Instead, the researchers assumed that physical activity was what the assessment tools were measuring. Other approaches have used a restricted definition of physical activity by examining the amount of time in vigorous activity. Active individuals have also been defined as individuals with an increased rate of body movement. It is interesting that in the more than 50 articles on activity and heart attack, at least 25 different measuring tools and, therefore, definitions of activity have been employed. The second concept that needs to be explained is physical fitness.
Physical fitness is the capacity to perform prolonged heavy work.
Individuals who are fit can sustain a higher maximum work load and thus, a higher energy expenditure than nonfit individuals. The greater the exercise intensity, the larger the amount of oxygen an individual requires. Physical activity with a frequency of three times per week for 20 minutes at 80 per cent maximal heart rate is said to be necessary for cardiovascular fitness generally do not occur in normal populations. Also, this level is needed to maintain fitness. With these definitions, it is possible to construct the model of physical activity.