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Regional hypertrophy, or the controversial idea that we can change the shape of a muscle through training, is a concept that still seems to cause headaches. Can we really change the shape of a muscle by using different exercises and/or methods of resistance training or does it grow in the same way irrespective of what we do to train it? Chris Beardsley (@SandCResearch) reviews the research.
What is the background?
What does regional hypertrophy mean?
Muscular hypertrophy is the increase in cross-sectional area or size of a muscle. Regional hypertrophy was first reviewed by Antonio (2000), where it was defined as a change in the shape of a muscle for the purposes of bodybuilding.
More recently, the term has been used to refer to differences in hypertrophy along the length of a muscle, where proximal or distal sections may display greater or less increases in size from one another. Such differences in hypertrophy over sustained periods of time may lead to changes in the shape of a muscle. It is thought that some exercises lead to growth in certain parts of a muscle while different exercises lead to growth in other parts.
How could regional hypertrophy occur?
Antonio (2000) proposed two main ideas by which regional hypertrophy might occur. Firstly, he suggested that the compartmentalization of muscles could mean that certain areas were activated to perform certain ranges of motion of a joint action or certain movements at a joint where multiple movements are possible (such as at the hip or shoulder). Secondly, Antonio observed that researchers have noted differences in muscle fiber type between one region of a muscle and another. This could mean that different rep ranges or muscle actions could provide different stimuli to the varying regions.
Compartmentalization – compartmentalization of muscles could mean that certain areas are activated to perform certain ranges of motion of a joint action or certain movements at a joint where multiple movements are possible (such as at the hip or shoulder). Indeed, most individual muscles are not comprised of single compartments in which all the muscle fibers run from one end to the other (the arrangement of the fibers is known as the muscle architecture). Instead, each muscle is made up of several segments, which display different features. For example, a recent study by Flack (2014) reported that on the basis of anatomy and innervation the gluteus medius has 4 compartments (anterior, anterior-middle, posterior-middle, and posterior). These compartments have different nerve branches and varying pennation angles (+33.1, +13.2, -9.9, and -29.5 degrees), which means that they best suited to slightly different tasks. Indeed, on the basis of the anatomy, Flack (2014) proposed that the horizontal arrangement of the posterior fascicles relative to the femoral neck, means that they probably act primarily to stabilize the head of the femur in the acetabulum. In contrast, the other fibers, which are arranged more vertically with respect to the femur, are better positioned to perform hip abduction.
Muscle fiber type differences – researchers have indeed also noted differences in muscle fiber type between one region of a muscle and another. Lexell (1983) found that type I fibers were predominant in the deep vastus lateralis, while type II fibers were predominant in the superficial vastus lateralis. Similarly, Sola et al. (1992) found a greater proportion of type I fibers in the deep latissimus dorsi and more type II fibers superficially. Other research has also found differences between regions from proximal-to-distal as well as between regions from deep-to-superficial. This means that where different repetition ranges are used and thereby target different muscle fiber types, this could lead to preferential growth in certain parts of a muscle. More recently, Wakahara (2013) found that differences in EMG activity in certain parts of a muscle correlated with the increases in muscular size. Miyamoto (2013) found differences in muscle tissue oxygenation saturation was between distal and middle regions of the vastus lateralis during knee extension exercise.
It should be noted that these observations are purely useful for understanding how regional hypertrophy COULD occur and are not to be taken as evidence that it DOES occur. For that, we need to look at the long-term trials, which are reviewed in the next section.
Can regional hypertrophy happen?
The following studies differences in hypertrophy between different regions of the same muscles (not between individual muscles of the same muscle group). The table below shows where there were significant differences observed between regions of the same muscle following a resistance-training program:
Narici (1989) assessed the effects of 60 days of unilateral resistance-training and 40 days of detraining in 4 young male subjects who trained 4 times a week with 6 sets of 10 maximal isokinetic knee extensions at 2.09 rad/s. Before and after each phase, the researchers measured quadriceps muscle cross-sectional area at 7 fractions of femur length proximally to distally using MRI scans. The researchers found that the increase in quadriceps muscle cross-sectional area was greatest at 2/10 femur length (12.0 ± 1.5%) and least at 8/10 femur length (3.5 ± 1.2%).
Housh (1992) assessed the effects of concentric isokinetic training on the muscular cross-sectional area of the forearm extensor and flexor muscles at three levels (proximal, middle, or distal) in 13 untrained male college students over an 8-week intervention. The subjects performed 6 sets of 10 repetitions of extension and flexion, 3 times per week using an isokinetic dynamometer. The researchers found significant hypertrophy in all trained muscle groups as well as preferential hypertrophy of individual muscles at specific levels.
Roman (1993) assessed changes in the muscle volume and muscular cross-sectional area of two elbow flexors in 5 elderly males using MRI scans following 12 weeks of heavy-resistance training. The subjects performed four exercises, including isokinetic dynamometer elbow flexion, barbell curls, dumbbell curls, and hammer curls for 4 sets of 8 repetitions on the dynamometer at 60, 180, 240, and 300 degrees/s and 3 sets of 8 repetitions on the other exercises. The researchers found that muscle volume and cross-sectional area of the biceps brachii and brachialis significantly increased by 13.9% and 22.6%, respectively. The researchers found that the greatest percentage increase in combined cross-sectional area of both muscles was observed at the point of maximal girth of the muscle (around 7 – 10cm along a 25cm muscle length).
