The structure of your weight lifting workouts will directly influence the effects the training has on your muscles. A workout that is designed to elicit gains in strength is different than a regimen meant to build size. The number of sets and reps you should do of each exercise depends on your training goals. Be sure that you're training smart by following a regimen that is designed to stimulate that type of development that you want to see in your muscles.
Keep in mind that while the number of sets and reps that you complete are an important component to your training plan, the amount of weight that you use will also play an instrumental role. The proper amount of weight is the load that allows you to complete the minimum number of assigned reps using correct technique, but not any more than the maximum number of reps without reaching failure.
For Building Strength: Low Reps
If your goal is to build strength, then you want to train using heavier loads. Lifting heavier weights provide enough of a stimulus to increase the amount of force your muscles are capable of producing.
The optimal training regimen for building strength is to complete 3 to 5 sets of 1 to 6 reps of each exercise. The weight you use should be incredibly challenging. You should be able to do at least 1 rep, but no more than 6. Because you'll be lifting heavy weights, it's a good idea to train with a partner so that he or she can serve as a spotter. Rest 3 to 5 minutes after every set so that your muscles are completely recovered before the next one.
When training for strength building, be sure to give your muscles at least a day off in between workouts. For example, you could target all of the major muscle groups on Mondays, Wednesdays and Fridays.
For Developing Size: Mid-Range Reps
To increase muscle size, you've got to routinely complete high volume workouts that are designed to overload your muscle tissue. High volume workouts will cause trauma to your muscle fibers, which in turn stimulates a recovery process that involves building up the volume and density of muscle tissue.
If you're lifting weights hoping to build muscle size, complete 3 to 5 sets of 8 to 12 reps of each exercise and rest just 30 to 90 seconds in between each set. Be sure to select a weight that allows you to complete at least 8, but no more than 12, repetitions. If the weight you use is too low, even if you do 8 to 12 reps, you won't provide put your muscles under enough stress to ultimately overload the tissue. If the weight you use is too high, so much so that you're unable to do at least 8 reps, the volume of your session isn't going to be adequate for causing overload and stimulating the muscle-building process.
To train even more effectively for muscle building, fill your workout will primarily large muscle, multi-joint exercises like the squat, lunge, bench press, row, deadlift and overhead press. Research has shown that multi-joint exercises elicit greater muscle size gains than single-joint exercises like biceps curl and leg extension.
When training for size, it's a good idea to give your muscles two days off in between workouts to ensure that your tissue is given enough time to heal and recover. For example, a schedule that involves working out on Mondays and Thursdays works well. If you want to train more than twice a week, consider splitting your muscle groups into separate workouts. You could do upper body exercises on Mondays and Thursdays, and focus on lower body on Tuesdays and Fridays, and you'd still give your muscles the two days off they need.
For Improving Endurance: High Reps
While most people are interested in building either strength or size through weight training, some individuals, including marathon runners and long-distance swimmers, may be most interested in improving their muscle endurance. Muscle endurance is the ability of a muscle to either hold a contraction for time or perform continuous contractions at a submaximal intensity. By building endurance, your muscles will be able to keep working over a long period of time.
The best way to build muscle endurance is to complete exercises at a volume of 2 to 3 sets of 15 to 18 repetitions. Use relatively light weights that cause a "burning" sensation at the end of each set, but never cause failure. Rest periods in between sets should be kept to no more than 30 seconds.
When training for endurance, you can lift weights up to three days per week. Give your muscles a day off in between workouts.