Myofibrillar Hypertrophy vs Sarcoplasmic: What Actually Differs

Myofibrillar hypertrophy is muscle growth from adding more myofibrils—the contractile protein strands that produce force. Sarcoplasmic hypertrophy is growth from expanding the fluid and non-contractile material around them. Both have been measured, but in most training the two grow together, and you can’t reliably choose one with rep ranges.

What is myofibrillar hypertrophy?

Myofibrillar hypertrophy is an increase in contractile protein—mainly actin and myosin, packed into myofibrils—inside a muscle fiber. More myofibrils mean more machinery to produce force, so this is the kind of growth most closely tied to strength.

A muscle fiber contains myofibrils, the rod-like structures built from actin, myosin, and titin, with sarcoplasm surrounding them. For the broader term, see what hypertrophy is and what triggers it.

• A muscle fiber contains myofibrils.
• Myofibrils contain actin and myosin, which generate force.
• Myofibrils occupy about 85% of a fiber’s intracellular space; sarcoplasm takes about 9%, and mitochondria about 5–6% (Roberts et al., 2020).

What is sarcoplasmic hypertrophy?

Sarcoplasmic hypertrophy is a disproportionate expansion of the sarcoplasm—the fluid, enzymes, glycogen, and other non-contractile material around the myofibrils. The fiber gets bigger, but the concentration of force-producing protein inside it goes down.

That does not make it fake muscle or simply water. Sarcoplasm includes metabolic proteins that support work, and one study of trained lifters who grew found 40 upregulated sarcoplasmic proteins, with glycolysis pathways enhanced (Haun et al., 2019).

• Researchers also describe conventional, proportional growth and myofibril packing, where contractile protein is added before the fiber enlarges.
• Sarcoplasmic hypertrophy means the sarcoplasm expands disproportionately relative to myofibril protein (Roberts et al., 2020).

Is sarcoplasmic hypertrophy real? What the evidence shows

It appears to happen, mainly in short, very high-volume training blocks—but most evidence says ordinary training grows contractile and non-contractile parts together. The “bodybuilder vs. powerlifter muscle” story comes from a few small studies, and it’s weaker than the internet makes it sound.

A short, high-volume block showed a shift. Haun and colleagues studied 30 trained young men over six weeks, ramping volume from 10 to 32 sets per week per exercise. Among the 15 biggest responders, fiber area grew 23% while actin and myosin concentration fell about 30% (Haun et al., 2019). It’s evidence of a possible short-term shift, not proof that high-rep training builds a separate kind of muscle.

Bodybuilders’ fibers were bigger, but produced less force per area. In a small cross-sectional study, 12 bodybuilders’ fibers were 67% larger than power athletes’ and 88% larger than controls’. Their specific tension—force per unit of fiber area—was 62% and 41% lower, respectively, though their fibers still produced 32% more total force than controls’ (Meijer et al., 2015). The study shows a difference, not what caused it.

Most load-induced growth looks proportional. A scoping review found specific tension preserved after growth, while the share of fiber area occupied by myofibrils barely changed: 84.2% versus 82.6% (Jorgenson et al., 2020). Roberts and colleagues note that evidence for sarcoplasmic expansion mostly comes from high-volume training in trained people, while evidence against it often comes from untrained people over 6–12 weeks (Roberts et al., 2020).

The fairest read is that sarcoplasmic expansion is real at the margins, but it is not a switch you can reliably control with your rep range.

What is the difference between myofibrillar and sarcoplasmic hypertrophy?

Myofibrillar growth adds force-producing protein; sarcoplasmic growth adds the fluid and metabolic material around it. The first raises how much force each bit of muscle can make; the second adds size and work capacity with less force per unit of area. In practice, most growth includes both.

Specific tension means force per unit of cross-sectional area: in plain terms, how much force a given area of muscle produces. The distinctions and limits are summarized here.

How do you build myofibrillar hypertrophy?

You don’t need to target it separately. Hard, progressive training builds contractile protein across a wide range of loads, and research shows similar muscle growth from roughly 30% of your max upward. If your goal is strength on specific lifts, add regular heavy practice on those lifts—that’s what actually converts size into force (Schoenfeld et al., 2021).

• For size, use hard sets close to failure, enough weekly sets, and progression. Mechanical tension is part of the training picture.
• For strength, heavy loads have a clear edge for one-rep-max gains; practice the lift you want to improve.
• Very high-volume blocks may skew toward sarcoplasmic expansion in the short term, but that is no reason to fear volume.
• Don’t chase low reps for “dense” muscle or high reps for “pump” muscle. Both can build muscle when sets are hard; see how many reps build muscle.

Why are some lifters big but not strong (or strong but not big)?

Mostly skill and specificity, not muscle type. Strength on a lift depends on muscle size plus practice with heavy loads on that exact movement. If you train mainly with moderate weights and machines, you can carry plenty of muscle yet test poorly on a heavy barbell single; the fix is practice, not a different hypertrophy.

• If you’re strong but small, you may need more hypertrophy volume.
• If you’re big but untested, add a heavy top set of a few reps on the lift you care about, once or twice a week, while keeping your hypertrophy work. Heavy loads have a clear advantage for one-rep-max gains (Schoenfeld et al., 2021); see how to get stronger after year one.

If your size and strength have drifted apart—or both have stopped—the cause is usually specific. Find out what’s behind your size–strength gap with a quick diagnosis that checks your training, recovery, and diet, then ranks what to fix first. Explore more topics in the lifting glossary.

FAQ

What is the difference between myofibrillar and sarcoplasmic hypertrophy?

Myofibrillar hypertrophy adds contractile protein—the actin and myosin that produce force. Sarcoplasmic hypertrophy expands the fluid and non-contractile material around it. The first maintains or raises force per unit of muscle; the second adds size with less force per unit area. Most real-world growth involves both.

How do you build myofibrillar hypertrophy?

Train hard and progressively: use sets close to failure, enough weekly sets, and add load or reps over time. That builds contractile protein across a wide range of loads (Schoenfeld et al., 2021). If you want strength on a lift, practice it regularly with heavy loads.

Is sarcoplasmic hypertrophy real?

It appears to happen, mainly during short, very high-volume training blocks. In one study, trained men’s fibers grew 23% in six weeks while actin and myosin concentration fell about 30% (Haun et al., 2019). Most load-induced growth looks proportional, so it isn’t something you need to chase or avoid.

Do bodybuilders have weaker muscle than powerlifters?

Possibly per unit of fiber area. One small study found bodybuilders’ fibers were larger but produced 41–62% less force per unit area than controls’ and power athletes’ fibers (Meijer et al., 2015). Their bigger fibers still made more total force than controls’ fibers, and heavy-lift skill helps explain platform performance.