Comparison of acute physiological responses between one long and two short sessions of moderate-intensity training in endurance athletes
Participants
n=14 male endurance athletes, VO2max 69.2 ± 4.2 ml/kg/min (well-trained; Tier 3)
Protocol
2 conditions (counterbalanced crossover), running in the laboratory on two separate days:
SINGLE: one 6×10 min moderate-intensity interval session
DOUBLE: two 3×10 min sessions separated by 6.5 h recovery (time- and intensity-matched)
Each day analysed as first part/session (intervals 1-3) and second part/session (intervals 4-6)
Measures: respiratory variables, HR, blood lactate (BLa), RPE during sessions; supine HR over 60 min of recovery; perceived training stress (1-10) the next morning
Key Findings
| Parameter | SINGLE (1 long) | DOUBLE (2 short) |
|---|---|---|
| HR, part 1 → part 2 | 168 ± 7 → 173 ± 7 bpm (↑, p<0.05) | 171 ± 9 → 166 ± 9 bpm (↓, p<0.05) |
| Blood lactate, part 1 → part 2 | 2.60 ± 0.75 → 3.01 ± 0.81 mmol/L (↑, p<0.05) | 2.72 ± 0.96 → 2.14 ± 0.65 mmol/L (↓, p<0.05) |
| RPE, part 1 → part 2 | 13.4 ± 1.0 → 14.8 ± 1.1 (↑, p<0.05) | not reported as changed |
| Supine HR in 60-min recovery | 65.4 ± 2.5 bpm | 60.7 ± 2.5 bpm (p<0.05) |
| Session RPE | 7.0 ± 1.0 | 6.0 ± 1.3 (p=0.001) |
| sRPE training load | 929 ± 112 | 743 ± 98 (p<0.001) |
| Next-morning perceived fatigue / muscle soreness | higher | lower (p=0.049 / p=0.002) |
Primary finding: one long session showed a duration-dependent "drift" in physiological responses (HR, lactate, RPE rising from first to second half at the same external intensity), which the authors read as a higher overall training stimulus at identical volume
DOUBLE reset: with 6.5 h between halves, HR and lactate were LOWER in the second session than the first — the second session started fresh
The authors' other reading: the lower cost of two short sessions could allow more accumulated time at this intensity (the "double threshold" logic)
Blood glucose and RER were measured: BG was 0.54 ± 0.70 mmol/L higher in the second vs first part of SINGLE and 0.32 ± 0.44 mmol/L lower in the second vs first session of DOUBLE, and RER was 0.025 lower in the second part of SINGLE than in the second session of DOUBLE; fat oxidation was not calculated and Type II fibre recruitment was not measured — any fibre-recruitment explanation is speculation (corrected 2026-10-12, full text)
Mechanisms
Measured: rising HR, lactate and RPE within the long session; lower HR and lactate in the second short session after 6.5 h.
Inference, not tested here: glycogen depletion with progressive Type II recruitment, cumulative metabolic stress and stronger molecular signalling (PGC-1α, AMPK) are the usual explanations for drift; 6.5 h presumably allows partial metabolic recovery. None of these were measured.
Practical Application
The same 60 min of moderate-intensity work done as one 6×10 min session vs two 3×10 min sessions 6.5 h apart in n=14 well-trained male endurance athletes (VO2max 69.2, running): in the single session HR (168→173 bpm), lactate (2.60→3.01 mmol/L) and RPE (13.4→14.8) rose from the first to the second half, while in the split format HR and lactate fell in the second session; the single session left a higher session RPE (7.0 vs 6.0), sRPE load (929 vs 743), higher supine HR in recovery (65.4 vs 60.7 bpm) and more next-morning fatigue and soreness — acute responses only; no adaptation was measured.
Where the point of a moderate session is the drift itself — durability, working in a progressively fatigued state — keep it continuous; splitting it resets the physiology and removes that stimulus.
