A few days ago my friend and I were discussing the muscle soreness after an intense workout. As she has a degree in food science, I was a bit surprised when she said: “My muscles feel so sore, I have to get rid of lactic acid from my legs!”. Then I realized the notion that lactic acid build up in our muscles is a cause of muscle fatigue and soreness after an intensive workout is still present up today, although this myth has been debunked somewhere in the 1980s. The truth is nobody knows why exactly we feel muscle soreness and pain after strenuous exercise, but we do know not only lactate does not cause muscle fatigue, but lactate is actually metabolic fuel for the muscle. And if we want to be more accurate, our body produces lactate, not lactic acid as chemically lactate is lactic acid missing one proton.
ATP -“The Energy Currency of the Cell”
On a daily basis, our skeletal muscles perform a large range of activities from simply sustain the body in an upright position to more complex and fast movements like exercise or running for a bus or a train. To be able to do that, our body requires sustainable energy-efficient systems for both light/ moderate to intense physical activity. As a basic energy source for muscular contraction, the human body uses organic chemical adenosine triphosphate (ATP).
The phosphate tail of ATP is the actual power source. The bonds between the phosphates are rich in energy which is released when the bonds are broken. For the muscle to continuously produce the force of work, a systematic regeneration of ATP is required in each cell.
Any reduction in the quantity of ATP in the muscles is manifested as muscle fatigue and decreased muscle ability to create the force needed to work. That is why ATP is continuously regenerated with the help of three metabolic energy systems.
These three metabolic pathways are:
(1) Phosphagen system,
(2) Glycolytic system,
(3) Mitochondrial Respiration or aerobic system
Three Metabolic Energy Systems
The first, phosphagen system, ensures that our muscles have the energy for activities which last no longer than a few seconds and of high intensity. For example, when we get up from a chair and start moving, the ATP required for these movements comes primarily from the phosphagen system. To restore ATP levels, this system uses a substance called creatine phosphate as the amount of ATP present in the skeletal muscles is not sufficient to provide a continuous supply of energy. In sports, activities like heavy weightlifting or sprinting relay mostly on this system. This one of the reasons why creatine is a popular sports supplement as it is believed it helps with improvement of performance during short periods of athletic activity.
If activity continues more than 10 seconds, the anaerobic glycolytic system kicks in to provide a sustainable amount of the ATP. This system uses glucose in the blood or glycogen in our muscles to regenerate ATP rapidly without oxygen. It is also called the lactic acid system or the anaerobic glycolysis system as the end product of this system is lactate. The glycolytic system can restore ATP as much as 2.5 times faster than the oxidative system in mitochondria. On the other hand, the rate of ATP regeneration in the glycolytic system is about half of the rate found in the phosphagen system. Under optimal conditions, this system can provide maximum muscular activity for 1.3 to 1.6 minutes (with 10-15 seconds of the phosphagen system).
And the third system, aerobic system, provides energy for low-intensity activities that can last anywhere from two minutes to a few hours. Regeneration of ATP in the oxidative system happens in the powerhouse of the cell -mitochondria, and involves the oxidation of nutrients. This means the system uses glucose, fatty acids, and amino acids and utilizes oxygen for work. When this system is activated, a large amount of energy is released. Complete oxidation of the substrates in the aerobic system gives more energy than the release of energy in other two systems: for instance, oxidation of one molecule of glucose in the oxidative system gives approximately 38 molecules of ATP, while only 2-3 molecules of ATPs are produced in the glycolytic system.
During low-intensity activities, non-esterified fatty acids (from adipose tissue) are utilized. In more intense exercise, carbohydrates serve as the main substrate for ATP replenishment, but as glycogen levels within the muscle diminish, the fatty acids are used for muscle work. In terms of endurance, this system lasts as long as there are enough nutrients in the body. When blood glucose drops below 2.8-3 mmol/l (below the limits required to maintain CNS activity), we lose consciousness.
So why is Production of Lactate Important?
In the glycolytic system, to re-synthesise ATP, no oxygen is required so the energy for muscle work is provided quickly. But, as no oxygen is used in the process, lactate is produced as an end product. In our bodies, all carbohydrates we have ingested turn into glucose. In the process called glycolysis, each molecule of glucose is converted to a substance called pyruvate. In a presence of oxygen, pyruvate is further converted into another molecule (acetyl-CoA) which enters the Kerb cycle and new ATP is generated. But in anaerobic conditions, pyruvate is converted to lactic acid.
The production of lactate during exercise is important for the effective regeneration of ATP in the glycolysis process. Some authors claim that we would not be able to endure a high-intensity exercise for longer than 10-15 seconds without the production of lactate.
The formation of lactate reduces high concentrations of pyruvate (the end product of glycolysis) in the cell, and the cofactor nicotinamide adenine dinucleotide (NAD +), which is involved in the reaction of conversion of pyruvate to lactate. Restoration of NAD + is important for the normal course of the glycolysis process (the decisive step in the process is indicated in the picture below). If glycolysis would continue indefinitely, all of the NAD+ would be used up, and glycolysis would stop. So to allow glycolysis to continue, our bodies must be able to oxidize NADH back to NAD+. The human body simply cannot regenerate NAD + faster during intense exercise than in the shown manner.
As we continue with the intensive exercise, the lactate levels become very high. Increased lactate levels can be metabolized in the body in two ways: lactate can be converted back to pyruvate in the presence of oxygen in the mitochondria,
it can be converted into the glucose by the process called gluconeogenesis (through this process we generate glucose from non-carbohydrate substrates, like proteins) in the liver and returned to a circulation which then returns to the muscles. Sometimes, lactate is used for the formation of the liver’s glycogen. This process is also known as Cory cycle, named after its discoverers.
Recent research has shown lactate is actually produced and used by cells continuously. Lactate formation is the link between oxidative and anaerobic metabolism. So we can say lactate is fuel for our body, or it can be transported via the blood to other tissues and used for energy.
So why Do We Feel Muscle Soreness after Intensive Exercise?
Delayed Onset Muscle Soreness or DOMS, the pain we sense in the muscles hours to days after strenuous exercise, is most probably caused by microtrauma to muscle fiber tissue, not by lactate. Another theory for the pain associated with DOMS is the “enzyme efflux” theory which is associated with the accumulation of calcium in the damaged muscles.
However, the mechanism of DOMS is still not yet completely understood.