Why does muscle soreness show up the next day?
― The culprit has already left the scene
You may have heard that "muscle soreness comes from a build-up of lactic acid." It's a story that's been around a long time, but the timing doesn't add up. Lactic acid is almost entirely gone from the body within an hour or two of exercise. Yet muscle soreness doesn't appear until nearly a full day later. Something strange is going on: the culprit has already left the scene, but the pain arrives long after.
The day after exercising for the first time in a while, climbing stairs is agony. Yet right after the workout, you probably felt surprisingly fine.
Many of us have been told it's "because lactic acid builds up" and left it at that. But this explanation runs into a timing problem.
Lactic acid rises during intense exercise. But once you stop, it returns to normal levels in a relatively short time. It doesn't linger until the next day.
In other words, by the time the pain sets in, lactic acid is already back to its usual level. So what's actually causing the pain?
Lactic acid that rises during exercise returns to close to normal within about 1-2 hours after you stop. It's not a substance that stays around until the next day.
In particular, movements where the muscle produces force while lengthening create tiny tears in muscle fibres. The body's response to those tears takes time to build up.
The delay isn't your body being lazy. It's that the repair response takes time to kick in, step by step. Let's look at it in order.
Lactic acid isn't the direct cause of the pain
Lactic acid is a substance that builds up in muscles during intense exercise. The phrase "lactic acid builds up" spread the idea that it lingers until the next day and causes pain.
But once you stop exercising, lactic acid is cleared relatively quickly. Moving gently while resting (a light walk, say) clears it even faster, and even if you stay still, it returns to near-normal levels within a few hours.
Muscle soreness, on the other hand, only intensifies half a day to two days after exercise. That's why this phenomenon is called "delayed onset muscle soreness."
There's a large gap in time between when lactic acid disappears and when the pain really sets in. That gap is exactly what the lactic-acid story can't explain.
the lactic acid is already gone.
What's really happening: tiny tears in the muscle
So what is going on? The key lies in how the muscle is being used.
Muscles can produce force in two ways: while shortening, and while lengthening. Climbing stairs mostly uses the former; walking downstairs, or slowly lowering something heavy, uses the latter. Think of it as bracing against a load like a brake.
This "lengthening while bracing" motion is thought to concentrate stress on the muscle. Some of the fine fibres that make up the muscle develop tiny, invisible tears.
The tear itself doesn't cause pain right away. It takes time after the tear forms for the body to notice it and begin its repair response.
The body's response takes time — and happens in stages
As we saw in the article on cuts, the body's repair process unfolds in stages. Something similar happens with these tiny muscle tears.
- Right after the tear forms, there's no noticeable response yet.
- Over about half a day, the immune system notices the site, and cells start to gather there.
- The cells that gather release substances that stimulate nearby nerves. This is when it first gets registered as "pain."
- The response peaks after 1-2 days.
- Over the following days, repair progresses and the pain fades.
Because so many steps sit between "the tear forming" and "feeling the pain," the pain ends up arriving late.
During intense exercise, you might feel a burning sensation in your muscles. This is a separate phenomenon from delayed onset muscle soreness.
During exercise, conditions inside the muscle change dramatically for a short time, and nerves are thought to respond to that. This sensation eases relatively quickly once you stop exercising. It's different — both in timing and in what's happening inside the body — from the "delayed" soreness that lingers into the next day.
If you treat "the burning feeling during exercise" and "next-day soreness" as the same thing caused by lactic acid, the mismatch in timing becomes impossible to explain. That's the central point of this article.
Why does it get easier the second time?
The first time you try a new movement (exercising after a long break, a new activity), soreness tends to be especially strong. But do a similar exercise again after some time, even without holding back, and the soreness is much milder.
This is called the "repeated bout effect," and it's thought to last for several weeks. After that first experience, the muscle and surrounding tissue are thought to become more resistant to that same kind of stress.
However, exactly why the effect is so large, and why it kicks in so quickly, isn't fully explained yet. We'll go into this further in the final expandable section.
Something you can check for yourself
- Pick a movement you don't normally do (skip a stair on the way down, do extra slow squats, etc.)
