After a 24-hour race, the blood of elite runners shows massive spikes in six tiny molecules that come from muscle tissue—some up to over 100,000 times higher than normal.

These tiny molecules in the blood might help doctors tell how badly a runner’s muscles are damaged after an ultra-race—better than current tests.

After a 24-hour race, the levels of certain tiny muscle molecules in the blood match up better with how weak the runners still feel the next day than the usual muscle damage markers do.

When you do a single tough workout like running or cycling, your muscles produce more of two specific tiny molecules called miR-1 and miR-133a—but if you train like that regularly over weeks or months, those same molecules go down. It’s like your muscles hit a reset button after long-term training.