Lactate production increases with workload, and the associated hydrogen ions, not lactate itself, cause acidosis and muscle fatigue by paralyzing actin-myosin filaments.

“as lactate goes up, it's married to a hydrogen ion. And that's what creates the acid part of lactic acid. And it's that hydrogen ion that's causing all the trouble. It's not the lactate. We can tolerate endless amounts of lactate. We just can't tolerate the hydrogen that comes with it. And it's that hydrogen that actually paralyzes the actin meosin filaments, prevents them from disengaging and that's what leads to that seizing up that you feel when you exceed your lactate threshold.”