The most typical advice on how to exercise in the gym is to carry out the same exercise in three to four series, so that each series consists of 8 to 12 repetitions or “reps”. But is there any real science behind that or is it just some arbitrary make-belief?
Consider this. In order to stimulate a muscle to grow, you need to exercise it using weight that is pretty heavy given its current strength. It should not be impossible for you to do the lift, but you do need to feel every rep really hard. If the resistance is chosen right, you will not be able to do more than approximately ten repetitions in a series.
Now an interesting fact: what turns out to be really important for muscle growth is actually not the absolute number of repetitions, but instead the time interval over which your muscles will be forced to scream for help but still be able to not give up totally. It really depends on your mental strength how hard and how long in each series you will be willing to push your body. The stronger the will, the stronger the body. But if you have managed to do more than 20 slow reps without compromising the form, that doesn’t mean that you are one insanely tough #^&%#, it just means that you’ve used too little weight. Because biology.
So, let us draw two important conclusions out of this. First, you don’t need to be extremely precise when selecting weight for a particular exercise. If you have started a series and you feel that you’ve picked a bit too light a weight, that’s ok; simply do a few more faster than usual reps until your muscles start slowing down and eventually stop responding to your commands. On the other hand, if you’ve picked too much weight, carry on with those really slow reps until you can not keep the proper form anymore. Hopefully, you will know your body enough not to miss weight by much… Now comes the interesting part – while it might not be obvious to you, bystanders would attest that in both previous cases a series lasted somewhere around a minute.
Our second conclusion concerns the continuity of tension throughout a series. A minute under continuous tension means a minute under uninterrupted tension. So if you really want to stimulate your muscles to become stronger and grow, it is important not to let them rest even for a second between two consecutive reps, until the entire series is complete. It means that you should not let e.g. your arms dangle freely at the bottom while doing biceps curls. Keep some minimum tension instead by not alowing your elbows to become fully extended. This universal advice applies to all exercises, no exceptions.