The author reports the results of experiments in which long bone fatigue is produced in 30 pairs of dog ulnas by applying opposing forces at both extremities thereby causing a strain. On a force-deformation curve the force axis indicates a zone of load, an intermediate zone of fatigue and a zone of overload; the deformation axis shows an elastic zone and a plastic zone. These zones are defined in the text.
The fatigue test in the load zone does not indicate either weakening of the bone or histological signs of fatigue. The fatigue test in the intermediate zone is accompanied by a progressive bone deformation, a diminution of resistance and microscopic lesions of fatigue. The fatigue test in the load zone with brief applications of overload produces the same histological lesions. These lesions, which are slip lines and microfissures striating the compressed cortex, are the cause of progressive bone weakness and of stress fractures.