Purpose
The aim of this study was to examine fatigue in the plantar flexor muscle group after a prolonged running exercise.
Methods
Eight healthy, habitually active male subjects ran on a motorized treadmill during 2 hours at a speed chosen to promote 70-75% VO2max. To evaluate fatigue the twitch interpolation technique was employed in the plantar flexor muscle group before and immediately after running. To achieve this, the isometric maximal voluntary contraction (MVC) as well as the electrically induced twitch produced during the MVC (IT) and with the muscle at rest (RT) were measured. The level of activation (LOA) during each MVC was calculated as LOA (%) = 100 • (1 – IT • RT-1). Fatigue was defined as a decrease in MVC after running. Central fatigue was defined as a decrease in the LOA, and peripheral fatigue as a decrease in the RT.
Results
MVC decreased significantly from before to after running (from 148.3 16 to 120.8 30.7 Nm; -19 19%). There was central fatigue with a reduction of 19 15% in the LOA (from 83 16 % to 68 20 % after running). Peripheral fatigue did not occur as the RT did not change significantly between before and after running (61.2 10.0 and 57.4 9.5 Nm, respectively). The changes in MVC were correlated to the changes in LOA (r = 0.90; p = 0.02), and could explain 82% (r2 = 0.82) of the changes in MVC. On the contrary, no significant relationship existed between the changes in MVC and changes in RT (r = -0.35; p = 0.39). However, when using multiple regression analysis the changes in LOA and RT together were able to explain 83% (r2 = 0.83; p = 0.01) of the changes in MVC.
Conclusion
In contrast to some previous studies using the twitch interpolation technique to investigate fatigue during whole body prolonged exercise, this study employed supramaximal electrical stimulation over a nerve that activates only agonists and no antagonist muscles. The results demonstrates that after 2 h of continuous treadmill running the reduction on isometric maximal voluntary contraction during plantar flexion was caused by central fatigue.