The terms lactic acid and lactate, despite biochemical differences, are often used interchangeably. The answer is that lactic acid and lactate are related but are technically different molecules. During high intensity exercise , skeletal muscle can produce large amounts of lactic acid . Once produced in the body, lactic acid rapidly ionizes by releasing a hydrogen ion; the remaining ionized molecule is called lactate. Fitness professionals have traditionally linked lactic acid or ‘the burn’ with an inability to continue an intensive exercise bout at a given intensity. Although the conditions within the exerciser’s muscle cells have shifted towards acidosis, lactate production itself does not directly create the discomfort (acidosis) experienced at higher intensities of exercise. It is the proton (H+) accumulation, coinciding with, but not caused by lactate production, that results in acidosis, impairing muscle contraction, and ultimately leading to the ‘burn’ and associated weariness . The increased proton accumulation occurs most notably from the splitting of ATP (the body’s energy liberating molecule) by the muscle protein filaments, in order to sustain vigorous muscle contraction. Interestingly, the lactate production is proposed to be a physiological event to ‘neutralize’ or ‘retard’ the exerciser’s muscle acidic environment. Thus, lactate accumulation, which for years has been associated with the cause of the burn, is actually a beneficial metabolic event aimed at diminishing the burn. Scientists denote conditioning at this physiological state as lactate threshold training.
Therefore, in simple terms lactic acid is the parent molecule and lactate is the offspring.
Fitness professionals can utilize this knowledge to enhance the cardiovascular endurance performance of their athletes and clients. All world and Olympic endurance athletes incorporate lactate threshold training into their workouts. This article will explain and discuss how lactate threshold training principles can be incorporated into a training program.
First we need to discuss what fuels our exercise sessions and thus we need to understand the Muscle Energy Systems .
All forms of exercise involve bodily movements. These movements are changes in the alignment of bones brought about by contraction of muscles – most often accompanied by intense contraction of muscles in other parts of the body without much apparent change in the angle of joints involved .
The more complex the movements (and quicker or harder), more the benefits – stronger bones, joints and muscles – not to mention the numerous physiological benefits as in improved cardiovascular and respiratory health.
ATP – The Energy Molecule
So for effective muscle contraction, the energy is derived from a high energy molecule called ATP ( ATP stands for ‘adenosine triphosphate’). As the name suggests, ATP contains three (tri) phosphate radicals.
The breakdown of this ATP molecule into ADP (adenosine diphosphate) – by splitting of the bond with the third phosphate radical – will release large amount of energy. This energy is utilized to cause muscle contraction .This muscle contraction is just one of thousands of physiological reactions where-in energy released by ATP breakdown is utilized).
ATP in Muscles
During short bursts of intense exercise, there is a 1000-fold increase in ATP demand in muscles. Despite this, the muscle tissue has the ability to maintain a relatively stable concentration of ATP molecules. This conveys muscle tissue the ability to greatly vary its metabolic rate as compared to other tissue. Thus, taxing muscles to bump up our basal metabolic rates and burn more calories does appear to make total sense.
Despite the high demand of ATP in muscles, especially during bouts of intense exercise, the amount of ATP stored per muscle cell is rather small – about 8mmol/Kg of muscle. And, at all times, the level of ATP never gets below 5mmol/Kg – at which point, muscles fails to produce the necessary power or contraction – a phenomenon called fatigue.
Furthermore, there is a very logical reason for the low concentration of ATP in muscles and the onset of fatigue. There is no denying the fact that fatigue is a protective reflex. If it weren’t for fatigue, we would continue to contract our muscles forcefully to cause irreversible muscle damage or rigor . Low level of ATP thus ensures that fatigue sets in within a short time during intense activity so as to protect muscles.
Energy Systems for Re-synthesis of ATP
Training energy system to effectively resynthesize ATP – as quickly as possible – so that muscle contraction continues without onset of fatigue – forms the basis of most exercise protocols.
There are various energy systems in place within the human body which cause regeneration of ATP. Depending on the substrate used and the rate at which these systems resynthesize ATP, these energy systems are:
ANAEROBIC ATP PRODUCTION
1. Phosphagen System – Is the most rapid system for regeneration of ATP molecules, utilizes phosphates from Creatine Phosphate (CrP) in the presence of enzyme Creatine Kinase. It does not require oxygen hence also called the anaerobic system.
Anaerobic Substrate Utilization
Carbohydrates-
-Glycogen is utilized in repeated high intensity intervals
-Blood glucose is utilized during initial high intensity intervals
ADP + glycogen –> lactic acid + ATP
1 glucose molecule produces 2 ATP in the anaerobic system
During the first 10 seconds of a high intensity exercise (100 meter sprint), the phosphagen system is responsible for ATP regeneration . Recent research, however, tends to support the view that there is rarely a total reliance of the phosphagen system and that the rate of CrP degradation to regenerate ATP begins to decline within 1.3 seconds of intense exercise– meaning other systems to regenerate ATP begin to kick in as early as 1.3 seconds after initiation of intense exercise.
Sports which typically depend on this system are weight lifting, shot put and jumping events. All of these activities require only a few seconds to complete and thus need a rapid supply of ATP
2. Glycolytic System – Another metabolic pathway capable of producing ATP rapidly without the involvement of O2. Glycolysis involves the breakdown of glucose or glycogen to form two molecules of pyruvic acid or lactic acid. Exercise lasting longer than a few seconds will utilize blood glucose and glycogen stored in the liver to regenerate ATP. This system is slower than the phosphagen system however produces more ATP molecules.
AEROBIC ATP PRODUCTION
Aerobic production of ATP occurs inside the mitochondria and involves the interaction of two cooperating metabolic pathways
1) Krebs cycle and 2) Electron transport chain
As the name suggests, this occurs in the mitochondria. It utilizes fatty acids from the blood, fat depots and muscles, glucose from diet or that stored in the liver and glycogen within the muscle. Since it requires the use of oxygen, it is also called the aerobic system.
Carbohydrates (stored glycogen)- utilized in moderate
to high intensity or over-distance activity:
ADP + Glycogen + Oxygen –> ATP + CO2 + water
Thus the end result is the formation of ATP and WATER. Hence the reason we breathe oxygen is to use it as the final acceptor of electrons in aerobic metabolism.
Fats and protein also undergo aerobic metabolism. Fats are broken down to form fatty acids and glycerol. However, glycerol is not an important direct muscle fuel source during exercise.
Fat- utilized under low intensity activity:
ADP + Fat + Oxygen –> ATP + CO2 + water.
Protein is not considered a major fuel source during exercise as it contributes only 2% to 15% of the fuel during exercise. – So NOT a GREAT energy source for training purposes
In summary, the aerobic metabolism of 1 glucose molecule produces 32 ATP through the aerobic system.
.Although the overview of energy system provided here is a very simplistic one, it will help you understand the complex interplay that exists between these systems at all intensities of exercise training – especially during incremental and maximal efforts.
To conclude, although it was always traditionally thought that training the system most suited for your sporting discipline will likely benefit your performance – more recently, it is being recommended that improving the efficiency of all of these systems will likely be more beneficial