Proper strength training not only rebuilds muscles and tendons but also rebuilds bones (1). Millions of people, especially those aged 50 and above, are affected by osteoporosis, which is a loss of bone mass (2).
According to research (3), in order to have a significant increase in bone mineral density, 75–80% of subjects required one repetition of maximum weight used during an exercise. Anything less would likely be insufficient to stimulate the body to make changes to bone mineral density. Endurance training alone does nothing to prevent this disease as it does not reach the required intensity to stimulate an adaptive change.
Strength training not only increases bone density, but building strength also helps prevent falls that result in the type of fractures that are common in those with osteoporosis. In addition, even when these falls occur, having more muscle will help by absorbing the impact and protecting the bones. This is why strength training is an optimal form of exercise, especially for senior citizens.
NET training principles are ideal for people suffering from osteoporosis due to the slow tempo (i.e., less force on the bones and more on the muscle) and an emphasis on HIT principles, which ensures sufficient muscle/bone tension load.
(1) A. Menkes, S. Mazel, A. Redmond, et al., "Strength Training Increases Regional Bone Mineral Density and Bone Remodeling in Middle-Aged and Older Men," Journal of Applied Physiology 74 (1993): 2478-84. link
(3) D. Kerr, et al., "Exercise Effects on Bone Mass in Postmenopausal Women Are Site-Specific and Load-Dependent," Journal of Bone and Mineral Research 11, no. 2 (February 1996): 218-25. link
(other) Kelley G.A., Kelley K.S. & Tran Z.V. "Resistance training and bone mineral density in women: a meta-analysis of controlled trials," American Journal of Physical Medicine and Rehabilitation 80, (2011): 65-77. link