Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression
Scale
Systematic search in PubMed, Web of Science and SPORTDiscus up to 2 February 2022, no date restriction
5,973 participants pooled; 353 articles in the mitochondrial synthesis and 131 articles in the capillary synthesis
Primary outcomes: skeletal-muscle mitochondrial content and capillarization. Secondary: VO₂max, muscle fibre cross-sectional area (CSA), fibre-type proportion
Included populations range from untrained and diseased to trained; analyses adjusted for training frequency, number of intervention weeks and initial fitness
Methodology — 3 training intensity categories
ET (Endurance Training): low- or moderate-intensity continuous training
HIT (High-Intensity Training): high-intensity interval or continuous training
SIT (Sprint Interval Training): sprint intervals
Key Findings
1. Mitochondrial content — similar gains across modalities once covariates are adjusted.
ET +23 ± 5%, HIT +27 ± 5%, SIT +27 ± 7% (P > 0.138)
Not influenced by age, sex, menopause, disease or the amount of muscle mass engaged
Higher training frequency (6 > 4 > 2 sessions/week) → larger increases
2. Per hour of training — large advantage to higher intensity (mitochondrial content).
| Modality | Mitochondrial gain per total hour of exercise |
|---|---|
| SIT | ~3.9× ET, ~2.3× HIT |
| HIT | ~1.7× ET |
| ET | reference (1×) |
For VO₂max the abstract gives no per-hour ratios; it states only that SIT displayed the largest increase per hour of exercise.
3. Capillarization — modality matters less than training stage and fitness.
Capillaries per fibre: ET +15 ± 3%, HIT +13 ± 4%, SIT +10 ± 11% (P = 0.556)
Capillaries per mm²: increased only after ET (+13 ± 3%) and HIT (+7 ± 4%); ET > HIT and SIT (P < 0.05). This coincided with fibre-CSA growth after ET (+6.5%), HIT (+8.9%) and SIT (+11.9%) — larger fibres dilute capillary density per mm²
Capillary gains occurred primarily in the first < 4 weeks and only in untrained to moderately trained participants
4. VO₂max — similar across modalities; SIT most efficient per hour.
ET, HIT and SIT improved VO₂max similarly; HIT showed a tendency for greater improvement than both ET and SIT (P = 0.082)
Higher training frequency (6 > 4 > 2 sessions/week) → larger VO₂max gains
Women showed greater percentage gains in VO₂max than men (P = 0.008)
5. Time course.
SIT was particularly effective for mitochondrial content and VO₂max in the early stages of training
ET and HIT showed slower but steady improvements over a greater number of training weeks — the paper groups HIT with ET here, not with SIT
The abstract reports no HIT plateau and no crossing of HIT below ET; any such trajectory is not in this paper
6. Initial fitness is the main moderator.
Lower initial fitness → greater percentage improvements in mitochondrial content, capillarization and VO₂max
The ability to adapt is maintained throughout life, irrespective of sex and presence of disease
Training load (volume × intensity) is a suitable predictor of mitochondrial and VO₂max changes; the relationship is less clear for capillary adaptations
Fibre-type: proportion of type I fibres unchanged overall (P > 0.116), with ET and SIT exhibiting opposing effects (P = 0.041)
Practical Application
**Systematic review + meta-regression of 353 (mitochondria) and 131 (capillary) articles, 5,973 participants: after adjusting for frequency, weeks and initial fitness, mitochondrial content rose similarly with ET (+23%), HIT (+27%) and SIT (+27%), and VO₂max rose similarly across the three; per hour of training SIT was ~3.9× and HIT ~1.7× as efficient as ET for mitochondria.Lower initial fitness and more sessions per week meant larger gains.** The intensity you choose changes how fast you get there per hour, not how far you get.
Time-crunched athlete: per hour, higher intensity buys more mitochondrial adaptation — SIT most, then HIT. This is the paper's per-hour result; how many such sessions a week is tolerable is not from this paper.
Athlete with time: total gains converge across modalities, so extra hours can be low-intensity without giving up mitochondrial or VO₂max adaptation — and frequency (6 > 4 > 2 sessions/week) adds more than the intensity label.
Do not sell a "HIT plateau" from this review: SIT front-loads its gains, while ET and HIT are the slow-and-steady modalities that keep improving over more weeks.
Capillary growth is a beginner's and early-block phenomenon (< 4 weeks, untrained to moderately trained); a trained athlete should not expect measurable capillary gains from another block of any modality.
Real-world observational support (concurrent literature)
Marathon runners (REF-197 Muniz-Pumares 2025): faster finishers accumulate MORE Z1; absolute MIT/HIT time is similar across performance levels
U23 cyclists → pro (REF-501 Leo 2022): athletes who progressed to the elite ranks accumulated MORE low-intensity training; HIT/MIT volume was similar to those who didn't progress
Female WorldTour cyclists (REF-502 Roete 2025): higher volume of low-intensity training is the only training variable that consistently distinguishes highly successful from less successful seasons
Inference, not tested in this review: these patterns are consistent with total adaptation converging across intensities once volume is allowed to differ — the observational studies are the evidence for that, not Mølmen's time-course result.
