According to the principle of balance, a workout should consist of three components: cardiovascular exercise, strength training and stretching. Considering that this is one of the most important principles for achieving overall fitness, it’s surprising that many people only focus on cardiovascular or muscle-building exercises. The principle of balance is based on the assumption that each exercise needs to have a counter-balance in order for it to be effective.
If you focus only on one type of exercise, then your body will not get stronger and more flexible at the same time; therefore, all types of exercises need to be included with proper rest intervals between them. In other words, someone who runs five miles every day could use some strength training as well or risk injury from overuse.
As long as there’s a balance among these three components – cardiovascular exercise, muscle building/stretching and strengthening, people can achieve their fitness goals without needing an excessive number of hours per week (or month) spent doing any single activity.