Smith and Rutherford (1995) compared the effects of unilateral concentric and eccentric contractions of the quadriceps in 10 young, healthy male and female subjects over a 20-week intervention. The eccentric group used weights which were 35% higher than the concentric group. The researchers measured muscular cross-sectional area proximally (three-quarters femur length) and distally (one-quarter femur length) using computed tomography. The researchers found significant increases in muscular cross-sectional area for both concentric and eccentric groups proximally (4.6 ± 5.1% and 4.0 ± 4.3%) but not distally (3.6 ± 18.5% and 2.6 ± 14.7%).
Kawakami (1995) assessed changes in muscle thickness using B-mode ultrasound and increases in muscular cross-sectional areas using MRI scans following unilateral resistance-training of the elbow extensors in 5 males over a 16-week intervention. The subjects performed the French Press exercise, standing, with the left hand for 5 sets of 8 repetitions at 80% of 1RM. The researchers found that muscle thickness and muscular cross-sectional area both increased after training. They found that muscular cross-sectional area increased significantly in the middle sections but not at the proximal and distal ends.
Narici (1996) assessed changes in quadriceps muscular cross-sectional areas in 7 healthy males following a 6-month resistance-training intervention using 6 sets of 8 unilateral leg extensions at 80% of 1RM. The researchers found that quadriceps muscular cross-sectional area increased by 18.8 ± 7.2%, 13.0 ± 7.2%, and 19.3 ± 6.7% in the distal, central and proximal regions, respectively.
Starkey (1996) compared the effects of different volumes of resistance-training on muscle thickness in 48 untrained subjects, training 3 times per week, for 14 weeks. The subjects performed either 1 set (low volume) or 3 sets (high volume) of variable-resistance bilateral knee extension and flexion exercise to fatigue for 8 – 12 repetitions. Before and after the intervention, the researchers measured anterior, lateral, and right thigh muscle thickness, as well as vastus medialis and vastus lateralis muscle thickness at different sites with B-mode ultrasound. They measured the muscle thickness at 3 different sites proximally to distally, measuring 20, 40 and 60% of the distance from the greater trochanter of the femur to the lateral epicondyle. The researchers detected increases in muscle thickness for the low-volume group at 60% of the vastus lateralis and at both 40% and 60% for the posterior thigh, while the high-volume group increased muscle thickness in the vastus medialis, and at both 40% and 60% for the posterior thigh.
Tracy (1999) assessed the effects of unilateral leg resistance-training in 12 healthy older men and 11 healthy older women. The subjects performed 4 sets of knee extension exercise, 3 days per week for 9 weeks. The researchers found that the men displayed greater absolute increases in quadriceps muscle volume measured by MRI (1,753 ± 44 to 1,955 ± 43cm3) than the women (1,125 ± 53 vs. 1,261 ± 65cm3) but the percentage increase was similar (12% for both). The researchers noted that there appeared to be differences in the extent of regional hypertrophy in that the greatest increases in cross-sectional area seemed to be at the mid-thigh for both males and females but they did not specifically test this.
Häkkinen (2001) assessed the effects of a 21-week resistance-training intervention in 10 elderly females on muscular cross-sectional area of the quadriceps at 3/12-to-12/15 of femur length. The researchers found that muscular cross-sectional area of the whole quadriceps muscle group increased significantly over the whole length of the femur. They also found that there were significant increases at 7/15-to-12/15 for the vastus lateralis, at 3/15-to-8/15 for the vastus medialis, at 5/15-to-9/15 for the vastus intermedius, and at 9/15 only for the rectus femoris. Thus, increases in muscular cross-sectional area at different points along the length of the femur differed between individual muscles.
Kanehisa (2002) compared the effects of two different 10-week interventions comprising isometric, unilateral elbow extension training, 3 times per week in 12 young adult males. The subjects performed the same volume of training but the relative load differed between the groups. One group performed maximal voluntary contractions (MVCs) for 6 seconds per set, 12 sets per session, while the other group performed contractions at 60% of MVC for 30 seconds per set, 4 sets per session. The researchers found that the resultant hypertrophy occurred mainly in the middle portion of the muscle, while much smaller increases in muscular size were observed at the proximal and distal ends.
Seynnes (2007) assessed changes in muscle size in the central and distal regions of the quadriceps during a 35-day high-intensity resistance-training program in 7 young, healthy volunteers who performed bilateral leg extensions 3 times per week using a flywheel ergometer. The researchers found significant increases in the central and distal regions (6.5 ± 1.1% and 7.4 ± 0.8%). However, the differences between sites were not significant.
