Muscle hypertrophy refers to an increase in muscle size as a result of resistance training. Hypertrophy occurs when individual muscle fibers increase in size. As you gradually increase the load you place on a muscle over time through training, it adapts by becoming bigger and stronger. As a muscle grows, the cross-section area of the muscle becomes larger in size and the muscle looks bigger and more defined. Another form of proposed muscle “growth” you hear less about is called regional muscle hypertrophy.
What is Regional Hypertrophy?
Regional hypertrophy refers to non-uniform increases in muscle size. You probably think of muscle growth as being equal along the length of a muscle in response to training, but regional hypertrophy suggests there may be areas of a muscle where more growth occurs relative to other portions of the muscle. It’s a controversial idea that isn’t fully accepted by most fitness professionals.
If regional muscle hypertrophy is a reality, it has implications. For example, you may be able to modify your training to promote the growth of a particular part of a muscle and change its shape. Regional hypertrophy could be a way to not only promote growth but “mold” a muscle.
Is There Any Proof that You Can “Shape” a Muscle?
The structure of muscles suggests that regional muscle hypertrophy may be possible. Muscles are not completely uniform. Research shows different regions of a muscle have varying distributions of fiber types. For example, the superficial part of a muscle might have a higher density of type 1 fibers relative to type 2 while the deeper portion could have a greater ratio of type 2 to type 1 fibers. The same is true of the distal and proximal parts of a muscle.
Type 1 fibers are those recruited initially during a resistance training movement. These fibers are resistant to fatigue but are unable to generate a great deal of muscle tension. If you’re lifting a heavy weight, type 2 fibers will be recruited after type 1 to take up the slack. Type 2 fibers have the ability to generate a lot of tension but they fatigue more quickly than type 1 fibers. Although this is the usual sequence of recruitment, when you train a muscle intensely, type 2 fibers can be selectively activated. If certain exercises selectively target type 2 fibers and they aren’t evenly distributed throughout the muscle, you may have more growth in those parts of the muscle rich in type 2 fibers.
In addition, different parts of a muscle may be innervated by different branches of a nerve. If that’s the case, each branch could selectively activate a portion of the muscle, leading to unequal muscle activation. What’s less clear is whether this selective activation leads to greater hypertrophy in that area.
In addition, muscles are compartmentalized, meaning they’re composed of distinct segments rather than being uniform from end to end. Some segments may be better adapted to certain types of motion than others and be preferentially activated during some exercises. So, the growth of a muscle isn’t necessarily uniform.
Does Regional Muscle Hypertrophy Really Occur?
Is there science to support the idea of regional muscle hypertrophy? A number of studies have looked at whether muscles grow regionally in response to various types of training. These studies have used measurements of the cross-sectional area of muscles, MRI, CT scans and ultrasound to assess muscle hypertrophy in various regions of a muscle in response to training. The muscles looked at included muscles in the thigh as well as biceps and triceps muscles. Almost all of the studies showed muscles can grow non-uniformly in untrained individuals in response to resistance training. For example, one study showed differences in muscle growth between the proximal and distal ends of the biceps muscle after 12 weeks of resistance training.
Research has also shown non-uniform areas of muscle growth in other muscle groups including the quadriceps. This suggests it’s possible to change the size AND shape of a muscle by adjusting training variables such as volume, load, and the range-of-motion of an exercise. According to an article published on Strength and Conditioning Research website, muscles are likely to increase more in size at the point where their cross-sectional area is greatest.
If regional muscle hypertrophy occurs, you may need a variety of exercises to get maximal, uniform growth of a particular muscle. Doing a single type of exercise won’t necessarily stimulate growth equally along the entire length of the muscle. Plus, regional muscle hypertrophy opens up the exciting possibility that you can change the appearance of a muscle by altering its shape.
Despite some studies supporting regional muscle hypertrophy, it’s still a somewhat controversial topic in the fitness world. You’ve probably heard it said more than once that you can’t selectively target a portion of a muscle, like the upper abs and lower abs – you can only work the muscle in its entirety. Some studies are calling this idea into question.
What Does This Mean?
If regional muscle hypertrophy is possible, you can potentially change the shape of a muscle, not just increase its size, through exercise selection and by varying the angle of the movements and the resistance you use. Compound exercises are best for increasing muscle size and for creating a greater anabolic stimulus for growth, but isolation exercises would be better for shaping the muscle.
Recently, research with EMG, a technique that measures the degree of muscle activation when a muscle contracts, showed a muscle isn’t activated uniformly along its length. Some areas are more strongly activated than others. If your goal is to get uniform hypertrophy and not change the shape of a muscle, it’s best to do a variety of different exercises that target a particular muscle or muscle group. This would help compensate for exercises that activate one part of the muscle more than another and could lead to unbalanced growth.
The Bottom Line?
It’s an intriguing idea that we can reshape our muscles through exercise and there’s some evidence that you can. You may have more control over the shape of your body than you think.
Strength and Conditioning Research. “Does Regional Hypertrophy Really Happen?”
Med Sci Sports Exerc. 2013 Nov;45(11):2158-65. doi: 10.1249/MSS.0b013e3182995349.
J Strength Cond Res. 2000;14(1):102-13.
T-Nation. “Made to Order Muscle”
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