In this month’s MSSE, I am highlighting two articles that deal with physical activity level and mortality risk, and a third that suggests a surprising effect of chronic resistance training.
First, Matthews et al. investigated the mortality benefits associated with replacing sitting with different types of physical activity. In a large prospective study of older adults, the authors found that less active adults who replaced one hour of sitting per day with an equal amount of physical activity such as exercise, household chores or lawn and garden activities, had lower mortality. Highly active adults, in contrast, had to replace sitting time with purposeful exercise for benefit, and for these people, replacing sitting time with nonexercise activities appeared to provide no reduction in mortality. These findings provide evidence that replacing sitting time with a variety of physically active pursuits can influence longevity, with the benefits being most pronounced in least active people.
Certainly, some physical activity is better than none; however, it has been heretofore unknown whether or not there is a difference in mortality risk among men who fall into the category of very low cardiorespiratory fitness (CRF). In this month’s journal, Farrell et al. reported a steep decrease in all-cause mortality with increasing time on a maximal treadmill exercise test among 6251 Cooper Clinic men, all of whom were in the overall lowest quintile of CRF within the ages of 40-49, 50-59, and 60-69 years. For every one minute increment in treadmill time, there was a 9%-15% reduction in all-cause mortality risk. The practical message is that even small improvements in CRF can decrease mortality risk in very low fit men.
One of the changes that accompanies improved CRF is an increase in skeletal muscle oxidative capacity. Since the pioneering work of John Holloszy in the 1960s, endurance training has been accepted as the primary mode for this improvement. More recently, resistance training (RT) has gained popularity as an exercise modality that increases muscle mass and strength (increased myofibrillar protein), but reports of mitochondrial adaptations observed with RT have been inconsistent. In healthy untrained men, Porter et al. found that a 12-wk RT program increased skeletal muscle mitochondrial respiratory capacity and coupling control. In particular, coupled (ATP producing) respiration per milligram of muscle increased by 37%. Moreover, when normalized to mitochondrial respiratory capacity, they found that coupled respiration supported by complex I of the electron transport chain increased after RT, while there was a concurrent decrease in complex II function after RT. Collectively, these results suggest that, in healthy untrained people, chronic RT has metabolic benefits beyond increased mass and strength.
L. Bruce Gladden