# Training Intensity Distribution of a 7-Day HIIT Shock Microcycle: Is Time in the "Red Zone" Crucial for Maximizing Endurance Performance? A Randomized Controlled Trial

PMID: 39235639
Journal: Sports medicine - open
Published: 2024 Sep 5
Authors: Strepp T, Blumkaitis JC, Sareban M, Stöggl TL, Haller N

## Question

During a 7-day running HIIT shock microcycle, how do velocity, power, heart rate, and oxygen uptake classify training intensity distribution, and which red-zone measure relates to endurance-performance change?

## Summary

In endurance-trained runners, a 7-day block of 10 running HIIT sessions improved several endurance-performance measures, but the main article focused on how intensity measurement changes the apparent training distribution. Heart-rate data underestimated time in the high-intensity red zone compared with running velocity and power, and only time in zone 3 by velocity correlated with peak power and 5-km time-trial improvement.

## Population

- Endurance-trained runners or athletes with substantial running volume.
- Sample size: 29
- Age: 30 +/- 6 years
- Sex: 23 male, 6 female
- Fitness level: Endurance-trained; baseline VO2max about 59-62 mL/kg/min
- Health status: Screened athletes; no functional cardiac abnormalities reported

## Methodology

- Randomized controlled parallel-group trial
- Four weeks total with a 7-day intervention and 14-day post-intervention testing phase
- University of Salzburg exercise laboratory plus outdoor running sessions
- Randomized and controlled study design.

## Protocol

- HIIT shock microcycle.
- Modality: Running.
- Work intervals: 4 minutes.
- Recovery: 2.5 minutes: 1 minute passive and 1.5 minutes active.
- Sets or repetitions: 5 intervals per session.
- Intensity: 90-95% HRmax-associated intensity.
- Session duration: About 42.5 minutes including warm-up.
- Frequency: 10 sessions in 7 days.
- Program length: 7 days.
- Progression: No progression; repeated dense sessions.
- HSM + LIT and regular-training control.
- Modality: Running and regular endurance training.
- Work intervals: HSM + LIT used same 4-minute intervals.
- Recovery: Same 2.5-minute HIIT recoveries plus 30 minutes LIT after each session.
- Sets or repetitions: 5 intervals per HIIT session.
- Intensity: Same HIIT target plus LIT not exceeding intensity at 1.5 mmol/L lactate.
- Session duration: About 72.5 minutes for HSM + LIT; control varied.
- Frequency: 10 sessions in 7 days for HSM + LIT; regular training for control.
- Program length: 7 days.

## Outcomes

### Zone classification
Status: mixed
Velocity and power showed polarized distributions, while HR showed a pyramidal distribution and less zone 3 time.

Zone, group, zone x group p<0.001; zone x intensity measure p=0.037.

### Heart-rate zone 3
Status: worse/safety concern
HR-derived zone 3 time declined across the HIIT block, especially with added LIT.

HSM + LIT time x intensity measure p<0.001.

### Performance
Status: improved
VO2max, peak power output, lactate threshold, economy, and 5-km time trial improved over time in intervention groups, with some improvements also in control.

Main time effects reported for performance table; time x group effects were p=0.076 for VO2max and p=0.053 for 5-km time trial.

### Velocity red zone
Status: improved
More velocity-based zone 3 time was associated with greater peak power and time-trial improvement.

PPO rs=0.52, p=0.028; TT rs=-0.47, p=0.049.

## Practical Insights

- HR can underestimate high-intensity exposure during dense HIIT blocks.
- Velocity or power can be useful alongside HR for advanced running intervals.
- Shock microcycles are athlete tools and should not be translated directly to general users.

## Limitations

- Small athlete sample.
- Uneven distribution of female participants.
- Control training was less comparable for velocity and power.
- RPE was not monitored.
- VO2 was measured in only four of ten HIIT sessions.

## Safety And Adherence

- No adverse events were reported in the main text viewed.
- Participants were screened, monitored, and were trained endurance athletes.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/39235639/) (pubmed)
- [DOI](https://doi.org/10.1186/s40798-024-00761-1) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11377407/) (full text)

## Agent Guidance

Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.