After strength training, whole muscles increase in volume, and therefore also in mass. In humans, this happens largely because of an increase in the volume of individual muscle fibers, rather than by an increase in the number of fibers.
Individual fibers can increase in volume either by increasing in length, or by increasing in diameter. Increases in length occur through the addition of new sarcomeres in series, which are probably added to the end of existing fibers, while increases in diameter occur by the addition of myofibrils in parallel.
Changes in the shape and structure of the muscle accommodate these increases in size, such that the origin and insertion of the whole muscle do not need to be altered.
So what stimulates a muscle fiber to increase in diameter or in length?
How is hypertrophy stimulated?
The stimulus for hypertrophy is mechanical tension. This mechanical tension must be generated by the fiber itself, but it can be produced either by active contraction or passive resistance to stretch.
When the mechanical tension experienced by the muscle fiber is produced more by the passive elements (these are the structural parts of the fiber, including the giant molecule titin), the fiber seems to increase in volume mainly by increasing in length, by adding sarcomeres in series.
This effect might be stimulated by titin sensing the stretch that is imposed upon it, as the fiber is deformed longitudinally.
In contrast, when the mechanical tension experienced by the muscle fiber is produced more by the active elements (the actin-myosin crossbridges), the fiber seems to increase in volume mainly by increasing in diameter, by adding myofibrils in parallel.
This effect might be stimulated by the outward bulging of the muscle fibers that occurs when actin-myosin crossbridges form, which deforms the muscle fiber in a transverse direction.
What determines how much of the mechanical tension is produced by active or passive forces?
The contribution of passive and active force to overall mechanical tension is determined by the length of the muscle, the contraction mode, and the lengthening speed.
#1. Length of the muscle
Strength training using exercises that involve larger ranges of motion (ROM) increase the proportion of mechanical tension that comes from passive elements, because the structural elements are stretched after fibers reach a certain length.
Contrary to popular belief, full ROM and partial ROM concentric-only strength training each cause similar hypertrophy, if perform the same amount of work in both training programs, and so long as we measure hypertrophy by changes in muscle volume, rather than by changes in muscle cross-sectional area.
However, the type of hypertrophy is slightly different after each type of strength training. Full ROM training mainly causes hypertrophy by increasing fascicle length, while partial ROM training predominantly causes increases in cross-sectional area.
#2. Contraction mode
Strength training using lengthening (eccentric) contractions increases the proportion of mechanical tension that comes from passive elements, because titin is activated once the fiber starts to lengthen, and automatically begins to contribute to force production.
Titin contains two elements in series with each other (Ig domains, and a PEVK segment), which are separated by a small N2A segment. When elongated passively, the highly elastic Ig domains increase in length, which provide only a small amount of resistance to stretch. When elongated actively, N2A binds to thin myofilaments, and this limits how much of the change in titin length can be achieved by lengthening of the Ig domains. Consequently, the much stiffer PEVK segment must elongate instead (and may also wind onto) the thin myofilament, and this provides a great deal of passive resistance to stretch, but only in active lengthening (eccentric) contractions.
Although it was once believed that eccentric-only strength training could produce greater muscle growth than concentric-only or conventional strength training, it is now widely accepted that when programs are volume- or work-matched, the amount of hypertrophy that occurs after each eccentric-only and concentric-only strength training is very similar.
Importantly, the latest research shows that while eccentric-only and concentric-only strength training produce similar increases in muscle volume, eccentric-only training mainly increases fascicle length, while concentric-only training mainly increases muscle cross-sectional area.
#3. Lengthening speed
Strength training with eccentric contractions can involve different lengthening speeds, and since the force-velocity relationship is much flatter on the eccentric side, compared to on the concentric side, these lengthening speeds do not substantially influence the amount of force exerted.
However, the lengthening speed does affect the proportion of the force that is produced by passive and active elements, respectively.
Faster speeds reduce the proportion of mechanical tension that comes from passive elements, because the detachment rate of actin-myosin crossbridges is faster, thereby reducing the amount of active force produced. Similarly, they increase the proportion of mechanical tension that comes from active elements, because of the viscoelastic properties of titin and the other, structural elements of the muscle fiber that resist lengthening.
Consequently, fast eccentric-only strength training produces greater increases in fascicle length than slow eccentric-only strength training, while slow eccentric-only strength training probably causes greater increases in muscle cross-sectional area.
What is the takeaway?
Muscle fibers increase in volume either by increasing in length, or by increasing in diameter. Increases in length occur through the addition of sarcomeres in series, while increases in diameter occur through the addition of myofibrils in parallel.
During strength training, when the mechanical tension experienced by a fiber is produced more by the passive elements, the fiber seems to increase in volume mainly by increasing in length. In contrast, when the mechanical tension experienced by the fiber is produced more by the active elements, the fiber seems to increase in volume mainly by increasing in diameter.
The contribution of passive and active force to overall mechanical tension, and therefore whether hypertrophy occurs through increases in either fiber length or diameter, is determined by the length of the muscle, the contraction mode, and the lengthening speed.