Lactic Acid and Muscle Soreness: Myth vs Chemistry

Few molecules have a worse reputation among athletes than lactic acid. It’s blamed for the burning feeling in your thighs during a hard climb, for the stiffness the morning after, and for “poisoning” muscles until they give up. Most of that reputation is undeserved. Over the past few decades, exercise biochemists have rewritten the story, and the real chemistry is more interesting than the myth.

First, a quick reminder of where lactate comes from. In the cytoplasm of a muscle cell, glycolysis turns each glucose into two pyruvate molecules and, along the way, converts the coenzyme NAD⁺ into NADH. For glycolysis to keep running, NADH must be turned back into NAD⁺. The enzyme lactate dehydrogenase does this by reducing pyruvate:

pyruvate + NADH + H⁺ ⇌ lactate + NAD⁺

With that in hand, let’s look at the myths.

Myth 1: “Muscles make lactic acid”

What actually happens: lactic acid (2-hydroxypropanoic acid, CH₃CH(OH)COOH) has a pKa of about 3.9. Inside a cell at pH around 7, the Henderson–Hasselbalch equation tells us the ratio of ionised to un-ionised form is roughly 10^(7.0 − 3.9), which is over 1,000 to 1. So almost all of it exists as the lactate ion, CH₃CH(OH)COO⁻. Physiologists therefore talk about lactate, not lactic acid. (If the pKa link feels rusty, see pH of weak acids.)

This is more than pedantry: as the next myth shows, the question of where the hydrogen ions come from depends on getting this right.

Myth 2: “Lactate production is what makes muscles acidic”

What actually happens: muscle pH does fall during very intense exercise, from about 7.0 to as low as about 6.5. But look at the equation above: converting pyruvate to lactate actually consumes a hydrogen ion. The hydrogen ions that lower pH come mainly from other reactions — above all, from the breakdown of ATP when it isn’t being regenerated by the mitochondria at the same pace, and from some steps of glycolysis itself.

Lactate and hydrogen ions rise together during hard exercise, because both are signs of a high glycolytic rate. That correlation is why lactate got the blame. Blood lactate is still a very useful marker of exercise intensity, but it’s a marker, not the culprit.

Myth 3: “Lactate is a waste product”

What actually happens: lactate is a fuel. It leaves working muscle fibres and is taken up by:

• Neighbouring muscle fibres, especially the slow-twitch fibres rich in mitochondria, which convert it back to pyruvate and oxidise it in the Krebs cycle.
• The heart, which happily burns lactate — during hard exercise it can be one of the heart’s main fuels.
• The liver, which turns lactate back into glucose (gluconeogenesis) and sends it back out in the blood. This loop between muscle and liver is called the Cori cycle.

Researchers describe this as a “lactate shuttle”: lactate moves energy from cells that make it to cells that can use it. Far from being rubbish, it’s one of the ways the body distributes carbohydrate energy quickly.

Myth 4: “Lactate is only made when there’s no oxygen”

What actually happens: muscles produce some lactate even at rest, and plenty during moderate exercise when oxygen supply is fine. Lactate forms whenever glycolysis produces pyruvate faster than the mitochondria take it up. Oxygen shortage is one reason this can happen, but a high rate of glycolysis — driven by adrenaline and by fast-twitch fibres being recruited — is enough on its own.

That’s why “anaerobic threshold” is a slightly misleading name. The point during rising effort when blood lactate starts to climb steeply is better called the lactate threshold: it’s where production begins to outpace clearance, not where oxygen runs out.

Myth 5: “The burn during a hard effort is lactic acid eating your muscles”

What actually happens: the burning sensation is real, and it’s linked to chemistry, but not to lactate attacking tissue. Sensory nerves in muscle respond to a combination of signals: hydrogen ions, phosphate, ATP released from cells, and other metabolites. Experiments suggest that it’s the combination of these, rather than any single one, that produces the strongest sensation of fatigue and discomfort.

Nothing is being “eaten”. Within a minute or two of stopping, the burn fades, which is exactly what you’d expect from metabolites being cleared and buffered, not from damage.

Myth 6: “Lactic acid causes the soreness you feel the next day”

What actually happens: blood lactate returns to resting levels within about 30 to 90 minutes after exercise. Yet delayed-onset muscle soreness (DOMS) typically peaks 24 to 72 hours later. The timing alone rules lactate out.

DOMS is associated with microscopic disruption of muscle fibres and the inflammation and repair that follow. It’s especially common after eccentric exercise, where a muscle lengthens under load — running downhill, lowering a heavy weight slowly, walking down stairs. Downhill running produces less lactate than uphill running at the same effort, yet causes far more soreness the next day. That’s a neat natural experiment against the myth.

Myth 7: “Acid build-up is the main cause of fatigue”

What actually happens: for decades, falling pH was thought to be the main reason muscles weaken during intense exercise. Newer experiments on isolated muscle fibres at body temperature found that acidity has a smaller effect than earlier studies (often done at cooler temperatures) suggested. Other factors appear to matter as much or more:

• Inorganic phosphate building up from ATP and phosphocreatine breakdown, which interferes with how myosin generates force and how calcium is released.
• Changes in calcium handling by the sarcoplasmic reticulum, the store that triggers contraction.
• Potassium leaking out of fibres, which affects the electrical signals that start each contraction.
• Signals from the brain, which may reduce drive to muscles before real damage occurs.

Some studies even suggest that moderate acidity can protect muscle excitability under certain conditions. Fatigue is a multi-cause phenomenon, and no single molecule deserves full blame.

Myth 8: “Massage or stretching flushes out lactic acid”

What actually happens: since lactate clears on its own within about an hour, there’s nothing left to “flush” by the time most people get a massage. The best-supported way to speed lactate clearance is active recovery — light exercise such as easy jogging or cycling — because working muscles and the heart oxidise lactate as fuel. Massage may still feel good and may reduce the perception of soreness, but not because it removes lactate.

Why the myth took hold

The story has a respectable history. In the early twentieth century, physiologists such as Archibald Hill and Otto Meyerhof studied frog muscles stimulated until exhaustion without oxygen. The muscles filled with lactic acid and stopped contracting, so it seemed natural to connect the two. That work won a Nobel Prize in 1922 and shaped textbooks for decades. Only later, with better techniques for measuring what happens inside living human muscle, did the picture change. It’s a good reminder that correlation — lactate rises when muscles fatigue — isn’t causation.

What lactate testing is still good for

Coaches still measure blood lactate from a finger-prick sample, and for good reason. The lactate threshold is a reliable indicator of endurance fitness: training moves it to a higher speed or power, because the athlete’s muscles build more mitochondria and clear lactate faster. So a falling lactate reading at the same pace is a sign the aerobic system is improving — lactate is the messenger, not the enemy.

Key takeaways

• In the body it’s lactate, not lactic acid; at cell pH, over 99.9 % is ionised.
• Converting pyruvate to lactate consumes H⁺; the acidity of hard exercise comes largely from other reactions.
• Lactate is a fuel shuttled to the heart, other muscles and the liver (Cori cycle).
• The burn is caused by a mix of metabolites acting on nerves; next-day soreness (DOMS) is linked to muscle damage and repair, not lactate.
• Fatigue has several causes, including phosphate and calcium handling; active recovery clears lactate best.

For the bigger picture of how muscles make energy, read what happens chemically when you exercise and aerobic vs anaerobic respiration.