What Drives Muscle Growth & What Doesn't | Dr. Mike Roberts
In a Nutshell
Mechanical tension from resistance training is the primary driver of muscle growth through myofibril addition and mTOR signaling; metabolic stress and muscle damage are not required. Response varies widely due to genetics, satellite cell activation, and ribosome content, with true non-responders under 5% and most people able to improve by increasing volume to ~20 sets per muscle group per week. Myonuclei added during training persist, enabling muscle memory, while older adults retain hypertrophy capacity but at reduced magnitude due to impaired proteostasis.
These notes were generated by AI and may contain inaccuracies.
The percentage of non-responders to strength training is low across studies. Most people will see at least some growth, though some individuals experience significantly more growth than others. There are clusters of response levels described as tertiles: low, moderate, and high. Less than 5% of the population, and possibly an even smaller percentage, will not grow muscle from resistance training.
This episode is part of the Perform podcast hosted by Dr. Andy Galpin, professor and scientist at Parker University. The guest is Dr. Mike Roberts, who holds a PhD in muscle biology. His research at Auburn University focuses on how skeletal muscle responds to strength training and resistance exercise. Dr. Roberts is considered the go-to reference for understanding how and why skeletal muscle grows in response to exercise.
Topics covered include responders versus non-responders to exercise, how the hypertrophy process changes with age and sex, nutritional and supplementation influences, and molecular mechanisms of muscle growth. Dr. Roberts' work is frequently misrepresented in discussions of training recommendations.
Mechanical overload is the primary driver of muscle growth. This refers to consistent resistance training in humans. Studies lasting 10, 12, and 16 weeks with 2-3 full-body sessions per week show measurable hypertrophy.
Hypertrophy occurs mainly as radial hypertrophy, meaning an increase in the cross-sectional area of muscle tissue. Ultrasound imaging shows approximately 15% increases in cross-sectional area of muscles such as the rectus femoris and vastus lateralis. Muscle biopsies from the vastus lateralis reveal 20-25% increases in type 2 fiber cross-sectional area. Type 2 fibers show preferential growth. Type 1 fibers may show no hypertrophy or around 10% increase depending on training status.
Until recently, the internal changes within muscle fibers during growth were unclear. The conventional view held that as fibers increase in size, there is a proportional increase in myofibrillar protein content. Research with Professor Troy Hornberger at Wisconsin using fluorescent microscopy on mouse models and human training study samples showed that fiber cross-sectional area increases result from an increase in the number of myofibrils rather than existing myofibrils simply getting larger. Myofibrils maintain a relatively conserved size, and resistance training produces larger radial increases through addition of myofibrils.
Resistance training produces proportional expansion of all cellular components. Some studies using citrate synthase activity as a marker showed decreased activity after 10 weeks of resistance training, suggesting a dilution effect where myofibrillar growth outpaced mitochondrial expansion. However, immunohistochemistry work by Brad Ruple in a 2021 paper demonstrated proportional expansion of mitochondria with hypertrophy, which aligns with the need for energy production in the growing cell.
The vastus lateralis contains approximately 400,000 to 600,000 individual muscle cells. Each muscle is composed of hundreds of thousands of individual myofibers rather than functioning as a single unit.
A myofiber is synonymous with a muscle cell. These cells are extremely long and multi-nucleated, with nuclei located on the periphery. This peripheral nuclear arrangement relates to the myonuclear domain theory of cell size regulation. As cells grow, additional myonuclei are required to regulate larger areas and produce mRNAs for protein synthesis.
Additional myonuclei come from muscle stem cells called satellite cells. Resistance training is a reliable stimulus for activating satellite cells, which then fuse to growing fibers. This mechanism is conserved across rodents and humans.
Myofibrils occupy 80-85% of muscle cell volume. Within myofibrils, sarcomeres run from Z-line to Z-line and contain the contractile proteins actin and myosin.
The myonuclear domain concept suggests that nuclei control specific regions of the muscle cell. When mechanical stretch occurs, existing nuclei can no longer adequately control the expanded area. Satellite cells are recruited to provide additional myonuclei, functioning like adding managers as an organization grows.
