REF-1376 · Rosenblat et al., 2022 · Sports Medicine · Paper · PubMed

Does HIIT build the heart more than sprint intervals?

In 32 small studies of healthy adults, only HIIT raised max stroke volume (+14%) and cardiac output (+13%); SIT raised capillaries in active people. Markers, not VO2max. Abstract, B+

Main caveat: Read from the abstract only; included studies, protocols, definitions and risk of bias could not be checked.
moderate evidenceMeasured on non-athletes to trained athletes

Rosenblat, Granata and Thomas asked how HIIT and SIT change the central and peripheral factors that set VO2max.

They searched SPORTDiscus and MEDLINE to 13 June 2020 for prospective English-language studies in healthy 18-65-year-olds with at least three interval sessions or one week of training that measured at least one of blood volume, plasma volume, haemoglobin mass, left ventricular mass, maximal stroke volume, maximal cardiac output, capillary density, maximal citrate synthase activity or mitochondrial respiration.

Thirty-two studies (369 participants, 49 women) entered the quantitative analyses, with 18 HIIT and 17 SIT interventions. Among blood measures, only plasma volume changed significantly, after HIIT. Left ventricular mass rose after HIIT (7.4%, p < 0.001) and SIT (5.3%, p = 0.007) in inactive people, the SIT change flagged as possibly misleading.

Maximal stroke volume (+14.1%, p = 0.015) and maximal cardiac output (+12.6%, p = 0.002) rose only after HIIT. Capillary density rose after SIT in active people (+13.8%, p < 0.001), and maximal citrate synthase activity rose after HIIT (+20.8%) and SIT (+15.7%, I² = 97%) in active people; mitochondrial respiration after HIIT did not change significantly (+5.0%, p = 0.585).

The authors conclude that the adaptations "were dependent on the interval type", with only HIIT improving cardiac function.

Our reading: useful mechanism background — HIIT acted on the heart, sprint work more on the muscle — but the abstract neither compares long with short HIIT nor reports VO2max, so it does not by itself support the claim that longer work intervals raise VO2max more.

Protocol

Design: systematic review and meta-analysis; SPORTDiscus and MEDLINE up to and including 13 June 2020.
Inclusion (abstract): English; prospective; at least three interval sessions or at least one week; HIIT or SIT; participants 18-65 years; at least one listed central or peripheral factor.
n: n = 369 participants (49 women) in 32 studies; 18 HIIT and 17 SIT interventions.
Subgroups named in the abstract: inactive and active individuals.
What was matched: not stated; HIIT and SIT results come from separate pools of studies.
Not in the abstract: the definitions of HIIT and SIT, work-interval lengths and intensities, whether studies had control groups, VO2max results, risk of bias, registration, which studies were included.
Reference list (Crossref metadata, not the abstract): cites Helgerud 2007 (REF-990) among an alphabetical run of training studies and the two 30/15 trials (REF-045, REF-1050) further on; whether any of them was pooled cannot be seen from the abstract.
Funding and interests: no funder in the Crossref or Europe PMC records; not in the abstract.

Key Results

Factor (abstract)Interval type and groupChangep or I²
Plasma volumeHIITthe only significant haematological changenot given
Left ventricular massHIIT, inactive+7.4%p < 0.001
Left ventricular massSIT, inactive+5.3% ("may be misleading")p = 0.007
Maximal stroke volumeHIIT only+14.1%p = 0.015
Maximal cardiac outputHIIT only+12.6%p = 0.002
Capillary densitySIT, active+13.8%p < 0.001
Maximal citrate synthase activityHIIT, active+20.8%p < 0.001
Maximal citrate synthase activitySIT, active+15.7%p < 0.001, I² = 97%
Mitochondrial respirationHIIT+5.0%p = 0.585
Authors' conclusion: "Improvements in the central and peripheral factors influencing VO2max were dependent on the interval type. Only HIIT led to a statistically significant improvement in cardiac function. Both HIIT and SIT increased maximal citrate synthase activity, while changes in other peripheral measures (capillary density, mitochondrial respiration) only occurred with SIT."
Long versus short work intervals: not addressed in the abstract; the comparison is HIIT against SIT, between studies.
A sentence that does not match its results: the conclusion says mitochondrial respiration changed only with SIT, while the results give only the non-significant HIIT figure (+5.0%) and no SIT figure.

Mechanisms

Measured: after HIIT, larger maximal stroke volume and cardiac output and more plasma volume; after SIT in active people, more capillaries; after both, more citrate synthase activity.
Authors' interpretation: the central and peripheral adaptations depend on the interval type, with only HIIT improving cardiac function.
Inference: a rise in maximal stroke volume and cardiac output is the classic route to a higher VO2max, so a format that improves cardiac function is likely to move VO2max; the abstract does not show VO2max itself, nor that work-bout length, rather than intensity or volume, is what separates HIIT from SIT.

Practical Application

Does HIIT build the heart more than sprint intervals? In 32 small studies of healthy adults, only HIIT raised max stroke volume (+14%) and cardiac output (+13%); SIT raised capillaries in active people. Markers, not VO2max. Abstract, B+
Mechanism background, not a format rule: HIIT and SIT appear to act on different links of oxygen delivery — heart and blood with HIIT, muscle capillaries with SIT in active people (pre-to-post changes, from the abstract).
Not evidence on 30/15 versus long reps: the abstract compares no work-interval lengths and reports no VO2max; for trained cyclists the format choice stays with SYNTH-33.

Limitations

Read from the abstract only; included studies, protocols, definitions and risk of bias could not be checked.
No control group was required; the percentages read as changes from before to after training.
Markers, not VO2max or performance; HIIT and SIT compared across different studies.
Small: 369 participants in 32 studies; 49 women.
Heterogeneity up to I² = 97% (SIT, citrate synthase).
The conclusion's line on mitochondrial respiration with SIT is not supported by any figure in the abstract.
Funding and conflicts not available from the abstract or the records.
How this entry was checked and changed
Why this level

a meta-analysis of 32 small studies would sit at A-; held lower because the inclusion asked only for prospective studies (no control group required) and the results read as changes after training, the outcomes are physiological markers rather than VO2max or performance, heterogeneity reached I² = 97% for one outcome, the HIIT-versus-SIT contrast comes from different studies, and the abstract is all that could be read