1.Physical activity is simply any bodily movement performed by the muscles that expend energy. This includes all the movement one does throughout the day, whether it be intentional, part of one's job/occupation or simply for transporting one's self from one place to another. All activity and steps count toward your health.
2.Exercise is a subset of physical activity that is planned and structured with the purpose of improving or maintaining physical fitness or health. In general, exercises are divided into activities that are predominantly aerobic (like running) or resistance (like weight lifting), although most sports and physical activities have components of both.
3.Fitness is the ability to perform activity at a moderate to vigorous level of intensity without tiring easily. Metabolic Equivalent of Tasks, or METs, are commonly used as the unit attributed to different activities to describe the intensity and the amount of energy spent in that activity. The more fit you are, the more physical activity you can do and then the more oxygen the body uses to meet the energy demands. METs can be measured during an exercise (treadmill or bicycle) stress test, and they are commonly performed at a doctor's office to investigate cardiac or pulmonary disease. Fitness also depends on your age, sex, body size and genetic background. But it's clear that regular physical activity improves your fitness level.
