Muscle Equilibrium

 

Every cell and organ in our body, including our muscles, operate thanks to proteins.  Proteins are large biological molecules made from specific arrangements of different amino acids.  Each protein carries out specific tasks to aid the operation of cells in our body.  A huge array of proteins perform many functions within living organisms and the largest proteins consist of over 30,000 individual amino acids [4], interacting with one another to form the specific shapes needed to operate as a biological machine.

Muscle protein is formed from several types of long filamentous proteins.  The thick filaments consist of a protein called myosin, whilst the thin filaments contain actin, troponin and tropomyosin.  In skeletal muscles, it is the arrangement of these filaments, plus their proximity to other body architecture (such as blood vessels and mitochondria) that divides them into either Type I and Type II muscle.

Type I muscle (slow twitch) is rich in blood vessels, is able to contract for long periods of time and is associated with endurance.  Type II muscle (fast twitch) contracts with more force but does not have the endurance of Type I.  Rapid recovery and strength gains to both types of muscle tissue are vital for athletes seeking to increase their physical performance.

Skeletal muscles are maintained through a constant state of muscle breakdown and muscle production, driven by key biological processes.  Whilst this constant turnover is essential for the muscle to remain operational, the balance can be readily influenced by external factors such as nutrition, levels of activity, injury and ageing, all of which result in muscle loss or gain.

The rate that new muscle fibres are synthesised will usually keep pace with the breakdown of old, damaged muscle fibres.  Because of this, carrying out normal daily activities and eating an appropriate diet will keep muscle cross-section size relatively constant through most of normal adult life [5] [6].

Whilst this constant turnover of muscle proteins usually maintains an equilibrium, it is also readily influenced by a variety of factors.  For muscle size to remain the same, muscle protein synthesis (MPS) and muscle protein breakdown (MPB) need to be balanced.  If MPS is promoted and exceeds MPB then a positive protein balance is achieved.  This enables hypertrophy, an increase in the cross-sectional size and strength of the targeted muscles.  Conversely, if MPB occurs at a faster rate than MPS, the result will be a negative protein balance and, ultimately, muscular atrophy – a decrease in muscle size and strength [7] [8].

Poor nutrition, ageing, inactivity and injury are all factors that can result in a negative protein balance [9], whilst resistance exercise, physical exertion and an availability of essential amino acids in the blood at the right time will shift the equilibrium towards a positive protein balance and MPS.  For that reason, modern training regimens centre on the behaviours that maximise MPS and minimise MPB.

Under normal physiological influence, skeletal muscle proteins regenerate with predictable regularity, with as many as 1–2% of proteins being broken down and re-synthesised daily [10].  This constant recycling of proteins provides ample opportunities to boost MPS.  This makes hypertrophy readily achievable in the right conditions.

The most important factor in MPS is a molecule called mTOR, which acts as the master controller responsible for coordinating all new muscle growth.  It is therefore imperative that any strategies aimed at achieving hypertrophy consider the conditions in which mTOR can best initiate MPS [11].

The simplest and most effective way to increase MPS is through optimising the diet.  All human bodily proteins are constructed from combinations of 20 different amino acids, and the presence of an excess of amino acids in the blood alone (known as hyperacidaemia) is able to boost MPS.  These necessary amino acids are detected by a nutrient-sensing molecule called Vps34 which in turn activates mTOR, the master controller of MPS.  This process drastically swings the balance towards muscle generation [12] [13].

In one study, providing a direct dose of amino acids into the blood led to an increase in the rates of MPS within half an hour, peaking after 90 minutes [14].  However, this amino acid-induced increase in the rate of MPS was not long-lived, with the results showing a return to normal levels of MPS after four hours [15].

A second strategy to increase MPS is to force muscles to recover from physical exertion by stimulating the molecular mechanisms responsible for muscle protein generation.

Muscle cells are designed primarily to contract and, during resistance exercise or competition, these contractions cause individual muscle cells and surrounding supportive cells to communicate with each through a variety of molecular signals [16].  One of these molecular signals is called 5′ adenosine monophosphate-activated protein kinase (AMPK), and performs multiple functions within our cells, switching various molecules on and off so that some of the cell’s processes can be controlled.  During exercise, AMPK becomes greatly elevated, and these extra AMPK molecules supress the muscle-building activities of mTOR [17].  Without mTOR working at full capacity, the normal rate of MPS drops, whilst MPB levels remain unchanged.  For the duration of the exercise this results in a temporary negative protein balance [18] with resulting negative impacts on muscle size and strength.

Once the period of resistance exercise is over, the levels of AMPK drop back to normal and mTOR is able to reassert itself.  It does so, and at a greatly elevated level, boosting the rate of MPS.  At the same time MPB is also increased, leading to a period of rapid protein recycling that lasts several hours [8] [19-21].