Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression.
Level 1 - systematic review of randomized trials
Systematic review and meta-regression of intervention trials
PubMed 39390310 · doi:10.1007/s40279-024-02120-2
What was done
A systematic review and meta-regression of exercise intervention studies (searched across PubMed, Web of Science, and SPORTDiscus up to February 2, 2022) compared the effects of low- or moderate-intensity continuous endurance training (ET), high-intensity interval or continuous training (HIT), and sprint interval training (SIT) on skeletal muscle mitochondrial content (353 articles) and capillarization (131 articles) in 5,973 human participants. Secondary outcomes included VO2 max, muscle fiber cross-sectional area, and fiber type proportions.
What was found
Adjusted percentage increases in mitochondrial content were similar across ET (23 ± 5%), HIT (27 ± 5%), and SIT (27 ± 7%) (P > 0.138) and were not modified by age, sex, menopause, disease status, or muscle mass engaged. Per hour of exercise, SIT was ~2.3 times more efficient than HIT and ~3.9 times more efficient than ET in expanding mitochondrial content. Capillaries per fiber increased similarly across ET (15 ± 3%), HIT (13 ± 4%), and SIT (10 ± 11%) (P = 0.556), whereas capillaries per mm² increased only after ET (13 ± 3%) and HIT (7 ± 4%), with ET showing larger increases (P < 0.05). Fiber cross-sectional area increased across ET (6.5 ± 3.5%), HIT (8.9 ± 4.9%), and SIT (11.9 ± 15.1%). VO2 max improved across all modalities (trend toward greater gains with HIT, P = 0.082; larger percentage gains in women vs. men, P = 0.008). Lower baseline fitness was associated with greater relative adaptations across all primary metrics.
Why it matters
This large meta-regression shows that mitochondrial and capillary adaptability to exercise is preserved across the lifespan irrespective of sex or chronic disease, with sprint interval training providing the highest time-efficiency for mitochondrial gains.
Limits
The abstract pools heterogeneous training interventions and durations from 353 distinct articles without specifying individual study risk-of-bias scores. Capillary gains were restricted to untrained or moderately trained individuals and plateaued early (<4 weeks), limiting generalizability to well-trained athletes.
Cited by
- supports Cardiovascular and endurance adaptations can be achieved in less time by doing higher-intensity exercise compared to longer-duration moderate-intensity exercise.