A guy in the gym finishes a hard, focused set of barbell squats and re-racks the weight. His legs are wobbly, his heart is racing and he feels light headed as he takes a big swig from his water bottle. He looks down at his watch and presses the start button to begin counting down backwards from 2 minutes.

Somebody told him that 2 minutes is the ideal rest time between sets in the gym, and he wants to get it exact. Every time the watch beeps, he'll be back in the squat rack to perform another set. He stands up tall and paces around trying to catch his breath in preparation for his next battle.

When thetime is up he doesn't feel really ok. His legs still feel weak, but it doesn't matters how he feels, because his 2 minutes have passed and he have to go back to perform another set of exercises.

He unracks the weight and squats down. His legs still burn and he wishes that he could have had more time to prepare for this set. He puts forth a mediocre effort, re-racks the bar, and sets his watch for another 2 minutes. Bobby, just like a ton of other aspiring lifters in the gym, is making a deadly, critical mistake.

The only thing he is doing is to force his body to train at an effort level really far for his maximum potential, and his muscles are rarely being stimulated to grow.

A muscle respond to stress. When you lift an amount of weight doing a number of reps your body adapts to this level of stress. If you want your muscles grow, then you have to force your body to higer levels.

In other words, building muscle is all about progression in both weight and reps. It is about lifting as much weight as you possibly can for the greatest number of reps that you possibly can (within a given rep range of course) and then continually striving to improve.

If you don't rest enough between sets, you will not reach your maximum strenght potential sacrificing the amount of weight you can lift, and also sacrificing the amount of muscle you can build. So, to begin the next set qhen you are still tired is the big mistake you should not make.

So, return to your exercise set when you feel you are again at 100% of your strenght capacity. When will it be? When your body tell you so, listen to it an you will know when it is time to the next set.

A deadlift and a tricep pressdown aren't exactly in the same boat here. After a heavy set of deadlifts to failure I'll usually be resting for at least 5 minutes, often even more. A set of tricep pressdowns is obviously not as taxing and may only require a rest period of 2.5 minutes for me to feel fully recovered.