You'll need a way to measure intensity and you'll need to decide how hard to exercise. Let's discuss intensity first.
So, how do you measure exercise intensity? Well, there are three methods to find intensity¿lungs, heart and brain.
The first way to find exercise intensity is through the lungs. It works by measuring the amount of air you use when exercising. While accurate, this method requires an oxygen mask so it's not very practical.
The second way to find exercise intensity is through the heart. It works by measuring your heart rate when exercising. While it's fairly accurate, the number isn't all that useful because it varies with age, gender, and even medicines you take.
The third way to find exercise intensity is through the brain. It works¿not by sticking sharp probes in the brain¿but by self evaluation. Amazingly, studies show it's as accurate as the first two methods. No equipment is needed, just come up with a number from one to ten, where one is very low intensity and ten is full intensity.
Now that you understand intensity, how hard should you exercise?
This depends on a number of factors, like what kind of shape you're in, what medications (including over-the-counter) you take, and what your goals are. All things considered, start with an intensity level of "moderate".
So what is moderate? Using the "brain" method, simply work out at the 3-5 level.
Summary: We took a whirl wind tour of different ways to measure exercise exertion, ultimately revealing the accurate, yet simple, 1-10 method.