Three proposed mechanisms include mechanical tension, metabolic perturbation, and muscle damage. A 2023 collaborative paper involving researchers across five continents concluded that mechanical tension is the prevailing driver of hypertrophy. Karen Esser at the University of Florida is recognized as a foundational researcher in skeletal muscle hypertrophy who trained several prominent scientists in the field.
Lactate accumulation was investigated as a potential driver. Rodent and in vitro studies showed some increases in muscle protein synthesis with lactate exposure. However, a human study infusing lactate during exercise versus buffered saline placebo found no enhancement of post-exercise muscle protein synthesis. This challenges the hypothesis that metabolic stress alone drives hypertrophy and reinforces mechanical tension as the primary mechanism.
Muscle damage is not required for hypertrophy. Research by Matt Stock at UCF comparing concentric-only versus mixed contraction protocols showed concentric training is equally effective based on ultrasound measurements. Meta-analyses support that the eccentric portion is not necessary for hypertrophy. There is almost no relationship between muscle soreness and muscle growth. Soreness can occur as a byproduct of mechanical tension but is not an independent growth mechanism. Pure eccentric training is not recommended specifically for maximizing hypertrophy, though it may serve other purposes such as joint stabilization.
Average hypertrophy responses are approximately 15% at the tissue level and 20-25% at the cellular level after 10-12 weeks of training. Response variation exists across individuals with low, moderate, and high responder clusters.
When examining muscle cell size increases after resistance training, individual muscle cells may show 20-25% growth while whole muscle measurements like the vastus lateralis typically show 10-15% hypertrophy after three months. The group average of 15% hypertrophy masks significant individual variation, with some individuals achieving 25% growth and others only 5%.
Marcus Bamman brought attention to responder heterogeneity at the molecular level. Priscilla Clarkson and Monica Hubal published the first paper in the mid-1990s showing individual response variation across hundreds of subjects. The distribution follows a bell curve where 60-70% of individuals fall in the middle range, approximately 10-15% are high responders, and another 10-15% show no hypertrophic response.
Bamman and Jay Kim's work demonstrated that highest responders to 16 weeks of resistance training exhibit exaggerated satellite cell activation. High responders also begin training with more satellite cells already present in their muscle tissue, providing a genetic advantage. These individuals activate satellite cells that fuse to myofibers, adding myonuclei that enable further growth. In Bamman's study, extreme responders achieved approximately 60% increase in fiber cross-sectional area.
Ribosomes are macromolecules composed of 80 proteins that translate messenger RNA into protein, driving muscle protein synthesis. Research by Brooks Mobley showed that high responders to 12 weeks of resistance training exhibited the greatest increases in muscle ribosome content.
Within normal physiological ranges in young adults aged 18 to peak muscle mass around 35-40 years, neither total testosterone nor free testosterone levels predict differential hypertrophic responses. Individuals with 500 ng/dL versus 900 ng/dL total testosterone show no difference in muscle growth when following identical training protocols. The same holds true for estrogen levels in women.
Baltimore aging studies show a relationship between testosterone levels and muscle mass across the lifespan from ages 18 to 90 in non-training individuals. However, testosterone levels do not predict the hypertrophic response to resistance training. Mechanical tension drives approximately 80% of muscle growth, while hormones and metabolic byproducts account for roughly 20% of the response.
If testosterone falls below normal physiological range, such as a 20-year-old male with 100 ng/dL total testosterone, hypertrophic response is blunted. Phil Atherton's research used pharmacology to suppress endogenous testosterone production in humans, demonstrating reduced hypertrophy when levels drop below 100 ng/dL. However, some hypertrophy still occurs even at these low levels due to mechanical tension mechanisms.
Exogenous testosterone supplementation produces significant hypertrophy regardless of training status. Training recommendations should not be modified based on normal-range testosterone levels from blood tests.
Muscle growth is conserved across young men and women. Meta-analyses by Brandon Roberts, Greg Nuckols, and Andy Galpin show young females can achieve similar or slightly greater relative hypertrophy compared to males with equivalent training stimuli. Estrogen receptor content in muscle does not predict hypertrophy.
No blood markers predict hypertrophic response, including growth hormone, free testosterone, total testosterone, cortisol, or myostatin. Stu Phillips' research confirmed these markers do not correlate with either hypertrophic outcomes or acute muscle protein synthesis responses.
