Centrally-mediated regulation of peripheral fatigue during knee extensor exercise and consequences on the force-duration relationship in older men

The aim of the present study was to investigate the existence of a critical threshold beyond which peripheral fatigue would not further decrease during knee extensor (KE) exercise in older men, and the consequences of this mechanism on the force-duration relationship. Twelve old men (59 +/- 2 years) randomly performed two different sessions, in which they performed 60 maximum voluntary contractions (MVC; 3s contraction, 2s relaxation). One trial was performed in the unfatigued state (CTRL) and one other following fatiguing neuromuscular electrical stimulation of the KE (F-NMES). Peripheral and central fatigue were quantified via pre/post-exercise decreases in quadriceps twitch-force (Delta P-tw) and voluntary activation (Delta VA). Critical torque (CT) was determined as the mean force of the last 12 contractions while W ' was calculated as the area above CT. Compared with CTRL, pre-fatigue (Delta P-tw = -10.3 +/- 6.2%) resulted in a significant (p < 0.05) reduction in W' (-18.2 +/- 1.6%) in F-NMES. However, CT (similar to 964 N), Delta VA (similar to 15%) and Delta P-tw (similar to 25%) post-MVCs were similar between both conditions. In CTRL, W' was correlated with Delta P-tw (r(2) = 0.78). Moreover, the difference in W' between CTRL and F-NMES was correlated with the level of pre-fatigue induced in F-NMES (r(2) = 0.76). These findings document that peripheral fatigue is confined to an individual threshold during KE exercise in older men. Furthermore, correlative results suggest that mechanisms regulating peripheral fatigue to a critical threshold also restrict W', and therefore play a role in exercise capacity in older men.