Did you know that after three weeks of moderate to high intensity exercise that requires weight bearing you start improving your bone density?... and if you start early in life results will persist even in old age.
Bone can increase its mass and strength rapidly, in response to mechanical loading during the early years through the process of modeling. Peak mass and strength are usually attained around the second and third decade.
However, with the ageing process there is a net loss of bone mineral properties: density, mineral content, structure and strength, increasing risk of osteoporosis and fractures.
It is well-known the ability of exercise to stimulate bone ‘remodeling; exercise limits the normal bone demineralization happening during aging, naturally determined by the dynamic rate at which the load is applied and duration of the loading session. In general, activities that produce ground reaction forces higher that one or two times respect to the body weight are more effective: in simple words, walking and running are better than swimming and cycling.
Because of its importance in osteoporosis prevention, the Physical Activity Guidelines for Americans encourage children and adolescents to include bone-strengthening exercises in their 60min of daily PA on at least threedays per week. (http://www.ncbi.nlm.nih.gov/pubmed/25076937). However, physical activities achieved in condition of body discharge (e.g. swimming, cycling) or in static condition (e.g. stretching, balance) do not stimulate (or very weakly) osteogenesis.
The osteogenic function of aerobic training (e.g. walking, running) is effective only if the intensity of exercise is high and that of strength training is effective only if executed with heavy loads. Besides, it is advisable to vary the directions of mechanical constraints during physical activity to strengthen the resistance of the bone in all the plans.
In order to obtain significant effects in terms of bone remodeling, the optimal duration of training programs should last at least four to six months. The positive effects of regular exercise begin from two - three weekly sessions (http://www.ncbi.nlm.nih.gov/pubmed/25245313).
Some researcher aimed to understand if skeletal benefits of physical activity executed in young age persist with aging. An interesting study compared bone properties between the throwing and non throwing arm of a 94-year-old former Major League Baseball player who had ceased throwing 55 years earlier. Analyses indicated that the subject’s throwing arm (non dominant) had greater total cross-sectional area of bone and estimated strength than the other arm (dominant in all his daily activities), despite muscle indices favor the dominant one. (http://www.ncbi.nlm.nih.gov/pubmed/24879028)
Another study evaluated bone mineral density (BMD) changes in 46 male athletes with a mean age of 22 years during their active career and again when their mean age was of 39 years and they had long-term retired. Results showed that there were no changes in bone density scores from activity to retirement. (http://www.ncbi.nlm.nih.gov/pubmed/22806559)
Thus, we can emphasize the importance of physical activity at every age, because adult and old bone quality is affected by optimization of skeletal growth through childhood, adolescence and young age. In particular, strength exercise can be considered one of the most important positive stimulus for the bone health.