Initial research by Stu Phillips suggested a correlation between muscle biopsy androgen receptor content and hypertrophic response. However, subsequent studies in trained individuals by Cody Han and a 40-person study by Clayton Lents' lab in Brazil found no association between androgen or estrogen receptor content and hypertrophy after 10 weeks of training.
Yuji Tsuchiya's research shows androgen receptor content decreases across the age span, and the relationship between receptor content and muscle growth becomes more evident in older populations compared to young adults.
A genome-wide association study conducted with Stu Phillips and Chris Vann examined whether genetic variants predict muscle fiber hypertrophy or lean mass changes after 10-12 weeks of resistance training. The study identified one gene, GLI3, that showed weak association with response, potentially explaining under 10% of variance and relating to satellite cell response. This finding requires validation in larger cohorts.
Direct-to-consumer genetic testing cannot currently provide clinically relevant information about training response to optimize muscle growth. A single nucleotide polymorphism is insufficient to predict nuanced training responses.
Josh Godwin conducted a two-year project examining cell membrane proteins in trained individuals using proteomics on pre- and post-training biopsies. Resistance training increased total cell membrane protein pool, suggesting membrane-level changes may be important for transducing mechanical tension signals to ribosomes for protein synthesis.
Research identified Vimentin (VIM) as a protein associated with myofiber hypertrophy through small sample size regression analysis. Vimentin is an intermediate filament typically found in cells, but studies showed it deposited in the extracellular matrix, potentially secreted by satellite cells.
Using John McCarthy's Pax7-DTA model of satellite cell ablation in mice, researchers removed satellite cells and applied mechanical overload. Without satellite cells, vimentin was barely present in the extracellular matrix and hypertrophy was reduced. While lack of satellite cells prevents myonuclear accretion (another factor in hypertrophy), the loss of vimentin coincided with reduced hypertrophy.
Additional research in embryonic fibroblasts has linked vimentin to stimulating mTORC1, which is required for bolstering muscle protein synthesis (MPS) in any cell. This connection suggests vimentin may be part of the mechanical tension story that requires further investigation.
Marty Bopart's research examines integrins spanning the cell membrane. These proteins communicate with the extracellular matrix, and when tugged upon, initiate a phosphorylation cascade that transduces the tension signal into anabolic signaling in muscle cells. This mechanism has been considered for the past decade as a potential driver of how mechanical tension translates into anabolic signaling.
Other candidate mechanisms include calcium signaling. When muscles contract, stretch-activated calcium channels allow influx of intracellular calcium. Beyond activating calpain proteases (which produce a proteolytic response), calcium influx also ties into mTOR activation.
Despite these various mechanisms, the core principle remains: mechanical tension feeds into anabolic signaling, which bolsters muscle protein synthesis, leading to deposition of more myofibrils and expansion of the myofiber. Once myofiber size reaches a critical threshold (approximately 10% increase), satellite cells fuse to add additional nuclei, enabling more protein production.
John Petrella, a PhD student with Marcus Bamman, contributed to early landmark hypertrophy molecular research by Jay Kim and Marcus showing modest to low extreme responders over 16 weeks of training in muscle fiber cross-sectional area increases. Their data indicated differences in myostatin signaling between extreme and low responders, with the myostatin response partly diminished in extreme responders.
Myostatin is a negative regulator of muscle growth. Overexpressing follistatin (which inhibits myostatin) in mice, genetically knocking out myostatin, or having mutations in myostatin (as seen in selectively bred dogs or Belgian Blue cattle) produces extremely muscular phenotypes. Similar natural mutations have been documented in humans, including a German child with documented myostatin-related muscle hypertrophy.
Resistance exercise downregulates myostatin at the mRNA level and upregulates follistatin (the myostatin inhibitor) at both mRNA and protein levels. A 2023 paper by Mason Macintosh demonstrated that this pathway represents a conserved response to training bouts. This phenomenon is a general physiological response observed across multiple labs, not a random finding specific to humans.
The percentage of true non-responders to resistance training has been refined through work by multiple labs including Abby Mackey's lab, Marcus Bamman's lab, Tannon's work, Clayton Labardi's work, and Stu Phillips' research. Recent reviews indicate non-responders are quite low, with most people seeing at least some growth. However, response heterogeneity exists with clusters in tertiles (low, moderate, high responders).
