Muscle metabolism

By Nick Webb from London, United Kingdom (Yohan Blake) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

​Every process in the human body requires energy. Muscle contraction is no exception. Let’s explore the three different pathways that the body can take when creating and using energy for muscular actions.
 
The energy releasing molecule in the body is called ATP (adenosine triphosphate). It creates energy by breaking one of the bonds between the phosphates leaving one ADP (adenosine diphosphate) molecule, one P (phosphate molecule).
ATP --> ADP + P + energy
 
The body stores just enough ATP and other high-energy compounds to begin contraction. But the muscle fibres can generate ATP through various metabolic processes.  

The phosphagen system
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​The ATP that muscle fibres produce at rest transfers energy to another high-energy compound called creatine phosphate. Creatine can then combine with the free phosphate and with the help of energy create creatine-phosphate (CP) and ADP.
ATP --> ADP + P + energy and then C + P + energy --> CP + ADP
 
CP can then be broken down when necessary to release energy. This can then be used to synthesise ATP.
CP --> C + P + energy    and then   ADP + P + energy = ATP

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

And now we are clearly where we started, with a molecule of ATP. This cycle of creating energy within muscles is called the phosphagen (or the ATP-CP) system. It is the quickest way for the body to synthesise energy. This pathway is primarily used during short, intense activities requiring lots of power. Shot put, high jump, short-distance sprinting, and powerlifting are examples of sports that rely heavily on this system.
 
This process requires no carbohydrates or fat for fuel nor any oxygen, hence it is anaerobic. ATP is re-created solely by relying on stored CP. Given these stores are limited, the CP system can usually supply energy for the body for bursts of up to approximately 10-15 seconds. Some estimates suggest that it can generate energy at a rate of 10 kcal per minute.
 
Activities that train the CP system are usually under 10s in duration and close to maximum intensity. Such levels of intensity use and tax heavily the central nervous system (CN). Therefore, longer rest periods are recommended.
 
Examples of training this system may look like this:
 
For powerlifting:

• 5 sets of 3 repetitions at 90% the maximum weight a person can lift
• 3-5 minutes rest between sets

 
For sprinting:

• 5 sets of 10-second all-out sprints
• 3-5 minutes rest between sets

 
A scientifically and clinically proven way to increase the capacity of the phosphagen system is through supplementing with creatine monohydrate (CM). CM has been proven beneficial for increasing strength and power output in various populations. Its long-term safety is still questionable.
 
If a fibre has to contract for longer than 10-15 seconds it starts synthesising energy in alternative ways.

The anaerobic system 
 
Another way for the muscle to manufacture energy without oxygen is through glycolysis, also referred to as the anaerobic metabolism. This type of metabolism uses glucose, which is simple sugar circulating in the blood and stored in muscles as glycogen. Glucose is broken down in the cytoplasm of the cell to produce pyruvic acid to produce ATP.
 
During intense activity, the muscle is deprived of the stored ATP and CP, so it starts breaking down glycogen to release glucose. The glucose is then broken down further into 2 pyruvic acid molecules and 2 ATP molecules.
Glucose --> 2 pyruvic acid + 2 ATP

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

When the rate of pyruvic acid production exceeds the cell’s ability to use it aerobically it is converted to lactic acid.
Pyruvic acid --> lactic acid
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Accumulating lactic acid enters the circulation and where it converts to lactate and hydrogen ions. This then alters the pH of body fluids causing the characteristic muscle burning after intense exercise. This eventually leads to an inability to contract the muscle anymore. Soreness in the muscles (DOMS – delayed onset muscle soreness) is likely to occur later on. Massage can be especially beneficial with assisting the body in clearing the built up of lactic acid. This speeds up recovery from exercise allowing the person to return to activity sooner at a higher intensity.
 
Glycolysis is the second-fastest way of re-creating ATP. It is the main energy system used for high-level intensity activities lasting from approximately from 30 seconds to 2 minutes. It is estimated that glycolysis can produce energy half as quickly as the ATP-CP system at a rate of 16 kcal per minute. It still produces energy relatively quickly but with low efficiency (only 2 molecules of ATP). 50m freestyle swimming, middle distance running, and hypertrophy based resistance training are examples of activities fuelled mainly by the glycolytic system.
 
