Impact of Aging on Fatigue and Corticomotoneuronal Excitability during Small Muscle Mass Exercise
Authors: Weavil Joshua C , Sidhu Simranjit K , Mangum Tyler S , Thurston Taylor S , Garten Ryan S , Richardson Russell S , Amann Markus 
Published: November 2023

BACKGROUNDThe impact of aging on exercise induced fatigue is inconclusive with many studies ascribing a greater fatigue resistance to older individuals.PURPOSETo examine the effect of aging on fatigue resistance during dynamic and isometric small muscle mass exercise, both performed at a given absolute work rate and relative exercise intensity.METHODSEight young (Y; 25 ± 1 yr) and 8 older (O; 70 ± 1 yr) males performed a) dynamic single leg knee extensor exercise (KE) to exhaustion at a given relative intensity (80% Wpeak; DYN), and b) intermittent maximal isometric KE (8 times 15 s on, 20 s off, ISO). The Y then replicated both exercises, matching the absolute work rate (DYN), torque (ISO), and duration achieved by the O. Quadriceps fatigue was quantified as pre‐post exercise decrease in maximum voluntary contraction (MVC), twitch force (QTW; electrical femoral nerve stimulation), and voluntary activation (VA). Excitability of the corticomotoneuronal tract (CM) was assessed by transcranial magnetic stimulation, electric transmastoid and femoral nerve stimulation which elicit electromyographic responses [motor (MEP) and cervicomedullary (CMEP) evoked potentials, and M‐wave, respectively] in the quadriceps.RESULTSDYN: The Y were characterized by a higher Wpeak compared to O (58 ± 3 W vs 45 ± 2 W; P < 0.01). Although DYN exercise at the same relative intensity induced significantly greater reductions in MVC (−37 ± 4% vs −23 ± 5%) and VA (−11 ± 2% vs −6 ± 1%) in the Y, the exercise induced decrease in QTW was similar in both groups (~50%; P = 0.42). However, when the Y repeated DYN exercise, matched with the O in terms of absolute work rate and duration, the reduction in MVC, QTW, and VA were 26%, 28%, and 85% lower, respectively, (P < 0.05) compared to O. CM excitability remained unaltered from pre to post exercise in both groups and across exercise modalities (P > 0.2). ISO: Quadriceps MVC was higher in Y vs O (244 ± 20 Nm vs 167 ± 7 Nm; P < 0.01). ISO exercise at the same relative intensity induced significantly greater reductions in MVC (−28.5 ± 5.3% vs −13.9 ± 3.4%), QTW (−43.9 ± 5.2% vs −23.1 ± 7.4%) and VA (−11.7 ± 2.7% vs −1.4 ± 2.1%) in Y compared to O. However, when Y repeated the ISO exercise, matched with the O in terms of absolute torque, the reduction in MVC and QTW were ~34% and ~53% lower (P < 0.05) compared to the O, with no significant change in VA from baseline. Finally, while MEPs remained unaltered from before to after the relative intensity trial in Y, CMEPs decreased (P < 0.05) which augmented the MEP/CMEP ratio (P < 0.05). There were no changes in the same absolute intensity trial or in the O (P > 0.2).CONCLUSIONThese data reveal a detrimental impact of aging on fatigue resistance during small muscle mass exercise when assessed at a given absolute work rate and absolute torque. In contrast, when matched for relative intensity, the O fatigue less during exercise than the Y, however, it is critical to consider that the associated absolute workload was substantially lower compared in the O. Furthermore, while fatiguing exercise utilizing a small muscle mass has no effect on the central motor pathway (i.e., CM) in the O, maximal ISO exercise in the Y impairs motoneuronal function, but facilitates motor cortical excitability with no overall net effect on the CM.