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claim that exercise-induced cardioprotection following few exercise sessions is
similar to long-term physical training [ 24 , 62 ]. Given that exercise-induced cardio-
protection is a multifactorial process, it is tempting to assume that it involves differ-
ent mediators and magnitude according to the exercise protocol duration. Clarifying
this issue would be important considering that the potential mechanisms involved in
this response are still largely debated.
4.3 Exercise Training Intensity
When it comes to exercise intensity, the first question we ask is how much exercise
is needed to protect the heart. Is there a minimum amount of exercise to achieve
cardioprotection? Many researchers have been trying to answer this question, as the
dose-response impact of aerobic exercise intensity on cardioprotection is extremely
important. And the truth is that although findings from several studies provide
insight into this matter [ 24 ], definitive answers remain unknown.
Before reviewing existing data on the effect of exercise intensity over cardiopro-tection, it is important to revisit a concept of exercise prescription. There are two
different methodologies to deal with aerobic exercise intensity: continuous or inter-
val exercise. Continuous exercise consists of maintaining submaximal power output
and VO 2 constant throughout the entire session, whilst interval exercise alternates
periods of greater and lower intensity within an exercise session [ 84 ].
In one of the first investigations into the role of long-term exercise intensity inproviding cardioprotection [ 85 ], rats were treadmill trained for 11–16 week at low
intensity (20 m/min, 0% grade, 60 min/day), moderate (30 m/min, 5% grade,
60 min/day) or intense (10 bouts of alternating 2-min runs at 16 and 60 m/min, 5%
grade). After submitted to 25 min of global ischemia, all trained groups presented
significantly greater post-ischemic cardiac output and work compared to sedentary
rats. However, as intensity increased the greater was myocardial recovery.
Interestingly, a previous study [ 67 ] exercised rats daily for 6 week at low (20 m/min, 0% grade, 60 min/day) or high intensity (5 bouts of alternating 1-min runs at
75 and 20 m/min, 15% grade, 10 min/day). The high-intensity protocol improved
myocardial functional recovery following 20 min of global ischemia in an isolated
Langendorff perfusion model, but the low-intensity program did not. The same
result was found by Starnes et al. [ 81 ], who showed that exercise training for 16
week, 5 days/week 40 min/day below 55–60% VO2max did not achieve protection
against IRI. Unfortunately, the earlier studies did not provide information regarding
exercise intensity used in terms of VO2max. Regardless, it is feasible that there may
be an exercise intensity threshold above which cardioprotection is achieved and that
the amount of cardioprotection is proportional to exercise intensity [ 24 , 86 ].
Controversially, Lennon et al. [ 87 ] concluded that both moderate- (i.e., 60 min/day at 50% VO2max) and relatively high-intensity exercise (i.e., 60 min/day at 70%
VO2max) performed during three consecutive days appear to be equally protective
against IR-induced myocardial stunning. The discrepancy in results may be due to
10 Cardiac Ischemia/Reperfusion Injury: The Bene cial Effects of Exercise