EMG signs of neuromuscular fatigue related to the ventilatory threshold during cycling exercise.

We questioned whether electromyographic (EMG) signs of neuromuscular fatigue accompany the changes in respiratory variables measured at the ventilatory threshold (VTh) during exercise on a cycloergometer. This was based on the assumption that the activation of muscle afferents sensitive to accumulation of lactate and potassium is suspected to elicit both the EMG signs of fatigue and hyperventilation. In 39 subjects performing an incremental cycling, the EMG estimates of neuromuscular fatigue in vastus lateralis were a non-linear increase in root mean square (RMS), a decrease in median frequency (MF), a non-linear increase in low-frequency EMG energies (EL), and/or a decrease in high-frequency energies (EH). VTh was determined from a non-linear increase in VCO2 [VTh(VCO2 slope)] and an increased value of the respiratory equivalent for oxygen [VTh(VE/VO2)]. We measured a significant increase in venous blood concentration of lactate and potassium, and a significant pHv fall at VTh. One EMG estimate of fatigue was detected in 33/39 individuals and two EMG estimates in 17 subjects. Highly significant positive correlations were found between the oxygen uptakes corresponding to each EMG estimate and to each detection criterion of VTh. These observations suggest that the activation of muscle sensory pathways contribute to the mechanism of VTh.