REF-995 · Reuter et al., 2023 · Sports Medicine - Open, Vol. · Paper · PubMed

Does Higher Intensity Increase the Rate of Responders to Endurance Training When Total Energy Expenditure Remains Constant? A Randomized Controlled Trial

B+moderate evidenceMeasured on non-athletesWritten before October 2026, being re-checked

The question was whether raising training intensity can boost how cardiorespiratory fitness responds to moderate-intensity endurance training. Thirty-one healthy, untrained participants (BMI 25.4 ± 3.3 kg m-2, age 46 ± 8 years, VO2max 34 ± 4 mL min-1 kg-1) first did 10 weeks of moderate training (3 days a week, 50 min a session at 55% HRreserve).

Stratified randomization (age, gender, VO2max response) then split them into CON, which kept up moderate intensity for 16 more weeks, and INC, which did energy-equivalent training at 70% HRreserve for 8 weeks followed by 8 weeks of high-intensity interval training (4 × 4). A responder was anyone whose VO2max gain was larger than the technical measurement error.

At 26 weeks, VO2max response was 3.4 ± 2.7 mL kg-1 min-1 in INC versus 0.4 ± 2.9 mL kg-1 min-1 in CON, a difference with P = 0.020. After the first 10 weeks, 16 of 31 participants (52%) were VO2max responders; CON added no responders after that, whereas INC reached 13 of 15 (87%).

Participants

Thirty-one healthy, untrained participants: BMI 25.4 ± 3.3 kg m-2, age 46 ± 8 years, VO2max 34 ± 4 mL min-1 kg-1. INC n = 15 (from "13 of 15"); the CON group size and the sex split are not given in the abstract.

Protocol

Phase 1 (all participants): 10 weeks of moderate-intensity training, 3 days a week, 50 min per session at 55% HRreserve.
Allocation: randomization stratified by age, gender and VO2max response.
CON (continuous moderate intensity): another 16 weeks at moderate intensity.
INC (increased intensity): 8 weeks of energy-equivalent work at 70% HRreserve, followed by 8 more weeks of 4 × 4 high-intensity interval training.
Responder definition: VO2max increase above the technical measurement error.
Analysis: between-group comparison of VO2max change and of responder counts; within INC, the change in the number of responders over time.

Key Results

At 26 weeks, VO2max had risen by 3.4 ± 2.7 mL kg-1 min-1 in INC and by 0.4 ± 2.9 mL kg-1 min-1 in CON (P = 0.020).
The first 10 weeks of moderate training left 16 of 31 participants (52%) counted as VO2max responders.
CON gained no further responders over the next 16 weeks of continuous moderate training.
Energy-matched training with rising intensity lifted the responder count in INC significantly, to 13 of 15 (87%; P = 0.031).
The higher intensities were more effective at raising the responder rate than continuing at moderate intensities (P = 0.012).
Authors' conclusion: with total energy expenditure held constant, high-intensity interval training still raises the VO2max response rate; sticking with moderate intensities might not be the best way to optimize training gains.

Practical Application

"My VO2max did not respond to steady moderate training — more of the same, or change something?" In a randomized trial of 31 healthy untrained adults (46 ± 8 years, VO2max 34 ± 4), 16 of 31 (52%) were VO2max responders after 10 weeks of moderate training; another 16 weeks at the same moderate intensity added no further responders, while energy-matched training at rising intensity ending in 4 × 4 intervals raised responders to 13 of 15 (87%) and VO2max by 3.4 ± 2.7 vs 0.4 ± 2.9 mL kg-1 min-1 (P = 0.020). For an untrained adult who is flat on moderate work, raising intensity at the same energy cost worked and repeating the block did not.
Scope: untrained adults. Inference, not tested here: applying "done and flat → change the stimulus toward higher intensity" to trained athletes. Their room to improve and their existing intensity mix are different.
The trial changed intensity only. It does not show that higher intensity is the only exit — more volume, or simply a different stimulus, was not tested.
The responder label is a threshold on a noisy measurement (a VO2max change above technical error). Not from this paper: a single block's individual response reproduces poorly (REF-498), so read the group shift (52% → 87%), not any one person's label.

Limitations

Untrained, middle-aged adults; no athletes.
n = 31 in total; the responder comparison rests on about 15 per arm.
Responders are defined by a single VO2max change above technical error; no repeated testing to separate true response from day-to-day variability is described in the abstract.
INC changed intensity twice (70% HRreserve, then 4 × 4), so the effect cannot be assigned to either step; a volume increase was not tested.
The number of responders in CON at 26 weeks is not given as a count in the abstract.
Retrospectively registered (DRKS00031445, registered 08 March 2023).
Written from the abstract and record metadata only (full text not read).
How this entry was checked and changed
Why this level

parallel RCT with n = 31; UNTRAINED adults, so anything said about athletes is transfer, not a finding; retrospectively registered