Get more from your workout by understanding when and how to use more/less reps and weight
When we work out, the two factors we should consider are (1) the number of reps when working out a muscle group (2) and the amount of weight/resistance used during that set. The goal of the workout will dictate how we should structure the number of reps and how much weight is used in each set.
High Reps with Low Weight:
Common knowledge in fitness circles is if we’re looking to build muscle endurance and lose body fat, then we should use low weights (30% - 50% of the one rep max) with a high number of reps (10-12+ repetitions) in order to achieve that goal.
For example, if you want to lose body fat, you might set up a workout that looks similar to the one below:
1 - 3 sets of a weight that would have you fatigued between 10 - 12 reps So if my one rep max for a bicep curl is 40lbs (meaning, I could only lift this weight one time before the muscle is fatigued), I would lift between 12lbs-20lbs and do 3 sets of 10-12 reps.
If you want to build muscle endurance then the workout would look similar to this one here:
1 - 3 sets of a weight that would have you fatigued between 12 - 16 reps
So if my one rep max for a leg press is 150lbs I would lift anywhere between 45lbs-75lbs and do 3 sets of 12 - 16 reps.
The rest period for these types of workouts should be anywhere between 30 - 90 seconds between sets.
High Weights with Low Reps:
For a long time, bodybuilders and weight lifting enthusiasts have believed that if you’re looking to build muscle mass (size) and strength, then you have to lift heavy weight (75% - 90% of your one rep max) with a low number of reps (4 - 8 repetitions) in order hit the target.
For example, if you were looking to build body mass then a workout you perform might look similar to the one below:
2-3+ sets of weight that have you fatigued between 4 to 8 reps
So if my one rep max for the bench press is 160lbs, then I’ll do 4 to 8 reps of 120lbs - 144lbs and do 2-3+ sets of 4 to 8 reps.
The rest period for these types of workouts should be anywhere between 2 - 6 minutes between sets
New Studies
Recent studies show that although the information listed above (on muscle mass and strength) is true, it may not be the only way to reach our goal.
Using any weight you’re comfortable lifting and doing sets where you lift to “failure” (lift until you can’t lift anymore) has shown to have the same benefits as someone who lifts a weight at least 75% of their one rep max for a low number reps on their muscle size and strength.
Lifting to “failure” also has its risk because as you become fatigued your form often breaks down which can raise the chances of injury. One alternative to this is lifting until you’re about two reps shy of “failing” to avoid possible injury (you won’t get the max benefits as someone lifting to “failure” but it's a much safer option).