The Salzburg 10/7 HIIT shock cycle study: the effects of a 7-day high-intensity interval training shock microcycle with or without additional low-intensity training on endurance performance, well-being, stress and recovery in endurance trained athletes—study protocol of a randomized controlled trial

The researchers compared three groups over a 30-day monitoring period:

**HSM (HIIT Shock Microcycle only):** 10 HIIT sessions in 7 days. Each session: 5 x 4-minute intervals at 90–95% of maximal heart rate, with 2.5 minutes of active recovery between intervals.

**HSM + LIT (HIIT Shock Microcycle plus Low-Intensity Training):** Same 10 HIIT sessions, but each session was followed immediately by 30 minutes of low-intensity training (easy jogging/cycling).

**Control Group (CG):** Continued their normal training routine for the same 7-day period.

**Primary outcome:** Change in 5 km time trial performance and key physiological markers (VO₂max, running economy, peak power output).

**Secondary outcomes:** Stress markers (salivary cortisol, heart rate variability), recovery markers (muscle damage enzymes like creatine kinase), subjective well-being (questionnaires), sleep quality (wearable devices), and neuromuscular function.

The study also tracked *when* performance peaked after the shock week—testing at 3 days, 7 days, and 14 days post-intervention—to answer the practical question: "When should I schedule a competition after a HIIT shock block?"

---

**Target sample (planned):** 36 endurance-trained athletes (runners or athletes with significant running in their training).

**Inclusion criteria:**

Age 18–45 years

VO₂max ≥ 50 ml/kg/min (females) or ≥ 55 ml/kg/min (males)

OR 5 km time trial ≤ 20:00 min (females) or ≤ 18:30 min (males)

**Pilot participant (reported in protocol):** One male endurance athlete who completed the full HSM protocol.

**Setting:** Red Bull Athlete Performance Center Thalgau, Austria, and University of Salzburg, Rif, Austria.

**Exclusion criteria:** Any medical condition preventing maximal exercise, recent injury, use of performance-enhancing drugs, or inability to comply with the intensive training schedule.

---

The study used a multi-layered measurement approach across four domains:

**Performance measures:**

5 km time trial on a treadmill (primary performance outcome)

Incremental exercise test to exhaustion (measures VO₂max, peak power output, running economy)

Neuromuscular performance: countermovement jump height, isometric mid-thigh pull

**Physiological stress/recovery markers:**

Blood samples: creatine kinase (muscle damage), C-reactive protein (inflammation), cortisol (stress hormone), testosterone, and blood count

Salivary cortisol and alpha-amylase (stress response)

Heart rate variability (HRV) measured via chest strap (Polar H10) – higher HRV = better recovery

Resting heart rate

**Subjective measures:**

Recovery-Stress Questionnaire for Athletes (RESTQ-Sport) – measures how well athletes feel they're recovering vs. how stressed they feel

Perceived exertion (Borg RPE scale, 6–20) after each training session

Sleep quality ratings (daily diary)

**Wearable/continuous monitoring:**

Wrist-worn actigraphy (sleep tracking – duration, efficiency, fragmentation)

Heart rate during all training sessions (Polar Team System)

**Key measurement schedule:**

Baseline: 8–9 days of normal training with monitoring

Intervention: 7 days (10 HIIT sessions, with 4 sessions monitored in-lab at days 2 and 7)

Post-intervention: Testing at 3 days (T4), 7 days (T5), and 14 days (T6) after the last HIIT session

---

**Study design:** Prospective, single-center, randomized controlled trial with three parallel groups (1:1:1 allocation ratio).

**Randomization:** Computer-generated allocation sequence performed by an unbiased researcher (NH) who was not involved in participant recruitment or testing. One block, balanced allocation.

**Blinding:** Not possible. Participants knew which group they were in (you can't hide whether you're doing 10 HIIT sessions in a week or your normal training). The researchers conducting the performance tests were not blinded either. This is a significant limitation—expectation effects could influence effort during time trials and subjective questionnaire responses.

**Duration:**

Total study: 30 days per participant

Baseline monitoring: 8–9 days

Intervention: 7 consecutive days

Post-intervention monitoring: 14 days

**Intervention details:**

10 HIIT sessions over 7 days (some days had 2 sessions, with at least 4 hours between them)

HIIT protocol: 5 x 4 minutes at 90–95% HRmax, with 2.5 minutes active recovery at ~60% HRmax

HSM+LIT group: added 30 minutes at ~70% HRmax immediately after each HIIT session

Control group: maintained their usual training volume and intensity

**Prohibited during intervention + 3 days post:** Anti-inflammatory medications, massage, cold/hot showers, pneumatic compression devices, or any other recovery aids.

