Neural and bioenergetic mechanisms of human skeletal muscle fatigue resistance in old age

Abstract

There is strong evidence that greater resistance to muscle fatigue in older adults, most often observed during low‐velocity or isometric contractions, has a basis in the bioenergetic response to contractions. However, the role of age‐related changes in motor unit (MU) behavior has not been evaluated in this context. To test the hypothesis that fatigue resistance in old resuls from adaptations in both MU behavior and energetics, we quantified dorsiflexor muscle fatigue (torque‐time integral, %initial); mean MU discharge rate (MUDR) by in‐dwelling electrode and cytosolic pH and inorganic phosphate (Pi) by magnetic resonance spectroscopy during parallel experiments using a protocol of 12 maximal isometric contractions (12s on, 8s off) in 10 young (35±4 yr, mean±SD) and 8 older (74±5) women. Older fatigued less than young (68±7%initial vs 59.7 ±9, p<0.05), had a smaller decrease in MUDR (Δ= −20±10 %initial vs −34±18, p=0.08), and tended to have less acidosis at fatigue (6.88±0.13 vs 6.77±0.16, p=0.18), with no difference in end‐exercise Pi. These data provide the first in vivo evidence of the combined roles of MUDR and energetic responses as mechanisms of fatigue resistance in older adults. NIH AG21094