Be Active Your Way - A guide for Adults prepared by the department of health and Human Services.
Part 1. Getting Started
Part 2. Making Physical Activity a Part of Your Life
Part 3. Keeping It Up, Stepping It Up
Part 4. Being Active for Life
So how does one decide what activity or exercise to do? Two considerations are: 1) is it something you enjoy - a hobby or time with friends and family and/or 2) the effect it has on health and longevity.
The "enjoy" piece is pretty easy to figure out. The effect on health and longevity can be a little more complex.
Physiologically we should consider the effect of exercise/activity on:
1) minimizing muscle loss (sarcopenia)
2) minimizing bone loss (osteopenia/osteoperosis)
3) maintains the ability of the neurological system to effect good balance and stability (the neurological system improves or degrades based on imposed demands and creates routines in the brain that it remembers or forgets based on exposure)
4) effect the ability of our bodies to intake and utilize oxygen (our cells need oxygen to make their energy) - and so consequently vascular, respiratory and metabolic systems are important.
5) effect the production of endorphins favorable to mood and mental well being.
6) effect metabolic health - our cells up regulate and down regulate their capacity to utilize energy sources (fatty acids, glucose, amino acids) and produce energy. The effect of physical exercise affects muscle cells such that it pulls glucose from the blood stream without reliance on insulin - very important to those with low insulin sensitivity.
So there you go... find activity/exercise that meets the above physiological requirements ... and you enjoy!