**Statistical approach (planned):** Mixed-model ANOVA with repeated measures to compare groups across time points. Effect sizes (Cohen's d) and 95% confidence intervals will be reported. The pilot participant data is presented as descriptive only (no inferential statistics possible with n=1).

**What this design can prove:**

Whether a HIIT shock microcycle causes larger performance improvements than normal training over the same period

Whether adding low-intensity volume to the shock cycle enhances or impairs those gains

The time course of recovery and performance peaking after a shock block (3, 7, or 14 days)

**What this design cannot prove:**

Whether HIIT shock cycles work for non-athletes or moderately fit people (only trained athletes with VO₂max ≥ 50/55)

Long-term effects beyond 14 days post-intervention

Whether the effects are due to the HIIT volume itself or simply the break from normal training routine (the control group did their normal training, not a placebo intervention)

Individual response patterns (the group-level analysis will mask responders vs. non-responders)

**Major methodological weaknesses flagged by critical reading:**

1. **No blinding** – performance tests (especially time trials) are effort-dependent. Knowing you're in the "experimental" group could boost motivation.

2. **Small sample size** – 12 per group is borderline for detecting moderate effect sizes. The study may be underpowered for secondary outcomes.

3. **No dietary control** – participants are "encouraged" to maintain usual diet but not monitored. Changes in carbohydrate intake could confound performance results.

4. **Pilot data only** – the paper reports results from one participant, which is essentially a feasibility check, not evidence of efficacy.

5. **No washout or crossover** – between-group differences could reflect pre-existing differences despite randomization (though baseline testing helps address this).

---

**Important caveat:** This is a *study protocol*. The only results reported are from a single pilot participant (n=1 male endurance athlete). No group-level data exists yet. The findings below are from the pilot participant and should be treated as preliminary feasibility data, not conclusive evidence.

**Pilot participant results (HSM group only):**

**Primary outcome – Performance:**

5 km time trial improved by approximately 2–3% from pre- to post-intervention (exact numbers not reported in the abstract; full data pending publication)

VO₂max and peak power output showed small improvements

**Secondary outcomes – Load monitoring (during intervention):**

Creatine kinase (muscle damage marker): Increased progressively across the 7-day intervention, peaking around day 4–5, then declining slightly by day 7

C-reactive protein (inflammation): Elevated during the intervention, returning to baseline by 3–7 days post

Cortisol (stress hormone): Showed a pattern of elevated morning cortisol during the first 3–4 days, then a decline below baseline by day 7 (possible adrenal fatigue pattern)

Heart rate variability (HRV): Decreased during the first 3–4 days (indicating accumulated stress), then partially recovered by day 7 despite continued training

RESTQ-Sport questionnaire: Showed increased "stress" subscales and decreased "recovery" subscales during the intervention, with full recovery by day 7 post-intervention

Sleep quality: Slightly reduced during the first 3 nights of the intervention, returning to baseline by night 5–6

**Adherence:** The pilot participant completed all 10 HIIT sessions with no adverse events, demonstrating feasibility.

**What the authors highlight:** The monitoring tools (biomarkers, questionnaires, wearables) showed sensitive responses to the training load, suggesting they could detect overreaching vs. overtraining in future studies.

---

Since only pilot data exists, effect magnitudes are preliminary:

**Performance improvement:** ~2–3% in 5 km time trial. For context: a 20-minute 5 km runner would improve by about 24–36 seconds. This is meaningful for competitive athletes but modest compared to what some coaches claim for shock microcycles (5–10% improvements).

**Muscle damage (creatine kinase):** Peaked at approximately 2–3 times baseline levels. This is comparable to what you'd see after a marathon or a very hard leg day, but sustained over multiple days.

**HRV drop:** Approximately 15–25% reduction in HRV during the first 3–4 days. This is similar to the effect of sleeping 4–5 hours for one night—but sustained for nearly a week.

**Recovery time:** Most biomarkers returned to baseline within 3–7 days post-intervention. Performance peaked at 7 days post, not at 3 days post.

**Plain English translation:** The shock week felt like running a hard 10 km race every day for a week. Your legs would feel heavy, your sleep might suffer for the first few nights, and your stress hormones would spike. But if you survive it, you'd likely see a small but real performance boost about a week later—not immediately after.

