Introduction High-intensity interval training and sprint interval training significantly increase maximal oxygen uptake (VO_2max), which enhances endurance performance and health status. Whether this response is due to increases in central cardiovascular function (cardiac output (CO) and blood volume) or peripheral factors is unknown. Purpose This study aimed to conduct a systematic review and meta-analysis to assess the effects of high-intensity interval training and sprint interval training (referred to as intense interval training) on changes in central cardiovascular function. Methods We performed a systematic search of eight databases for studies denoting increases in VO_2max in which CO, stroke volume (SV), blood volume, plasma volume, end-diastolic/systolic volume, or hematocrit were measured. Results Forty-five studies were included in this analysis, comprising 946 men and women of various health status (age and VO_2max, 20-76 yr and 13-61 mL·kg-1·min-1) who performed 6-96 sessions of interval training. Results showed an increase in VO_2max with intense interval training that was classified as a large effect (d = 0.83). SV (d = 0.69), and CO (d = 0.49) had moderate effect sizes in response to intense interval training. Of 27 studies in which CO was measured, 77% exhibited significant increases in resting CO or that obtained during exercise. Similarly, 93% of studies revealed significant increases in SV in response to intense interval training. Effect sizes for these outcomes were larger for clinical versus healthy populations. Plasma volume, blood volume, and hematocrit had small effect sizes after training (d = 0.06-0.14). Conclusions Increases in VO_2max demonstrated with intense interval training are attendant with increases in central O2 delivery with little contribution from changes in hematocrit, blood volume, or plasma volume.
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