Study analysis · PLOS One · 2026
Exercise doesn’t create new mitochondria—it rebuilds the ones you already have.
Doing regular moderate exercise like brisk walking makes the energy factories in your muscles bigger and better, but doesn’t make more of them.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study looks at 14 other studies that measured changes in people's muscles and fitness before and after doing moderate exercise. It found that on average, people's muscle health and fitness markers got better, but we can't say for sure that the exercise caused the changes because there was no comparison group.
What’s the bottom line?
This study looks at how regular, not-too-hard exercise changes the tiny energy factories in your muscles, called mitochondria, and helps your body use oxygen better.
How strong is this study?
The study was done carefully by following strict rules and checking the quality of the research it included. However, the original studies were small and didn't compare exercisers to non-exercisers, so we have to be cautious about how much trust we put in the results.
0 / 100
- COI disclosureconflicts of interest not disclosed
- Data availabilitydata not shared
- Code availabilitycode not shared
28 / 100
- Randomizationrandomization unclear
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=184)+12.0/20
- Follow-up+10/10
100 / 100
77 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registration+15/15
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 549 / 100
Probability of being correct
The highest quality evidence. Systematic reviews and meta-analyses that pool randomized controlled trials, giving the most reliable summary of experimental evidence.
This design cannot establish causation — the findings describe an association, not a cause. The included studies are pre-post designs without control groups or randomization, and the overall certainty of evidence is low to very low per GRADE. Therefore, causation cannot be established.
Key takeaways
- 01
Exercise increased muscle mitochondria density by a large amount (SMD 1.04), boosted oxygen use by 9–19%, slightly increased a protein that helps mitochondria merge (MFN2, SMD 0.40), but didn’t change key genes thought to make new mitochondria.
- 02
Yes, these changes mean muscles can produce energy more efficiently, which helps people feel less tired and improves health, especially for those who don’t exercise much or are older.
Surprising findings
- Exercise improved mitochondrial density and fitness without increasing PGC-1α or TFAM—the so-called 'master regulators' of mitochondrial biogenesis.For years, scientists believed PGC-1α was the key trigger that made new mitochondria after exercise. This study shows those markers didn’t change, suggesting the adaptation happens through different pathways—like fusion (via MFN2) rather than creation.
- Mitochondria may get better by merging, not multiplying.Most textbooks emphasize 'mitochondrial biogenesis' as the main way exercise improves energy production. But here, MFN2—a protein that helps mitochondria fuse together—increased significantly (SMD 0.40), while fission and biogenesis markers stayed flat.
Practical takeaways
Do 150 minutes of moderate exercise per week—like brisk walking, cycling, or swimming—to improve your energy levels and heart health.
The benefits are likely due to improving existing mitochondria, not creating new ones, and results may vary based on age, fitness level, and training consistency.
medium confidenceFocus on consistency over intensity—moderate, steady workouts may be enough to trigger meaningful cellular and fitness improvements, especially if you're new to exercise.
Most studies were short (2–16 weeks), so long-term effects are unclear. Also, muscle biopsies were only taken from the vastus lateralis (thigh), so results may not apply to all muscles.
medium confidenceWhy this study matters
Your Muscles Get More Efficient, Not More Crowded
The study found that moderate-intensity continuous training (MICT) increased mitochondrial volume density by a large amount (SMD 1.04), meaning the mitochondria take up more space in muscle cells. This suggests existing mitochondria are growing or fusing, not multiplying.
This means even light, steady exercise—like walking or cycling—can make your muscles better at using energy, helping you feel less tired and improving long-term health.
Heart and Lung Fitness Actually Improves
MICT boosted VO2 max by 9–19%, a clinically meaningful gain, with a pooled effect size of 0.75. This improvement was seen across sedentary and older adults after 6–16 weeks of training.
Better VO2 max means your heart and lungs work more efficiently, which lowers your risk of early death and chronic disease—even if you're not athletic.
The 'Master Switch' for New Mitochondria Didn’t Flip
Despite improvements in mitochondrial density and fitness, the study found no significant changes in PGC-1α, TFAM, or DRP1—key proteins thought to trigger the creation of new mitochondria or their division.
