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A Critical Evaluation of the Integration of High-Intensity Interval Training

Updated: May 25


High-intensity interval training (HIIT) is considered one of the most effective methods for

improving cardiorespiratory fitness and metabolic function (Helgerud et al., 2007; Poole &

Jones, 2023). A key resulting adaptation is an improvement in the maximal rate of oxygen

uptake (VO2max) (Buchheit & Laursen, 2013a). While popular definitions often describe HIIT

as exercise reaching 90% of maximal heart rate (the “red zone”), research contexts define it

more precisely as repeated bouts performed in the severe-intensity domain at or near

VO2max (Edwards et al., 2023; Seiler, 2024). In addition to balancing the overall training

load, the specific characteristics of HIIT sessions—such as the duration, intensity, and

recovery periods of the intervals—can be manipulated to target different physiological

adaptations (Seiler, 2024). Therefore, optimizing performance depends not only on the

appropriate mix of high- and low-intensity training but also on the precise manipulation of the

HIIT structure itself.

Addressing a gap in the literature, Odden et al. (2024) conducted the first longitudinal

study to measure the fraction of VO2max achieved in every session of an interval training

intervention and its relationship to performance adaptations in well-trained cyclists.

Participants who attained a higher percentage of VO2max during the intervals demonstrated

greater performance improvements, as reflected by larger increases in VO2max, higher

power output at a 4 mmol·L⁻¹ blood lactate concentration and improved maximal 1-minute

incremental power output. These findings suggest that accumulating more time close to

VO2max during exercise may lead to greater improvements in endurance performance. This

raises the important question of which HIIT structure elicits the greatest physiological

adaptations and therefore represents the most effective strategy for optimizing performance.

Longer intervals risk being too demanding to sustain the intensity required to achieve a high

fraction of VO2max. Conversely, shorter intervals may be too brief for VO2max to rise

sufficiently before muscular fatigue becomes the limiting factor (Buchheit & Laursen, 2013a).

A meta-analysis by Wen et al. (2019) examined the effects of different HIIT protocols on

VO2max. The researchers concluded that interval durations exceeding two minutes per bout

are superior for maximizing VO2max improvements, although shorter intervals also yielded

benefits. Furthermore, the study suggests that a higher total work volume per session—

particularly volumes exceeding 15 minutes—leads to greater improvements, independent of

the specific interval structure. These effects are most pronounced when the training program

is sustained for 4–12 weeks or longer.

In contrast, a recent study by Urianstad et al. (2024) found that short intervals yielded a

superior physiological response compared to longer, continuous bouts. The protocols were

matched for total work duration, which consisted of six 8-minute blocks. The short-interval

protocol, which alternated 30 seconds of work with 15 seconds of active recovery, resulted in

a higher fraction of VO2max and more accumulated time above 90% VO2max. These

intervals were performed at 118% of the power output from a 40-minute all-out trial

(PO40min), with recovery phases at 60% of PO40min The continuous work bouts, by

comparison, were performed at a steady 100% of PO40min. Notably, the study compared

two protocols from opposite ends of the HIIT spectrum—extremely brief intervals and

comparatively long, 8-minute bouts—without investigating any intermediate durations.

Despite significant differences in time spent near VO2max, the average oxygen consumption

during the sessions was remarkably similar between the groups: 86.7% of VO2max for the

short-interval protocol versus 85.0% for the continuous-interval protocol. However, the short-

interval group also accumulated a significantly greater amount of time above 90% of

VO2max. A critical consideration is that individuals exhibit different physiological responses

even when working at the same relative external load. For example, at a given percentage of

PO40min, athletes can operate at different fractions of their VO2max. Odden et al. (2024)

leveraged this variability in their study. Although all participants trained at the same relative

intensity, the researchers could differentiate between two groups based on their physiological

response. One group accumulated significantly more time above 90% of VO2max than the

other. This group also demonstrated a higher average fractional utilization of VO2max during

the intervals (86.2%) compared to the "low-responder" group (79.9%). However, the intensity

of an interval must be carefully managed, as higher intensities can induce premature fatigue

and therefore reduce the total time spent near VO2max. For instance, a study by Kemi et al.

(2019) demonstrated this trade-off: participants performing 4 × 4-minute intervals at 100% of

VO2max could only complete a maximum of 70% of each interval before exhaustion. In

contrast, those exercising at 80–95% of VO2max were able to complete the entire session.

This distinction is critical because, as established by Odden et al. (2024), the total

accumulated time at or near VO2max —not just the peak intensity reached—is a crucial

driver of endurance adaptations.


Interestingly, in the study by Odden et al. (2024), despite the standardized external load

and no differences in perceived exertion (RPE) or heart rate, one group exercised at a

significantly higher fraction of their VO2max than the other. This finding suggests that

common markers like RPE and heart rate can be misleading indicators of the true

physiological stimulus, potentially leading to suboptimal training prescriptions. Consequently,

direct VO2 measurement is likely a superior method for prescribing an appropriate training

stimulus. A fundamental strategy to improve training accuracy would therefore involve first

determining an individual's VO2max, and then measuring the fractional utilization achieved

during various interval protocols to identify the most effective structure. Nevertheless, while

the specific protocol is a key consideration, it is still established that generally HIIT, in its

various forms, provides a potent stimulus for cardiorespiratory adaptations (Seiler et al.,

2013; Seiler, 2024).

However, short-interval protocols are generally performed at a much higher power output

than long-interval protocols. This higher intensity is coupled with greater neuromuscular

activation and recruitment of fast-twitch muscle fibres, which in turn is associated with higher

peak blood lactate concentrations and a greater anaerobic energy contribution (Urianstad,

2024). This highlights a key concept from Buchheit and Laursen (2013b), who concluded that

while different HIIT protocols may elicit similar cardiorespiratory responses, they can be

associated with vastly different peripheral adaptations, particularly at the neuromuscular level

and in the anaerobic energy system. This appears to be an important factor concerning the

second lactate threshold (LT2). A higher LT2, located closer to VO2max, would allow an

athlete to sustain exercise at intensities near VO2max for a longer duration. Supporting this,

Odden et al. (2024) found a significant positive relationship between an athlete's baseline

fractional utilization of VO2max at LT2 and the percentage of VO2max they could achieve

during interval training. Therefore, a high lactate threshold may be a key determinant of the

ability to accumulate time at a large fraction of VO2max, potentially leading to superior

training adaptations. Additionally, low-intensity training is a primary stimulus for structural

cardiac adaptations, such as increased cardiac volume and mass (Seiler, 2024) These

adaptations can, in turn, enhance the effectiveness of subsequent high-intensity interval

training (HIIT) by improving the heart's stroke volume (Dausin et al., 2024). Consequently,

the physiological benefits achieved through low-intensity training are critical for supporting

the adaptive responses to HIIT and should not be overlooked.


Finally, the optimal HIIT protocol cannot be evaluated solely on its ability to maximize time

spent at a high fraction of VO2max. A truly effective prescription must also account for other

critical factors, including the associated physiological strain, the athlete's individual

prerequisites, and the protocol's integration within the broader training program. Therefore,

the choice of a HIIT protocol is highly specific, and no single protocol is universally superior.

In addition to the direct effects of a given session, an optimal training structure must integrate

supporting variables, the athlete's specific characteristics, and the desired long-term

adaptations.





References

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