Hypertrophy Science: Range of Motion (ROM), “muscle shape” and Non-Uniform Muscle Growth

Full ROM is associated with significant greater strength and hypertrophy gains than a shorter ROM. One study found for full ROM a greater increase in strength (18% vs. 4%) and a greater increase in hypertrophy (60 vs. 15%) at the distal cross-sectional area closest to the joints (knee or elbow), or insertions (1).

The average hypertrophy across the full length of the muscle belly was more than double for the full ROM (44% vs. 21%). Also muscle fiber pennation angles (fiber directions) increased more with full ROM (11% vs. 7% but not statistically significant).

However shorter ROM can in some instances still produce significant hypertrophy to the same extent as full ROM (2), persons with injuries or diminished ROM may benefit from this.

Muscle fibers don’t just span from origin to insertion, that an over-simplistic way to put it. Jose Antonio did a review on the Non-Uniform Muscle Growth and regional adaptation in skeletal muscle (3). Skeletal muscle is a heterogeneous tissue that exhibits numerous inter- and intramuscular differences: architecture, fiber composition, and muscle function (3).

With different exercises selective recruitment of different regions of a muscle can be achieved, so that there’s no single exercise that can maximize the hypertrophic response of all regions of a particular muscle (3). Several muscles are compartmentalized so that fibers terminate intrafascicularly (within the fascicle) and each subdivision is in turn innervated by its own nerve branch with different motor unit territories.

A few examples:

Schoenfeld et al (4) investigated muscle activation for two hamstrings exercises: the stiff leg deadlift and the lying leg curl. Activation of the upper hamstrings was similar between exercises, but the activation of the lower hamstrings, both medially and laterally, was significantly greater in the lying leg curl (170% and 65% respectively).

In one study, researchers went on to examine and confirmed the regional difference in muscle hypertrophy (MRI) corresponding to the regional difference in muscle activation (EMG) in a multijoint exercise for triceps (5). The area of the triceps with the most muscle activation had more hypertrophy after 12 weeks. Also for this case the areas closest to the elbow (distal cross-sectional area) grew the most as opposed to the area closer to the shoulder (6).

The same authors in other study correlated muscle activation (MR) for elbow extensors after one training session for one group with the hypertrophy from another group performing 12 weeks of training. Significantly lower activation in the distal region which was correlated with significantly less hypertrophy in the distal region compared with other areas (6). 

For maximal hypertrophy of an entire muscle various exercises must be executed to purportedly stimulate growth in a regional- specific manner. In other words, exercise selection and variety is necessary.

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1. McMahon GE, Morse CI, Burden A, Winwood K, Onambélé GL. Impact of range of motion during ecologically valid resistance training protocols on muscle size, subcutaneous fat, and strength. J Strength Cond Res. 2014 Jan;28(1):245-5
2. Pinto RS, Gomes N, Radaelli R, et al. Effect of range of motion on muscle strength and thickness. J Strength Cond Res 2012; 26(8): 2140-5.
3. Antonio, J. Nonuniform response of skeletal muscle to heavy resistance training: can bodybuilders induce regional muscle hypertrophy? J. Strength Cond. Res. 14(1):102–113. 2000
4. Schoenfeld BJ, Contreras B, Tiryaki-Sonmez G, Wilson JM, Kolber MJ, Peterson MD. Regional Differences in Muscle Activation During Hamstrings Exercise. J Strength Cond Res. 2014 Jun 24.
5. Wakahara T, Fukutani A, Kawakami Y, Yanai T. Nonuniform muscle hypertrophy: its relation to muscle activation in training session. Med Sci Sports Exerc. 2013 Nov;45(11):2158-65
6.  Taku Wakahara,  Naokazu Miyamoto,  Norihide Sugisaki,   Koichiro Murata,  Hiroaki Kanehisa,  Yasuo Kawakami,  Tetsuo Fukunaga,  Toshimasa Yanai. 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 April 2012, Volume 112, Issue 4, pp 1569-1576