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Can you change the shape of your muscles?

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Chris Beardsley is a biomechanics researcher. He is also the editor of Strength and Conditioning Research. He is based in Loughborough, Leicestershire.

It used to be the case that educated lifters would scoff at the idea you could change the shape of a muscle by using different exercises or training techniques. Message board threads bear testimony to hundreds of novice lifters who were ridiculed for asking how to isolate their upper chest. After all, everyone knows you can’t isolate part of a muscle, right?

Actually it isn’t quite that simple, not least because isolation isn’t necessary for more muscle growth to occur in one part of a muscle than in another.

First of all, let’s be clear that an increase in muscular cross-sectional area doesn’t generally happen equally over a whole muscle anyway. Researchers have known this since as far back as the late 1980s. These early studies found that muscle growth tended to be greater at one end of a muscle than at the other[1].

More recently, researchers have also found that there are differences from side-to-side as well[2]. So the idea that a muscle grows evenly across its length and width is not really true, meaning that muscles do change shape as they increase in size. However, his doesn’t mean that we can alter which parts of a muscle grow most, just that different parts of a muscle grow more than others in response to the same stimulus.

What we do know
The good news is that it appears that we can alter which parts of the muscle grow. The slightly less good news, however, is that there is scant information regarding what training variables we can use to influence the degree to which muscle growth occurs more in one part of a muscle than in another. Only a small number of studies have assessed differences in regional hypertrophy between different training variables and so our current picture is incomplete.

Studies to date have looked at whether differences in regional hypertrophy arise from using either eccentric and concentric muscle actions[3], high or low percentages of 1RM[4], high or low volumes[5], large or small ranges of motion[6], and different types of external resistance, such as isokinetic, in which speed is constant throughout the exercise (such as when using accommodating resistance or a pneumatic resistance machine), versus isoinertial, in which load is constant throughout the exercise, like when using free weights[7].

While that seems like a long list, the number of studies investigating each training variable is tiny in each case, and that means the risk of error is high. It also means we have no idea at all how proximity to muscular failure, training frequency, exercise selection, or rest periods affect regional hypertrophy.
What we do know, however, is that there do seem to be real differences in regional hypertrophy between large and small ranges of motion and between the different types of external resistance (isoinertial and isokinetic).

The role of exercise selection
You may have noticed that exercise selection wasn’t on that list. Although exercise selection has not yet been studied in this way, it may actually be the factor that makes the biggest difference to where muscles grow most. Multiple studies have found that electromyographic activity in different parts of a muscle varies markedly between exercises and even between variations of the same exercise. While this sounds purely speculative, as such studies don’t actually measure changes in muscular size, researchers have made an important connection.

The highest levels of electromyographic activity in a muscle during an exercise have been found to correspond with exactly where muscle growth occurs most extensively, in both single-joint[8] and multi-joint[9] exercises.

That opens up a whole new world of knowledge for the scientifically-literate lifter, as studies exploring differences in electromyographic activity in different parts of a muscle during different exercises or exercise variations are far more easy to track down.

For example, take the case of whether we can make the upper chest grow to a different extent than the lower chest. A very recent study found that by performing the push-up with a 30-degree bend at the waist caused an increase in electromyographic activity in the clavicular head of the pectoralis major, but a reduction in the activity of the sternocostal head[10].

The clavicular head runs between the humerus and the clavicle, while the sternocostal head runs between the humerus and the sternum. Increasing the size of the clavicular head will likely increase the appearance of the upper chest, as it runs in an oblique line higher up the torso from the sternocostal head, which runs more horizontally across the body. So using a push-up with a bend at the waist would likely have been a good recommendation for any novice lifters with weak upper chests.
Other biomechanically-similar exercises like incline presses may also prove beneficial.

Weak point breakthrough
It is not just different anatomical parts of muscles that can be developed in this way. A recent study found that the upper part of each of the hamstrings muscles displayed different levels of electromyographic activity from the lower parts during two different exercises[11].

