At one time, I dabbled with timing my sets with a stopwatch. It was a Vince Gironda thing and I will say it was a HARD workout. The problem was, I was so focused on the stopwatch I lost all focus on the actual exercise. I was just pumping through the movements with very little concern about training the muscle. So yes, it was HARD work. But since my goal was hypertrophy, i cannot say it was smart work. So like most on here...I try to go at a brisk pace, but not so much that I feel like my cardio will tap me out before I hit muscular failure.