Where the point is quality minutes at a set intensity with a low fatigue cost (a session the athlete must repeat tomorrow), the split buys the same volume cheaper — the authors' own suggestion for "double threshold training", though the intensity tested here was moderate.
The split needs a real gap — 6.5 h was the interval studied. It is not licence for back-to-back sessions.
Not from this paper: start long continuous sessions at the lower end of the moderate domain — the drift carries the athlete upward through the session (house advice built on the measured drift).
Limitations
Acute physiology only, one session per condition. Lower drift and lower session RPE in the split format do not establish better adaptation from splitting — no training outcome was measured.
n = 14 well-trained males (VO2max 69, Tier 3), running in the laboratory: a recreational athlete's recovery capacity and time budget differ, and other modalities were not tested.
Total volume was matched by design, so this cannot answer the question athletes actually have — whether two shorter sessions let them do MORE.
6.5 h between sessions is a specific and generous window; a lunchtime-and-evening split is not the same intervention.
Moderate intensity between thresholds only; nothing here applies to splitting high-intensity work or long low-intensity work.
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: crossover, n=14
Content correction 2026-09-27
"Elite, Tier 4-5" -> well-trained male endurance athletes (VO2max 69.2), Tier 3; Sport general -> running (lab running).
Blood glucose, fat-oxidation and Type II recruitment rows/bullets -> not in the abstract; removed from findings and marked speculation. Superseded 2026-10-12: the full text reports blood glucose and RER (see Key Findings); Type II recruitment was not measured (corrected 2026-10-12, full text).
Abstract numbers added (HR 168→173, BLa 2.60→3.01, RPE 13.4→14.8 in SINGLE; HR/BLa fell in DOUBLE; supine HR 65.4 vs 60.7; sRPE 7.0 vs 6.0; load 929 vs 743).
Level A -> B (crossover, n=14).
Cross-reference descriptions checked against the cited entries' current content; wrong years, wrong entries and claims the cited entry does not make were corrected.
Full text (checked 2026-10-12)
The fourteen trained males were national-level cross-country skiers (n = 11) and runners (n = 3), aged 23.1 ± 4.1 y, who did at least 5 endurance sessions per week, had more than 5 years of endurance training and had not done "double-threshold training" before (full text, Table 1).
All intervals were run on a 10.5% incline at 90% of each athlete's speed at 4 mmol·L−1 (9.6 ± 0.5 km·h−1), with 2 min of recovery between 10-min intervals and a 20-min standardized warm-up (full text, Table 1).
During sessions, athletes drank sports drink at 1 L·h−1 with 70 g·h−1 of CHO, matched between days, and ate a 185-kcal energy bar and a 100-kcal banana 15 min into the 60-min recovery (full text).
Correction: the entry says "blood glucose, fat oxidation and Type II fibre recruitment were not reported"; the paper measured blood glucose and RER, with BG 0.54 ± 0.70 mmol·L−1 higher in the second vs. first part of SINGLE and 0.32 ± 0.44 mmol·L−1 lower in the second vs. first session of DOUBLE (full text).
VO2 did not differ significantly between or within SINGLE and DOUBLE, whereas VE was 6.7% ± 3.5% higher in the second vs. first part of SINGLE (full text).
Hedges g for HR between the second part of SINGLE and the second session of DOUBLE was 0.90, 0.96 and 0.72 across the three interval stages, and for Bla it was 1.04, 1.25 and 0.22 (full text, Table 2).
Supine HR was 7.8% ± 12.3%, 9.4% ± 13.4% and 9.0% ± 13.7% higher 30, 45 and 60 min after SINGLE than the DOUBLE average, and sRPE training load was 19.6% ± 9.3% higher for SINGLE (full text).
Motivation was 1.0 ± 0.5 points lower before the second DOUBLE session than before the first (p = .041), and perceived training quality did not differ between SINGLE and DOUBLE (full text, Table 3).