The level of exertion is determined by the combination of the following 4 factors: INTENSITY, NATURE, DURATION and FREQUENCY. These factors are intrinsically connected to one another.
The nature of the activity has a huge effect on the level at which it is carried out. For example, a trained cyclist will become quickly tired if he runs or swims. The explanation is obvious. During certain movements, certain muscle groups are used.
Other muscle groups are not used or there is little impact from the training. When people do exercises that they are not used to, and new muscles are used, it is more tiring than when they perform their usual sport.
Intensity is the level of effort required for an activity. At low intensity, e.g. slow cycling, walking or swimming, there is also a low level of intensity. But this means it can be maintained for long periods. A sprint until you’re out of breath demands maximum effort from even the best trained sports people.
Very intensive effort can only be maintained for short periods. The greater or more intense the effort, the more intense the breathing and the higher the heart rate, until it reaches its maximum. The heart rate is therefore a useful benchmark for measuring the intensity of exercise.
For untrained people, short periods of exercise can be very tiring. In general, you are advised to engage in moderately intensive exercise. A general rule is: the lower the intensity, the longer the effort can be maintained in order to obtain a training effect.
Intensive, extended efforts require a longer recovery period than lighter, short efforts. If a subsequent training session is started too quickly, the body will not have had the chance to fully recover. The new effort will be weaker. If you wait too long between two activity periods, the training effect of the previous exercise may have been lost.