Your doctor was right, just not complete. S/he may have intended to tell you to do weight bearing, moderate impact activities - it really doesn't matter if aerobic or anaerobic. It is the impact that stresses bones and so builds up architectural density. BUT you will need to focus on all limbs - which is why your doc may have mentioned anaerobic. Sorry CQ but, whilst brisk walks are great for bone density of the legs, hips and spine, cycling and aqua are of little value as they are not weight bearing and are extremely low impact and would not stress the bones and so would not help with oseopenia or osteoporosis, prevention or management! Upper body, arm bones, will be best stressed with weights, unless you can walk on your hands for a while!!. So a half hour weights routine would be good for both prevention and management! You could use resistance bands if you don't like machines or free weights. But almost any pilates/yoga class would be great too! I know you don't know me from Adam but this is my specialist subject (I work with the NHS with a wide range of conditions when I am not teaching). If you want any more info feel free to pm me. I use a range of websites with my clients, I'll share some, so you don't have to trust the word of a stranger. I am assuming you are trying to prevent osteoporosis, hence the osteopenia stuff. http://www.osteopenia3.com/osteopenia-exercise.html American lady and very simple exercise to do at home (they also show what your GP meant). http://www.livestrong.com/article/356947-weight-bearing-exercises-for-osteopenia/ Has a nicer look if you want to read around more. If you have osteoporosis there isn't much difference, but some extra precautions http://www.mayoclinic.com/health/osteoporosis/HQ00643 I hope that is helpful, your GP was right and you have a wide range of possibilities. Try out a range and see which you like best! (sorry, enthusiastic verbiage will now cease).