Does zone 2 training build mitochondria?

other primary study · 2022

“These results indicate an increase in mitochondrial respiratory capacity in gastrocnemius muscle after training without significant changes in mitochondrial volume density (MitoVD) (Table 1).”

Endurance Training Increases the Running Performance of Untrained Men without Changing the Mitochondrial Volume Density in the Gastrocnemius Muscle. Int J Mol Sci
Who: healthy untrained young non-smoking men aged 20-25 (mean 21.9), Poland
PMID 36142755

pooled analysis · 2026

“This meta-analysis provides evidence that MICT leads to meaningful mitochondrial adaptations including increased mitochondrial volume (MitoVD), upregulation of key regulatory factors in mitochondrial fusion (e.g., MFN2), and increased citrate synthase activity.”

Markers of clinical and mitochondrial adaptation in response to moderate intensity continuous training: A systematic review and meta-analysis. PLoS One
Who: adults aged 20-74 (predominantly male; sedentary, healthy, older and obese) with vastus lateralis biopsies before and after moderate-intensity continuous training
PMID 41481647

pooled analysis · 2025 · landmark

“Using pooled data from the five mitochondrial content markers (mito pooled ; comprising MvD, CS, SDH, COX, and HADH measurements), and adjusting for covariates (intervention weeks, training frequency, initial fitness level, active muscle mass while training, disease status, sex, and age) with appropriate weighting, all training intensity categories displayed significant increases in mitochondrial content (ET, 22.7 ± 4.6%; HIT, 27.0 ± 5.1%; SIT, 27.0 ± 6.7%; all P < 0.001; model 3).”

Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Med
Who: Pooled human exercise-training groups (untrained, moderately trained and well-trained; healthy and diseased, mean age ~25-39 years, 74-97% men) doing low/moderate-intensity continuous endurance training (ET, below second ventilatory threshold/4 mmol/L lactate), high-intensity interval training (HIT) or sprint interval training (SIT), with muscle biopsies
PMID 39390310

trial or observed group · 2017

“By contrast, mitochondrial function was unaffected by either training protocol, suggesting that the observed adaptations reflect an increase in mitochondrial content with short‐term training.”

Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single-leg cycling matched for total work. J Physiol
Who: 10 healthy, habitually active young men (23 +/- 1 yr), one leg trained with moderate-intensity continuous training
PMID 27396440

other primary study · 2012

“COX activity, CS activity and the rate of capillarization were all increased in both age groups as a result of regular physical activity (Table 1).”

The effects of aging, physical training, and a single bout of exercise on mitochondrial protein expression in human skeletal muscle. Exp Gerontol
Who: 24 healthy men: young and old, sedentary vs habitually physically active (cross-sectional)
PMID 22449457

other primary study · 2017

“In contrast to SIE, CE (24 min at 63% of W Peak ) did not provide a sufficient stimulus to induce a significant increase in nuclear PGC-1α protein content immediately post-exercise (1.4-fold), or after 3 hours of recovery (1.0-fold).”

Sprint-interval but not continuous exercise increases PGC-1α protein content and p53 phosphorylation in nuclear fractions of human skeletal muscle. Sci Rep
Who: 20 healthy, moderately trained men aged 18-35, non-smokers, not in cycling sports; 19 completed (between-subjects, n = 10 per group)
PMID 28281651

controlled trial · 2018

“In summary, while the similar PGC‐1α mRNA responses to CM and SE are in line with previous findings (Psilander et al. 2010) and underline the ability of high‐intensity exercise‐induced metabolic stress to counteract a reduced volume of exercise, the lower elevation in PGC‐1α mRNA detected in response to RS than both SE and CM is novel and indicates that the metabolic perturbations induced by RS did not provide sufficient stimuli to compensate for the low work volume.”

Metabolic stress-dependent regulation of the mitochondrial biogenic molecular response to high-intensity exercise in human skeletal muscle. J Physiol
Who: 12 endurance-trained men (amateur cyclists/triathletes, mean age 31.9, VO2max 61 mL/kg/min)
PMID 29727016

controlled trial · 2025

“Thus, while only MICT increased mitochondrial content and CI activity, only SIT—which was associated with an increase in mitochondrial stress and quality control pathways (see Fig. 4E )—increased mitochondrial respiratory rates.”

Sprint interval exercise disrupts mitochondrial ultrastructure driving a unique mitochondrial stress response and remodelling in men. Nat Commun
Who: 28 healthy young males (mean age 26.5, VO2max ~52 mL/kg/min), Victoria University, Australia
PMID 41326383

pooled analysis · 2025 · landmark

“However, for a given exercise training volume (i.e., number of exercise training hours), training with a high exercise intensity is the most efficient way of increasing mitochondrial content, as SIT in this context showed larger increments than both HIT and ET, and HIT was more efficient than ET.”

Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Med
Who: Pooled human exercise-training groups (untrained, moderately trained and well-trained; healthy and diseased, mean age ~25-39 years, 74-97% men) doing low/moderate-intensity continuous endurance training (ET, below second ventilatory threshold/4 mmol/L lactate), high-intensity interval training (HIT) or sprint interval training (SIT), with muscle biopsies
PMID 39390310

trial or observed group · 2017

“The main finding was that interval compared to continuous single‐leg cycling elicited superior mitochondrial adaptations in human skeletal muscle, as indicated by higher CS maximal activity and mass‐specific J O2 (PCI and PCI&CII) after training.”

Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single-leg cycling matched for total work. J Physiol
Who: Ten young active men, within-subject design (one leg HIIT, other leg MICT)
PMID 27396440

controlled trial · 2014

“Over 24–28 h postexercise, MitoPS was significantly greater after the HIGH vs. LOW exercise trial (P < 0.05).”

Influence of aerobic exercise intensity on myofibrillar and mitochondrial protein synthesis in young men during early and late postexercise recovery. Am J Physiol Endocrinol Metab
Who: 8 healthy recreationally active young men (21 ± 1 yr, VO2peak 46.7 ml/kg/min)
PMID 24595306