True non-responders (those showing minimal or no growth) represent less than 5% of the population, possibly even smaller. Notably, even low responders still get stronger through neurological adaptations, as demonstrated in studies comparing VL (vastus lateralis) ultrasound measurements with 1RM changes.
Clayton Labardi and Marcus published work approximately one year ago showing that lower responders to low-volume training could respond to increased training volume. This provides a practical implication: if someone is not growing, they may not be training with sufficient volume to maximally stimulate hypertrophy.
Cody Han's dissertation work examined volume escalation in well-trained college-age men. The study used a 6-week training protocol starting with typical weight room volumes (3-4 sets per exercise, 3 days per week) and progressively increased volume. By mid-study, participants reached 20 sets per week per exercise; by week six, they reached 32 sets per week per exercise.
The program included back squats (full range), bench press, deadlifts, and cable pull-downs at approximately 10RM intensity (roughly 80% effort, 1-2 reps in reserve). Mood disturbance questionnaires showed the high volume affected participants psychologically.
Biopsies were taken pre-intervention, at week three, and week six. A deload week followed where half the participants reduced volume by 25% and half did nothing (75% volume reduction). Measurements included DEXA scans and bioelectrical impedance analysis for total body water, intracellular water, and extracellular water, conducted with Jordan Moon.
Data analysis indicated that approximately 20 sets per week per exercise maximizes hypertrophy without extraneous increases in extracellular water. This aligns with Eduardo De Souza's data from Tampa, which also points to around 20 sets per week for optimization.
For practical application, splitting 20 working sets per muscle group across 2-3 days per week makes sense. A two upper/two lower split allows the math to work favorably without requiring three-hour gym sessions. Ten working sets per muscle per day is also feasible—for example, three sets of 10 across three exercises.
Not every rep of every set needs to be taken to absolute failure to achieve these volumes. Training at 3-4 reps in reserve according to the RIR scale is sustainable.
Research comparing 30% 1RM training (taken to near volitional fatigue, requiring 40+ reps) versus 80% 1RM training (10-12 reps to near failure) shows both can produce hypertrophy. Stu Phillips' data and lab findings confirm that 30% 1RM training close to failure or 80% close to failure both stimulate muscle growth. However, higher loads (80%) produce greater strength gains as an added benefit.
Casey Sexton's dissertation work examined acute bouts in well-trained individuals. Participants performed either 30% or 80% squat and leg extensor training. Biopsies taken 3 and 6 hours post-exercise showed identical global mRNA expression signatures, identical mTORC1 signaling, and identical myostatin pathway downregulation between conditions. Stu Phillips has shown similar muscle protein synthetic responses between conditions.
Volume load calculations (reps multiplied by weight) showed no statistical difference between 80% failure and 30% failure conditions, suggesting both reach a sufficient mechanical tension threshold.
The mechanical perturbation involves integrins spanning the cell membrane being tugged by the extracellular matrix, transducing signals to the interior portions of integrin complexes, ultimately leading to mTOR activation and muscle protein synthesis. Both 80% failure and 30% failure training appear to meet a minimal threshold at the cellular level, triggering an all-or-none signaling cascade.
Stretch-under-load protocols produce extremely massive hypertrophy. Animal models, including quail wing studies where weights are attached to wings stretching the anterior latissimus dorsi (ALD) muscle, demonstrate significant hypertrophy. In human terms, this would involve holding stretched positions under load for extended periods (days to weeks), which would produce substantial muscle growth if the load could be maintained without tissue rupture.
Troy Hornberger at Wisconsin is conducting research at the forefront of understanding these physiological mechanisms.
In addition to radial hypertrophy, sarcomeres can be added in series, producing longitudinal hypertrophy. Troy Hornberger's work examines how myofibrils lengthen. A preprint indicates that making myofibrils bigger in diameter occurs through canonical mTOR signaling, but lengthening occurs through a different signaling cascade. Hornberger has produced images showing the addition of sarcomeres with longitudinal hypertrophy and the addition of myofibrils with radial hypertrophy.
Brad Schoenfeld's research examined interset stretching in humans and found a decent effect on hypertrophy outcomes measured by ultrasound. Participants would lift weights and then stretch the muscle during rest periods. Immobilization boots placed in dorsiflexion can also cause hypertrophy through tension and tugging on the extracellular matrix that leads to anabolic signaling.
