Effects of old age and contraction mode on knee extensor muscle ATP flux and metabolic economy in vivo

Key points We used 31‐phosphorus magnetic resonance spectroscopy to quantify in vivo skeletal muscle metabolic economy (ME; mass‐normalized torque or power produced per ATP consumed) during three 24 s maximal‐effort contraction protocols: (1) sustained isometric (MVIC), (2) intermittent isokinetic (MVDC IsoK ), and (3) intermittent isotonic (MVDC IsoT ) in the knee extensor muscles of young and older adults. ME was not different between groups during the MVIC but was lower in older than young adults during both dynamic contraction protocols. These results are consistent with an increased energy cost of locomotion, but not postural support, with age. The effects of old age on ME were not due to age‐related changes in muscle oxidative capacity or ATP flux. Specific power was lower in older than young adults, despite similar total ATP synthesis between groups. Together, this suggests a dissociation between cross‐bridge activity and ATP utilization with age. Abstract Muscle metabolic economy (ME; mass‐normalized torque or power produced per ATP consumed) is similar in young and older adults during some isometric contractions, but less is known about potential age‐related differences in ME during dynamic contractions. We hypothesized that age‐related differences in ME would exist only during dynamic contractions, due to the increased energetic demand of dynamic versus isometric contractions. Ten young (Y; 27.5 ± 3.9 years, 6 men) and 10 older (O; 71 ± 5 years, 5 men) healthy adults performed three 24 s bouts of maximal contractions: (1) sustained isometric (MVIC), (2) isokinetic (120°·s –1 , MVDC IsoK ; 0.5 Hz), and (3) isotonic (load = 20% MVIC, MVDC IsoT ; 0.5 Hz). Phosphorus magnetic resonance spectroscopy of the vastus lateralis muscle was used to calculate ATP flux (mM ATP·s –1 ) through the creatine kinase reaction, glycolysis and oxidative phosphorylation. Quadriceps contractile volume (cm 3 ) was measured by MRI. ME was calculated using the torque‐time integral (MVIC) or power‐time integral (MVDC IsoK and MVDC IsoT ), total ATP synthesis and contractile volume. As hypothesized, ME was not different between Y and O during the MVIC (0.12 ± 0.03 vs . 0.12 ± 0.02 Nm . s . cm –3 . mM ATP –1 , mean ± SD, respectively; P = 0.847). However, during both MVDC IsoK and MVDC IsoT , ME was lower in O than Y adults (MVDC IsoK : 0.011 ± 0.003 vs . 0.007 ± 0.002 J . cm –3 . mM ATP –1 ; P < 0.001; MVDC IsoT : 0.011 ± 0.002 vs . 0.008 ± 0.002; P = 0.037, respectively), despite similar muscle oxidative capacity, oxidative and total ATP flux in both groups. The lower specific power in older than young adults, despite similar total ATP synthesis between groups, suggests there is a dissociation between cross‐bridge activity and ATP utilization with age.