The first phase is the initial Stimulus. This is also often called the primary mechanism. Since muscle growth happens primarily through an increase in the size of individual muscle fibers, this must be detected by receptors inside muscle cells.
The mechanisms that stimulate muscle growth are:
These mechanisms are environmental conditions that can be detected by single muscle fibers, which then stimulate the molecular signalling events that increase muscle protein synthesis rates, and subsequently cause the accumulation of protein inside individual muscle fibers.
When we think about mechanical tension in the role of muscle growth we have to think about:
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 (Isometric). 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.
Muscle fibres can detect the difference between mechanical tension provided by active contractions and by passive loading. This is reflected in the long term adaptations to weight 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 mechanical tension signal that leads to hypertrophy is detected by single muscle fibers and not by the muscle as a whole. This is an important factor because it means that we need to define the mechanical tension stimulus in relation to the forces experienced by each individual muscle fibre, and not by the whole muscle.
There are two points we need to cover here.
Firstly, in an active muscle contraction, the tensile force sensed by the muscle fibre 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 fibre can contract.
Secondly, muscle fibers interact with one another, 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 is probably why muscles do not adapt uniformly after weight training, some regions increase fibre diameter and or length more than others.
Effect 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. 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. This reduction in contraction velocity is closely linked to the amount of metabolic stress in the muscle.
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, this produces mechanical tension on the fibre, which stimulates growth.
Metabolic Stress & Muscle Damage
The role of metabolic stress and muscle damage in building muscle is hard to investigate independently from mechanical stress.
Currently metabolic stress can be explained well by the effects of fatigue on increasing mechanical tension. Similarly, any potential effects of muscle damage is just as easily explained by greater mechanical loading.
Currently we have a clear model for how the primary mechanism of mechanical tension can produce muscle growth, and it is determined by the tension produced and detected in each muscle fibre.
Because all roads for muscle building lead to mechanical tension I wouldn’t worry too much about the role of metabolic stress and or muscle damage.