An observation I made during my plyometric hiit routines is that my cock significantly retracts. These are extreme routines, while running can also be pushed to the extreme. At that time the focus is on the cardiovascular system and blood is surging to all other parts of the body where it is immediately required, result, at least for me, my cock retracts as my blood is being driven into my major muscle groups, hence the pump.
I think it might be counter productive to stress the cock when your blood flow is being currently vested into other areas of the body that are the primary focus of the exercise at hand. So now the cock is being stressed while competing for blood supply when it needs it most.
After my routine and a hot shower, things are hanging nicely, good time to maybe do a PE routine!
Just my thought, but others may have another experience.