Miriam E. Nelson, Ph.D Responds:

Miriam E. Nelson, PhD, of Tufts University, and the best-selling author of the Strong Women book series, responds: Strength training is good for your muscles, your metabolism, and your bones. When you strength train properly, you actually cause microscopic tears in the muscle tissue. This process is part of what causes the muscles to grow in size and strength. It is important to take a day off between exercising each muscle group. If your muscles continue to be very sore, wait one more day. 

The American College of Sports Medicine Guidelines suggest that you strength train two or three days a week with a day off between exercise sessions. This applies only to strength training, not aerobic or flexibility exercises. When you first start strength training, you will notice more soreness after exercise. If your muscles are not tired at the end of a set of exercises, you may need to increase the weight you are moving or lifting. Make increases of 10% each time and determine how your body responds.