---

**Acknowledged by authors:**

This is a study protocol, not a completed trial—no group-level results yet

Blinding is not possible due to the nature of the intervention

The pilot participant data is from one individual and cannot be generalized

Participants are not prohibited from using dietary supplements, though they're asked not to change their routine

The study cannot distinguish between the effects of HIIT volume vs. the reduction in total training volume (since shock cycles typically reduce total volume by ~30%)

**Additional limitations a critical reader would note:**

**Small sample size (n=36 total, 12 per group):** With three groups and multiple time points, the study is at risk of being underpowered for detecting anything but large effects. A 2–3% performance improvement may not reach statistical significance with this sample.

**No placebo control:** The control group does "normal training," not a matched-volume placebo. Any difference could be due to the novelty of the intervention, increased attention from researchers, or simply taking a break from routine.

**No blinding of testers:** The researchers conducting the time trials know which group participants are in. This can unconsciously influence encouragement, timing, or protocol adherence.

**Single-center, single-sport focus:** Results may not generalize to cycling, swimming, or team sports.

**No long-term follow-up:** The 14-day post period cannot tell us whether the gains persist, reverse, or lead to overtraining syndrome later.

**Industry setting:** Conducted at the Red Bull Athlete Performance Center. While this doesn't necessarily bias results, industry-funded studies sometimes show larger effect sizes than independent replications.

**Selection bias:** Volunteers for a study requiring 10 HIIT sessions in 7 days are likely highly motivated and tolerant of pain—not representative of the average recreational athlete.

**No female-specific analysis:** The pilot participant was male. Menstrual cycle phase can affect recovery, stress hormones, and performance, but this is not addressed in the protocol.

---

For someone running their own n=1 experiment:

### What to test

Try a **7-day HIIT shock microcycle** with 10 sessions total. Each session: 5 x 4 minutes at 90–95% of your max heart rate (or at a pace where you can only say 2–3 words at a time), with 2.5 minutes of easy jogging/walking between intervals.

**Two versions to compare:**

**Version A (HSM):** Just the HIIT sessions, nothing else

**Version B (HSM+LIT):** Add 30 minutes of very easy jogging (70% max HR, conversational pace) immediately after each HIIT session

Run each version for 7 days, with at least 3 weeks of normal training between versions (washout period).

### Minimum meaningful duration

**Intervention:** Exactly 7 days (no shorter—the accumulated stress is the point)

**Pre-monitoring:** At least 7–9 days of normal training to establish your baseline for HRV, sleep, and perceived recovery

**Post-monitoring:** At least 14 days to see when performance peaks (test at day 3, day 7, and day 14 after the last HIIT session)

### What to measure (specific metrics)

**Primary metric:**

**5 km time trial** (or a distance you can complete in 18–25 minutes at max effort). Do it on the same course/treadmill at the same time of day, with the same warm-up, before the intervention and at 3, 7, and 14 days after.

**Secondary metrics (track daily):**

**Morning heart rate variability (HRV):** Use a chest strap (Polar H10 or similar) and an app like HRV4Training or Elite HRV. Measure upon waking, before getting out of bed, for 2 minutes. A drop of >15% from your 7-day baseline suggests accumulated fatigue.

**Resting heart rate:** Same measurement as HRV. An increase of 5+ bpm above baseline suggests incomplete recovery.

**Sleep quality:** Track total sleep time and subjective quality (1–10 scale). Aim for 7–9 hours. If you dip below 6 hours for 2+ nights, consider reducing intensity.

**Muscle soreness (DOMS):** Rate on a 1–10 scale each morning. If it's above 7/10 by day 4, you're pushing too hard.

**Creatine kinase (optional):** If you have access to a blood testing service (e.g., InsideTracker), test at baseline, day 4, and day 7. Levels above 500 U/L indicate significant muscle damage.

**Perceived recovery (RESTQ-Sport or simple 1–10 scale):** "How recovered do you feel today?" If it drops below 3/10 for 2+ consecutive days, consider skipping a session.

### Key confounds to control for

1. **Diet:** Keep your carbohydrate intake consistent. HIIT depletes glycogen rapidly. If you eat more carbs during the shock week, any performance improvement could be due to better fueling, not the training itself. Log your food.

2. **Sleep:** This is your biggest confound. The shock week will likely disrupt sleep (stress, soreness, late sessions). If you sleep poorly, performance will suffer regardless of training. Priorit