Cortical voluntary activation testing methodology impacts central fatigue

Currently, cortical voluntary activation (VA(TMS)) is assessed by superimposing transcranial magnetic stimulation (TMS) on a maximal voluntary contraction (MVC), 75% MVC and 50% MVC, each contraction being interspersed with 5-10 s of relaxation. Here, we assessed whether this traditional approach (TRADI) underestimates central fatigue due to this short recovery compared to a continuous method (CONTI). VA(TMS), motor-evoked potential (MEP), and cortical silent period (CSP) of the vastus lateralis were determined in 12 young healthy adults before and after a 2-min sustained MVC of knee extensors in two randomly assigned sessions. In TRADI, evaluations comprised a 7-s rest between the three contractions (100, 75, and 50% MVC) and evaluation following the 2-min sustained MVC started after a minimal rest (3-4 s). In CONTI, evaluations were performed with no rest allowed between the three levels of contraction, and evaluation after the 2-min sustained MVC commenced without any rest. MVC was equally depressed at the end of the 2 min in both conditions. Post 2-min sustained MVC, VA(TMS) change was greater in CONTI than in TRADI (-29 (15)% [-42, -17] vs. -9 (4)% [-13, -5], respectively, P < 0.001). Differences were also observed between TRADI and CONTI for MEP and CSP immediately after the fatiguing exercise. All differences between the two methods disappeared after 2 min of recovery. After a 2-min sustained MVC, a few seconds of recovery change the amount of measured VA(TMS) and associated parameters of central fatigue. The continuous method should be preferred to determine deficits in voluntary activation.