TMBT 50K: What I Learned About Fueling and Cramps

My TMBT 50K took 8:29:40, and the cramps that started around km 38 were the story of the last two and a half hours. The main thing I learned is that those cramps were probably not a salt problem I could have swallowed my way out of, and that my fuelling plan needed to be built around hours, not checkpoints.

I finished 14th overall and 2nd in Men Veteran. The watch file tells a more useful story than the result, so this is mostly about that.

What do the splits show?

The splits show a steady first 38 km and a last 11 km that took more than an hour longer than any other section. My watch split the race into five laps:

Look at the heart rate column before the times. For the first four sections it sat between 148 and 153, a narrow, steady band. In the last section it dropped to 141, on the hardest climbing of the day. That isn’t a fitness gain arriving at km 45. It’s what heart rate does when the legs won’t let you work hard enough to raise it.

The last 4 km to the finish, which climb about 670 m, took me 1 hr 28 min. Strava counted only about 20 minutes of that as moving time. Some of the gap is the watch failing to tell slow hiking from standing still on a slope that steep. Some of it was me standing still, waiting for a cramp to let go.

Why I don’t think more salt would have fixed the cramps

The cramps started right after I nearly went down on the descent off the high point, and that timing matters more to me than anything I ate. A sudden, awkward contraction in a muscle that has already done 38 km and 2,000 m of climbing is a very different thing from a slow electrolyte drain.

The research leans the same way. Martin Schwellnus’s review of exercise-associated muscle cramps found that support for the dehydration and electrolyte-depletion explanations came mainly from anecdotes, small case series and one small case-control study, while four prospective cohort studies didn’t support them. The evidence that has built up points instead to altered neuromuscular control in fatigued muscle (Schwellnus, 2009).

The best-known of those cohort studies followed 210 Ironman triathletes. The 43 who cramped had the same changes in blood electrolytes and body weight as those who didn’t. What predicted cramping was racing faster than expected and a history of cramping in recent races (Schwellnus et al., 2011).

I’m one runner, and I can’t prove what caused my cramps any more than anyone else can prove theirs. But “near-fall on tired legs, then cramps in the same legs” fits the fatigue picture far better than a story about running out of salt. If you want the longer version of the evidence, we covered it in Leg Cramps on Long Runs: What Really Causes Them.

That doesn’t mean sodium and fluid don’t matter in a 50K. In the Sabah heat they do, for reasons that have nothing to do with cramps. It means I’d stop treating every cramp as a sign I forgot my salt.

Why I’d plan fuel by the hour, not the checkpoint

A fuelling plan built around aid stations breaks the moment your pace does. I’m not publishing a gel-by-gel log here, because the useful part isn’t the exact number. It’s the shape of the problem.

Most of us plan an ultra the same way: eat something between each checkpoint, top up the bottles, repeat. That works while the gaps between checkpoints take roughly the time you expected. My last section took 2 hr 40 min. Any plan that said “one or two things between stations” would have left me short there, at exactly the point where I was moving slowest and spending the most time out in the afternoon sun.

Your carbohydrate need doesn’t fall just because you’re walking. Heart rate was lower, but I still had 11 km and 866 m to cover. If anything, a lower intensity is the easier time to eat, since gut problems tend to get worse as effort and heat go up (Costa et al., 2017).

Next time I’ll carry fuel for time, not distance: enough for at least an hour more than the slowest split I can imagine for each section. The amounts are in our carbs per hour guide, and the race fueling calculator will turn hours into totals so you know what to pack.

What about the heat?

The heat arrived just as I slowed down. My watch’s temperature sensor, which sits against a warm wrist and reads high, showed about 25°C in the first hours and mid-30s through the afternoon. Ranau’s altitude buys you a cool start. It doesn’t stop the sun once you’re out on exposed ridges after midday.

That shift lined up with the part of the race where I was slowest, so the last section was long, hot and steep all at once. For anyone running TMBT or a similar Sabah course, I’d plan fluid for the afternoon you’ll actually be racing in, not the morning you started in. Our guide to hot, humid trail ultras in Malaysia has the practical detail.

What I’d change next time

• Descending on tired legs. The near-fall is what started everything. More downhill running late in long training runs, when the legs are already gone, is the closest thing to a fix I can control.
• Fuel for the slowest split. Pack each section as if it’ll take an hour longer than planned.
• Write down what I eat. A quick note at each checkpoint costs ten seconds and makes the next race report far more useful.
• Stop chasing cramps with salt. When a cramp hits, walk, ease off, and let it pass. Keep sodium and fluid on their own schedule, set by sweat rate and heat, not by the last muscle that seized.

Second in category with a cramping last 11 km means there’s time sitting in that final section. I’d rather go and find it with better legs on the descents than with a pocket full of salt. For the rest of the ultra fuelling picture, from carbs to drop bags, start with our ultra marathon nutrition guide.

References

1. Schwellnus MP (2009). Cause of exercise associated muscle cramps (EAMC): altered neuromuscular control, dehydration or electrolyte depletion? British Journal of Sports Medicine, 43(6), 401-408. doi:10.1136/bjsm.2008.050401
2. Schwellnus MP, Drew N, Collins M (2011). Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes. British Journal of Sports Medicine, 45(8), 650-656. doi:10.1136/bjsm.2010.078535
3. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR (2017). Systematic review: exercise-induced gastrointestinal syndrome, implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246-265. doi:10.1111/apt.14157

{“@context”: “https://schema.org”, “@type”: “Article”, “headline”: “TMBT 50K: What I Learned About Fueling and Cramps”, “description”: “My TMBT 50K in 8:29:40: splits, heart rate and the cramps from km 38, and what they taught me about pacing, fuelling by the hour and why cramps are rarely just salt.”, “datePublished”: “2026-10-15T03:00:00+08:00”, “dateModified”: “2026-10-15T03:00:00+08:00”, “author”: {“@type”: “Organization”, “name”: “Stamina Sports Malaysia”, “url”: “https://staminasports.my/”}, “publisher”: {“@type”: “Organization”, “name”: “Stamina Sports Malaysia”, “url”: “https://staminasports.my/”}, “mainEntityOfPage”: {“@type”: “WebPage”, “@id”: “https://staminasports.my/tmbt-50k-fueling-cramps/”}}