This challenges the long-held belief that exercise works by turning on 'biogenesis' genes. Instead, it may work by upgrading existing mitochondria through fusion and enlargement.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This study looks at how regular, not-too-hard exercise changes the tiny energy factories in your muscles, called mitochondria, and helps your body use oxygen better.
Research results
Exercise increased muscle mitochondria density by a large amount (SMD 1.04), boosted oxygen use by 9–19%, slightly increased a protein that helps mitochondria merge (MFN2, SMD 0.40), but didn’t change key genes thought to make new mitochondria.
What this means - more context
Yes, these changes mean muscles can produce energy more efficiently, which helps people feel less tired and improves health, especially for those who don’t exercise much or are older.
This study aims to evaluate the effects of moderate intensity continuous training (MICT) on molecular transducers of mitochondrial biogenesis and cardiorespiratory fitness in adults, focusing on mitochondrial adaptations and functional outcomes.
The systematic review and meta-analysis found that MICT significantly increases mitochondrial volume density (MitoVD) and VO2 max, with modest increases in citrate synthase (CS) activity and mitofusin 2 (MFN2) expression. No significant changes were observed for PGC-1α, TFAM, or DRP1, suggesting mitochondrial adaptations may occur through fusion and volume enlargement rather than biogenesis or fission. Evidence certainty is low to very low.
Methods Used
Fourteen studies (n = 184) involving adults undergoing MICT for ≥2 weeks were included. All studies collected pre- and post-intervention vastus lateralis muscle biopsies. Meta-analyses used inverse-variance random effects models for outcomes reported in at least three studies, focusing on mitochondrial markers and VO2 max.
Main Finding
MICT significantly increased MitoVD (p < 0.00001) and VO2 max (p < 0.0001), with a pooled standardized mean difference of 1.04 and 0.75 respectively. MFN2 increased modestly (SMD 0.40, p = 0.01), CS showed a non-significant trend (SMD 0.48, p = 0.05). No changes in PGC-1α, TFAM, or DRP1 were observed.
Confidence Level
The overall risk of bias is low to moderate, but the certainty of evidence is very low to low due to heterogeneity, small sample sizes, and variability in training protocols. Results should be interpreted with caution despite statistical significance for key outcomes.
Study Flags
Red Flags
- •Low to very low certainty of evidence
- •High heterogeneity across studies
- •Small total sample size (n = 184) and limited studies per outcome
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Exercise improved mitochondrial density and fitness without increasing PGC-1α or TFAM—the so-called 'master regulators' of mitochondrial biogenesis.
For years, scientists believed PGC-1α was the key trigger that made new mitochondria after exercise. This study shows those markers didn’t change, suggesting the adaptation happens through different pathways—like fusion (via MFN2) rather than creation.
Practical Takeaways
Do 150 minutes of moderate exercise per week—like brisk walking, cycling, or swimming—to improve your energy levels and heart health.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 549 / 100
Probability of being correct
The highest quality evidence. Systematic reviews and meta-analyses that pool randomized controlled trials, giving the most reliable summary of experimental evidence.
Human Meta-Analysis
Subject
Moderate probability
on the GRADE evidence scale
This study looks at 14 other studies that measured changes in people's muscles and fitness before and after doing moderate exercise. It found that on average, people's muscle health and fitness markers got better, but we can't say for sure that the exercise caused the changes because there was no comparison group.
Strengths
- PRISMA-compliant systematic review with meta-analysis
- Use of skeletal muscle biopsies (gold standard for mitochondrial assessment)
- Risk of bias assessed using ROBINS-I and RoB-2 tools
Weaknesses
- All included studies are pre-post without control groups
- Small sample sizes (n=184 total, often <15 per study)
- High heterogeneity in participant characteristics and training protocols
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
This study looks at how regular, not-too-hard exercise changes the tiny energy factories in your muscles, called mitochondria, and helps your body use oxygen better.
Research results
Exercise increased muscle mitochondria density by a large amount (SMD 1.04), boosted oxygen use by 9–19%, slightly increased a protein that helps mitochondria merge (MFN2, SMD 0.40), but didn’t change key genes thought to make new mitochondria.
What this means - more context
Yes, these changes mean muscles can produce energy more efficiently, which helps people feel less tired and improves health, especially for those who don’t exercise much or are older.
