If pressure is applied across the weakest point of a small bone, it takes about 25 pounds of pressure to cause a fracture. The force it takes to break a human bone is contingent on which bone it is.
Muscle usage strengthens bones. Bones increase and decrease in increments a little at a time. Bones lose mass with age, even moreso for people with calcium deficiencies, people who smoke cigarettes or people who drink alcoholic beverages. Bones become brittle from a lack of flexible cartilage and sturdy bone. The bones of males tend to be denser when compared to the bones of females.