The researchers compared the stiff-legged deadlift and the lying leg curl. They found that the lying leg curl displayed greater electromyographic activity in the lower hamstrings (both lateral and medial) compared to the stiff-legged deadlift.

This is really interesting, as the medial and lateral hamstrings groups have different attachment points at the tibia (on each side) and might be expected to display differences. But they don’t. It is the upper and lower parts of the same individual muscle that differ.

For the serious trainer up to professional bodybuilders, this means that with some careful research they can identify the exact exercises they need to bring up their weak points. For the educated lifter, the take-home message is more simple: use a variety of exercises, exercise variations, and loading types (variable resistance and conventional weights) and your muscle growth will be more consistent over each muscle, meaning your overall muscular development will be greater.

1. Narici, M. V., Roi, G. S., Landoni, L., Minetti, A. E., & Cerretelli, P. (1989). Changes in force, cross-sectional area and neural activation during strength training and detraining of the human quadriceps. European Journal of Applied Physiology and Occupational Physiology, 59(4), 310-319.
2. Wells, A. J., Fukuda, D. H., Hoffman, J. R., Gonzalez, A. M., Jajtner, A. R., Townsend, J. R., & Stout, J. R. (2014). Vastus Lateralis Exhibits Non-Homogenous Adaptation to Resistance Training. Muscle & Nerve, published ahead of print, February 24, 2014.
3. Smith, R. C., & Rutherford, O. M. (1995). The role of metabolites in strength training. European Journal of Applied Physiology and Occupational Physiology, 71(4), 332-336.
4. Kanehisa, H., Nagareda, H., Kawakami, Y., Akima, H., Masani, K., Kouzaki, M., & Fukunaga, T. (2002). Effects of equivolume isometric training programs comprising medium or high resistance on muscle size and strength. European Journal of Applied Physiology, 87(2), 112-119.
5. Starkey, D. B., Pollock, M. L., Ishida, Y., Welsch, M. A., Brechue, W. F., Graves, J. E., & Feigenbaum, M. S. (1996). Effect of resistance training volume on strength and muscle thickness. Medicine & Science in Sports & Exercise, 28(0), 10.
6. Bloomquist, K., Langberg, H., Karlsen, S., Madsgaard, S., Boesen, M., & Raastad, T. (2013). Effect of range of motion in heavy load squatting on muscle and tendon adaptations. European Journal of Applied Physiology, 113(8), 2133-2142.
7. Matta, T. T., Nascimento, F. X., Fernandes, I. A., & Oliveira, L. F. (2014). Heterogeneity of rectus femoris muscle architectural adaptations after two different 14-week resistance training programmes. Clinical Physiology and Functional Imaging, published ahead of print, 21 April, 2014.
8. Wakahara, T., Fukutani, A., Kawakami, Y., & Yanai, T. (2013). Nonuniform muscle hypertrophy: its relation to muscle activation in training session. Medicine & Science in Sports & Exercise, 45(11), 2158-2165.
9. Wakahara, T., Miyamoto, N., Sugisaki, N., Murata, K., Kanehisa, H., Kawakami, Y., & Yanai, T. (2012). Association between regional differences in muscle activation in one session of resistance exercise and in muscle hypertrophy after resistance training. European Journal of Applied Physiology, 112(4), 1569-1576.
10. Kang, D. H., Jung, S. Y., Nam, D. H., Shin, S. J., & Yoo, W. G. (2014). The Effects of Push-ups with the Trunk Flexed on the Shoulder and Trunk Muscles. Journal of Physical Therapy Science, 26(6), 909.
11. Schoenfeld, B. J., Contreras, B., Tiryaki-Sonmez, G., Wilson, J. M., Kolber, M. J., Peterson, M. D., & Arbor, M. I. (2014). Regional Differences in Muscle Activation During Hamstrings Exercise. Journal of Strength & Conditioning Research, published ahead of print, 24 June, 2014.


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