Physiological basis for recommending changes to the physical activity guidelines.

The rapid drop in mortality risk as people become more active has been known for some time. While the public health message is clear (i.e., that any physical activity [PA] is better than no PA), no acceptable explanation as to why this rapid drop in mortality occurs is available. Without an explanation, providing specific answers as to what sedentary people should do to optimize health remains difficult. Research findings from studies on exercise physiology and inactivity physiology show why a lack of understanding of the importance of regular exercise or for becoming physically active exists. These findings also show how new knowledge can modify present PA guidelines. With sedentary people, being less inactive is more critical than becoming more active. Active people can also reduce their inactivity and achieve greater health benefits. Adults doing more vigorous aerobic and strength training achieve greater health benefits. No more than two days should elapse between exercise sessions 1) to improve insulin sensitivity and improve glucose tolerance, and 2) to activate muscle enzymes such as lipoprotein lipase to reduce postprandial lipemia. Older adults should do more strength exercises to minimize the effects of aging on sarcopenia. Subjective perception of effort is an easy, useful way to prescribe exercise. Being less inactive and more active are both important lifestyle considerations, and both must be promoted to all age groups.