Background and Objectives: During physical exercise, the electrical signal of the muscle fibers decreases due to repeated muscle contractions held at different intensities. The measured signal is strongly related to the motor unit activation rate, which is dependent on the chemical mediators and the available energy. By reducing the energy availability, adenosine triphosphate (ATP) production will decrease and therefore the muscle fibers activation rate will be negatively affected. Such aspects become important when taking into account that the training intensity for many young athletes is rather controlled by using the heart rate values. Yet, on many occasions, we have seen differences and lack of relationship between the muscle activation rate, the heart rate values and the lactate accumulation. Materials and Methods: We conducted a prospective analytical study conducted during a 4-month period, on a sample of 30 participants. All study participants underwent an incremental exercise bike test to measure maximum aerobic capacity as well as the muscle activation rate in the vastus lateralis by using an electromyography method (EMG). Results: With age, the EMG signal dropped, as did the electromyography fatigue threshold (EMGFT) point, as seen through p = 0.0057, r = -0.49, CI95% = -0.73 to -0.16, and electromyography maximum reached point (EMGMRP) (p = 0.0001, r = -0.64, CI95% = -0.82 to -0.36), whereas power output increased (p = 0.0186, r = 0.427). The higher the power output, the lower the signal seen by measuring active tissue EMGFT (p = 0.0324, r = -0.39) and EMGMRP (p = 0.0272, r = -0.40). Yet, with changes in median power output, the power developed in aerobic (p = 0.0087, r = 0.47), mixed (p = 0.0288, r = 0.39), anaerobic (p = 0.0052, r = 0.49) and anaerobic power (p = 0.004, r = 0.50) exercise zones increased. Conclusions: There has been reported a relationship between aerobic/anaerobic ventilatory thresholds (VT1 and VT2) and EMGFT, EMGMRP, respectively. Each change in oxygen uptake increased the power output in EMGFT and EMGMRP, improving performances and therefore overlapping with both ventilatory thresholds.
Keywords: VO2; muscle; physical exercise intensity; ventilatory threshold.