. The purpose of this study was to determine whether the frequency of formal training sessions is an important variable affecting the magnitude of improvement in cardiorespiratory fitness during phase II cardiac rehabilitation. METHODS: The effect of exercise training frequency on cardiorespiratory fitness was evaluated during a 5-week early (phase II) cardiac rehabilitation program in 50 low-risk, male patients recovering from acute MI. Baseline graded treadmill tests to fatigue endpoints, with direct measurement of maximal oxygen uptake (VO2max), were administered 4 weeks after MI. The subjects were then randomly assigned to either a control group (n = 12) and restricted to "very light" physical activity (requiring < 50% of VO2max) at home, or to one of three training groups which, in addition to very light home activity, performed moderately intense (approximately 70% of VO2max) aerobic exercise for 30 to 35 minutes either once per week (n = 13), twice per week (n = 13), or three times per week (n = 12) in the hospital-based phase II program. The four groups were similar in age, clinical status, and use of beta-blockers and calcium channel blockers. RESULTS: Submaximal and maximal cardiorespiratory responses were initially similar in all four groups. Each of the four groups demonstrated significant (p < 0.05) increases in maximal treadmill duration at follow-up. However, VO2max increased significantly only in the three training groups. The spontaneous improvement in treadmill duration in the control group, in the absence of formal exercise training, may simply reflect recovery from the acute cardiac event. Those training two and three sessions per week also showed significant, comparable decreases in submaximal exercise heart rate and rate-pressure product and similar increases in maximal treadmill duration and VO2max. CONCLUSIONS: Results suggest that two exercise sessions per week is as effective as three per week for cardiorespiratory conditioning in the early weeks of phase II cardiac rehabilitation.
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