Hyperplasia refers to adding new muscle fibers rather than only making existing fibers thicker. For the last 50 years, the prevailing view has been that hyperplasia does not occur in humans in response to exercise, though it has been observed in other models such as aging-related fiber loss and with exogenous testosterone.
The speaker believes hyperplasia does occur in humans in response to normal exercise, though it is not the primary driver of muscle growth. Troy Hornberger and Stu Phillips maintain that hyperplasia does not occur. Kevin Murach's work with the synergistic ablation model in rodents showed nearly a doubling of plantaris muscle mass, with both increased fiber thickness and increased fiber number via fiber splitting.
Troy Hornberger and Kent Jorgenson published a rebuttal using a mathematical model showing that changes in pennation angle during massive hypertrophy alter the number of fibers visible in cross-section. This mathematical adjustment could explain nearly all of the apparent increase in fiber number observed in synergistic ablation studies.
Biopsies from individuals who have lifted weights for 10 years show muscles approximately 30% larger than untrained individuals, yet fiber size differences do not fully account for the increased muscle size, suggesting more fibers exist in trained individuals. A Norwegian study found histological evidence in powerlifters on steroids, including fissures, central nuclei, and invagination of the cell membrane consistent with myofibers splitting into daughter cells. Joey's work with the LD muscle demonstrated branching fibers that appear to be splitting.
The Norwegian study showed approximately 1% of fibers splitting in controls versus 2-3% in bodybuilders. While this percentage appears small in a single biopsy snapshot, over years of training the compounding effect could be meaningful. The speaker concludes that hyperplasia exists and contributes to some portion of the variance in muscle growth, though it does not explain half or anywhere near the majority of muscle growth.
Lengthened partials involve performing the lengthened portion of a range of motion without completing the full shortening phase. In a leg press, this means lowering the weight until the knees are near the chin and then extending only halfway rather than reaching full lockout. Danny Plotkin is conducting a within-subject dissertation study comparing lengthened partial leg press and leg extension on one leg versus full range of motion on the other leg. MRI scans cover the entire quadriceps from knee to hip, and mid-thigh biopsies are being analyzed for satellite cells, ribosomes, fiber cross-sectional area, and myonuclear number.
Preliminary analysis shows ribosome content increased numerically in the lengthened partial leg. Japanese papers and a 2025 Brad Schoenfeld study found that lengthened partials conferred added hypertrophy benefits for both triceps and biceps. For individuals seeking maximum hypertrophy, integrating lengthened partial training into programs may be beneficial.
Dorsiflexion boots that maintain a stretched position have been studied for their effects on plantar flexors including the gastrocnemius. A disuse study placed participants with and without resistance training history in a knee brace for two weeks, followed by eight weeks of recovery resistance training. Biopsies were taken upon arrival, after bracing, and after retraining.
The rate of vastus lateralis atrophy during two weeks of bracing was identical regardless of prior training status. However, previously trained individuals maintained higher absolute muscle mass even after losing the same relative percentage. This supports the strategy of maximizing muscle mass before planned periods of unloading such as surgery.
After eight weeks of recovery resistance training, relative muscle mass increases were actually greater in previously untrained individuals. This is consistent with newbie gains outpacing recovery gains in those with training histories. Individuals with long training histories approach a ceiling of adaptation; after disuse, retraining returns them near that ceiling. Previously untrained individuals who experience disuse can still make substantial gains upon beginning resistance training.
A two-week full bracing model differs from typical detraining where someone stops exercising for six weeks. Research by Adam Sharples and others has examined longer detraining periods. Ricky Yokozawa and Jeremy Loenneke studied six-month bench press training comparing continuous training versus protocols with one to two weeks off, finding minimal negative impact from brief training breaks.
Muscle physiologists define muscle memory at the level of the muscle fibers themselves rather than neural or motor learning components. When muscle fibers hypertrophy through resistance training, they add myonuclei which function like additional nuclei within the fiber. These myonuclei persist even after training ceases and the fiber size decreases back toward baseline. The myonuclei remain in place and do not disappear during periods of detraining.
The theory of myonuclear permanence proposes that once myonuclei are added during training, they remain even when the muscle atrophies. This creates a retained cellular infrastructure that facilitates faster regrowth upon retraining. The analogy compares this to a store manager who remains even after the store itself has closed, allowing rapid reopening when needed. The manager retains knowledge from previous operations and can apply it to new locations.
