It seems likely to consider that the bone density (BD) of athletes is higher than that of control subjects. But recently, many authors reported lower BD of amenorrheic female athletes, and suggested that strenuous exercise could decrease BD through, probably, endocrinological disorder.
So, we compared BD of second metatarsal in female athletes with irregular menstrual cycle (IM group) or with stress fracture (SF group) to that of normal menstrual athletes (NM group) or those without bony injuries (NF group), by MD/MS method.
MD/MS method, which is the screening method of BD by scanning mid-diaphysis of second metacarpal (or metatarsal) for 3cm by 20times in X-ray films, was developped from microdensitometry whose scanning was only once at the same part of the bone.
BD of IM group was lower than that of NM group, but without significant difference. BD of SF group was significantly lower than that of NF group.
The index of mechanical strength of the bone, “I”, that is area moment of inertia, did not increase in accordance with decrease of BD. We supeculate this as one of the factor of stress fracture in a sense of decreased mechanical strength.

The magnitude of mechanical stress at the shoulder and elbow appears to be directly correlated with the degree of maximum shoulder external rotation (MER) during throwing. Therefore, it is very important to prevent excessive MER to minimize the risk of throwing injuries. The purpose of this study was to investigate the relationships between MER during throwing and the kinematic parameters of throwing mechanics, shoulder muscle strength, and shoulder range of motion in high school baseball players. The subjects were 40 male high school baseball players with no elbow or shoulder joint problems. Three-dimensional analysis was performed to calculate the MER angle. Then, the shoulder and elbow angles at initial foot contact (IFC) were computed. ROM and muscle strength of shoulder joint were also measured in each subject. Multiple linear regression analysis was used to relate the MER angle to these factors. Significant correlations were observed between the MER angle and the external rotation (ER) angle (r=−0.51, p<0.001) at IFC, and the ER range of motion (r=0.84, p<0.01). The MER angle significantly correlated with shoulder internal rotation (IR) at IFC. This finding suggests that stress on the shoulder and elbow could be increased by the degree of shoulder IR angle at the moment of IFC. Further, excessive ER range of motion may also be a risk factor.