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Studies are somewhat contradicting as to what exercise intensity level is most beneficial for one's health. For healthy young to mid-age individuals, the optimum seems to be moderate to heavy exercise. For those with compromised health, or over ~50 years old, it is between light and moderate.
How do you determine what is light, moderate or heavy exercise? The simplest way is doing the sing/talk test. If you can sing while exercising, the activity is light. If you can't sing, but can carry on conversation, it is moderately intense. And if physical activity prevents you from carrying on conversation, it is heavy.
Or you can base it on your heartbeat rate, measured either at your wrist, or on the side of your neck, just below the jaw angle. Average resting rate is 75 beats per minute for women, and 70 for men. Conventional formula for your maximum heart rate (MHR) is 220-(your age) heartbeats per minute.
According to the American College of Sports Medicine, light exercise is below 0.55 of your MHR, moderate exercise from 0.55MHR to 0.69MHR, heavy exercise 0.7-0.89MHR, and very heavy at 0.9-1MHR.
Keep in mind though, that these are only average values, and that your individual MHR can vary up to 10% - or even more - up or down.
Probably best overall method for determining exercise intensity level is the Rate of Personal Exertion (RPE). With it, you determine how hard on your body is the activity, forming a scale from no exertion level at the low end, to unbearably hard on the high end. You rate your overall feeling of effort, based mainly on the degree of muscle fatigue and respiration rate. The conventional scale is from 6 for no exertion to 19 for extremely hard.
There is no reason why we couldn't simplify it to, say, a scale from 1-10, with three degrees of difficulty - low, mid, and high - for each, light, moderate and heavy exercise. In that case, the 2-4 range would be light, 5-7 moderate, and 8-10 heavy exercise. It seems to be near-general consensus that for either weight control, or cardiovascular benefit, one needs to exercise in the moderate range, at least 3-4 times a week, for about half an hour, or more, at the time.
EXERCISE INTENSITY CHART (maximum heart rate, MHR~220-age)
Regardless of which level of exercise you choose as your goal, the only proper way is to gradually, as it feels comfortable, build it up to the desired level, and keep it there. If you have to choose between exercising rarely, or not at all, the latter is probably safer.
One thing you need to pay special attention to is
how quickly your heart rate returns to its resting level.
If you are fit, exercising regularly, your heart should be getting back to its resting rate at the rate of about 20 beats per minute. That gives about 5-6 minutes for the recovery time from heavy exercise, and up to a couple minutes less from moderate exercise.
If you are not quite there in this time frame, it probably isn't a reason for concern. However, significantly slower rate is associated with exponentially higher risk of dying within several years. If your downturn rate is about 12 heartbeats a minute, or lower, after you've been exercising regularly for months (or longer),
exercising may not be benefiting you - to the contrary!
This, in general, indicates inefficient oxygen delivery. If so, your heart probably has hard time to respond to demands imposed by exercising, and may get damaged as a result. The problem may be an existing cardiovascular disease, or some form of anemia. If you smoke excessively, it can deprive your body's oxygen supply.
Or your body may be inefficient in production of nitric oxide, a vasodilator it uses to expand blood vessels for better blood flow, which is needed at moderate-to-heavy exercise levels.
The reason for low nitric oxide is usually overproduction of asymmetrical dimethyl arginine (ADMA), which inhibits the enzyme needed for nitric oxide synthesis. If early diagnosed (by a simple ADMA blood test), it is easily corrected with arginine supplementation. However, if your body was high on ADMA - and low on nitric oxide - for a longer period of time, your risk of developing cardiovascular and other diseases skyrockets12.
This is another reminder that the effect of exercise on your health, just as any other factor, is highly individual. It needs to be in agreement with your overall health, diet and lifestyle.