The role of muscle pump in the development of cardiovascular drift

This study examined the role of muscle pump in the development of cardiovascular drift (CV_drift) during cycling. Twelve healthy males (23.4±0.5 years, mean ±SE) exercised for 90 min with 40 and 80 pedal revolutions per minute (rpm) at the same oxygen consumption, in two separate days. CV_drift was developed in both conditions as indicated by the drop in stroke volume (SV) and the rise in heart rate (HR) from the 20th min onwards (ΔSV = -16.2±2.0 and -17.1±1.0 ml beat^-1; ΔHR = 18.3±2.0 and 17.5±3.0 beats min^-1 for 40 and 80 rpm, respectively, P < 0.05) but without difference between conditions. Mean cardiac output (CO₂ rebreathing) was 14.7±0.3 l min^-1 and 15.0±0.3 l min^-1, and mean arterial pressure was 100.0±1.0 mmHg and 96.7±0.8 mmHg for 40 and 80 rpm, respectively, without significant changes over time, and without difference between conditions. Electromyographic activity (iEMG) was lower throughout exercise with 80 rpm (35.6±1.2% and 11.0±1.0% for 40 and 80 rpm, respectively). Similarly, total hemoglobin, determined with near-infrared spectroscopy (NIRS) was 58.0±0.8 (AU) for 40 rpm and 53.0±1.4 (arbitrary units) for 80 rpm, from 30th min onwards (P < 0.05), an indication of lower leg blood volume during the faster pedal rate condition. Thermal status (rectal and mean skin temperature), blood and plasma volume changes, blood lactate concentration, muscle oxygenation (NIRS signal) and the rate of perceived exertion were similar in the two trials. It seems that muscle pump is not an important factor for the development of CV_drift during cycling, at least under the present experimental conditions.