Leg Cramps on Long Runs: What Really Causes Them (and What Helps)

Most cramps on long runs come from tired muscles working too hard for too long, not just from low salt. Studies of marathon, Ironman and ultra-trail athletes found crampers were no more dehydrated than everyone else. Pacing, strength work and fuelling matter most. Sodium becomes part of the plan once you’re out for two hours or more in heat.

Why do muscles cramp during long runs?

The leading explanation is muscle fatigue. After hours of hard work, the nerve signal telling a muscle to contract gets louder and the signal telling it to relax gets weaker. Eventually the relax side loses, and the muscle locks.

That’s why cramps love the second half. Nobody cramps at KM 3 of a park run. They cramp at KM 32, on the last climb of a trail race, or in the final minutes of a futsal match that went to extra time.

Is it really not about salt?

For most athletes, salt isn’t the main cause. Among 210 Ironman triathletes, the 43 who cramped had the same blood electrolytes and the same weight loss as those who didn’t. What predicted cramps was racing faster than usual and a history of cramping (Schwellnus et al., 2011).

Spanish researchers found the same pattern twice. In a road marathon, 20 of 88 finishers cramped, with sodium and hydration matching the non-crampers but clearly higher muscle damage (Martínez-Navarro et al., 2022). A 2026 ultra-trail study found it again (Martínez-Navarro et al., 2026).

So the salt tablet you swallowed in a panic at KM 30 was never going to undo the pace you chose at KM 5.

Then why does sodium still matter?

Because plain water alone can make things worse after heavy sweating. In two small lab studies, men ran in 35°C heat until they lost about 2% of body weight. Those who drank plain water afterwards became more prone to cramping. Those who drank an electrolyte solution became less prone (Lau et al., 2019; Lau et al., 2021).

Each study had only 10 men, and the cramps were triggered electrically rather than in a race. The lesson still holds up: sweat out salt for hours, drink only water, and you dilute what’s left. In Malaysian heat, a two-hour run is enough to get you there.

When should you start planning sodium?

Plan sodium for any effort over about two hours in heat, especially if you’re a heavy or salty sweater. White crust on your cap or shirt after a long run is a clue.

A rough guide by distance:

• 10K: water is usually enough.
• Half marathon (around 2 hours): start planning sodium, and measure your sweat rate once.
• Full marathon like KLSCM: your sodium plan should come from your own numbers.
• Trail and ultra: sodium is standard race-kit.

Our guide on how much sodium you lose running in Malaysian heat shows you how to estimate your own losses in one session.

How do you work out your own sodium gap?

Do the arithmetic once and you’ll never guess again. Here’s an example with made-up numbers. Plug in your own.

That’s where we built Elec Stamina to fit. Each caplet carries 220 mg of sodium, made for use during or after sustained physical activity. In the example above, the gap works out to between two and three caplets’ worth of sodium per hour. You know your numbers; you decide.

Runners tell us they like caplets for one boring, practical reason: there’s nothing to mix, and no second bottle to carry. Sixteen caplets in a pack slip into a race-vest pocket.

What else cuts down cramps and late-race fatigue?

Sodium is one piece. These are the others, in rough order of impact:

1. Pace to your training, not your hopes. Racing faster than you’ve trained for was the strongest cramp predictor in the Ironman study.
2. Do lower-limb strength work. In the ultra-trail study, 88% of non-crampers trained strength regularly, versus 56% of crampers. Squats, lunges and calf raises twice a week are cheap insurance.
3. Fuel with carbohydrate. The guideline is up to about 60 g per hour for efforts of 2 to 3 hours, and around 90 g per hour for ultra-distance, using mixed carb sources (Jeukendrup, 2014). Practise it in training; race day is not the time to find out your stomach disagrees.
4. Have a plan for the cramp itself. Stop, stretch gently, and sip something sour. In one study, a small sip of pickle juice shortened induced cramps by about 49 seconds compared with water, faster than any salt could be absorbed (Miller et al., 2010). It works through a reflex from the mouth, not through electrolytes.

One honest note: no caplet fixes a pacing mistake or a skipped strength block. Elec Stamina handles the sodium side of your plan. The legs are your job.

Your 2+ hour race kit

• Fuel you’ve tested in training (gels, chews or real food)
• Fluids with a known sodium content
• Elec Stamina for the sodium gap your numbers show
• Something sour for emergencies
• A pace plan you’ll actually stick to for the first 10 km

FAQ

Does magnesium stop muscle cramps?

There’s no good evidence for exercise cramps. A Cochrane review of 11 trials found magnesium made no meaningful difference to cramps in older adults, and found no trials at all on exercise cramps (Garrison et al., 2020).

Should I take salt for a 10K?

Usually not. Under an hour, most people don’t lose enough sodium to need it. Focus on pacing.

Why do I cramp in races but not in training?

Race pace. Most of us run harder on race day than on any training run, which matches the Ironman finding.

When should I take Elec Stamina?

During or after sessions over about two hours, especially in heat or if you sweat heavily. Follow the label directions and your own sodium numbers.

Running two hours or more this weekend? Work out your sodium gap, then pack Elec Stamina. Delivery is free in Malaysia on orders from RM80.

Written by the Stamina Sports team · Last updated 13 October 2026

References

• Schwellnus MP, Drew N, Collins M (2011). Br J Sports Med 45(8):650–6. doi:10.1136/bjsm.2010.078535
• Martínez-Navarro I et al. (2022). J Strength Cond Res 36(6):1629–35. doi:10.1519/JSC.0000000000003713
• Martínez-Navarro I et al. (2026). J Strength Cond Res 40(8):e894–e900. doi:10.1519/JSC.0000000000005440
• Lau WY, Kato H, Nosaka K (2019). BMJ Open Sport Exerc Med 5(1):e000478. doi:10.1136/bmjsem-2018-000478
• Lau WY, Kato H, Nosaka K (2021). J Int Soc Sports Nutr 18(1):22. doi:10.1186/s12970-021-00414-8
• Miller KC et al. (2010). Med Sci Sports Exerc 42(5):953–61. doi:10.1249/MSS.0b013e3181c0647e
• Jeukendrup A (2014). Sports Med 44 Suppl 1:S25–33. doi:10.1007/s40279-014-0148-z
• Garrison SR et al. (2020). Cochrane Database Syst Rev 9:CD009402. doi:10.1002/14651858.CD009402.pub3