I thought I’d write up a quick piece about this question, as I hear it a lot, and although this is just a quick overview, it will hopefully make things a bit clearer.
There’s a lot of factors that will influence the weight to be used for an exercise, not least of which is the exercise itself. For the purposes of this article I’m going to ignore power training to avoid complicating things, and just focus on ‘normal’ strength and hypertrophy type training.
The three variables we’ll focus on are reps, tempos and rest periods.
Ok, so how do reps influence what weight you use?
If you pick a weight that you can lift for 12 reps, and use it to do sets of only 6 reps, then those sets are going to feel very easy, and if you pick a weight that you can only lift once and try to do a set of 8 reps, it’s not going to happen. The weight used needs to match the reps, otherwise you’ll either be working well below your capacity and the program will feel way too easy, or you’ll be trying to work way above your capacity and be unable to complete the program as prescribed.
And what about the tempo?
Tempo will affect the duration of a set and your time under tension. If you take a set of 10 reps and perform it with a tempo of 4211 (4 second eccentric, 2 second pause, 1 second concentric and 1 second pause), your time under tension will be around 80 seconds. Now lets do that same set of 10 reps but with a tempo of 2010 this time. That change of tempo has brought the time under tension down to 30 seconds. This will have a huge effect on how the set feels, even though you’re doing 10 reps both times, and so the weight used needs to take this into account.
And lastly but not leastly, how do rest periods affect the weights used?
Your rest periods are what you’re using to allow your body to recover for the next set. If your rest periods are very short, and you’re not allowing your body time to fully recover, then weight used will generally have to be lighter. When longer rest periods are used, your body is allowed to recover more completely between sets, and so higher weights would generally be able to be used.
As I said at the beginning, this is a pretty brief overview, but hopefully it helps you out a little bit when trying to navigate your way through your next training program.