There might be something to the Navy SEALs’ 40% rule that when you’re completely exhausted, you’re really only 40% done. US researchers have found the two components of the neuromuscular system, nerves and muscles, tire at different rates. It’s not the muscle that tires first, it’s the neural pathways that control the muscle. “What we call muscle fatigue is a lot more likely to be neural fatigue,” the lead researcher said.