Lack of grip strength would certainly be an indication of corresponding lack of overall body strength. While it is possible to develop strength in isolated mus

Lack of grip strength would certainly be an indication of corresponding lack of overall body strength. While it is possible to develop strength in isolated muscle groups with specific exercises, it is not easy to develop full system strength without involving the hands in gripping form. If overall physical strength has some relationship to health and longevity (which I believe is generally accepted to be some indication of), then lack of grip strength would be a simple measure of fitness. I don't think it's a huge mystery why wealthier groups are physically weaker (and less healthy, and more overweight). There is less need or requirement to do manual labor compared to people in developing countries. Jobs tend to be more white collar than blue collar. Kids tend to have more indoor entertainment options (electronics, computers, toys). Science fiction has often portrayed advanced civilizations as soft, physically weak beings (gelatin creatures in glass tubes and such) that use their mental gifts more than their physical ones. However, humans will probably not reach a point of not needing their bodies in order to maintain health and long life. So we really should all be regularly active. Besides, there's ample evidence that good physical fitness also benefits the mind as well as the mood.