This study aims to evaluate the effects of moderate intensity continuous training (MICT) on molecular transducers of mitochondrial biogenesis and cardiorespiratory fitness in adults, focusing on mitochondrial adaptations and functional outcomes.
The systematic review and meta-analysis found that MICT significantly increases mitochondrial volume density (MitoVD) and VO2 max, with modest increases in citrate synthase (CS) activity and mitofusin 2 (MFN2) expression. No significant changes were observed for PGC-1α, TFAM, or DRP1, suggesting mitochondrial adaptations may occur through fusion and volume enlargement rather than biogenesis or fission. Evidence certainty is low to very low.
Methods Used
Fourteen studies (n = 184) involving adults undergoing MICT for ≥2 weeks were included. All studies collected pre- and post-intervention vastus lateralis muscle biopsies. Meta-analyses used inverse-variance random effects models for outcomes reported in at least three studies, focusing on mitochondrial markers and VO2 max.
Main Finding
MICT significantly increased MitoVD (p < 0.00001) and VO2 max (p < 0.0001), with a pooled standardized mean difference of 1.04 and 0.75 respectively. MFN2 increased modestly (SMD 0.40, p = 0.01), CS showed a non-significant trend (SMD 0.48, p = 0.05). No changes in PGC-1α, TFAM, or DRP1 were observed.
Confidence Level
The overall risk of bias is low to moderate, but the certainty of evidence is very low to low due to heterogeneity, small sample sizes, and variability in training protocols. Results should be interpreted with caution despite statistical significance for key outcomes.
Study Flags
Red Flags
- •Low to very low certainty of evidence
- •High heterogeneity across studies
- •Small total sample size (n = 184) and limited studies per outcome
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Exercise improved mitochondrial density and fitness without increasing PGC-1α or TFAM—the so-called 'master regulators' of mitochondrial biogenesis.
For years, scientists believed PGC-1α was the key trigger that made new mitochondria after exercise. This study shows those markers didn’t change, suggesting the adaptation happens through different pathways—like fusion (via MFN2) rather than creation.
Practical Takeaways
Do 150 minutes of moderate exercise per week—like brisk walking, cycling, or swimming—to improve your energy levels and heart health.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 549 / 100
Probability of being correct
The highest quality evidence. Systematic reviews and meta-analyses that pool randomized controlled trials, giving the most reliable summary of experimental evidence.
Human Meta-Analysis
Subject
Moderate probability
on the GRADE evidence scale
This study looks at 14 other studies that measured changes in people's muscles and fitness before and after doing moderate exercise. It found that on average, people's muscle health and fitness markers got better, but we can't say for sure that the exercise caused the changes because there was no comparison group.
Strengths
- PRISMA-compliant systematic review with meta-analysis
- Use of skeletal muscle biopsies (gold standard for mitochondrial assessment)
- Risk of bias assessed using ROBINS-I and RoB-2 tools
Weaknesses
- All included studies are pre-post without control groups
- Small sample sizes (n=184 total, often <15 per study)
- High heterogeneity in participant characteristics and training protocols
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study was done carefully by following strict rules and checking the quality of the research it included. However, the original studies were small and didn't compare exercisers to non-exercisers, so we have to be cautious about how much trust we put in the results.
0 / 100
- COI disclosureconflicts of interest not disclosed
- Data availabilitydata not shared
- Code availabilitycode not shared
28 / 100
- Randomizationrandomization unclear
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=184)+12.0/20
- Follow-up+10/10
100 / 100
77 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervalsno confidence intervals
- Pre-registration+15/15
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 549 / 100
Probability of being correct
The highest quality evidence. Systematic reviews and meta-analyses that pool randomized controlled trials, giving the most reliable summary of experimental evidence.
This design cannot establish causation — the findings describe an association, not a cause. The included studies are pre-post designs without control groups or randomization, and the overall certainty of evidence is low to very low per GRADE. Therefore, causation cannot be established.
Standing
The people behind it
The researchers who wrote the study this analysis is built on.
Authored by
3 researchersIf this is your work, this is how we attribute it on Fit Body Science. Veronica Vabishchevich is listed as the lead author.