25 Jan Muscle Growth Part 1
Strength training causes muscle growth through 3 separate phases: –
Firstly, there is an initial stimulus, 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 the muscle cells. Secondly, there are molecular signalling events inside the muscle. These last several hours, and result from the initial stimulus. Thirdly, there is a temporary increase in the rate of muscle protein synthesis, which is triggered by the molecular signalling events. This is what leads to increases in overall muscle size, which can then be measured in various ways;
- Initial stimulus
There are three mechanisms that stimulate muscle growth:
(1) Mechanical tension
(2) Metabolic stress
(3) Muscle damage
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.
The main difficulty faced when trying to understand the independent effects of mechanical tension, muscle damage, and metabolic stress is that it is very difficult to stimulate a muscle with either muscle damage or metabolic stress while not providing the mechanical tension stimulus at the same time.
Some studies have also suggested that muscle fiber activation, resulting from motor unit recruitment, is a primary mechanism through which muscle growth is stimulated. But this cannot be the case, because full motor recruitment is approach at high velocity contractions, which cause little muscle growth.
Hence, when muscle activation is maintained constant at high levels, increasing the velocity of contraction reduces the hypertrophy. This seems to happen because faster contraction velocities reduces the mechanical tension produced by each muscle fiber, and therefore the amount of mechanical loading detected by its receptors.
- Molecular signalling
Signalling pathways that are activated in response to strength training workouts seem to be involved in elevating muscle protein synthesis, and found to then cause the accumulation of protein inside muscle fibers.
However in cases of over training, which can lead to muscle loss, the oxidative stress seems to inhibit muscle protein synthesis from being increased. With these results in mind we should not assume that a greater increase in motor signalling will “always” cause a greater increase in muscle protein synthesis rates, and a subsequent long-term increase in muscle size.
- Muscle protein synthesis
Even though the protein content of a muscle fiber is determined by the ongoing balance of muscle protein synthesis and muscle protein breakdown rates, there was good evidence to suggest that it was the increase in muscle protein synthesis rate that was responsible for the change in muscle size over time.
Consequently, it was a real breakthrough when identified that elevations in muscle protein synthesis could be related to long-term gains in muscle size after taking away the uplift in muscle protein synthesis required to repair damaged muscle tissue.
This information was important find, not only because it confirmed the central role of increasing muscle protein synthesis rates for causing hypertrophy, but also because it hinted that while muscle damage repair and muscle growth are very similar processes, repairing muscle damage probably does not enhance increases in muscle size.