Rodent evidence from researchers Kevin and Corey Dunan suggests some potential loss of myonuclei may occur, but the prevailing consensus among most researchers supports myonuclear permanence. When retraining occurs, these retained myonuclei become reinvigorated and support accelerated hypertrophy.
The duration of myonuclear permanence depends on training history and stimulus consistency. Population-level research across age groups suggests that consistent training, even if intermittent, likely preserves myonuclei until approximately age 50 and beyond. The exact timeline remains uncertain and varies based on training duration and intensity.
A seven-week training block followed by seven weeks of detraining and then another seven weeks of retraining demonstrates that individuals not only regain initial hypertrophy levels rapidly but exceed them. This conferred advantage stems from both increased myonuclear number and three-dimensional chromatin changes within nuclei that allow more efficient expression of exercise-responsive genes upon retraining.
Full disuse studies lasting two weeks show no retraction in myonuclei per fiber in trained groups. However, normal daily activities such as walking, standing, and moving provide sufficient stimulus to maintain myonuclei for months to years. Return to baseline may occur after decades, though the precise timeline remains unclear.
One study examined older participants who trained for six months at either 80% or 55% of one-repetition maximum, followed by a one-year hiatus. After the hiatus, the high-load group remained stronger than baseline while the lower-load group did not. This suggests neuromuscular retention of motor drive may be better preserved with higher-load training.
Anabolic resistance describes the reduced ability of muscle tissue to grow in response to training and protein intake with advancing age. Research proposals have examined whether retraining adaptations are impaired in older populations compared to younger cohorts. Average age 55 individuals show reduced adaptability responses compared to college-aged participants.
Above age 80, strength adaptations occur but hypertrophy becomes debatable at the single-fiber level. Multi-omics research from Stanford's Michael Snyder group identified two critical inflection points where molecular changes accelerate aging effects: age 44 and age 60. Age 60 represents the primary point where diminished training responses emerge across endurance and resistance modalities due to chromatin changes, ribosome function alterations, and nutrient-sensing protein dysfunction.
Postmenopausal women retain the ability to grow muscle. Research using two-day-per-week training programs with 12-16 sets per exercise shows that gains occur but at approximately 50% the magnitude seen in college-aged cohorts. This attenuation occurs independent of hormonal changes such as testosterone or estrogen levels, and receptor changes do not appear to be the primary driver.
Global proteomics analysis reveals that young cohorts show robust muscle proteome changes coinciding with enhanced hypertrophy, including increased protein turnover and expression of new proteins. Older cohorts exhibit attenuated hypertrophy alongside a lack of proteostatic changes. The absence of ribosomal protein alterations and other proteostasis-related proteins in older individuals correlates with diminished growth capacity.
Traditional muscle protein synthesis measurements capture only synthesis rates without accounting for breakdown, making net protein balance difficult to determine. Proteomics examines the entire protein complement within tissue, revealing turnover dynamics, protein function, and pathway alterations. Bioinformatics analysis of protein lists identifies functional clusters and predicts active pathways, showing that younger individuals demonstrate enhanced proteostatic mechanisms while older individuals show their absence.
The classical interference effect, established in 1980 research, demonstrates that endurance exercise potentially interferes with muscle hypertrophy while strength training does not impair aerobic capacity gains. Molecular mechanisms proposed include AMPK activation from endurance exercise potentially blunting mTOR signaling from resistance training.
However, resistance exercise can expand the mitochondrial reticulum within muscle cells. This mitochondriogenesis occurs particularly in older and deconditioned populations. Acute bouts of resistance training elevate markers such as PGC-1 alpha, indicating mitochondrial biogenesis pathways can be activated through strength training alone.
Epigenetic markers such as PGC-1 alpha and TFAM drive mitochondrial biogenesis. A study conducted with Robbie Seaborne and Adam Sharples examined six-week training data from older participants using epigenetic pyrosequencing. Across the genome, methylation changes were minimal, but consistent demethylation occurred at the mitochondrial DNA region responsible for biogenesis in all older individuals. Demethylation at this mitochondrial DNA region signals increased mitochondrial DNA replication and therefore more mitochondria. This occurred after only 12 sessions of resistance training in older, deconditioned individuals.
