Cramps aren't a mystery. Usually.
In endurance sport, most exercise-associated muscle cramps line up with a simple pattern: sweat loss that outran replacement, especially sodium. A generic tab or gel gives you the population average. If you're a heavy sweater in the heat, the average isn't enough — and that's the workout you cramped in.
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Because your sweat rate isn't constant. Heat, humidity, effort, and how salty your body specifically sweats each move the target. A tab formulated for the average endurance athlete might deliver 400–800 mg of sodium per hour. If your body loses 1,800–2,500 mg/hr on a hot day — and many people's do — you were running a chronic deficit before your calf ever knotted up.
The calculator does the math for your body, that specific workout. It's the number the sweat lab would give you, without the lab.
Two things to check before the next long effort
- Your sodium target for the conditions you'll face. Not the average. Your target for the sport, the duration, and the sweat type. The calculator will give it to you in 30 seconds.
- Whether your current drink actually hits it. Pop your target next to the label of what you're using; the gap is usually large. That gap is the mid-race problem.
Then a small on-your-body test: run one long effort on the target and see whether the cramp shows up. It's the cheapest lab you can do.
Not every cramp is sodium. Fatigue, altitude, medication changes, unusually intense efforts, and neuromuscular factors all matter — and researchers still debate the exact mechanism. But the sodium-math version is the one you can act on tonight, without buying anything.
If you've had recurrent unexplained cramping, or you have blood-pressure, heart, or kidney concerns, that's a conversation for a clinician — not a calculator.
What you'd do next
Run the calculator, note your target, then either tweak your current mix or take the recipe it gives you (Skratch plus a pinch of pink salt, KCl, and Mg) and try one long session on it. Cost: about $1.50–$2 a session versus $6.50 for premixed tabs. That's the whole build.
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