**2.2 Limitation of human studies**

To summarize the above, distance runners do not seem to acquire much benefit from their training when densitometric measurements are used to determine the bone health. However, as shown by serum bone marker assays, distance runners did not reveal an inadequate bone metabolism status. Actually, over the past decades, an increasing number of reports suggest that BMD does not accurately predict bone health or bone strength. Patients with fractures also show normal BMD values.36 The BMD and BMC measures apparently correlate strongly with body mass and body size.11, 37-39

Today, "bone quality" is used as a new term to represent bone health, which is composed of various parameters, including tissue architecture, turnover, microfracture and mineralization, of a healthy bone.40 Further, the organization of the bone matrix may also play an important role in bone strength.41 Unfortunately, many of the bone quality measurements are too invasive to be feasible in human subjects. Thus, animal studies have been frequently used for further clarifying related issues.
