Energy Systems Part 2

In Part 1 we saw that the body uses 3 distinct but interlaced energy systems:

Alactic CP last about 4 sec (250-420  kJ/min immediate but only lasts 2-4sec) recharged during recovery  

Anaerobic lactic  last between sec4  - 2 min at maximum intensity (around 300 kJ/min almost immediately but it’s availability depends on how quickly it is used) 

Aerobic main system over 2 min + (63-84 kJ/min and takes around 60 sec to 90 sec to fully activate but can last for a considerable amount of time)

You can think of  the three energy systems operating how a Hybrid car engine produces power to the wheels. 

The Alactic system works like the cars electric battery which provides immediate fast energy to provide max acceleration away from stationary or a short term boost. It is recharged from the other two systems when power demand is lower.

The Aerobic system is the body’s internal combustion engine which takes care of the long distance cruising.

And the Anaerobic Lactic system is the auxiliary turbo boost to deal with hills and overtaking abd at start of exercise before the aerobic system gets up to speed.

But as seen earlier all three systems are working together to seamlessly provide power to our running engine.

We routinely refuel by eating food containing  Carbohydrate, fats and protein in various proportions. The body breaks them down initially by the digestive system and stores the carbs in the form of Glucose and Glycogen (and fat); the fats in the form of Free fatty acids and triglycerides; and the Protein as amino acids.  The carbs and fats the primary nutrients used to produce the energy for the muscles. The protein is mainly used during recovery after exercise to rebuild the muscles and the body’s energy systems. However when carbs start running low they can also be used to create energy. This is not ideal though as it means it may require breaking down essential muscle for fuel.

All three systems must still go through a process to create the only form of energy that the muscle can actually use. The body’s energy currency is ATP (adenosine tri phosphate) This conversion takes place in the individual muscle cells largely within tiny organelles in the cell called mitochondria. These are the true power stations of the body which utilise numerous enzymes and proteins to create the ATP as efficiently as possible and keep the muscles supplied with the energy required to ensure our muscles continue to contact and and relax to power our body in both day to day living and our to carry out our chosen exercise  

Some ATP is also stored in muscle but only a couple of seconds, perhaps enough to jump out of way of runaway bus not any use in athletic activity.  So the mitochondria are continually operating to maintain the amount of ATP in the muscle.

You should see then that the body has to perform a balancing act depending on the intensity of the exercise and the planned duration. From Part 1 the energy systems can produce large amounts of energy for only a relatively short duration but can keep a lower level of energy going for much longer duration. Your body does this balancing act seamlessly and with little deliberate interference from you. It turns up the Alactic and Anaerobic lactic systems for short fast exercise and turns them down for slower long duration exercise while progressively bringing in the Aerobic system.

For the endurance type exercise I will ignore the Alactic system for the moment and look in more depth at the Anaerobic lactic and Aerobic systems. Both systems rely on a great extent on a process called Glycolysis - the breakdown of carbohydrates (Glycogen/glucose). The Anaerobic system can only use Carbohydrate to make ATP and not Fats or Protein. These can only be processed in the Aerobic system.

The above diagram illustrates glycolysis with carbs being processed (through numerous steps not shown) very quickly to produce ATP Energy and pyruvate/lactate. Depending on the relative strength of the aerobic system the pyruvate is burnt in the Aerobic system or enters the muscle or blood as Lactate where it produces more ATP.

The Aerobic system uses as much of the Pyruvate as it can handle along with some of the fats. What pyruvate that it can't process is immediately transformed to Lactate

As we saw above this provides a lot of ATP particularly if the Aerobic system can handle it. There is a limit however in the total supply of carbohydrates contained in the body. Perhaps 1.5 hours depending on the effectiveness of  the Aerobic system. The down side of this system is that it does increase the levels of hydrogen ions in the muscle and blood which lowers the pH and Calcium ions which are a likely factor in fatigue. The lactate produced is used in adjacent muscle fibres or shuttled away to other parts of the body including the heart and brain as fuel, or to the liver to become a new supply of glycogen. If the concentration of these ions get too high either because too much pyruvate is produced or because the Aerobic system is not strong enough to use it, the glycolysis is turned down and energy levels start to fall and you slow down. The feeling of muscle burn appears in the legs and other parts of the body that used to be wrongly thought to be caused by lactic acid. The levels of lactate in the blood are a reflection of the balance between lactate being produced by Glycolysis and being used in the Aerobic system or used in adjacent muscle cells of other parts of the body. The level in the blood is a marker for when these two systems start to go out of balance and is known as the Lactate threshold. (Marker between exercise intensity that can be sustained for a long time and and an intensity that is time limited. Often quoted as about 10k race intensity but varies from individual and training history.

The Aerobic system (our Internal combustion engine) therefore uses the fuel from the Glycolysis and Fats to to do the base line supply of ATP and is progressively important the longer the duration of exercise at the lower intensities.  

As intensity of exercise increases,  the proportion of carbs use increases and as intensity gets low or even at rest the uses of fats is dominant. 

We’ll examine what implications this has on how we train and race in Endurance training factors