Time Courses of Changes In Perceptual, Respiratory, and Neuromuscular Responses in the Severe Intensity Domain

Abstract

The severe intensity domain zone 1 (SIZ1) includes intensities between critical velocity (CV) and 50%∆ (∆ = difference between CV and VO2peak), where exhaustion may occur below VO2peak. The severe intensity domain zone 2 (SIZ2) includes intensities > 50%∆ but < 175% CV, where VO2peak is reached at exhaustion. PURPOSE: This study examined the time course of changes in ratings of perceived exertion (RPE), breathing frequency (ƒb), electromyographic amplitude (EMG AMP) and EMG mean power frequency (MPF) during exhaustive treadmill runs within the SIZ1 and SIZ2. METHODS: Ten runners (Age: 23 ± 3 yrs) performed an incremental treadmill test to determine the velocity at VO2peak (vVO2p). The CV was determined from 4 constant velocity runs. The RPE, ƒb, EMG AMP, EMG MPF (from the vastus lateralis) and times to exhaustion (Tlim) were examined during SIZ1 and SIZ2 runs. Polynomial regression was used to examine the normalized (% change from the initial values) RPE, ƒb, EMG AMP, and EMG MPF versus %Tlim (10-100%) relationships for the SIZ1 and SIZ2 runs. Repeated measures ANOVAs and Student Newman-Keuls tests were used to determine the time course of changes from the initial 10% of Tlim. RESULTS: During the SIZ1 (86 ± 5% vVO2p, ~33%∆, Tlim=17.7 ± 2.6 min) run, there was a quadratic increase for RPE (R2=0.99, p < 0.001), significant from 30 to 100% of Tlim, a cubic increase for ƒb (R2=0.99, p < 0.001), significant from 30 to 100% of Tlim, a linear increase for EMG AMP (r2 = 0.85, p < 0.001), significant at 100% of Tlim, and a linear decrease for EMG MPF (r2 = 0.66, p = 0.004), but no differences among time points. During the SIZ2 (98 ± 4% vVO2p, 93%∆, Tlim=6.6 ± 0.8 min) run, there was a linear increase for RPE (r2=0.99, p < 0.001), significant from 30 to 100% of Tlim, a quadratic increase for ƒb (R2=0.99, p = 0.03), significant from 20 to 100% of Tlim, a quadratic increase for EMG AMP (R2 = 0.68, p = 0.02), but no differences among time points, and a cubic decrease for EMG MPF (R2 = 0.84, p = 0.01), significant at 100% of Tlim. CONCLUSIONS: These findings indicated RPE was more closely related to increases in ƒb, than to neuromuscular fatigue in both the SIZ1 and SIZ2. It is possible feedback from group III and IV afferents in the respiratory muscles contributed to increased perceptions of effort to a greater degree than those in the leg muscles during severe exercise intensities.