- Note how your legs feel right after exercising. In most cases, it won't hurt much yet
- That night, the next morning, and the day after, move the same area the same way and record how it feels
- Confirm that the pain peaks not on the day you exercised, but 1-2 days later
- A few weeks later, try the same exercise again and compare how sore you get
Step 4 is the heart of this observation. You can confirm for yourself that the pain peaks not "at the moment of exercise" but with a delay. If you don't usually exercise, or have concerns about your heart or joints, don't push yourself — start with a light load. If severe pain persists, or you notice heavy swelling or numbness, don't self-diagnose — consult a doctor.
Summary
Muscle soreness showing up the next day isn't lactic acid's fault. Lactic acid returns to near-normal levels within a few hours of exercise. The real cause is tiny tears in the muscle fibres and the repair response that follows. It takes half a day to two days for the tear to be noticed, for cells to gather, and for that to be felt as pain.
The pain isn't simply delayed.
The body's response itself needs steps, and those steps need time.
Want to know more? ― Terms, numbers, and how this connects to the textbookLabelled by level, from junior-high science to topics still being researched
- JHSCovered in junior high school science
- HSCovered in high school "Biology Basics" / "Chemistry Basics"
- HS+Covered in high school "Biology," or treated as advanced/sidebar material in textbooks
- Univ.Not covered in high school — content from a university-level course (exercise physiology)
- ResearchNot even settled as "established fact" at university — something researchers are actively investigating
JHSTerms: words used around muscle soreness
- Lactic acid: A substance produced in muscles as part of the energy-making process during intense exercise.
- Delayed onset muscle soreness (DOMS): Muscle soreness that intensifies about half a day to two days after exercise. This is what the article covers.
- Eccentric contraction: Muscle producing force while lengthening. Walking downstairs, or slowly lowering something heavy, are examples.
- Concentric contraction: Muscle producing force while shortening. Walking upstairs, for example.
- Inflammation: The body's defensive and repair response to tissue damage or abnormality. Comes with swelling, heat, and pain.
HSChecking with an equation: is lactic acid still around when the pain hits?
The article said "lactic acid disappears within a few hours." Let's actually check this with a calculation. We'll use the idea of a half-life — the time it takes for a quantity to fall by half.
fraction remaining = (1/2) ^ (elapsed time ÷ half-life)
| Fraction remaining | Ratio relative to the level right after exercise, taken as 1 |
| Elapsed time | Time since exercise ended [minutes] |
| Half-life | Time for the amount to fall by half. For blood lactate, about 25 minutes |
Here's how to read it: every 25 minutes, the remaining amount halves again. After 2 half-lives (50 min) it's a quarter; after 3 (75 min) it's an eighth.
| 25 min after exercise (1 half-life) | (1/2)¹ = 0.5 (50%) |
| 50 min after (2 half-lives) | (1/2)² = 0.25 (25%) |
| 90 min after (3.6 half-lives) | 90 ÷ 25 = 3.6 half-lives |
| Fraction remaining at 90 min | about 0.083 (about 8%) |
Within an hour and a half, lactic acid has dropped to under a tenth of its post-exercise level. Some reports suggest it clears even faster in practice, but this calculation alone already shows it's plenty fast.
| Soreness roughly starts intensifying | about half a day (720 min) after exercise |
| Soreness roughly peaks | about 1-2 days (1440-2880 min) after exercise |
| How many half-lives is 720 min? | 720 ÷ 25 = 28.8 half-lives |
| Fraction of lactic acid remaining then | (1/2)^28.8 ≒ essentially zero |
After 28.8 half-lives, the remaining amount is so small it barely registers in the calculation. By the time soreness is "just starting to build," lactic acid is already essentially at zero. By the peak, 1-2 days later, the gap is even wider.
"It hurts because lactic acid is built up" doesn't hold up against the timing. That's why the lactic-acid explanation has been ruled out.
※ The half-life of lactic acid varies with exercise intensity and recovery method (resting vs. light activity), and reported values differ. This calculation is meant to give a sense of scale, not a precise figure.
| From tear forming to immune cells starting to gather | said to be a few hours to about half a day |
| Period over which cells gather and nerve-stimulating substances build up | said to be about half a day to a day |
| When the response peaks | said to be 1-2 days later |
Add these up, and you get "peaking half a day to two days later" — which matches the actual observed time course of muscle soreness well.
In other words, it's not lactic acid's time course (gone within hours) but inflammation's time course (building over half a day to two days) that matches the timing of muscle soreness. This is the time-based evidence for why the cause is thought to be muscle-fibre tears and the response that follows, rather than lactic acid.
