Resistance Training for Increased Bone Mineral Density
Let's start by understanding the basics of how our bones respond to stress. Muscles and tendons interact with bone directly. The forces created by muscle contractions pull on the tendinous insertions into the bone, which places compressive and bending pressure on the bone structure. As a result of this mechanical loading, osteoblasts (the building cells for your bones) secrete proteins that eventually mineralize and increase bone diameter and strength.
The threshold stimulus at which new bone formation starts is also called the minimal essential strain (MES). Consistently surpassing the MES signals osteoblasts to form bone in the areas experiencing the stress. The MES is estimated to be roughly 1/10 of the force needed to fracture bone. As a result of the new bone formation, a force that was previously enough to surpass the MES will now be below the new MES threshold.
Resistance Training and Bone Growth
As muscle strength and size increase in response to training, so do the forces being put on the bones from subsequent muscle actions. Any increase in muscle strength or size may then result in a comparable increase in bone mineral density(BMD). Inactivity or immobilization appears to have the opposite effect and leads to an increased loss of BMD.
The time frame for bone adaptation is quite long. It usually takes at least six months and is heavily influenced by the program being implemented. However, the process of osteogenesis (new bone formation) begins within the first few weeks of training.
Training Principles to Increase Bone Strength
So what do you need to do?
We need to first pick the right kind of exercises. The exercises need to specifically load the region of the skeleton you want to strengthen. This is particularly important when looking at the areas of the body most commonly affected by osteoporosis. Studies have shown that multi-joint movements that direct force through the spine and hips are superior for osteogenesis. Therefore, exercises such as the squat, bench press, power clean, deadlift, and shoulder press are recommended and more effective at improving bone strength.
When training to increase bone strength, progressive overload is especially important. This is the practice of consistently increasing the stress placed on the system (bones in this case) over time to continually drive positive adaptation. Bone responds best to higher forces that are repetitively applied. The greatest skeletal development is made when the magnitude of mechanical load is high (80%-85% or more of one rep max) and exercises are performed at least two times a week.
Conclusion
As with most mechanisms in the body, stress leads to adaptation and growth. Bones are no different. Without moving, mechanical loading, impact, the body does not receive the signals to continue to build and preserve a strong skeletal foundation. Increased muscle strength and size, and increased bone strength as a result improve your resilience to injury, including falls. Building a consistent plan that incorporates higher intensities that load the spine and hips is the best way at continuing to build strength in your bones for all ages.