According to a research published in the journal Proceedings of the National Academy of Sciences lack of exercise or physical activity is one of the main reasons more and more people today have fragile bones compared to early human species. For the study the team of researchers from Pennsylvania State University included 59 adult humans and 229 non human primates for analysing non-invasive tomography scans of their hip joint ends of the femurs (the long bone in the thigh). They discuss three possible reasons explaining the relationship between lack of exercise and deteriorating bone health. (Read: We bet you didn