Blazevich (2007) assessed the effects of muscle action on the muscular cross-sectional area of each of the quadriceps proximally (25% from proximal end point) and distally (75% from proximal end point) using ultrasonography after a 10-week training period and a 3-week detraining period in 21 men and women. The subjects performed slow-speed (30 degrees/s) concentric-only or eccentric-only isokinetic knee extensor training. The researchers found no significant differences in regional hypertrophy between the concentric and eccentric groups. Using pooled data, the researchers found that changes in muscular cross-sectional area of the vastus lateralis and of the rectus femoris were relatively consistent along their lengths, but there was a trend for a greater increase distally than proximally in the vastus medialis, and a significantly greater increase distally than proximally in the vastus intermedius.
Melnyk (2009) assessed the effects of a 9-week resistance-training intervention and a 31-week period of detraining on 3 different regions of the quadriceps (proximal, middle, and distal) using MRI scans in 11 young males, 11 elderly males, 10 young females, and 11 elderly females. There was a significantly greater increase in the middle region than in the proximal and distal regions (5.9 ± 4.6 vs. 4.4 ± 4.6 cm2 and 4.1 ± 3.9cm2).
Matta (2011) compared muscle thickness at 3 different sites (50, 60, and 70% of arm length) of the biceps brachii and triceps brachii following a 12-week resistance-training intervention in 49 healthy untrained males. The subjects performed the bench press, lat pull-down, triceps extension, and biceps curl exercises. The researchers observed significant difference in the increases in biceps brachii muscle thickness at the proximal (12%) and distal (5%) sites. There was no significant difference between the increases in muscle thickness at the proximal and distal sites of the triceps brachii.
Wakahara (2012) and (2013) assessed the extent to which regional hypertrophy occurring following a 12-week training intervention corresponds to regional differences in muscle activation in the training session in 12 males. The researchers measured the muscular cross-sectional areas of the triceps brachii along its length using MRI scans. The researchers found that the middle area of the triceps brachii was significantly more activated than the most proximal region. Similarly, they found that the relative change muscular cross-sectional area following the training intervention was significantly greater in the middle than the proximal region.
Bloomqvist (2013) compared the effects and deep or partial squat training in 17 male students on muscular cross-sectional area of the thigh muscles, measured at 6 sites from proximal to distal on both the front and the back of the thigh. The researchers found that the deep squat group increased front thigh muscular cross-sectional area significantly in all regions but the partial squat group only increased front thigh muscular cross-sectional area significantly in the two most proximal regions. They found that the deep squat group increased back thigh muscular cross-sectional area only at the second-most proximal site and there were no other significant increases.
Ema (2013) compared resistance training-induced changes in muscle thickness of the quadriceps (vastus lateralis, vastus medialis, vastus intermedius, rectus femoris) in different regions of each muscle in 11 recreationally active men after a 12-week resistance-training program for the knee extensors. The researchers found that increases in the muscle thickness of the vastus lateralis and rectus femoris were significantly greater in the distal than in the proximal region but increases in the muscle thickness of the vastus intermedius was significantly greater in the medial than in the lateral region.
Wells (2014) assessed changes in the vastus lateralis at two measurement points (termed VL0 and VL5) after 15-weeks of periodized resistance training in 23 National Collegiate Athletic Association (NCAA) Division 1 female soccer athletes. VL0 was 50% of the straight-line distance between the greater trochanter and lateral epicondyle of the femur. VL5 was 5cm medial to VL0. The researchers found that the increase in muscle thickness was significantly greater at VL5 than at VL0.
Matta (2014) compared changes in the rectus femoris at different sites (30% and 50% of thigh length) following either isokinetic and isoinertial 14-week resistance-training programs in 35 untrained male subjects. The researchers found that isoinertial training led to significant increases in muscular cross-sectional area at both 30% and 50% of thigh length (proximal and distal), isokinetic training led to significant increases at only 50% of thigh length (distal). Thus, the increase in muscular cross-sectional area at the proximal site was greater following isoinertial training compared to following isokinetic training (47.4% vs. 14.4%). On the other hand, the increase in muscular cross-sectional area at the distal site tended to be greater following isokinetic training compared to following isoinertial training (64.7% vs. 19.5%).
Many studies that have investigated hypertrophy at different sites within the same muscle have found that some parts grow more than others, following a period of resistance-training. This suggests that resistance-training can lead to an altered shape of a muscle, as well as a change in its size.
What factors can influence regional hypertrophy?
A small number of researchers have compared the effects of certain training variables, including muscle action, relative load, training volume, and range-of-motion on regional hypertrophy. These studies are described above and are summarized in the following table:
Muscle action (i.e. concentric vs. eccentric) and relative load do not seem to have any effect on regional hypertrophy. However, there is some evidence that training volume, range of motion and external resistance type (i.e. isoinertial vs. isokinetic) do affect the extent to which regional hypertrophy occurs.
What are the practical implications?
Regional hypertrophy does appear to occur in a range of upper and lower body muscles. There are some indications that muscles tend to increase in size most at their points of greatest cross-sectional area.
There is some evidence that varying training volume, range of motion and external resistance type causes growth to occur in different parts of the same muscle. Therefore, variety of these training variables is recommended over the course of a training cycle.
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