# Impact of weekly frequency of high-intensity interval training on cardiorespiratory, metabolic, and performance measures in recreational runners - An exploratory study PMID: 40976973 Journal: Physiological reports Published: 2025-09-01 Authors: Lenk M, Matzka M, Lauber L, Kunz P, Sperlich B ## Question How do one, two, or three weekly sessions of 4 x 4-minute HIIT over 6 weeks affect VO2max, time to exhaustion, ventilatory threshold, running economy, submaximal heart rate, and metabolic variables in recreationally active individuals? ## Summary Recreationally active young adults completed 6 weeks of supervised treadmill HIIT using a 4 x 4-minute format once, twice, or three times per week. VO2max and time to exhaustion improved across groups, but the study did not find statistically significant differences between training frequencies. Effect sizes favored two or three weekly sessions, and adherence was high, but the exploratory small sample means the frequency conclusion is preliminary. ## Population - Recreationally active young adults; 26 completers from 44 initially recruited. - Sample size: 26 - Age: 23 +/- 3 years - Sex: 14 female; 12 male inferred from total sample - Fitness level: Highly active on a recreational level; baseline VO2max 51.3 +/- 7.1 ml/kg/min - Health status: Apparently healthy recreationally active individuals ## Methodology - Exploratory randomized parallel-group training intervention comparing three HIIT frequencies. - 6 weeks - Laboratory treadmill testing and supervised treadmill HIIT training - Randomized and controlled study design. ## Protocol - 4 x 4-min treadmill HIIT, 1-3 times weekly. - Modality: Treadmill running. - Work intervals: 4 min high-intensity running. - Recovery: 3 min active recovery at 4 km/h. - Sets or repetitions: 4 intervals per session. - Intensity: 85-90% Vmax and HRmax for first two sessions, progressing to about 95% Vmax and HRmax. - Session duration: At least 25 min for intervals and recoveries; warm-up/cool-down not reported. - Frequency: 1, 2, or 3 sessions/week. - Program length: 6 weeks. - Progression: Velocity increments of 0.2-0.5 km/h across sessions. - Alternative weekly frequencies of same HIIT protocol. - Modality: Treadmill running. - Work intervals: Same 4-min high-intensity intervals. - Recovery: Same 3-min active recoveries. - Sets or repetitions: Same 4 intervals per session. - Intensity: Same individualized HRmax/Vmax-based prescription. - Session duration: Same per-session structure. - Frequency: Comparator frequencies were 1, 2, or 3 sessions/week depending on reference group. - Program length: 6 weeks. - Progression: Same progression rules across groups. ## Outcomes ### VO2max Status: improved VO2max improved across all training frequencies, with largest descriptive effects in the 2x and 3x weekly groups. Significant main effect of time, P < 0.05; Cohen's d range 0.09-0.61 with 95% CIs including zero; no significant time x group interaction. ### Time to exhaustion Status: improved Time to exhaustion improved across all training frequencies, with more favorable descriptive effects in 2x and 3x groups. Significant main effect of time, P < 0.05; Cohen's d range 0.02-0.77 with 95% CIs including zero; no significant time x group interaction. ### Frequency comparison Status: no clear change Changes from pre to post did not differ significantly between one, two, and three weekly HIIT sessions. Time x group interactions were not statistically significant. ### Ventilatory threshold and running economy Status: no clear change VT1 and running economy did not significantly improve over the 6-week intervention. No significant changes reported; authors note intervention may have been too short for these submaximal adaptations. ### Submaximal heart rate Status: mixed Participants training two or three times weekly showed a trend toward reduced submaximal HR, but changes were not statistically significant. Effect estimates were imprecise and confidence intervals overlapped zero. ### Adherence Status: mixed Adherence was high but declined as frequency increased: 100% for 1xHIIT, 96% for 2xHIIT, and 91% for 3xHIIT. ## Practical Insights - For recreationally active users, two weekly 4 x 4-min HIIT sessions may be a practical frequency for improving VO2max and time to exhaustion. - A third weekly session did not show a clear additional benefit in this small exploratory study and had slightly lower adherence. - The protocol is better suited to users with a running base than sedentary beginners. - Frequency recommendations from this paper should be presented as preliminary because all effect-size confidence intervals included zero. ## Limitations - Small sample size. - Exploratory design with no a priori power calculation. - All effect-size confidence intervals included zero. - No significant interaction effects between time and group. - Six weeks may be too short for running economy, ventilatory-threshold, or neuromuscular adaptations. - Participant background activity varied. - No direct strength assessments. - Dietary monitoring was not integrated. ## Safety And Adherence - No training-related adverse events or injuries were reported in the extracted article text. - Some initially recruited participants withdrew due to illness such as colds or flu. - Intensity was individualized and monitored with HR sensors. ## Original Sources - [PubMed](https://pubmed.ncbi.nlm.nih.gov/40976973/) (pubmed) - [DOI](https://doi.org/10.14814/phy2.70573) (doi) - [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12451023/) (full text) ## Agent Guidance Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.