Lactic acid has been blamed for sore muscles for decades. The science says otherwise — and the real explanation is far more interesting.

In this episode, Jen, Chris, and Matt unpack the truth behind delayed onset muscle soreness (DOMS): what's actually happening inside your muscle fibres, why the pain peaks a day or two after exercise rather than straight away, and why the familiar "no pain, no gain" mantra is more complicated than it sounds.

Along the way they bust one of the most persistent myths in fitness, explain why running downhill causes more soreness than running uphill, and reveal which popular recovery methods are actually backed by evidence — and which aren't. (Stretching fans, brace yourselves.)

In this episode:

1. 00:58 — The lactic acid myth debunked
2. 02:21 — What's actually causing DOMS
3. 05:11 — Individual variation in soreness
4. 06:20 — The no pain, no gain myth
5. 07:43 — Should you exercise when sore?
6. 09:12 — What actually works for recovery

The Lactic Acid Myth Has Been Comprehensively Disproven

(00:58)

For generations, "feel the burn, that's the lactic acid" has been repeated in gyms, by coaches, and in fitness articles. There's one straightforward problem with it: lactic acid clears from your bloodstream within 30 to 60 minutes of stopping exercise. DOMS doesn't even begin until 12 to 24 hours later.

As Chris explains: "The lactic acid explanation has been comprehensively disproven. That timeline alone makes this theory impossible because muscle soreness typically doesn't begin until 12, even 24 hours post-exercise, sometimes longer."

The culprit that fitness culture has blamed for generations couldn't physically be responsible.

Your Body Has Builders In — And They Make a Lot of Noise

(02:21)

The real cause of DOMS is microscopic damage to muscle fibres and the surrounding connective tissue, followed by the inflammatory response your body launches to repair it. Specific hormones called prostaglandins and leukotrienes are released, causing swelling and activating pain receptors. The whole process takes time to develop — which is why soreness peaks one to three days after exercise, not immediately.

Jen adds that DOMS may also involve damage to the deep fascia — the connective tissue wrapping around muscles — which is densely populated with pain-sensitive nerve endings. This explains why even gentle pressure on sore muscles can feel disproportionately uncomfortable.

As Matt puts it: "The soreness is actually a repair job in progress. Like my body's got builders in. And they're making just an awful lot of noise."

Getting Less Sore Over Time Is a Good Sign

(06:20)

One of the most widespread myths in fitness is that soreness equals an effective workout. Research conclusively demonstrates that DOMS is neither necessary nor sufficient for muscle growth. Some muscle groups, like the shoulders, rarely experience significant soreness yet still grow perfectly when trained properly.

Chris explains the repeated bout effect: "Your body adapts to exercise through something called the repeated bout effect. That means you'll experience progressively less soreness for the same workout, even as your strength and muscle mass continues to increase."

Getting less sore over time isn't a sign you're not working hard enough. It's a sign your body is adapting and improving.

What Actually Works for Recovery (And What Doesn't)

(09:12)

An analysis of around 120 studies identified which recovery treatments have real evidence behind them:

1. Active recovery — light movement at 30 to 60% of maximum heart rate — outperforms complete rest for reducing soreness
2. Massage therapy increases blood flow and may stimulate endorphin release
3. Cold water immersion at around 10 to 15 degrees Celsius shows effectiveness, as does contrast therapy (alternating hot and cold)
4. Stretching reduces soreness by less than two millimetres on a 100-millimetre pain scale — effectively undetectable

Beyond specific treatments, the fundamentals matter most: seven to nine hours of sleep (growth hormone released during deep sleep stimulates muscle repair), 20 to 40 grams of protein per meal, and increasing training volume by no more than 10% per week.

About So That's Why

So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.