Resistance Training: How Muscles Grow at the Cellular Level

Resistance Training: How Muscles Grow at the Cellular Level

(Last Updated On: March 29, 2019)

Resistance Training: How Muscles Grow at the Cellular Level

Resistance training is challenging, but you’re doing it for a reason – to improve your body composition and health. When you put in so much effort, you expect to see results. Be forewarned – it takes patience. Change doesn’t happen overnight and it doesn’t happen without consistent training and good nutrition.

Muscles won’t grow unless you subject them to greater loads than they’re accustomed to. When you resistance train, you overload your muscles in a progressive, incremental manner. Over time, in response to training, muscles increase in cross-sectional area so they’re measurably larger in size. They also become stronger so they can handle heavier loads. After a few months, you begin to see the results of your hard work. Yay! You’ve become stronger and your muscles are more defined. Ever wonder what’s happening “behind the scenes” at the cellular level that allowed your muscles grow in size?

Muscle Fibers Grow By Increasing in Size

Muscles are dynamic, living tissue capable of growth and adaptation. When you subject muscles to heavy resistance – it creates micro-damage to muscle fibers, or muscle cells, that make up the muscle. In response to overload, the contractile proteins called actin and myosin, which make up the muscle fiber increase in size and number. Actin and myosin are the proteins responsible for muscle contraction.

Working behind the scenes are specialized cells called satellite cells. Though you don’t hear much about them, they play an important role in muscle growth. These “support cells” are normally inactive until you overload the muscle through resistance training. That’s when they’re “called to duty.”

Resistance training is the stimulus that wakes up muscle satellite cells. You call on these cells each time you work your muscles against heavy resistance.  Once turned on, satellite cells fuse with existing muscle fibers to help the muscle fiber make new contractile proteins. One way they do this is by donating their nuclei to these muscle fibers so they have more resources to build new proteins.

Muscles grow in cross-sectional area mainly through an increase in the size of existing muscle fibers. It’s doubtful whether muscle fibers increase in number in response to training, although this does take place in some animals under experimental conditions. This raises questions as to whether muscle fibers might grow, to some degree, by an increase in fiber number but is unproven at this point.

Can Muscles Grow in Size without Becoming Stronger?

As mentioned, muscle fibers increase in size by forming more contractile elements, actin, and myosin. As a result, they become stronger. Another way muscles increase in size is through a process called “sarcoplasmic hypertrophy.” Sarcoplasmic hypertrophy is the growth of the non-contractile components inside muscle cells, the liquid portions of the muscle cell that aren’t involved in generating force. When you lift mainly for strength, using a resistance of 80% to 90% of your one-rep max and a low number of reps, you primarily increase the size of the contractile elements, the portion of the muscle that generates force. Thus, you become stronger.

When you do a “hypertrophy” workout, using a weight you can lift 8 to 12 times before failure, you’re getting more sarcoplasmic hypertrophy, a growth of the non-functional portions of muscle fibers, relative to the contractile elements. So the muscle becomes larger without necessarily becoming stronger.

High-intensity resistance training also causes muscle cells to temporarily increase in size due to cellular swelling. That’s why your muscles look a little larger and more defined after a workout, but this effect only temporary. Interestingly, some research suggests cellular swelling may stimulate muscle protein synthesis and enhance actual muscle growth.

The Role Hormones Play in Muscle Hypertrophy

Hormones and hormone-like chemicals called cytokines work behind the scenes to regulate the activity of satellite cells and stimulate muscle growth. Although a number of hormones and hormone-like proteins influence hypertrophy, three of the most important and best understood are insulin-like growth factor (IGF-1), growth hormone and testosterone. Insulin also aids muscle hypertrophy by helping to move amino acids into muscle cells where the cells can use them to build and repair.

All four of these hormones have an anabolic effect on muscle tissue. Growth hormone also has catabolic effects in that it stimulates fat breakdown. These hormones all increase in response to resistance training and support muscle growth. Research suggests exercise intensity is the key to maximizing the release of hormones that build muscle. In other words, you don’t create an environment that promotes muscle growth by lifting light weights, doing lots of reps and stopping before your muscles are fatigued.

Factors That Affect Muscle Growth Potential

Why are some people capable of getting more muscle growth than others? You have control over some factors that affect muscle hypertrophy like how much tension you place on the muscle and the amount of protein and other macronutrients you get in your diet. It’s almost impossible to build muscle when you’re subjecting your body to a protein or calorie deficit.

Factors you have less control over are genetics, sex, and age. For example, some people genetically have a higher proportion of fast-twitch muscle fibers, the kind that generate strength and power, relative to slow-twitch fibers, muscle fibers optimized for endurance. Having more fast-twitch muscle fibers gives you an advantage when it comes to developing strength and power.

Men build muscle easier than women and have a greater potential for muscle growth, primarily due to greater levels of testosterone. Age is a factor too. As people age, levels of anabolic hormones decrease. That doesn’t mean you can’t build muscle after a certain age. Even elderly people have the ability to increase muscle strength and size. It just takes more work and focus on nutrition.

The Bottom Line?

After pumping iron, a lot goes on inside muscle cells. Satellite cells merge with muscle fibers to help the fibers grow in size, muscle fibers swell and a variety of hormones and growth factors are called into play. Despite all the activity, it takes several weeks to months to see the results. Be patient and consistent with your training and nutrition and the changes will come.



Journal of Strength and Conditioning Research. “Mechanisms of Muscle Hypertrophy”

Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of muscle regeneration. Physiological Reviews, Volume 84, 209-238.

IDEA Health and Fitness Association. “Muscle Hypertrophy”

Sports Medicine, 36 (2), 131-49. (2006)


Related Articles:

Is Muscle Damage Necessary for Muscle Growth?

Strength Training: 5 Rules for Training to Failure

What Does Research Show about Partial Reps vs. Full Reps for Strength Training?

The Repeated Bout Effect: Why You Don’t Always Get Sore When You Lift Weights

How Your Muscles Repair after a Workout and How It’s Linked with Hypertrophy


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