Time course of the cross-over effect of fatigue on the contralateral muscle after unilateral exercise

Abstract

We investigated the cross-over effect of muscle fatigue and its time course on the non-exercising contralateral limb (NEL) after unilateral fatiguing contractions of the ipsilateral exercising limb (EL). For this purpose, 15 males performed two bouts of 100-second maximal isometric knee extensions with the exercising limb, and neuromuscular function of both the EL and NEL was assessed before (PRE), after a first fatiguing exercise (MID) and after a second fatiguing exercise (POST). Maximal voluntary isometric torque production declined in the EL after the first bout of exercise (-9.6%; p<0.001) while in the NEL, the decrease occurred after the second bout of exercise (-10.6%; p<0.001). At MID, torque decline of the EL was strictly associated to an alteration of the mechanical twitch properties evoked by neurostimulation of the femoral nerve (i.e., peak twitch torque, maximal rate of twitch development). According to these markers, we suggest that peripheral fatigue occurred. At POST, after the second bout of exercise, the voluntary activation level of the knee extensor muscles was altered from PRE (-9.1%; p<0.001), indicating an overall central failure in both the EL and NEL. These findings indicate that two bouts of unilateral fatiguing exercise were needed to induce a cross-over effect of muscle fatigue on the non-exercising contralateral limb. Differential adjustments of the motor pathway (peripheral fatigue vs. central fatigue) might contribute to the respective torque decline in the EL and the NEL. Given that our unilateral fatiguing exercise induced immediate maximal torque reduction in the EL and postponed the loss of torque production in the NEL, it is also concluded that the time course of muscle fatigue differed between limbs.

Figure 1. Graphical overview of the experimental protocol.

Neuromuscular tests comprised single stimuli (single arrows), doublet stimuli (double arrows), MVC with superimposed doublet stimulus and followed by a doublet stimulus delivered at rest. Two MVCs of the exercising limb (EL; uninterrupted line) and the non-exercising limb (NEL; dashed lines) were performed at PRE, while only one was respectively realised at MID and POST. Testing order for the EL and NEL was randomly selected. The fatiguing exercise of the EL consisted of two bouts of 100-second MVC.

Figure 2. Maximal voluntary isometric torque.

Maximal voluntary isometric torque tests of the knee extensor muscles measured at PRE, MID and POST tests for the exercising limb (A) and the non-exercising limb (B). Columns represent group mean values, while triangles, squares black and white symbols show individual values. Error bars are the standard error of the group mean. Significant differences p<0.05 (*) and p<0.001 (***).

Figure 3. Voluntary activation level of the knee extensor muscles.

Voluntary activation level measured at PRE, MID and POST tests for the exercising limb (in grey) and the non-exercising limb (in white). Columns represent group mean values and error bars are the standard error of the group mean. Significant differences p<0.001 (***).

Figure 4. Torque production capacity and sEMG activity during the fatiguing exercises.

Torque production capacity measured during the 10 periods of the first (black circles) and the second (white circles) fatiguing exercise for the exercising limb (A). Pooled data of the considered period significantly lower from the pooled data of the first period: p<0.01 (**) and p<0.001 (***). sEMG RMS/Mmax ratios of the vastus lateralis (diamonds), the vastus medialis (triangles), the rectus femoris (squares) muscles and coactivation level (circles) of the semitendinosus (ST) muscle during the 10 periods of the first (B) and second (C) fatiguing exercise. Values are mean and standard error of the mean.