Sarcoplasmic hypertrophy vs myofibrillar hypertrophy: real or gym myth?

Sarcoplasmic hypertrophy vs myofibrillar hypertrophy is the gym's favorite way to sound scientific about rep ranges. The story goes: heavy, low-rep training builds dense, strong "myofibrillar" muscle, and high-rep bodybuilding builds puffy, weak "sarcoplasmic" muscle full of fluid. Pick your reps, pick your muscle.

Here's the honest version. Sarcoplasmic hypertrophy is not a myth: there's real evidence it can happen. It's also not proven to be a separate kind of muscle you can choose to build, and the research is genuinely unsettled. Either way, the training advice doesn't move: hard sets, enough of them, a sensible spread of loads. Which is the kind of decision a workout app that plans both lifting and running should be making for you, so you can stop arguing about it on forums.

Myofibrillar vs sarcoplasmic hypertrophy: the definitions

A muscle fiber is a long cell packed mostly with myofibrils: the contractile machinery, built from proteins like actin and myosin, that actually produces force. The myofibrils sit in a fluid called the sarcoplasm, which also holds glycogen, enzymes, mitochondria and the rest of the cell's working parts.

From there:

• ›Myofibrillar hypertrophy is a fiber getting bigger by adding contractile material, roughly in proportion to its growth. More engine.
• ›Sarcoplasmic hypertrophy is a fiber getting bigger mainly by expanding the sarcoplasm, so the space around the myofibrils grows faster than the myofibrils themselves. More of everything else.

A review from one of the main labs working on this defines sarcoplasmic hypertrophy as a disproportionate expansion of the sarcoplasm relative to myofibril protein gains. It also describes two other patterns: conventional hypertrophy, where myofibril protein grows in proportion to fiber size, and "myofibril packing", where myofibril protein is added before the fiber grows (Roberts et al., 2020).

The practical point to hold on to: whichever happens, the muscle gets bigger, and every version of it comes from the same thing. Training hard. Most programs, including a 3- or 4-day split you can copy, don't try to target one type, and there's no good reason they should.

What the research actually shows

The case that sarcoplasmic hypertrophy happens

The best-known study here put 15 resistance-trained young men who grew during a 6-week, very high-volume program under the microscope. Their average fiber size went up 23%, but actin and myosin concentrations went down by roughly 30%, while sarcoplasmic proteins tied to energy production went up. Eight days after training stopped, sarcoplasmic protein concentrations were still elevated (Haun et al., 2019). The authors' conclusion used the word "may": short-term high-volume training may reduce contractile protein concentration despite bigger fibers, and may promote sarcoplasmic expansion.

Another study had trained men do 6 weeks of leg training, one leg with higher volume and lighter loads, the other with heavier loads. The high-volume leg grew more on MRI and showed higher synthesis rates of non-myofibrillar proteins. The heavy leg gained more strength. Synthesis of myofibrillar proteins didn't differ between legs, and the relative amounts of the major muscle proteins didn't change differently either (Vann et al., 2022).

And in resistance-trained men, a leg trained with constantly varied loads, volumes and rest periods for 8 weeks showed an increase in the share of fiber area taken up by non-contractile space, while a leg on a standard program didn't. Both legs' fibers grew, with no significant difference between them (Fox et al., 2021).

The case that it's not the whole story

Plenty of studies find the boring answer. In untrained men doing 10 weeks of ordinary full-body training, fibers grew and myofibril area grew in proportion: no disproportionate sarcoplasmic expansion (Ruple et al., 2021). In a 6-week study comparing light loads with blood flow restriction against traditional heavy training, both produced growth with proportional changes in myofibrillar and non-myofibrillar areas (Libardi et al., 2024).

High-load training doesn't neatly produce the opposite, either. In trained men doing 10 weeks of low-volume, heavy training, there were no changes in sarcoplasmic protein concentration or muscle fluid, but actin also went down slightly and myosin trended down (Vann et al., 2020). Not exactly the "dense myofibrillar muscle" the folklore promised.

The measurement problem

This is the bit most articles skip. Hypertrophy gets measured with ultrasound, MRI, biopsies, protein assays and electron microscopy, and a review from the same research group found that the changes these methods report are often poorly associated with one another (Haun et al., 2019). When one technique says "more sarcoplasm" and another can't confirm it, nobody should be writing training programs off the result.

So the honest summary is: sarcoplasmic hypertrophy looks like it can happen, possibly more with very high-volume training, possibly temporarily. Whether it's a lasting, separate type of muscle you can deliberately build is not established. The review that started much of this conversation ends by proposing more research, not training advice (Roberts et al., 2020).

