3 Was The Magic Number in1990. Now It’s 15

A quick rest might be all you need to smash your sets and hit those goals.

If you're exercising, but not hitting the reps you want, don't give up, just take 15 seconds rest then go again. 

This will help you work significantly more of your muscles than if you try and stop each time your muscles fail.

Those who did it for 6 weeks got more endurance and muscle size, especially in the thighs. 

Source: Journal of Strength and Conditioning Research