The "All-Aerobic" Myth

One of the most prevalent misconceptions about fitness ESPECIALLY after 40 is that this word is synonymous with “aerobic fitness”. Whereas running, bicycling, swimming etc is an indispensable component of any fitness program at any age, it is in no way sufficient to ensure overall fitness, especially after 40.
Sorry for all of you 40-something marathoners who are reading these lines thinking of yourself as the ultimately fit homo sapiens. You've only got part of the equation right.

My observation is that many people in their 40’s, especially males, tend to overdo aerobic training (marathoners) while totally ignoring weight training, whereas the human body gradually start losing muscle mass from the early 30s . This phenomenon, called sarcopenia noticeably accelerates after the mid 40s with potentially disastrous consequences such as a deteriorating metabolism (muscles greatly contribute to the control of blood sugar level), an increased risk of accidents because of weakened muscles (especially falls), postural imbalances and eventually an inability to live independently later in life.

Therefore even a "fit" mature individual following an aerobic-only exercise routine will exhibit unmistakable signs of aging in his gaunt appearance caused by a drastic loss of muscle mass. Sarcopenia is actually one of the hallmarks of old age from a cosmetic viewpoint.

Sarcopenia is nonetheless not widely recognized yet as a major health issue probably because most mature people are also overweight. 
As a result, their huge muscle loss is completely concealed by an increase in body fat, giving them a “chubby” appearance that may be mistakenly perceived as youthful (because very few of our ancestors were overweight, especially in old age, we can be easily fooled by what appears to be "puppy" fat).

Unfortunately, the only known way of slowing down sarcopenia is through regular weight training. Running or riding a bicycle will do nothing to increase muscular mass and strength of the upper body. Even legs may be substantially affected by sarcopenia if they are only trained at moderate intensity during aerobic exercise.

The only way to maintain optimal health after 40 is to combine aerobic exercise with resistance (weight) training on a regular basis (i.e. a few times a week for each type of exercise). As in any other areas of your life, balance is key, so don’t put all your (fitness) eggs in one basket (aerobic or weight training).