We know that muscles increase in size after strength training, but it is less clear to what extent they also change shape, because of greater increases in one region than in another.
Even so, there is a fairly clear pattern in the data, if we look carefully.
How do muscles grow?
Muscles are made out of thousands of cylindrical muscle fibers, arranged in bundles, called fascicles.
In fusiform muscles, fascicles run longitudinally from one end of the muscle to the other. In pennate muscles, they run diagonally from one side of the muscle to the other, attaching to the fascia that surrounds it. The hamstrings provide good examples of the various different types of muscles, as the semitendinosus is fusiform, while the other hamstrings are pennate.
Muscles grow when individual muscle fibers increase in volume. Since they are cylindrical, muscle fibers can increase in volume either by increasing in length, or by increasing in diameter.
It can initially be confusing to think of muscle fibers as increasing in length, since muscles do not change where they are attached to the skeleton at their origin and insertion after training. Yet, fibers can easily increase in length for several reasons.
Firstly, fibers do not always run the entire length of muscle fascicles anyway. So an increase in muscle fiber length does not need to cause an increase in muscle fascicle length. Even so, fascicle length often does increase after certain types of strength training.
Secondly, since fascicles run from one side of a muscle to the other in pennate muscles, an increase in fascicle length does not necessarily need to increase the overall length of the muscle substantially when measured from their origin to their insertion.
Thirdly, even when a muscle does increase in its overall length, this does not change where it is attached to the skeleton. As it increases in length, the muscle simply bulges out in the middle, such that it takes a longer path from the origin to the insertion.
What determines whether muscle fibers increase more in length or diameter?
When strength training increases fiber length proportionally more than fiber diameter, this causes a different change in the muscle shape from when it increases fiber diameter proportionally more than fiber length.
When strength training increases fascicle length most, this tends to increase the size of the muscle more in the distal region, where new sarcomeres are added to existing fibers. In contrast, when strength training mainly increases fiber diameter (as well as pennation angle), this tends to cause larger increases in the middle region of the muscle.
The trigger for whether a muscle fiber increases more in length or diameter is the extent to which the passive and active elements contribute to overall force production. The passive elements are those parts of the muscle fiber that are deformed elastically during muscle lengthening, including the cytoskeleton, titin, and the outer collagen layer (endomysium). The active elements are the actin-myosin crossbridges, which use energy to produce force.
When active elements are solely responsible for force production, the fiber tends to increase mainly in diameter. When the passive elements elements contribute more to muscle force production, the fiber tends to increase more in length.
Do different types of strength training cause muscle growth to occur in different regions (part 1)
Several studies have compared the long-term effects of different types of strength training on the muscle growth that results in several regions of a muscle. Some of these studies have compared types of strength training that involve different contributions from active and passive elements to muscle force (contraction mode, range of motion, and external resistance type).
#1. Contraction mode
Eccentric-only strength training involves using only the lowering phase of a movement, while concentric-only strength training uses only the lifting phase of a movement. During eccentric contractions, we can exert approximately 30% more force than in similar concentric contractions, because the passive elements of the muscle fibers contribute substantially.
Eccentric-only strength training and concentric-only strength training cause muscle growth to occur in different regions. Eccentric-only strength training causes greater muscle growth in the distal region, while concentric-only strength training causes the middle region to increase more in size. Importantly, eccentric-only training also caused greater increases in fascicle length compared to concentric-only training.
#2. Range of motion
Full range of motion strength training involves the working muscles reaching a fairly long length in an exercise, while partial range of motion strength training involves stopping the exercise while the muscles are shorter. Reaching a longer muscle length by using a full range of motion allows the passive elements of the muscle fibers to contribute more to muscle force, in accordance with the length-tension relationship.
Researchers have found that full range of motion strength training and partial range of motion strength training cause muscle growth to occur in different regions. Full range of motion strength training causes muscle growth to occur to a greater extent in the distal region. Again, full range of motion training also caused greater increases in fascicle length, compared to partial range of motion training.
#3. External resistance type
Using a constant load (weight) as external resistance during strength training involves the muscles producing greater forces at long lengths in an exercise, while accommodating resistance (such as provided by elastic bands) involves the muscles producing greater forces while they are shorter. This means that the passive elements of the muscle fibers to contribute more to muscle force when using weight, compared to when using accommodating resistance.
There are indications that using a constant load (weight) or accommodating resistance during strength training might cause muscle growth to occur in different regions. Specifically, using a constant load seems to produce greater increases in distal region muscle thickness, but whether this is linked to greater muscle fascicle length is unclear.
Summary
Eccentric-only strength training, full range of motion strength training, and constant load (weight) strength training all involve a greater contribution of the passive elements of the muscle fibers to force production, compared to concentric-only strength training, partial range of motion strength training, and accommodating resistance. These types of strength training also produce greater increases in fascicle length and distal region muscle size. Therefore, it seems likely that some instances of regional muscle growth are caused by different changes in fiber length and diameter.
