Bone strength: for worse--and for better--it's somewhat relative.

researchers in Australia recently compared subjects who had at least one parent with a total knee replacement (for severe primary knee osteoarthritis, a.k.a. degenerative arthritis of the knee) with age- and gender-matched controls without a family history of knee osteoarthritis. Patella, medial tibial (inner shin bone) and lateral tibial (outer shin bone) compartment cartilage volume and bone size were measured among the 188 matched pairs, who had a mean age of 45, and a total range of 26 to 60 years. Radiographic osteoarthritis was also assessed for joint space narrowing and osteophytosis (a characteristic of degenerative joint disease). Knee pain was assessed by questionnaire.

The subjects with a family history of degenerative knee arthritis were found to have more bone area in the medial tibial region near the knee joint. They were also heavier, with a higher body mass index (BMI nearly 28 compared to 26 kg/[m.sup.2] on average for the controls). These subjects had 25% more knee pain, over twice that of the controls, who also had stronger lower limb muscles. There were no significant differences in height or endurance fitness between groups, and there was a non-significant but nevertheless observable trend to increased radiographic osteoarthritis in the subjects with a family history.

While BMI, shin bone area and leg strength are all somewhat influenced by inherited genes, and appear to play a significant role in the development of knee pain and osteoarthritis, the battle's outcome is far from preordained. Strategies like regular resistance training to strengthen the muscles around the knee are well-known, effective ways to help fight the dreaded knee problems that can plague runners in particular. But now, there is some evidence that the childhood phase of the battle, while still crucial, may be easier won than previously thought. The Iowa Bone Development Study, a study of 467 children (mean age 5 years) published over the summer, reports that even everyday levels of physical activity can make a significant impact on the bone strength of very young children--whether they are physically fit or not.

Current research suggests that the skeleton is most sensitive to mechanical loading during childhood, and that load-bearing strain to the musculoskeletal system is the most important characteristic of physical activity for bone health. But, exercise recommendations, though carefully weighed for optimal benefits, are not always realistic in the day-to-day lives of children.

However, the study found that in young children, bone adapts to everyday levels of physical activity by structurally remodeling. Targeted bone loading activities or extraordinary amounts of activity are not the only means to achieve stronger bone geometry. When compared to peers, children who engage in higher levels of daily activity are likely to have significant improvements in bone geometry.

Furthermore, the evidence suggests that every bit of activity not only helps more than was once believed, but may help a great deal because of previous inactivity. The study authors write, "Because bone adjusts to relatively higher levels of strain by remodeling, it seems possible that less active children ... require lower levels of loading to stimulate structural changes than more active children." Simply reducing inactivity and increasing moderate levels of activity could "incur a structural advantage for some children."

The researchers observed a 9% difference in mean bone cross-sectional area and section modulus (indicators of axial and bending strength) between the least and most active children in the study. After adjusting for age, body mass and height, activity level explained nearly 7% of the variability. They conclude, "Given the relatively large amount of bone strength believed to be genetically determined ... the small contribution via vigorous activity that we observed may be valuable." It seems we now have another good reason beyond weight control for children to exercise.

(Annals Rheum. Dis., 2004, Vol. 63, No. 10, pp. 1255-1259; Med. Sci. Sports Exerc., 2004, Vol. 36, No. 7, pp. 1124-1131)