Ballard, J., McKeown, B., Graham, H., & Zinkgraf, S. (1990). The effect of high level physical activity (8.5 METs or greater) and estrogen replacement therapy upon bone mass in postmenopausal females, aged 50-68 years. International journal of sports medicine, 11(3), S. 208-214. doi:10.1055/s-2007-1024793

Ballard, J.E., B.C McKeown, H.M Graham, and S.A Zinkgraf. "The Effect of High Level Physical Activity (8.5 METs or Greater) and Estrogen Replacement Therapy Upon Bone Mass in Postmenopausal Females, Aged 50-68 Years." International Journal of Sports Medicine 11, no. 3 (1990): S. 208-214.

Ballard, J.E., B.C McKeown, H.M Graham, and S.A Zinkgraf. "The Effect of High Level Physical Activity (8.5 METs or Greater) and Estrogen Replacement Therapy Upon Bone Mass in Postmenopausal Females, Aged 50-68 Years." International Journal of Sports Medicine 11.3 (1990): S. 208-214.