HS+Why does damage concentrate in "force-while-lengthening" movement?
When a muscle produces force while shortening, the muscle itself is generating the motion. When it produces force while lengthening, on the other hand, it's bracing against an outside force (like gravity), acting like a brake.
In this "bracing" motion, it's known that less nerve signal is needed to produce the same amount of force. Fewer fibres are thought to be recruited, while each one bears a greater share of the load — and this is considered one reason tiny tears form more easily.
Walking downstairs, descending a mountain, slowly lowering something heavy — if it involves "lowering" or "bracing," that's force-while-lengthening movement. This is why next-day soreness tends to be worse after these activities.
Univ.Pain and actual damage don't line up neatly
Compare the intensity of muscle soreness with the actual extent of muscle damage (strength loss, or levels of blood markers indicating damage), and the two don't always correspond cleanly.
Sometimes there's barely any pain even though strength has dropped a lot, and sometimes the reverse. The intensity of pain isn't a direct gauge of how much damage is happening inside the body.
This matters in practice, too. "No soreness" doesn't necessarily mean "not enough of a workout," and "severe soreness" doesn't necessarily mean "extra effective." Pain and what's actually changing in the body need to be considered separately.
ResearchWhat's still unclear
- The exact trigger that causes the pain itself still hasn't been fully pinned down. Several substances associated with inflammation are thought to stimulate nerves, but which one plays the leading role, and how they combine, is still under study.
- What's really behind the "repeated bout effect" (soreness easing from the second time on) is also unresolved. There are theories that the muscle fibres themselves get stronger, that surrounding connective tissue changes, and that nerve recruitment changes — and no single theory fully accounts for it.
- Whether measures like stretching, massage, and icing actually reduce muscle soreness is disputed across studies. There are reports showing an effect and reports finding no clear effect, and no widely agreed-upon method has been established so far.
- Why pain intensity and actual tissue damage don't match up is also not fully explained. Individual differences in perception and nerve sensitivity are thought to play a role, among other factors, but the full picture is still being worked out.
The "it's lactic acid's fault" misconception has largely been dispelled, but the question of exactly what does cause the pain still doesn't have a complete answer. It's a good example of how resolving one misunderstanding just reveals the next question waiting behind it.
Connections to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science ・ exercise and energy | The basics of how lactic acid forms |
| HS | Biology Basics ・ respiration and energy metabolism | The time calculation using lactic acid's half-life |
| HS | Biology Basics ・ immunity and inflammation | How immune cells gather and repair begins |
| HS+ | Biology ・ mechanism of muscle contraction | The difference between eccentric and concentric contraction |
| Univ. | Exercise physiology | The mismatch between pain and damage markers |
| Research | Muscle physiology (unresolved) | Identifying the pain trigger, mechanism of the repeated bout effect |
| ― | Health & exercise | When to see a doctor for persistent severe pain |
- Cheung, K., Hume, P. A. & Maxwell, L., Delayed onset muscle soreness: treatment strategies and performance factors, Sports Medicine 33, 2003.
- Schwane, J. A. et al., Delayed-onset muscular soreness and plasma CPK and LDH activities after downhill running, Medicine and Science in Sports and Exercise 15, 1983.
- Hyldahl, R. D., Chen, T. C. & Nosaka, K., Mechanisms and mediators of the skeletal muscle repeated bout effect, Exercise and Sport Sciences Reviews 45, 2017 (on the repeated bout effect).
- Nielsen, O. B. & de Paoli, F. V., Regulation of Na+-K+ homeostasis and excitability in contracting muscles, and other exercise-physiology textbook accounts of lactate clearance.
- General-audience materials on post-exercise muscle soreness and recovery from the Japanese Society of Clinical Sports Medicine (日本臨床スポーツ医学会) and related bodies.
※ The half-life of lactic acid and the time course of the inflammatory response vary by individual, exercise intensity, and conditions. The figures given here are commonly cited approximations.
※ This article is a general-audience science explainer. If severe muscle soreness persists, or is accompanied by swelling or numbness, don't self-diagnose — consult a doctor. When starting an exercise routine, work within a level appropriate to your health and any existing conditions. The figures given are approximations meant to help illustrate the underlying mechanism.