Factors underlying the perception of effort during constant heart rate running above and below the critical heart rate

Purpose: This study examined: (1) the sustainability of the critical heart rate (CHR) minus 5 b min⁻¹ (CHR − 5) and CHR plus 5 b min⁻¹ (CHR + 5); (2) the ratings of perceived exertion (RPE), velocity, $$ \dot{V}O_{2} $$V˙O2, minute ventilation ($$ \dot{V}_{E} $$V˙E), breathing frequency (f_b), and electromyographic amplitude (EMG AMP) and EMG mean power frequency (MPF) responses during treadmill running at CHR − 5 and CHR + 5 to determine what factors underlie the perception of effort when heart rate (HR) is held constant; and (3) the relationships among RPE, $$ \dot{V}O_{2} $$V˙O2, and HR, to determine which variable(s) reflect exhaustion during exercise performed at a constant HR. Methods: The CHR was determined in eight runners (mean ± SD; age = 24 ± 3 years) from a series of four exhaustive, constant velocity runs. The RPE, velocity, $$ \dot{V}O_{2} $$V˙O2, $$ \dot{V}_{E} $$V˙E, f_b, EMG AMP, and EMG MPF responses were recorded during runs at the CHR − 5 and CHR + 5. Results: At CHR − 5, RPE, f_b, and EMG MPF increased, while velocity, $$ \dot{V}O_{2} $$V˙O2, $$ \dot{V}_{E} $$V˙E, and EMG AMP decreased. At CHR + 5, RPE and f_b increased, velocity, $$ \dot{V}O_{2} $$V˙O2, $$ \dot{V}_{E} $$V˙E, and EMG AMP decreased, and EMG MPF remained constant. Conclusions: The close association between f_b and RPE throughout the run at CHR − 5 and during the last 50 % of the run at CHR + 5 indicated that muscle afferents may have provided feedback from metabolic and mechanical stimuli that contributed to the perceptual responses. In addition, only RPE consistently indicated exhaustion and the current findings supported its use to monitor exercise performed at a constant HR.