I havnt used clen in quite some time but today is around day 3 of my current go. Today I had to cut my chest training short because my left pec got so pumped that it started hurting incredibly badly like I had just trained it for an hour and it was only the 3rd set. Also should mention I am not on anything except trt at the moment. Suddenly I am a much bigger fan of clen.
Beta adrenergic stimulation = amplified response to hypoxia = much greater blood flow, at least that's my idea.