Fitness and respiration
During exercise, the muscle cells respire more than they do at rest. This means:
This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate.
During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration. The lactic acid that forms causes muscle fatigue and pain.
Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise.
But being fit is not the same as being healthy. Healthy people are free from disease and infection: they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.