Abstract
1. Thirty-seven specimens of the proximal third of the human femur were subjected to cyclically varying loads applied in a physiological direction to the femoral head, having maximum values of from four to thirteen times body weight.
2. Ten of these specimens sustained subcapital fractures of the femoral neck after numbers of cycles of loading varying from 123 to 8,193.
3. The maximum value of cyclic load needed to give fatigue fracture after 10,000 or fewer cycles was found to vary from about twelve times the body weight at ages twenty to fifty to about five times the body weight at age seventy or more.
4. In youth and in middle age the load levels mentioned above are greater than those encountered in normal living, but are comparable to those which may be applied to the femoral head during activities known to produce "fatigue" fractures clinically in young adults.
5. In the elderly the load levels mentioned above are within the range that can be applied in normal living. It is inferred that some femoral neck fractures in the elderly may be fatigue fractures caused by the cyclic loading of normal walking.
