We often get a lot of people asking us how much weight they should use for a specific exercise or workout. Of course, this is a good question as you need to know where to begin and when you should push yourself harder. However, it's impossible for us to give an answer that will work for everyone. Each person is unique in the amount of strength and endurance they have (particularly in reference to specific muscle groups), so even if you take two people that seem identical in height, weight, gender, and physical ability, there can still be vast differences in their strength from one muscle group to the next.
So, since this doesn't answer the question, there is a way to answer this question yourself — and it is by using the same technique that we used to choose proper pounds/kilograms with our personal training clients in the past. This does take some trial and error, but it is the best and most accurate way we have come across to choose how much weight you need to lift.
This is how it works — whether you are doing low repetition, high load, strength training or you are doing a high repetition, low load, toning routine. Start by selecting the number of repetitions you want to use. Remember, lower repetitions (6-12) leans more toward strength training/muscle building, and higher repetitions (14-20) leans more toward endurance and toning. Let's say you choose 12 repetitions, now you need to pick the number of sets, typically 2-4; let's say you choose 3 sets.
This is where the "hard" part comes in; you need to choose a weight that is difficult for you to finish the last few repetitions for the first two sets, and that you can only finish about 80-90% of the very last set before your muscles give out. In other words, pick a weight that is challenging enough that you may not be able to complete the last couple of reps of that very last set. Also keep in mind that you need to do all of the repetitions slowly and under control as to not "cheat" by using momentum, bouncing, or swinging to help. This is where most people go wrong, by choosing too many pounds/kilograms, they actually start recruiting other muscle groups (that are not supposed to be helping) to help lift the load by letting their form break down. For example, swinging their arms during a bicep curl moving from the shoulder and torso rather than keeping their shoulder and upper arm stationary and just moving from the elbow. If your form breaks down before the last 10-20% of your repetitions, then you are using too much weight and you still need to be fighting like mad to keep that form solid for those last few movements as well.
On the other hand, if you can finish all three set of 12 reps from the example above with clean, proper form and without difficulty, then the next time you do that exercise, you need to use a heavier dumbbell, kettlebell, medicine ball, etc. for at least the first set and possibly for all three. To try and make this as simple as possible, follow these rules:
1. Always start with a very light load if you have never done the exercise before. If it has been a few months since you last did a particular exercise start with 25% less than what you used last time.
2. Always focus on keeping your form perfect. Though you can lift more with bad form, you increase your chances for injury and don't effectively train the muscles you are supposed to be targeting.
3. If you want to make progress, you should always struggle with the last few repetitions of your last set. Remember the last 10-20% of your reps specifically on your last set should be very difficult to keep proper from and your muscles might even give out and not be able to continue.
4. If you can finish all of your sets and repetitions without struggling, increase your weight next time you do that exercise, for at least the first set, possibly all three.
5. Don't ever go off of what someone else is lifting. Listen to your own body as it will tell you all you need to know about what the right amount of weight/effort is.