Neural responses to fatiguing heavy-resistance loading before and after prolonged strength training

Abstract

Acute effects and recovery from exercise-induced fatigue is widely studied, but there is a lack of data concerning adaptations to acute fatigue with strength training. The purpose of this study was to examine neural responses to fatiguing heavy-resistance loading (HRL) and investigate neuromuscular adaptations with strength training. 75 healthy males (age 32 ± 8 years) divided randomly into the control (n=23) and training group (n=52) attained in 20 weeks of strength training. Before and after 20 weeks of strength training subjects carried out HRL comprising of 5 sets of 10 maximal bilateral knee extensions (10 RM). Maximal voluntary force, M-wave and twitch characteristics were measured before and after HRL and electromyographic (EMG) activity continuously during the HRL. Blood lactate concentrations were determined before, immediately after, and 15 and 30 minutes after the HRL. In the training group torque, power and load used during the HRL and maximal voluntary contraction (MVC) torque of knee extension and aEMG during MVC increased with strength training. MVC torque decreased on average 21 ± 12 % after HRL. There were no changes in lactate responses with training except pre-loading for the training group. In the control group lactate concentrations were significantly smaller at all time points at week 20 than at week 0. In the 4th and 5th sets mean power frequency of EMG was significantly lower (p<0.001) and in the 2nd set higher (p<0.05) after the training than before training. HRL resulted in significant decreases in peak tension (PT), contraction time (CT), rate of force development (RFD) and a significant increase in half-relaxation time (HRT) of the twitch. Relative, percentual changes (between pre-loading and post-loading state) in twitch characteristics were not significantly different between week 0 and week 20. HRT increased significantly due to HRL at week 0 (P<0.05) but not at week 20. No significant differences were observed in post-activation potentiation (PAP) between the corresponding week 0 and week 20 values. In the training group pre-loading PAP values were significantly different from post-loading values (P=0.01) at week 0, but not at week 20. An increase in duration of M-wave was observed after HRL in rectus femoris (P<0.05) and vastus medialis (P<0.05) before and after strength training. There were no significant changes in M-wave amplitude between pre- and post-loading values or any training induced changes in M-wave amplitude or duration. Torque, power and mean power frequency decreased during HRL. Neural input (indicated by aEMG) to muscle did not decrease during the HRL indicating that fatigue is mostly peripheral in origin in intermittent, multiple repetition loadings, such as the loading used in this study. All in all HRL induced fatigue changes were as expected from previous knowledge. Torque, power and neural input to muscles increased with strength training. Power was increased only for the two first sets and after that fatigue prevented usage of improved power. Strength training did not have effect on characteristics of the M-wave or twitch except HRT. Increase of HRT due to fatigue can be overcome by strength training. In general effects of strength training were in line with previous studies. In conclusion maximal strength oriented training is effective in improving torque and po