Strength training requires the use of resistance to tear down your muscles and rest to repair your muscles. This process of breaking down and rebuilding leads to improvements in your muscle's strength, tone and endurance. You can combine exercises targeting different body parts for training as long as you allow for at least a day of rest before you train those muscles again.
If you are new to strength training, the National Strength and Conditioning Association recommends that you train two or three days a week. Ideally you should select exercises on each day to strengthen the major muscle groups of your body. However, you can use a split routine in which you strengthen your chest and triceps on the first day, your back and biceps on the second day and your legs and shoulders on day three. Allow a day of rest between your workouts.
As your strength improves, the NSCA suggests training two to six days a week. This type of training routine reduces your days of rest to one a week. Use the same split routine as a beginner, and group your chest and triceps, back and biceps, and legs and shoulders together. However, you will exercise each group twice a week instead of once.
If your training goals are solely to improve the strength of your legs and shoulders, you can exercise these two muscle groups on the same day three times a week. Allow for a day of rest between for muscle recovery. The NSCA recommends a resistance amount heavy enough for you to complete at least three repetitions and light enough for you to complete 12. Most days, your resistance amount should tire your muscles after the eighth repetition. Aim to complete two to five sets of leg and shoulder exercises.
Another way to include leg and shoulder strengthening into your exercise program is with the use of circuit training. Circuit training alternates a cardiovascular exercise with a strength training exercise. For example, walk in place for three minutes and then perform 10 squats for your legs. Follow those stations with three minutes of stair climbing and then 10 shoulder presses. Your workout continues to alternate aerobic and strengthening exercises for your total workout time of approximately 30 to 45 minutes.