This systematic review found that exercise training had beneficial effects on LV remodeling after myocardial infarction but that the sizes of changes were dependent on time of instigation and duration of the exercise intervention. The largest changes in LV remodeling were obtained when programs began after around 1-week post MI hospital discharge and lasted for 6 months. For ESV, a strong predicator of mortality post-MI , each one-week delay in initiating exercise training would require an additional month of training to obtain a comparable reduction in ESV. Similarly, delaying exercise training by one week after a MI would require an additional month of training to attain the same change in LVEF.
As with all systematic reviews, the conclusions of this review are only as strong as the quality of the component studies. The trials included predominantly younger males with reduced systolic function though there is unlikely to be differences in effects by sex, future trials should include a greater proportion of women. In the trials, the type and location of infarction was not widely used to stratify results and few studies measured exercise capacity. Accordingly, future trials are needed to measure and assess the specific effects of these factors on LV remodeling. The trials included were not well described and did not report all of the information required to conduct optimal analysis in the case of repeated measures. Here, a conservative alternative nevertheless showed significant findings in the meta-regression. The effect sizes would have been larger if correlations between pre-and-post intervention scores had been available.
The findings are biologically plausible. Though the physiological mechanisms responsible for the anti-remodeling benefits of exercise training following MI are not well understood, they may arise from favorable improvements in coronary and peripheral vascular endothelial function, myocardial contractility, autonomic balance, or systolic and diastolic wall stress [24, 25]. Several studies have also shown that aerobic training can elicit improvements in diastolic function and wall stress [5, 12, 13, 26]. Post-training decreases in plasma pro-NT-BNP reduce EDV  and increase peak early mitral flow velocity [5, 12, 13] and peak early to late mitral flow velocity ratio [12, 13]. Aerobic-training improves autonomic balance  and peripheral vascular endothelial function . The anti-remodeling benefits arising from exercise programs with a longer training length may explain the effects of cardiac rehabilitation attendance on survival after MI .
There is no current recommendations  or consensus  as to when exercise training should commence after MI. Most cardiac rehabilitation and secondary prevention programs commence at least four to six weeks after hospital discharge . To achieve maximal anti-remodeling benefits, clinically stable patients after uncomplicated MI should begin aerobic exercise training earlier after hospital discharge (from one week) and should continue training for up to 6 months. As this conclusion represents a potentially significant change from current practice, it is important to take into account the size and safety of this.
Though understandable, there is no evidence from the trials in this review or other observational studies that early commencement of exercise training is harmful. Trials have indicated that risk of adverse events or complications (including: re-infarction, and revascularization ) and EF are not raised by earlier physical activity when compared to activity after 6 weeks - even when pre-discharge (Bruce protocol) stress tests are performed 1-week post-MI . Consistent with these findings, pre-discharge exercise (Bruce protocol) stress testing is safe and feasible in the majority of post-MI patients 3 days after infarction . In trials in this review, no adverse events occurred during the 6-month exercise training sessions initiated at the earliest juncture (around one week post-MI) in people with mild to moderate LV systolic dysfunction [5, 6]. Moreover, clinical events were significantly lower in the trained versus control group during the 6-month period.
In addition to benefitting mortality and being safe, earlier commencement of exercise training appears to markedly increase participation in secondary prevention services. Emerging evidence from observational studies indicates that commencement of cardiac rehabilitation after one week leads to a 90% increase in participation rates compared to a commencement after four weeks  and a faster return to work .
By providing more specific guidance on basic yet pivotal program design characteristics, these findings are also likely to render existing research evidence on exercise after MI more usable to health professionals and decision-makers . Despite convincing evidence of the benefits of exercise after myocardial infarction [1, 2, 36, 37], secondary prevention in clinical populations remains poor  and health services to promote exercise are under-utilized and poorly funded . Health services, including different forms of cardiac rehabilitation [36, 37, 40] should be used more widely to promote exercise earlier and for longer after MI.