Tl;dr: Studies show viagra/sildenafil can actually increase muscle protein synthesis, reduce muscle fatigue, reduce lactate concentration, and enhance exercise performance:
Apparently all the young kids at the gym these days are using daily cialis and viagra to get a better pump and achieve better gains as well.
I myself have noticed that when I take cialis, my HR is lower when I run at the same speed as I normally do. Has anyone here ever experimented with taking viagra or cialis and training? If so, did they notice any improvement in performance as well?