Wayne Westcott Ph.D. C.S.C.S.Men and women who do not perform regular resistance exercise experience progressive muscle atrophy.Research indicates that inactive adults lose about one-half pound of muscle per year during their 30s and 40s.(5,8)   The rate of muscle loss may double to one pound per year in people over fifty years of age. (13)  Unfortunately, the higher rate of fat gain masks the muscle loss.  Adults typically lose about five pounds of muscle and add about 15 pounds of fat each decade during midlife years.Importance of resistance trainingBased on body-weight  the average adult changes approximately ten pounds per decade.  But, based on body composition, the average adult changes approximately twenty pounds per decade (five pounds less muscle and fifteen pounds more fat).A woman in her 50s may have fifteen pounds less muscle and 45 pounds more fat than she had in her 20s.  This represents an unhealthy 60 pound change in her body composition, which increases her risk for a variety of devastating degenerative problems, such as cardiovascular disease, many types of cancer type 2 diabetes and low-back pain.While the focus is typically on fat loss (one out of two American adults is presently on a weight-reduction diet plan), more emphasis should be placed on muscle gain. This is due to the fact that the five pound-per- decade loss of muscle is largely responsible for up to a five-percent-per- decade reduction in resting metabolic rate. (5, 12)  a slower resting metabolism means that some calories previously used by high-energy muscle tissue are no longer needed, and are therefore stored as fat.Because one pound of muscle requires between 35 to 50 calories a day for tissue maintenance, a 10-pound muscle loss may reduce resting metabolism by 350 to 500 calories daily.

See more here – 

Strength Training for Seniors: Part 1 – Restart Retirement