Muscle Burn and Performance
I was doing pushups tonight, and did 15 on my first set. Rested about 90 seconds, did another set and got 14 on my second, rested about another 90 seconds, then only got 10 on my last.
Why is it such a drastic drop? I notice this on a lot of my pressing exercises.
My Answer: This is actually normal. What happens is that with higher repetitions, you accumulate a lot of lactic acid in your muscles. This is the burning sensation that you feel. The lactic acid interferes with your nerves, and as a result, you can't complete as many repetitions. That's why there's a drastic drop in reps. Once you hit that drastic drop in reps, it's time to stop performing that exercise. More sets of that exercise just hamper muscle growth.
In Training for the Busy Bodybuilder I go over the concept of the "critical drop-off point," which you can use to determine the optimal number of sets for an exercise. The critical drop-off point is when a muscle reaches a 5-7% decrease in performance either in weight or reps. Once this critical drop-off point is reached, then the exercise must be terminated. This 7% rule applies for loads of 85% of your one rep maximum (1RM) or more. For loads less than 85% of your 1RM, there should be no more than a 20 percent drop-off.
Most of the time, however, you will not have a calculator with you to determine this critical drop-off point. A simpler method to help you determine the critical drop-off point is to terminate an exercise if the decrease in reps is greater than 2 reps (if your target rep range is 3-12) or greater than 3 reps (if your target rep is greater than 12 reps). So if you perform 16 pushups on the first set, then you would continue to performing pushup sets until your reps fall below 12.