Lactic acid and exercise-related pain: everything you need to know

When you train intensively, it is common to experience muscle soreness in the days that follow, and it is often thought that lactic acid is to blame, as it builds up in the bloodstream after a very intense workout. But this isn’t always entirely accurate; let’s take a look at what lactic acid actually is.

Lactic acid is a by-product of the anaerobic metabolism of lactate, which causes muscle fatigue; its molecular structure leads to a drop in blood pH and thus creates a purely acidic environment that prevents further activity.

In this article, we’re going to debunk the myth that lactic acid is responsible for what is known as delayed-onset muscle soreness.

How is lactic acid formed?

The energy used to enable muscle contraction comes from a molecule called ATP. To have energy available, muscle cells must continuously produce ATP, which they do by using three energy systems:

• aerobic system
• lactacidanaerobic system
• alacticanaerobic system

When intense exercise lasts for more than 10 seconds, the alactic anaerobic system is no longer sufficient to produce energy (phosphate reserves are depleted).

The lactacid anaerobic system then takes over and, to obtain energy in the absence of oxygen through a series of processes within glycolysis, converts pyruvate into lactic acid.

If we also consider the initial phase, this system reaches its peak – that is, maximum lactic acid production – approximately 30 to 40 seconds after the start of the activity. Prolonged exertion, on the other hand, requires a reduction in exercise intensity or a break.

The anaerobic lactic system is the energy system used in strength and endurance activities for approximately one minute.

For example, during a bench press, the anaerobic lactate system is triggered when there is insufficient oxygen to produce ATP via the aerobic system and is activated after 7–8 seconds of intense exercise; its build-up in the muscles and bloodstream causes acidosis, which impedes the continuation of the bench press exercise and leads to physical fatigue.

The ability to sustain this system depends on the amount of lactic acid the muscle can tolerate, and this tolerance threshold can only be improved through training and, to some extent, with certain supplements, such as creatine, which acts as a buffer.

In a working muscle, lactic acid production occurs mainly in the white or fast-twitch fibres, i.e. those that work in the absence of oxygen and have reduced levels of haemoglobin and mitochondria.

But if lactic acid is produced during training, how can we explain the pain felt a few days later?

The pain felt after an intense workout is not caused by a build-up of lactic acid; it is completely cleared within 2–3 hours of the workout, whereas the peak of DOMS occurs 24–48 hours after the workout. There is therefore no correlation between lactic acid and muscle soreness.

DOMS is in fact caused by damage induced by muscle contractions, as well as by microtrauma perceived as a sensation of pain.

The significance of lactic acid

This raises the question: is the production of lactic acid important or not? It should be noted that lactic acid should not be regarded as a harmful substance, as it contains energy that is captured and utilised by various organs, particularly the liver – where it is converted into glucose via the Cori cycle – and by the heart, which uses it primarily for energy.

Furthermore, it acts as a powerful stimulus for the secretion of anabolic hormones such as GH and testosterone, which is why high-intensity weight training is recommended to combat muscle atrophy.

In addition to the Cori cycle, there are other buffering systems that prevent the build-up of lactic acid in the muscle and enable intense exertion to be sustained over longer periods: the most common of these are carbonic acid and bicarbonate.

Observations

When a blood test is carried out after or in the days following a training session, it can be observed that post-workout muscle inflammation may affect CPK levels , causing them to rise.

Creatine phosphokinase (CPK) is an enzyme found in muscle tissue, cardiac fibres, the brain and the lungs. For this reason, it is advisable to carry out routine blood tests after at least five days of rest without physical activity.