Resistance training therefore supports mitochondrial health, particularly in older populations. Antioxidants taken as mega-dose vitamin C and E together, or green tea catechins, can interfere with mitochondrial biogenesis adaptations during endurance training. The recommendation is to first use exercise to bolster endogenous muscle cell redox systems and stimulate mitochondrial biogenesis, rather than relying on prophylactic antioxidant supplementation. While some ultra-endurance studies show reduced lipid peroxidation markers with mega-dosing, the oxidative stress signal itself may be required to drive muscle adaptation.
A 14-week study with Andreas Kvamme compared two groups. One group performed only SIT (sprint interval training) using a protocol of one minute on and two minutes off, progressing from four to eight or ten sets, three days per week. This produced clear increases in mitochondrial biogenesis and VO2 max. The second group first completed six to seven weeks of exclusive resistance training to increase muscle cell size, ribosome number, and myonuclei per cell, then switched to endurance training. The resistance-training prime produced a hampered mitochondrial biogenesis response during the subsequent endurance block. The hypothesis is that exclusive resistance training rearranges myonuclear gene expression architecture for resistance adaptations, creating a lag before endurance stimuli can fully drive mitochondrial biogenesis.
Meta-analyses, including Jake Wilson's 2012 Gatorade Sports Science Institute paper, examined concurrent training variables. To maximize strength and hypertrophy while incorporating aerobic work, the optimal approach is SIT on a bike performed two days per week. This choice simultaneously addresses exercise modality, frequency, and intensity. High weekly endurance volume (for example, running 100 miles) leaves insufficient specific signal and metabolic capacity for hypertrophy.
SIT (true sprint intervals, such as 30-second Wingate efforts) outperforms long slow distance training for activating higher-threshold motor units. Higher-threshold motor units innervate large numbers of type II fibers, creating neural and fiber-type adaptations more compatible with resistance training. Long slow distance training does not recruit these motor units and therefore creates a mismatch when followed by resistance training. SIT also produces measurable power increases, consistent with practicing high-velocity movement.
Cycling and rowing lack the eccentric loading present in running. The absence of eccentric stress reduces physical load, recovery demand, and injury risk while still allowing high-intensity intervals. Papers comparing modalities confirm that non-eccentric options preserve more recovery capacity. When performing concurrent training, volume should remain low and intensity high; low-intensity, low-volume endurance work fails to stimulate mitochondrial adaptations, while excessive volume erodes hypertrophy potential.
Ribosomes drive muscle protein synthesis and growth; mitochondria supply energy. A proposed experiment would transplant mitochondria into cultured muscle cells and compare anabolic responses (IGF-1 stimulus) against vehicle-treated cells to test whether added mitochondria enhance hypertrophy. Related human data from Stu Phillips' group showed enhanced muscle growth after four to five weeks of low-intensity cycling prior to resistance training, although a subsequent replication study did not confirm the finding. Capillarization and mitochondrial reticulum expansion occur with endurance training, making it difficult to isolate mitochondrial contributions in vivo.
Older deconditioned individuals exhibit robust mitochondrial biogenesis after resistance training; this response may partly explain their hypertrophy gains. Modeling suggests muscle protein synthesis carries a high ATP cost (approximately three to four ATP per peptide bond), and muscle is 85% contractile protein by volume, underscoring the energetic demand of growth.
A review by Paulo Mesquita examined biopsy markers across resistance, endurance, and concurrent training studies. Results are highly variable. Resistance training generally elevates ribosome biogenesis markers more than mitochondrial markers, yet acute resistance bouts also increase PGC-1 alpha mRNA and protein phosphorylation. Endurance exercise reliably raises mitochondrial biogenesis and capillarization markers but does not stimulate ribosome biogenesis. High-volume resistance training can begin to engage muscular-endurance pathways that overlap with mitochondrial signaling, whereas low-repetition sets do not.
Early literature reported reduced mitochondrial density after strength training and concluded that resistance exercise impairs aerobic capacity. This interpretation failed to account for increased muscle fiber size: absolute mitochondrial content can rise even while density falls. Resistance training therefore does not harm mitochondrial function and can be beneficial for mitochondrial health in older, deconditioned populations. In already highly endurance-trained young athletes, adding resistance training is unlikely to raise VO2 max further.
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