REF-1538 · Morton et al., 2009 · Journal of Applied Physiology · Paper · PubMed

Does doubling HIIT beat the same four sessions spread out?

In 23 recreational men over 6 wk, VO2max and Yo-Yo rose in all with no clear difference. Only the double on placebo got the larger enzyme rise.

Main caveat: Read from the abstract. VO2max and Yo-Yo values, the gap between sessions and glycogen data are missing.
limited evidenceMeasured on recreational people

Three groups of recreationally active men did the same number of high-intensity running sessions, four a week for 6 weeks. Two groups did them as two sessions a day on 2 days a week, one with a 6.4% glucose drink before and during every second session, one with a placebo. The third trained once a day on 4 days a week and drank nothing during training.

VO2max and the Yo-Yo test improved in all three with no difference between conditions. Heat shock proteins, the primary aim, also rose in all with no difference. Succinate dehydrogenase activity rose in all, and significantly more in the placebo double in both muscles.

Our reading: the larger enzyme response followed the double only when the carbohydrate was also removed; the double with a glucose drink did not reproduce it, and performance did not follow the enzyme in any arm.

Protocol

What it isolates: a three-way split that few trials have. Glucose against placebo within the double tests the fuel. The glucose double against once a day comes closest to testing the double with a carbohydrate drink supplied, though the authors class both double arms as Low, only the drink separates them, and the once-a-day arm drank nothing during training. All three did four sessions a week, so doubling also means more rest days.
Who: recreationally active men, 8 (double + glucose), 7 (double + placebo), 8 (once a day).
Training: high-intensity intermittent running, 4 sessions a week, 6 weeks. Double arms: 2 sessions a day, 2 days a week. Once-a-day arm: 4 days a week. The gap between the two sessions is not given in the abstract.
Fuel: 6.4% glucose or placebo immediately before every second session and at regular intervals during it.
Measures: VO2max, Yo-Yo Intermittent Recovery Test 2, HSP70, HSP60, αB-crystallin and succinate dehydrogenase activity in gastrocnemius and vastus lateralis.
Funding and interests: PubMed: Research Support, Non-U.S. Gov't; sponsor and conflicts not readable in the abstract record.

Key Results

MeasureDouble + glucose (n = 8)Double + placebo (n = 7)Once a day (n = 8)Between arms
VO2maxroseroseroseno difference between conditions
Yo-Yo IR2 distanceroseroseroseno difference between conditions
HSP70, HSP60, αB-crystallin, both musclesroseroseroseno difference between conditions
SDH activity, gastrocnemius+27%+76%+53%larger in placebo double, P = 0.03
SDH activity, vastus lateralis+17%+70%+19%larger in placebo double, P = 0.04
Training effects: VO2max and Yo-Yo P = 0.001 across all groups; SDH P = 0.001 in both muscles.
No clear difference, not "the same". With 7 or 8 per arm, no equivalence test and no values for VO2max or the Yo-Yo in the abstract, a modest performance difference could be missed (inference).
Double with glucose against once a day: in the gastrocnemius the double with glucose sat numerically below once a day (+27 against +53%). The abstract does not report a test of that pair.

Mechanisms

Measured: a larger SDH rise in the arm the authors describe as low on both stored and drunk carbohydrate, with no parallel rise in heat shock proteins or performance.
Authors' interpretation: carbohydrate availability appears not to change the heat shock protein response; reduced availability from muscle and from drinks gives a stronger oxidative enzyme stimulus, without better high-intensity performance.
Inference: the enzyme gain here belongs to the second session done without carbohydrate rather than to the second session as such. That is one small study, in recreational men.
Not measured, or not in the abstract: glycogen, interval quality in the second session, threshold, a time trial longer than the Yo-Yo, recovery, sleep, women.

Practical Application

Does doubling HIIT beat the same four sessions spread out? In 23 recreational men over 6 wk, VO2max and Yo-Yo rose in all with no clear difference. Only the double on placebo got the larger enzyme rise.
Good for: the plain case of compressing a week: 4 sessions on 2 days showed no clear loss or gain in VO2max against 4 days, in recreational men, so a double can be a scheduling choice here (inference).
Good for: separating the double from the drink. The enzyme rise did not come with the glucose double; muscle glycogen in the double arms is not in the abstract.
Not good for: concluding that an enzyme gain improves performance. Neither test separated the arms.
Not good for: trained athletes or women; neither was tested.
Not from this paper: the gap to the next hard session is in SYNTH-70, which sets no same-day rule, and fuelling a close next session in SYNTH-85.

Limitations

Read from the abstract. VO2max and Yo-Yo values, the gap between sessions and glycogen data are missing.
Allocation not stated, so differences between arms may partly reflect who was in them.
7 or 8 per arm. Low power for a performance difference.
Drink not equal across arms: the once-a-day group took nothing during training; blinding of the glucose and placebo arms is not stated.
Primary aim was heat shock proteins, not performance or enzymes.
Recreational men only, 6 weeks.
How this entry was checked and changed
Why this level

(from the abstract) — 23 men would be B+ if randomised (rubric: parallel RCT 20-39); one step lower because allocation is not stated, with 7 or 8 per arm. Performance outcomes are present but their values are not in the abstract. Subscription article; no copy in PubMed Central or Europe PMC