Relationship between muscle fatigue and oxygen uptake during cycle ergometer exercise with different ramp slope increments

Summary

The surface electromyogram (EMG) from active muscle and oxygen uptake ($\dot VO_2$) were studied simultaneously to examine changes of motor unit (MU) activity during exercise tests with different ramp increments. Six male subjects performed four exhausting cycle exercises with different ramp slopes of 10, 20, 30 and 40 W · min⁻¹ on different days. The EMG signals taken from the vastus lateralis muscle were stored on a digital data recorder and converted to obtain the integrated EMG (iEMG). The$\dot VO_2$ was measured, with 20-s intervals, by the mixing chamber method. A non-linear increase in iEMG against work load was observed for each exercise in all subjects. The break point of the linear relationship of iEMG was determined by the crossing point of the two regression lines (iEMG_bp). Significant differences were obtained in the exercise intensities corresponding to maximal oxygen uptake ($\dot VO_{2 max}$) and the iEMG_bp between 10 and 30, and 10 and 40 W · min −1 ramp exercises (P < 0.05). However, no significant differences were obtained in$\dot VO_{2 max}$ and$\dot VO_2$ corresponding to the iEMG_bp during the four ramp exercises. With respect to the relationship between$\dot VO_2$ and exercise intensity during the ramp increments, the$\dot VO_2$-exercise intensity slope showed significant differences only for the upper half (i.e. above iEMG_bp). These results demonstrated that the$\dot VO_{2 max}$ and$\dot VO_2$ at which a nonlinear increase in iEMG was observed were not varied by the change of ramp slopes but by the exercise intensity corresponding to$\dot VO_{2 max}$ and the iEMG_bp was varied by the change of ramp slopes. In addition, the significant differences in the$\dot VO_2$ exercise intensity slopes for the upper half of the tests would suggest that the recruitment patterns of MU and/or muscle metabolic state might be considerably altered depending upon the ramp slope increments.