Influence of lactate accumulation of EMG frequency spectrum during repeated concentric contractions

One hundred and twenty consecutive maximal leg extensions at a constant angular velocity of 1.5 radians . s-1 were performed by nine physical education students. Integrated electromyographic (IEMG) activity and power spectrum density function (PSDF) of the EMG were recorded from m. vastus lateralis, vastus medialis and m. rectus femoris using bipolar surface electrodes. Muscle biopsies were obtained from m. vastus lateralis before and after exercise. Tissue samples were analyzed for muscle fiber type distribution and lactate and glycogen concentration. Muscle force and IEMG decreased in parallel over the exercise period. Thus, the IEMG/force ratio was unchanged. Mean power frequency (MPF) of PSDF of the three nucleus decreased by 10% (p less than 0.001) during the initial 25 contractions with no further decline during the latter part of exercise. The relative contribution of the highest bandwidth (130-500 Hz) of the PSDF decreased (p less than 0.001) between the first and final contractions. Muscle glycogen concentration decreased from 85 +/- 23 to 68 +/- 22 mmol . kg-1 w.w. during the exercise. Muscle and blood lactate concentration averaged 12.1 +/- 8.8 mmol . kg-1 w.w. and 3.8 +/- 0.8 mmol . l-1, respectively. The relative changes in MPF and in the highest bandwidth were correlated with muscle lactate concentration and fiber type distribution: in individuals with a high proportion of fast twitch muscle fibres and/or the greatest lactate accumulation, MPF and high frequency components of EMG PSDF decreased most markedly. Reductions in muscle force and IEMG are suggested to be partly due to a decreased motor neuron firing rate. It is discussed whether lactate or associated metabolic changes are influencing the motor unit action potential through feedback processes.