The second model is the indirect model in which physical activity produces changes in risk factors. The changes in the risk factors are associated with the reduced risk of heart attack. For example, the changes may occur through alterations in body composition or a reduction in blood pressure. There could be an independent contribution of physical activity to the second model separate from physical fitness. A primary candidate for this is alterations in high density lipoprotein cholesterol.
Increased activity is associated with an increase in high density lipoprotein cholesterol.
A critical question that needs to be addressed is whether fitness is a necessary component in the second model, particularly with reference to the physical activity-high density lipoportein cholesterol relationship. The cross- sectional results comparing high activity groups with low activity groups have been quite consistent, in which high activity groups have higher concentrations of high density lipoportein cholesterol. Physical activity as an index of heart attack risk in college alumni. Work activity and coronary heart disease mortality. Morris JN, Kagan A, Pattison DC, et al.