Sarcoplasmic vs myofibrillar hypertrophy: what bodybuilders vs powerlifters suggest

The folklore usually leans on a comparison: bodybuilders look bigger than powerlifters but aren't as strong for their size. There's one study that fits that story. Single fibers from bodybuilders were much larger than those of power athletes and controls, but produced less force per unit of fiber area. The authors suggested differences in myofibrillar density might be one explanation, alongside an apparent negative effect of hypertrophy itself (Meijer et al., 2015).

That's a cross-sectional study of 32 people. It can't tell you whether training style caused the difference, what the athletes did outside the gym, or whether the people who gravitate toward bodybuilding started different. It's a reasonable reason to keep studying the question. It's not proof you'll build "fake" muscle with sets of 12.

What the theory gets wrong about training

The pop version of sarcoplasmic vs myofibrillar hypertrophy says your rep range picks your muscle type. Three problems with that.

Rep ranges don't sort neatly by size and strength. Muscle growth is similar across a wide range of loads when sets are hard, while heavier loads build more maximal strength (Lopez et al., 2021). That strength gap is mostly skill and practice with heavy weights, which is the core of strength vs hypertrophy training, not proof of a different kind of tissue.

Volume is the bigger lever. If any style leans sarcoplasmic, the studies above point at very high volume, not at moderate reps. Most people aren't anywhere near those volumes. The ranges that actually drive growth are in more on how many sets per workout: roughly 10 to 20 hard sets per muscle per week.

Bigger is still bigger. Even the study that found the most sarcoplasmic expansion found bigger fibers (Haun et al., 2019). A bigger muscle has more room for contractile protein to follow. Treating early growth as wasted "puff" is a good way to talk yourself out of training that works.

There's also a separate argument about whether muscles grow by making fibers bigger or by making more of them. That one is hypertrophy versus hyperplasia, and it's a different question from what's inside each fiber.

Myofibrillar hypertrophy vs sarcoplasmic hypertrophy: what to actually do

If you want size, strength, or both, the plan looks the same whichever camp wins:

None of that requires choosing a fiber compartment. The details that do matter are effort and execution. A controlled lowering phase is worth keeping, and the eccentric vs concentric split covers what each phase does. Sets need to be hard enough to count, and how close to go is the question of what is training to failure. Short version: close is good, all the way is optional.

Myths worth dropping

"High reps give you a pump, and the pump is sarcoplasmic hypertrophy." The pump after a set fades after the session. Sarcoplasmic hypertrophy, if it happens, is a change inside the fiber measured days after training, not swelling you can see in the mirror.

"Sarcoplasmic muscle is useless." Fibers that are bigger by any route are still muscle. And the sarcoplasm holds the glycogen and enzymes that fuel hard sets.

"Powerlifters build real muscle, bodybuilders build fluff." Both groups build plenty of muscle. They also train for different things. Strength is partly skill on specific lifts, which is why heavy practice makes you stronger on those lifts.

"You need to train differently to target each type." No study shows you can reliably select one. Train hard, with enough sets, across a sensible range.

How exrsz handles muscle growth

exrsz builds a periodized plan from your goal, schedule, level and equipment, so a muscle goal gets more hard sets and a strength goal gets more heavy work, balanced against your running in the same plan. It adapts each week to how you actually trained and reshuffles missed sessions.

FAQ

What is the difference between sarcoplasmic and myofibrillar hypertrophy?

Myofibrillar hypertrophy is fiber growth driven by more contractile protein. Sarcoplasmic hypertrophy is fiber growth driven by a disproportionate expansion of the fluid and non-contractile parts of the cell (Roberts et al., 2020).

Is sarcoplasmic hypertrophy real?

There's some evidence it can happen. Trained men who grew on a 6-week high-volume program showed lower actin and myosin concentrations alongside more sarcoplasmic protein (Haun et al., 2019). Whether it's a lasting, separate type of growth you can target isn't established.

Do high reps cause sarcoplasmic hypertrophy?

Not shown. Studies hinting at sarcoplasmic expansion used very high volume, not simply higher reps, and muscle growth is similar across a wide range of loads when sets are hard (Lopez et al., 2021).

Myofibrillar vs sarcoplasmic hypertrophy: which is better for strength?

Contractile protein produces force, so myofibrillar growth matters more in theory. In practice, the bigger strength driver is heavy practice on the lifts you want stronger, alongside enough hard sets to grow.

Should I train differently for each type?

No. Train with hard sets, 10 to 20 per muscle per week, and include heavy work if strength matters to you. The research doesn't support a way to select one type of growth.