01 Feb Muscle growth Part 2
There are three important areas to address when thinking about the role of mechanical tension in muscle growth:
(1) the nature of active and passive mechanical tension
(2) the role of external resistance
(3) the effects of fatigue.
Active and passive mechanical tension
Muscles can experience mechanical tension either when they are contracting actively, or when they are passively stretched. When they are actively contracting, they can produce force either while shortening, lengthening, or remaining at a constant length. In all cases, the amount of mechanical tension has been related to the subsequent change in muscle size, thereby confirming the key role of this mechanism in hypertrophy.
While we are most accustomed to muscle growth happening after strength training using active muscle contractions, hypertrophy has also been reported after passive stretching of inactive muscle, in both humans and animals, and very likely involves somewhat similar molecular stimulation through the motor pathway.
Interestingly, however, it seems likely that muscle fibers can detect the difference between mechanical tension provided by active contractions and by passive loading. This is reflected in the nature of the molecular signaling through the motor pathway, and also in the long-term adaptations to strength training, which are often greater after combining both active and passive loading, even when muscle forces are equated. This suggests that muscular contractions and stretching provide independent, and additive stimuli that lead to muscle growth.
The role of external resistance
The way in which mechanical tension causes muscle growth is frequently misunderstood, because we tend to think of the external resistance as being the mechanical stimulus. While this is appropriate when thinking about passive stretching of muscle tissue, it is not valid when thinking about strength training in which active muscle contractions are involved.
The mechanical tension signal that leads to hypertrophy is detected by single fibers and not by the muscle as a whole, by mechanical receptors that are probably located on membrane of each muscle cell. This is an important factor, because it means that we need to define the mechanical tension stimulus in related to the forces experienced by each individual muscle fiber, and not by the whole muscle.
In this respect, there are two key points.
Firstly, in an active muscle contraction, the tensile force sensed by a muscle fiber is essentially the force it produces itself. Even so, in the absence of fatigue, it is the external resistance that determines the speed at which each fiber can contract. While the resistance must be external to the muscle, it can be internal in the body, such as when contracting the agonist and antagonist muscles simultaneously.
Secondly, muscle fibers interact with one another, bulging outwards and exerting force laterally, and the whole muscle bends and changes shape during a contraction. This means that a muscle contraction exposes its fibers to a variety of external constraints. This leads to different fiber shortening velocities, mechanical tension, and length changes, and this affects the fibers of some regions more than others.
The effects of fatigue
When doing multiple, repeated muscle contractions, fatigue develops. This means that the muscle fibers governed by the working motor units become unable to produce the required force, and this causes higher threshold motor units to be recruited, and their associated muscle fibers are then activated. In addition, the fatigue causes the working muscle fibers to reduce their contraction velocity over the set. Consequently, during fatiguing sets with any load, high threshold motor units that grow after strength training are activated, and their muscle fibers contract at a slow speed. Since the muscle fibers shorten at a slow speed, a large number of attached actin-myosin cross bridges are formed, and this produces mechanical tension on the fiber, which stimulates it to grow.