Limitations
Heterogeneous protocols pooled — definitions of ET/HIT/SIT vary across decades of studies
Biopsy-based mitochondrial/capillary measures have sampling variability
Populations range from diseased and untrained to trained; the elite ceiling is under-represented, and the abstract does not give the fitness distribution
Cannot fully separate volume from intensity in observational pooling — confounds remain
The primary outcomes are muscle biopsy markers, not performance; VO₂max is secondary and the abstract gives no per-modality VO₂max percentages
Sex-stratified analysis limited; most data on male cohorts
Sport-specific differences not fully addressed (cycling vs running vs cross-country skiing)
How this entry was checked and changed
Why this level
(systematic review + meta-regression: 353 + 131 articles, 5,973 participants)
Content correction 2026-09-27
"500+ studies" -> 353 (mitochondrial) + 131 (capillary) articles, 5,973 participants; primary outcomes are mitochondrial content and capillarization.
VO₂max per hour "SIT ~5× ET, HIT ~2× ET", total VO₂max "ET 10.2 / HIT 12.0 / SIT 8.9%", fitness strata "+8.0% vs +4.5%" -> not in the abstract; replaced with the mitochondrial per-hour ratios (SIT ~3.9× ET, ~2.3× HIT; HIT ~1.7× ET) and "VO₂max increased similarly, HIT tendency P = 0.082".
Time course "HIT flattens weeks 10-15 and falls below ET after ~20 weeks", the quoted sentence, and Archetype D "HIT plateau by week 15-20" -> the paper says SIT works best early while ET AND HIT show slower but steady improvements over more weeks; quote and archetypes removed.
Added abstract findings: capillary gains only < 4 weeks and only in untrained to moderately trained; frequency 6 > 4 > 2; women greater VO₂max % gains. Sport cycling/running/swimming -> general.
Cross-reference descriptions checked against the cited entries' current content; wrong years, wrong entries and claims the cited entry does not make were corrected.
Resync 2026-09-28: Level line carried editorial 'largest ever published' wording and Tier said N/A -> Level states the review's scale; Tier = mixed (untrained to trained).
2026-10-01: cross-reference repointed from a merged framework to the one that replaced it (SYNTH-01→31, 05→34, 09→70, 12/15→32, 19→26).
2026-10-01: citation of the superseded pointer SYNTH-19 repointed to SYNTH-26, which carries the cited content.
Full text (checked 2026-10-12)
ET and HIT were defined as training below or above the second ventilatory threshold/4 mmol/L blood lactate/87% of HRmax/87% of VO2max/75% of Wmax, and SIT as maximal or near-maximal efforts of 4–90 s with recovery periods > 1:1 (full text).
After covariate adjustment, VO2max increased by 10.2 ± 1.0% with ET, 12.0 ± 1.3% with HIT and 8.9 ± 2.6% with SIT (all P < 0.001; model 14) (full text).
Per total hour of training, SIT was ~4.9 times more efficient than ET and ~2.9 times more efficient than HIT for VO2max (both P < 0.001), while HIT was ~1.7 times more efficient than ET without reaching significance (P = 0.166) (full text).
Well-trained participants did not show any change in VO2max per hour of training for any intensity category (P > 0.136) (full text).
After only 2 weeks, mitochondrial content rose 13.5 ± 5.7% (ET), 18.7 ± 7.0% (HIT) and 21.5 ± 8.6% (SIT) and VO2max rose 3.6 ± 2.1%, 6.9 ± 3.1% and 7.9 ± 3.4%; ET and HIT increased further between 2–6 and 6–10 weeks, SIT did not (full text).
Well-trained participants (baseline VO2max 62.2 ± 6.7 mL/kg/min) still increased mitochondrial content by 6.6 ± 6.5% (P = 0.046) and VO2max by 4.5 ± 3.1% (P = 0.004), while untrained participants gained 7.6 ± 4.2 percentage points more VO2max than well-trained ones (full text).
Women gained more VO2max than men in percentage terms (Δ 3.0 ± 2.2% points; P = 0.008) but similarly in absolute terms (3.8 ± 0.8 versus 3.7 ± 0.4 mL/kg/min), and the per-hour VO2max model included 24 training groups of women and 183 of men (full text).
Absolute VO2max gains were larger in participants aged < 35 years (4.8 ± 0.4 mL/kg/min) than in those aged 35–55 (2.9 ± 0.6 mL/kg/min) and > 55 years (2.2 ± 0.6 mL/kg/min) (full text).