The way to train this type of metabolism employs higher work to rest ratios. This is because the glycolytic system recovers more quickly than the phosphagen one. Possible work to rest ratios may range. A 1:6 ratio (6 seconds rest for every second of work) can be used to improve the body’s capacity to use this pathway and hypertrophy muscles. A 1:1 or 2:1 ratio would be helpful to teach the body to tolerate lactate build up.
 
Examples of training this system may look like this:
 
Hypertrophy type resistance training using 1:6 ratio:

• 3 sets of 8 repetitions taking 4 seconds on each rep (total of 32 seconds work) at 50-80% the maximum a person can lift
• 3 minutes rest between sets
• 30 seconds work to 180 seconds rest gives us 1:6 ratio

 
Sprint intervals to improve tolerance to lactate:

• 10 repetitions of 20-second sprints at highest intensity possible for the target time range
• 20 seconds rest between repetitions
• 1:1 work to rest ratio

 
Carb loading is a strategy that attempts to force the muscles to store more or rather the as much as possible glycogen. This stored glycogen would then be more readily available to fuel glycolytic activity. This is an intervention aimed at and believed to improve performance in activities using the anaerobic system.
 
The aerobic system
 
For demanding activities lasting beyond 2 minutes and indeed for most of our everyday tasks we rely on the aerobic system. The metabolic reactions the body does in the presence of oxygen represent the majority of the cellular energy produced. It is the slowest type of metabolism releasing energy at about 10 kcal per minute. It is, however, also the most efficient in terms of energy yield as we are going to see.
 
The aerobic system provides all the energy required by the muscles at rest and during light to moderate activity. In the sarcoplasm of the cell, the mitochondria use glucose, fatty acids, ADP, phosphate ions, and oxygen to produce 17 ATP from 1 molecule of fat. This is clearly much higher yield compared to glycolysis.
 
The biomechanical process of producing ATP aerobically is complex and has numerous intermediary steps. It is called the Krebs cycle or tricarboxylic acid cycle or TCA for short. Here, carbon atoms of the substrate molecule get converted to carbon dioxide. Hydrogen ions, on the other hand, get converted to water.
Glucose + Oxygen --> Carbon Dioxide + Water + ATP

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

​Sporting activities that rely primarily on the aerobic system include distance running and distance swimming. The other two systems are best trained with a form of interval or intermittent exercise like previously described. The aerobic system, on the other hand, can be trained both with intervals and with continuous training. For example:

• 60 minutes light rowing at 50-60% of a person’s maximum heart rate (MHR)
  • this protocol will develop exclusively the aerobic system
• 2-minute run at moderate intensity followed by 2-minute at low intensity repeated 5-6 times
  • this protocol may also activate the glycolytic pathway depending on a person’s fitness level

 
The most widely used measure of the effectiveness of the aerobic system is the maximal consumption of oxygen a person can achieve per minute, also called VO2 max. In addition to being a symbol of high fitness, it is also correlated with better health in most populations.
 
It is possible to try to increase the body’s ability to metabolise fats for fuel in anticipation for an event such as a marathon that relies mainly on the aerobic system. Because fat contains 9 kcal per gram compared to 4 kcal per gram for carbohydrates, some endurance athletes favour a higher fat diet. The theory is that by increasing the proportion of dietary fats in one’s diet, the utilisation of fat can be improved and upregulated. Some studies confirm this theory, others deny it. In practice, there are a lot of successful endurance athletes relying on high carb, low-fat diet. Also, even if higher fat consumption was to improve performance short-term, the long-term consequences may not be worth it.
 
It is worth mentioning at this point that these three pathways of metabolism can be separated only conceptually. In practice, most activities, most of the time rely on a mixture of fuels and the body readily transitions between the systems in response to demands.
 
Some working knowledge of these systems is very helpful to a therapist in terms of advising a client on how they can be improved. Additionally, knowing the different demands on the body that the different types of metabolism place, can inform what form of soft tissue therapy will be most appropriate.