# Biological sex does not influence the peak cardiac output response to twelve weeks of sprint interval training PMID: 38151488 Journal: Scientific reports Published: 2023-12-27 Authors: Bostad W, Williams JS, Van Berkel EK, Richards DL, MacDonald MJ, Gibala MJ ## Question Does biological sex influence changes in peak cardiac output after 12 weeks of sprint interval training in previously untrained young healthy adults, and does SIT affect plasma volume, endothelial function, arterial stiffness, and carotid artery distensibility? ## Summary This uncontrolled 12-week sprint interval training study tested whether biological sex affected the peak cardiac output response in previously untrained young adults. Twenty participants, 10 males and 10 females, completed about 33 supervised cycle-ergometer SIT sessions. VO2peak and peak power increased, but peak cardiac output did not change, and there was no sex-by-time interaction for VO2peak or peak cardiac output. Estimated peak arteriovenous oxygen difference increased, suggesting that peripheral oxygen extraction or diffusion capacity may have contributed to the VO2peak improvement. The study also found limited changes in vascular outcomes, with lower resting heart rate and systolic blood pressure after training and no broad effect on endothelial function. ## Population - Previously untrained young healthy adults grouped by biological sex. - Sample size: 20 - Age: Females 21 +/- 4 years [18-28]; males 21 +/- 3 years [18-28]. - Sex: 10 female and 10 male participants. - Fitness level: Previously untrained; baseline VO2peak relative to fat-free mass did not differ between sexes. - Health status: Healthy young adults. ## Methodology - Two-factor repeated-measures mixed design comparing males and females before and after 12 weeks of supervised sprint interval training. - 12 weeks, with 4-week intermediate vascular and blood follow-up and 12-week post-training testing. - Human Performance Laboratory and Vascular Dynamics Laboratory at McMaster University; supervised in-person cycle-ergometer training. ## Protocol - 12-week cycle sprint interval training. - Modality: Cycle ergometer. - Work intervals: 20-second all-out sprint. - Recovery: 2 minutes recovery between sprints. - Sets or repetitions: 3 sprints per session. - Intensity: All-out; sprint-bout heart rate averaged 92 +/- 5% HRpeak. - Session duration: 10 minutes including warm-up, recoveries, and cool-down. - Frequency: 3 sessions per week. - Program length: 12 weeks. - Progression: Resistance individualized by CAROL Bike self-learning algorithm based on weight, power output, and fatigue index. ## Outcomes ### VO2peak Status: improved VO2peak increased after 12 weeks of SIT from 2.89 +/- 1.0 to 3.16 +/- 1.0 L/min, with no sex-by-time interaction. Main effect of time eta2p=0.53, p<0.001; sex-by-time p=0.61. ### Peak cardiac output Status: no clear change Peak cardiac output was unchanged after training, and biological sex did not affect the response. 15.1 +/- 3.0 to 15.2 +/- 3.3 L/min, eta2p=0.002, p=0.85; sex-by-time p=0.72. ### Peak arteriovenous oxygen difference Status: improved Estimated peak a-vO2 difference increased after SIT. 187 +/- 36 to 204 +/- 30 ml O2/L blood, eta2p=0.35, p=0.006. ### Vascular outcomes Status: mixed SIT had relatively little effect on arterial stiffness and endothelial function, but resting cardiovascular measures and some carotid stiffness findings suggested limited central cardiovascular adaptation. Central, leg, and arm PWV were not significantly changed; carotid distensibility time effect p=0.04; beta-stiffness sex-by-time interaction p=0.02. ## Practical Insights - A 10-minute cycle SIT session with only 60 seconds of all-out work can improve VO2peak in young untrained adults when performed three times weekly for 12 weeks. - Heart-rate response during very short all-out intervals can be high during sprints even when whole-session average intensity is lower. - This study does not justify sex-specific programming differences for VO2peak or peak cardiac output responses to this SIT format. - Mechanism claims should be cautious: VO2peak improved without detectable peak cardiac output change. ## Limitations - Small sample of 20 completers. - No non-exercise control group. - Young healthy untrained population limits clinical and older-adult translation. - Peak cardiac output was measured by inert gas rebreathing, which the authors note may underestimate peak cardiac output and may be limited for detecting changes. - Plasma volume was estimated indirectly rather than directly measured with a gold-standard method. - Adverse events were not reported in detail. ## Safety And Adherence - One recruited participant withdrew after a health status change made them ineligible to continue. - The article did not provide a systematic adverse-event count for the SIT sessions. ## Original Sources - [PubMed](https://pubmed.ncbi.nlm.nih.gov/38151488/) (pubmed) - [DOI](https://doi.org/10.1038/s41598-023-50016-4) (doi) - [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10752867/) (full text) ## Agent Guidance Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.