Do different types of strength training cause muscle growth to occur in different regions (part 2)
A couple of other studies have compared the effects of other strength training variables on muscle growth in different regions of a muscle.
It has been suggested that certain strength training methods might be able to load slow twitch (more oxidative) muscle fibers proportionally more than fast twitch (less oxidative) muscle fibers, at least in relation to other strength training methods.
Specifically, some researchers believe that by performing a greater volume of reps with light loads, the proportional load on the muscle fibers that are controlled by the low-threshold motor units might be increased, relative to the load on the fibers controlled by high-threshold motor units. However, this proposal is contentious, and some research groups regard it as highly unlikely, because only the muscle fibers of high-threshold motor units seem to respond to strength training by increasing in size, partly because these motor units control a greater number of fibers, and partly because fast twitch muscle fibers themselves are more responsive.
Even so, it is well-known that muscle fiber type does differ between regions of the same muscle, both in rodents and in humans.
Therefore, if strength training can produce proportionally different growth in fibers of different types, it is theoretically possible that regional muscle growth should occur in response to training with high and low volumes, and in response to using heavy and light loads. Since higher volumes and lighter loads both stress the more oxidative fiber types to a greater extent, regions that have a greater proportion of slow twitch fibers should be developed to a proportionally greater extent.
However, in the limited studies that have been published to date, it seems that training with either heavy or light loads, or high and low volumes, result in similar regional muscle growth, which may be because heavy and light loads both provide a similar stimulus to the muscle fibers of high-threshold motor units, and neither provide any meaningful stimulus to the muscle fibers of low-threshold motor units.
Why might different exercises lead to different muscle growth?
Different exercises might produce different regional muscle growth from one another, even when the same types of strength training are used, simply because of the functional compartmentalization of muscles.
Although we tend to think of muscles as homogenous structures that all contribute similarly to a joint action, they are actually made up of separate compartments that have different characteristics. Sometimes, these compartments are even separated from each other by tendinous inscriptions, such as in the semitendinosus.
For example, the gluteus medius has four compartments (anterior, anterior-middle, posterior-middle, and posterior), which are separately innervated, which have different muscle architecture (pennation angle, fascicle length, and physiological cross-sectional area), and internal moment arm lengths for a variety of hip movements. Each region is almost certainly activated separately by the brain in response to a need to produce force in different directions, and therefore contributes to different hip movements.
Even so, separate innervation may not be necessary for regional muscle growth to occur after strength training, particularly when the increase in muscle fiber size results from an increase in fiber length.
After eccentric-only, involuntary (electrically-stimulated) strength training in rabbits, the resulting increase in muscle fascicle length differs between regions, and it is associated with the amount that the fascicles elongate during exercise. This change in fascicle length during exercise was probably not affected by the level of motor unit recruitment in each region, since the fibers were stimulated electrically.
It seems likely that differences in muscle architecture between regions of the muscle will cause the fibers in each region to work at different points on the length-tension relationship, and therefore experience different levels of mechanical loading, and display different resulting changes in fiber length after training.
Some researchers have managed to link the activation of certain regions of a muscle during a single bout of a strength training exercise with the resulting change in muscle size after long-term training with the same exercise, and different exercises using the same muscle groups have been shown to involve different regional muscle activation from one another.
Yet, in both cases, this activation has been measured by magnetic resonance imaging scans, which more likely reflects the degree of muscle damage rather than the amount of muscle activation subsequent to motor unit recruitment. Therefore, we cannot be sure whether these results reflect differences in innervation between functional compartments, or simply differences in muscle architecture between regions.
What does this mean in practice?
Logically, training programs that develop a range of muscle regions will lead to greater muscle growth than training programs that only develop one region. Even so, the appropriate approach for each muscle group may differ.
Muscles with only one functional compartment will likely only respond to using different loading types. Exercises should therefore be chosen that stress either the passive elements (eccentric training, full ranges of motion, and constant loads) or the active elements (concentric training, partial range of motion, and accommodating resistance).
Muscles with multiple functional compartments (especially those with regions that are innervated separately) may benefit from a range of exercises that involve different movement patterns, as well as different loading types.
What is the takeaway?
Regional muscle growth can occur either because of differences in the amount that the muscle fibers increase in length or diameter after strength training, or because of differences in the growth of different functional compartments.
Eccentric-only strength training, full range of motion strength training, and constant load strength training involve a greater contribution of the passive elements of the muscle fibers to force production, compared to concentric-only strength training, partial range of motion strength training, and accommodating resistance. They also lead to different regional hypertrophy, probably because the greater involvement of the passive elements stimulates greater increases in fascicle length, while the greater involvement of the active elements stimulates greater increases in muscle fiber diameter.
Additionally, different exercises can cause different regions of a muscle to grow, depending on the exercises and the muscle. Where multiple functional compartments of a muscle exist, each can be stimulated to grow to a greater extent than the others after training with an exercise because of a need to produce force in a specific direction. This can occur because that region is innervated separately, or simply because the fascicles of that region are subjected to greater elongation in the exercise as a result of their specific architecture.