Hypertrophy: Myofibrillar vs. Sarcoplasmic Hypertrophy
No evidence supports selectively targeting myofibrillar vs. sarcoplasmic hypertrophy through rep range manipulation. Both compartments increase proportionally across load ranges. The distinction is mechanistically real but not independently programmable (Haun et al., 2019 — PMID 31448106).
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Myofibrillar fraction of muscle cross-sectional area | 75–85 | % of total CSA | Contractile proteins (actin, myosin) dominate; sarcoplasm (fluid, glycogen, organelles) fills remaining 15–25% |
| Sarcoplasmic expansion with high-rep training | modest | relative effect | Some evidence of preferential glycogen/fluid expansion with high-rep training; effect magnitude unclear in humans |
| Myofibrillar growth: heavy vs. light load | equivalent | when volume equated | Schoenfeld 2017 (PMID 28834797): no significant difference in muscle CSA gains across 8–12 vs. 25–35 rep ranges |
| Proportion of MPS that is myofibrillar | 60–70 | % of post-exercise MPS | Post-exercise MPS is predominantly myofibrillar regardless of load used (Phillips et al., 1997) |
| Sarcoplasmic volume change: glycogen supercompensation | 2–3 | % increase in fiber diameter | Glycogen loading expands sarcoplasm transiently; not true hypertrophy (reverses with depletion) |
| Practical rep range for hypertrophy | 5–30 | reps per set (all effective) | Range is wide; neither extreme (1–3 rep strength nor 40+ rep endurance) is hypertrophy-optimized at equated volume |
The common belief is that heavy, low-rep training builds dense, contractile (myofibrillar) muscle while high-rep, pump-focused training builds puffy, non-functional (sarcoplasmic) muscle. Here is what the research actually shows: both forms of training increase myofibrillar and sarcoplasmic compartments proportionally, and selectively targeting one type through rep range manipulation is not supported by current evidence.
This myth persists partly because the underlying biology is real. Myofibrils (contractile units containing actin and myosin) do constitute 75–85% of muscle cross-sectional area. The sarcoplasm (fluid, glycogen, mitochondria, enzymes) fills the remaining 15–25%. And these compartments could theoretically expand at different rates. The critical question is whether programmable training variables actually produce selective hypertrophy of one compartment — and the answer, based on current evidence, is no.
Myofibrillar vs. Sarcoplasmic: What Each Contains
| Component | Location | % of CSA | Function | Training-responsive? |
|---|---|---|---|---|
| Actin (thin filament) | Myofibril | ~35% | Force production via cross-bridges | Yes — primary hypertrophy target |
| Myosin (thick filament) | Myofibril | ~30% | Force production, ATPase activity | Yes — primary hypertrophy target |
| Titin, nebulin | Myofibril | ~5% | Structural, elastic energy | Yes — modest increase |
| Glycogen | Sarcoplasm | ~2–3% | Fuel for anaerobic glycolysis | Yes — transiently, reverses with depletion |
| Mitochondria | Sarcoplasm | ~2–4% | Aerobic ATP production | Yes — with endurance-like training |
| Sarcoplasmic reticulum | Sarcoplasm | ~5% | Ca²⁺ storage and release | Modest increases with training |
| Cytoplasmic fluid | Sarcoplasm | ~8–12% | Metabolic environment | Transient expansion with pump training |
What the Evidence Shows Across Load Ranges
Haun et al. (2019, PMID 31448106) reviewed studies measuring myofibrillar protein accretion and found that both heavy and moderate-to-light training protocols produce predominantly myofibrillar MPS responses post-exercise. Roberts et al. (2021, PMID 35115966) labeled pure sarcoplasmic hypertrophy a scientific “unicorn” — theoretically possible but never cleanly demonstrated in human resistance training studies.
Schoenfeld (2017, PMID 28834797) randomized trained men to 3 sets of 2–4 reps vs. 3 sets of 8–12 reps vs. 3 sets of 20–30 reps, equating total volume-load. All groups showed equivalent muscle CSA gains over 8 weeks, measured by ultrasound. The strength gains differed by specificity, but muscle size did not.
Practical Implication
You cannot selectively build myofibrillar or sarcoplasmic muscle through rep range manipulation. Program rep ranges based on practical factors: heavy loads (3–6 reps) build concurrent strength; moderate loads (6–15 reps) optimize the effort-per-set efficiency for volume accumulation; higher reps (15–30) reduce joint stress while preserving hypertrophic stimulus. All three produce genuine contractile hypertrophy. For rep range evidence across the spectrum, see the rep-ranges page.
Related Pages
Sources
- Haun, C.T. et al. (2019). A critical evaluation of the biological construct skeletal muscle hypertrophy. Frontiers in Physiology, 10, 247.
- Schoenfeld, B.J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. Journal of Strength and Conditioning Research, 24(10), 2857–2872.
- Roberts, M.D. et al. (2021). Sarcoplasmic hypertrophy in skeletal muscle: a scientific 'unicorn' or resistance training adaptation? Frontiers in Physiology, 12, 816718.
- Taber, C.B. et al. (2019). Muscle hypertrophy: a narrative review on training principles for increasing muscle mass. Strength and Conditioning Journal, 41(3), 48–58.
Frequently Asked Questions
Can you train specifically for myofibrillar vs. sarcoplasmic hypertrophy?
The evidence says no. While both compartments are physiologically distinct, no intervention has produced selective expansion of one without the other. Heavy training (1–5 reps) does not preferentially build myofibrils, and high-rep training (15–30 reps) does not preferentially expand sarcoplasm. Haun et al. (2019, PMID 31448106) reviewed the biological construct and concluded the distinction is real but independently programmable hypertrophy of each type is not supported by current evidence.
What is the difference between myofibrillar and sarcoplasmic hypertrophy?
Myofibrillar hypertrophy refers to growth of contractile proteins — actin and myosin thick and thin filaments that produce force. These make up 75–85% of muscle CSA. Sarcoplasmic hypertrophy refers to expansion of the sarcoplasm: the fluid, glycogen stores, organelles, and metabolic enzymes surrounding the myofibrils. Sarcoplasmic expansion increases muscle size without proportionally increasing force production capacity.
Does high-rep training produce bigger but weaker muscles than low-rep training?
This is the central claim of the myofibrillar/sarcoplasmic dichotomy — and the evidence does not support it. Schoenfeld et al. (2017, PMID 28834797) showed equivalent muscle CSA gains across 8–12 and 25–35 rep ranges with matched volume. Strength did differ (low rep trained showed more 1RM gains), but this reflects neural adaptations more than muscle composition. Higher rep ranges produce genuine contractile hypertrophy, not just fluid-filled 'fake' muscle.
Why does the myth persist about bodybuilders using high reps for sarcoplasmic size?
The myth has face validity: bodybuilders often use higher rep ranges and do have very large muscles with somewhat lower strength-to-size ratios than powerlifters. But this reflects training specificity — powerlifters optimize neural drive and tendon stiffness; bodybuilders optimize total muscle CSA via higher volume. Both approaches produce genuine myofibrillar hypertrophy. The sarcoplasmic 'pump' is real but transient; it does not account for chronic size differences.