Before we get into how muscles get bigger, it is important to understand the basic anatomy of a muscle. Muscles are divided by connective tissue (perimysium) into groups of fibres called fascicles. These fascicles can contain hundreds of muscles fibres and as you can see in the image below, each muscle fibre contains hundreds of rod-like myofibrils. Each myofibril then contains hundreds of thousands of sacromeres which are the building blocks of muscles and contain the actin and myosin molecules that form crossbridges and overlap to produce a contraction and generate force. Another thing to note is that muscle fibres have their own source of stems cells called satellite cells which play an important role in cell repair and growth.

For muscle growth to occur, sacromeres need to be produced by the muscle cell and can be added in parallel to other myofibrils which would increase the diameter of the fibre, or in series, increasing muscle length. And it is typically eccentric contractions and training at longer muscle lengths that promotes longitudinal hypertrophy.

There are thought to be three primary mechanisms for this exercise-induced muscle hypertrophy to occur. These are mechanical tension, metabolic stress and muscle damage.

Mechanical tension:

Schoenfeld (2010) has identified that this is the primary mechanism of muscle hypertrophy and it occurs through a process called mechanotransduction. When a muscle fibre contracts, the sarcomeres within it shorten in length and bulge out from the sides. This physically stretches the wall of the muscle cell which is detected by stretch receptors as a threat to it's structure. This tension then leads to the activation of several myogenic (muscle building) pathways. The downstream effects of this is an increased production of sarcomeric proteins. And when the mechanical tension is great enough, the satellite cells we mentioned earlier become active, giving the muscle more machinery to produce more protein and build more sacromeres.

Metabolic Stress:

Metabolic stress is another well known mechanism and occurs when lactate and other metabolites accumulate in the cell as a result of anaerobic glycolysis (using glycogen faster than oxygen can be delivered). In a resistance training context, this is training with higher reps and shorter rest periods to get that glorious pump. This is also how occlusion training works.

From a cellular level this is what is hypothesized to happen:

1. Metabolites such as lactate and hydrogen ions will accumulate and draw water into the cell through osmosis causing swelling. This swelling then stretches the wall of the muscle fibre and the same process as above occurs.
2. The accumulation of metabolites also causes fatigue in the muscle by reducing the release of calcium from the sarcoplasmic reticulum and also by reduced the sensitivity of actin and myosin to calcium (2). This increases the activation of higher threshold motor units and their associated larger muscle fibres which are capable of generating more tension and therefore more stretch on the cell walls.
3. Muscle fatigue also slows contraction velocity which allows more time for cross bridges to form and therefore more tension and again, more stretch.

Muscle Damage:

Trauma to muscle cells was long thought to be a major contributor to muscle hypertrophy. However, the research presented above has shown that the primary mechanism of muscle hypertrophy is in fact mechanical tension.

So there you have it! But what does this mean for your training if muscle hypertrophy is a goal?

Well it's probably best to use a combination of increasing mechanical tension as well as inducing metabolic stress. To increase mechanical tension you need to do something very simple but very overlooked by many... progressively overload your volume load (sets x reps x load). This means increasing the weight and/or volume you are lifting.

And to utilise metabolic stress, complete your final set with a finisher. These can be

• Drop sets- dropping the weight of consecutive sets with no rest
• Cluster sets- using a heavy weight and resting mid set until you complete all reps
• Pyramids- increasing and decreasing the weight
• 50's- doing 50 reps in a row

There are lots of options!

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Reference: Trust Me I'm a Physio