Submaximal exercise cardiac output is increased by 4 weeks of sprint interval training in young healthy males with low initial Q∙ -V∙ O2: Importance of cardiac response phenotype

Abstract

Cardiovascular adaptations to exercise, particularly at the individual level, remain poorly understood. Previous group level research suggests the relationship between cardiac output and oxygen consumption (Q_ -V_ O2) is unaffected by training as submaximal Q_ is unchanged. We recently identified substantial inter-individual variation in the exercise Q_ -V_ O2 relationship that was correlated to stroke volume (SV) as opposed to arterial oxygen content. Therefore we explored the effects of sprint interval training (SIT) on modulating Q_ -V_ O2 given an individual's specific Q_ -V_ O2 relationship. 22 (21±2 yrs) healthy, recreationally active males participated in a 4-week SIT (8, 20 second sprints; 4x/week, 170% of the work rate at V_ O2 peak) study with progressive exercise tests (PET) until exhaustion. Cardiac output (Q_ L/min; inert gas rebreathe, Finometer Modelflow™), oxygen consumption (V_ O2 L/min; breath-by-breath pulmonary gas exchange), quadriceps oxygenation (near infrared spectroscopy) and exercise tolerance (6-20; Borg Scale RPE) were measured throughout PET both before and after training. Data are mean Δ from bsl±SD. Higher Q_ (HQ_ ) and lower Q_ (LQ_ ) responders were identified post hoc (n = 8/group). SIT increased the Q_ -V_ O2 post-training in LQ_ (3.8±0.2 vs. 4.7±0.2; P = 0.02) while HQ_ was unaffected (5.8±0.1 vs. 5.3±0.6; P = 0.5). DQ_ was elevated beyond 80 watts in LQ_ due to a greater increase in SV (all P<0.04). Peak V_ O2 (ml/kg/min) was increased in LQ_ (39.7±6.7 vs. 44.5±7.3; P = 0.015) and HQ_ (47.2±4.4 vs. 52.4±6.0; P = 0.009) following SIT, with HQ_ having a greater peak V_ O2 both pre (P = 0.02) and post (P = 0.03) training. Quadriceps muscle oxygenation and RPE were not different between groups (all P>0.1). In contrast to HQ_, LQ_ responders are capable of improving submaximal Q_ -V_ O2 in response to SIT via increased SV. However, the increased submaximal exercise Q_ does not benefit exercising muscle oxygenation.