The Metabolic Systems and Their Role in Exercise

We get our energy from food and use that energy to perform work.  The carbohydrates, fats and proteins found in food are eventually converted into a molecule called ATP, the main energy source that allows humans to do work.  We get energy from ATP whenever this molecule loses one phosphate to become ADP.

There are three energy systems that we use to breakdown ATP which vary in dominance based on the intensity.  To determine which metabolic system is in dominance, especially with the lactic acid and aerobic system, heart rate monitors are often used to measure intensity.  Knowledge of heart rate training will allow the user to target specific metabolic systems to produce the desired training effect such as weight loss, reduced resting blood pressure, increased speed, power or overall endurance.  Many times, those with weight loss goals religiously train within the "fat burning" aerobic system and never get the results they want- this occurs mainly because the rate of energy burned is more important than the type of energy burned.  I will go into more detail about weight loss on a separate post.  (Remember that the energy systems don't operate independently, they all operate at the same time, but at different percentages based on the workout.)

1. ATP-Phosphocreatine Alactic Anaerobic System aka. ATP-PC or ATP-CP: Contributes to short-duration maximal exercises that require power.  This is a very important energy system to train if a sprint to the finish line is your strategy to the podium.
2. Lactic Acid "LA" system: This system is the dominant ATP manufacturer at the initial start of exercise (1-2 min), accelerations and any time pace is lifted.  It produces energy fast to sustain increased work demands.  Carbohydrate is the primary fuel source.  In racing, strategies called "attacks" or "accelerations" are used by competitive athletes to catch the competition off guard and force them to use CHO when it is least comfortable.  Athletes who can stay in this system longer are said to have a high lactic threshold.
3. Aerobic "O2" system:  The "cruise-control" system that dominates when the intensity is held at a steady state for (2-5 min).  Fat is the primary fuel source.  In racing, a strong aerobic system coupled with economy and consistency is an advantageous way to save glycogen stores for responses to attacks or to do the attacking.  This is a big reason why VO2max is one of the major contributing factor to success.

1. ATP-PC:

The ATP-PC system is similar to a recycling center because as ATP is broken down into ADP, PC almost instantaneously converts ADP back into ATP.  This system is often called "alactic anaerobic" because neither oxygen or lactic acid is produced.  Although hearing no lactic acid sounds like a dream come true, this system does have a time limit.  After a maximal muscle contraction, the ATP-PC system can only operate for about 10 seconds- it then loses efficiency almost entirely at 20 seconds (1).  

The ATP-PC system is dominant in muscle fibers that have a greater amount of PC compared to ATP.  In terms of muscle stores, there is about three times more PC compared to ATP (2).  Specifically, fast twitch muscle fibers have a greater ratio of PC to ATP compared to slow twitch muscle fibers.  This allows individuals who have a greater percentage of FT muscle fibers to excel in sports such as sprinting and jumping events.

2. Lactic Acid system:

When the ATP-PC system and the O2 system can't meet the energy requirements, the LA system takes over to quickly produce energy.  ATP is mainly produced through glycolysis and glycogenolysis.  Although this system can provide energy quickly, problems involve the rate of lactate production and lactate clearance.  If lactate production is greater than lactate clearance, lactic acid will accumulate and cause discomfort.

The LA system dominates at around one to two minutes of exercise; afterwards, the O2 system takes on more work to generate ATP.

How lactic acid is produced:

• In order for muscles to contract, calcium must be released from the sarcoplasmic recticulum.  Free calcium eventually activates an enzyme called glycogen phosphorylase, an enzyme that activates glycogenolysis- this process always results in the production of lactic acid with or without oxygen (3, 4).

• Enzyme activity and mitochondrial density.  Fast twitch muscle fibers have a greater concentration of the enzyme called lactic dehydrogenase.  This enzyme catalyzes pyruvate and NADH + H to produce lactate and NAD+.  As levels of NADH + H and pyruvate increases, so does the activity of this enzyme and the amount of lactic acid produced (5).

• Glycolysis.  As pyruvate is produced from glycolysis, it is reduced to lactic acid (6, 7,5).

• Stimulation of the sympathetic nervous system to release epinephrine and glucagon.  As a result of the breakdown of glycogen, a large amount of glucose-6-phosphate (G6P) is produced, a molecule that increases the rate of glycolysis and pyruvic acid.  The increased amount of pyruvic acid is then converted into lactic acid with the help of lactic dehydrogenase (6).

How lactic acid is cleared:

• Within the liver, lactic acid is converted into glucose through the process, gluconeogenesis.

• Gluconeogenesis also occurs within FT oxidative glycolytic and FT glycolytic muscle fibers.  Any lactic acid left over within the muscle enters gluconeogenesis to be converted back into glucose (3, 10, 9).

• Because of the normal PH level of the body, 99% of lactic acid is dissociated immediately into hydrogen protons and lactate anions (11).  Because lactate can pass easily through mitochondria, muscle, blood, active/ inactive muscles and skin, a small amount of lactic acid can pass through the skin with sweat (8).

2. Aerobic System:
ATP is generated through three processes: aerobic glycolysis, the Krebs cycle and the electon transport oxidative phosphorylation process.  The O2 system dominates after approximately two minutes of exercise.  Between one and two minutes, the O2 system and the anaerobic system produces approximately the same amount of ATP (1).  As exercise demands increase, oxygen consumption increases until reaching the maximum amount of oxygen that the O2 system can process.  The maximum amount of oxygen that the O2 system can consume is called the VO2 max.

Resources:
1. Gastin, P.B.: Energy system interaction and relative contribution during maximal exercise. Sports Medicine. 31(10):725-741 (2001).
2. Gollnick, P. D., & D. W. King: Energy release in the muscle cell. Medicine and science in Sports. 1(1):23-31 (1969).
3. Brooks, G.A., T.D. Fahey, T.P. White, & K.M. Baldwin: Exercise Physiology: Human Bioenergetics and Its Applications (3rd edition) Mountain View, CA: Mayfield (1999).
4. Fox, E.L.: Measurement of the maximal lactic (phosphagen) capacity in man. Medicine and Science in Sports (abstract). 5:66 (1973).
5. Spriet, L.L., R.A. Howlett, & G.J.F. Heigenhauser: An enzymatic approach to lactate production in human skeletal muscle during exercise. Medicine and Science in Sports and Exercise 32(4):756-763 (2000).
6. Brooks, G. A.: The lactate shuttle during exercise and recovery. Medicine and Science in Sports and Exercise. 18(3):360-368 (1986).
7. Gasser, G.A., & G.A. Brooks: Muscular efficiency during steady-rate exercise: Effects of speed and work rate. Journal of Applied Physiology. 38(6):1132-1139 (1975).
8. Brooks, G.A.: Intra- and extra-cellular lactate shuttles. Medicine and Science in Sports and Exercise. 32(4):790-700 (2000).
9. Gladden, L.B.: Muscle as a consumer of lactate. Medicine and Science in Sports and Exercise. 32(4):764-771 (2000).
10. Donovan, C.M. & M.J. Pagliassotti: Quantitative assessment of pathways for lactate disposal in skeletal muscle fiber types. Medicine and Science in Sports and Exercise. 32(4):772-777 (2000).
11. Gladden, L.B.: Lactate metabolism: A new paradigm for the third millennium. Journal of Physiology. 558:5-30 (2004).