Skeletal Muscle Mitochondrial Energetics Are Associated With Maximal Aerobic Capacity and Walking Speed in Older Adults

Lower ambulatory performance with aging may be related to a reduced oxidative capacity within skeletal muscle. This study examined the associations between skeletal muscle mitochondrial capacity and efficiency with walking performance in a group of older adults. Thirty-seven older adults (mean age 78 years; 21 men and 16 women) completed an aerobic capacity (VO2 peak) test and measurement of preferred walking speed over 400 m. Maximal coupled (State 3; St3) mitochondrial respiration was determined by high-resolution respirometry in saponin-permeabilized myofibers obtained from percutanous biopsies of vastus lateralis (n = 22). Maximal phosphorylation capacity (ATP(max)) of vastus lateralis was determined in vivo by P-31 magnetic resonance spectroscopy (n = 30). Quadriceps contractile volume was determined by magnetic resonance imaging. Mitochondrial efficiency (max ATP production/max O-2 consumption) was characterized using ATP(max) per St3 respiration (ATP(max)/St3). In vitro St3 respiration was significantly correlated with in vivo ATP(max) (r(2) = .47, p = .004). Total oxidative capacity of the quadriceps (St3*quadriceps contractile volume) was a determinant of VO2 peak (r(2) = .33, p = .006). ATP(max) (r(2) = .158, p = .03) and VO2 peak (r(2) = .475, p < .0001) were correlated with preferred walking speed. Inclusion of both ATP(max)/St3 and VO2 peak in a multiple linear regression model improved the prediction of preferred walking speed (r(2) = .647, p < .0001), suggesting that mitochondrial efficiency is an important determinant for preferred walking speed. Lower mitochondrial capacity and efficiency were both associated with slower walking speed within a group of older participants with a wide range of function. In addition to aerobic capacity, lower mitochondrial capacity and efficiency likely play roles in slowing gait speed with age.