Coaches have been programming training for decades based on heart rate zones, GPS data, and how hard athletes say they feel. There's just one problem. None of those metrics actually tell you what's happening inside the muscle itself.

A new case report by Martin Buchheit and Paul Laursen just changed that.

Using a portable electrical stimulation device called Myocene, researchers measured something called low-frequency fatigue — a direct readout of muscle contractile impairment — immediately after nine different training sessions. Zone 2 runs. Sprint intervals. Small-sided games. Gym sessions. All-out cycling efforts. Every single one produced a completely different biological signature.

The results were striking. Easy Zone 2 runs barely registered. All-out sprint intervals crushed contractility to below 80% of baseline. But here ...