Over the years I have been asked this question many times. It can be phrased in many different ways, however I want to discuss this as simply as possible. Let’s assume this is for strengthening, and you are past a rehab setting, as those numbers might be easier.
1. You need to measure how much you can lift a particular weight in one try, or “set”, with good, slow form. Your are looking to find the weight or resistance that you can lift, push or pull, 8-12 times. If you can do more than 12 reps, then the weight/resistance was too light, and vice versa. Most people go too fast. It should take 3-5 seconds for each repetition.
2. Next, you rest 60 seconds, and perform the set again.
3. For muscle strengthening, you will need to perform 3 sets of 8-12 repetitions, for each exercise. You can perform more than 3 sets, and make more gains, however 3 sets will be the most efficient use of your time. Gains start to taper off after 3 sets.
4. The muscle will be repairing itself over the next 24-48 hours, and start to grow stronger in response to the exercise. After 2 days of rest, it is time to perform 3 sets of that exercise again.
5. As you start getting stronger over the next couple weeks, you will need to slowly increase the weight or resistance. You will notice that you can start to perform more than 12 reps per set, and it is time to increase. Pick the smallest increase and re-evaluate.
6. You must rest 60 seconds between sets! Otherwise you are increasing muscle endurance, not strength. This is a very important distinction. Most people hate resting and waiting, as it’s boring, and they just want to get it done.
In the first few weeks, you are getting stronger because of a stronger neurological connection between your brain, and the muscle fibers. Your body is learning to fire a higher muscle fiber percentage to help you move that resistance.
After roughly 2 months, you will start to actually gain muscle size. This will continue, provided you have enough of the right nutrients; water, protein, extra calories, etc.
Resistance training can be infinitely more complicated and nuanced. What I’ve listed above is the simplest explanation, and you can make tremendous gains with those simple steps. Here is an article about protein intake. If you want to gain muscle, you may need to ingest more protein.