Muscle activation and hypertrophy

Muscle fibers only experience hypertrophy in response to mechanical tension. Yet, in the context of strength training exercises, mechanical tension can only be generated and experienced by the activated muscle fibers. Therefore, muscle activation is extremely important for hypertrophy.

.

#1. Active mechanical tension

Muscle activation causes the formation of actin-myosin crossbridges by means of a process called excitation-contraction coupling. The crossbridges are what generate force production. The force is then detected by mechanoreceptors inside the muscle fiber itself, which then signal for that fiber to experience growth. The mechanoreceptors do not signal to any other muscle fibers, nor do any forces that are transmitted laterally within the muscle produce any stimulus to other muscle fibers. Therefore, only muscle fibers that are activated can experience hypertrophy as a result of active mechanical tension (read more).

.

#2. Passive mechanical tension

Although many fitness influencers believe that passive mechanical tension is applied regardless of whether muscle activation is experienced, this is not true. Passive mechanical tension occurs when titin molecules are stretched, and the way in which titin is stretched differs depending on whether the muscle fiber is activated or not. Indeed, muscle fibers generate passive mechanical tension at much shorter lengths when they are activated compared to when they are inactive. This means that we can produce passive mechanical tension during strength training only in the active muscle fibers (since strength training exercises do not reach truly maximum muscle lengths). In contrast, we can produce passive mechanical tension in all of the muscle fibers of a muscle when we do static stretching because of the much longer muscle lengths that are reached.

.

Conclusions

Muscle fibers only grow after experiencing (active or passive) tension. Yet, in the context of strength training exercises, this mechanical tension can only be experienced by activated muscle fibers. Active tension is produced whenever a muscle fiber is activated, displays excitation-contraction coupling, and forms actin-myosin crossbridges that pull the active muscle fiber to shorter lengths. Mechanoreceptors located inside the muscle fiber detect the tension and signal for the addition of myofibrils, which increases the muscle fiber diameter. This process is limited to the muscle fiber that [1] formed the crossbridges that generated the active tension, and [2] detected that active tension with its mechanoreceptors. Hence, only activated muscle fibers can grow in response to the generation of active tension inside a muscle. Passive tension is produced when the titin inside of single muscle fibers is stretched. Mechanoreceptors located inside the muscle fiber detect the tension and signal for the addition of sarcomeres in series, which increases the muscle fiber length. However, the exact length that produces passive tension differs depending on whether the muscle fiber is activated or not. If the muscle fiber is activated, enough passive tension is generated without reaching the really, really long lengths that are experienced during static stretching. Hence, only activated muscle fibers can grow in response to the generation of passive tension inside a muscle during the vast majority of strength training exercises (or at least those exercises that do not reach the same muscle lengths as are reached during static stretches). ​