How Does Strength Training Increase Bone Density?

Strength training is one of the most effective ways to increase bone density. This pays off during your athletic career by lowering risk of injury and over the long-term by reducing risk of osteoporosis and other bone disorders later in life. How does it do it?

Mechanical Loading Stimulates Bone Growth

Bone is formed and strengthened by mechanical loading. Repetitive loading in the form of resistance training leads to the release of growth factors such as IL-1, IL-6, FGF, FGB, NGF, and IGF that can stimulate the growth of bone and connective tissue.

For example, a growth factor known as TGF-B stimulates collagen deposition, while NGF or nerve growth factor is involved in nerve development. A minimum of 6 to 8 months of repeated loading is necessary to see measurable results such as on a bone mineral density (BMD) scan. Impact forces such as jumping or stair climbing and non-impact muscle forces such as weight lifting are both important for increasing bone mineralization.

Although BMD scans are a great way to evaluate the effectiveness of exercise for bone health, it should be noted that exercise causes larger improvements in bone strength than can be measured by bone mineral density, as new bone formation is often at the bone surface. A study by Robling showed that 5.4 percent increase in BMD is equal to a 64 percent increase in ultimate force and 94 percent increase in energy to failure.

Strength & Power Athletes Have Higher BMD

Numerous studies show that athletes in strength and power sports have higher bone mineral density than age-matched controls or those in endurance sports (swimming, cycling, distance running). One study showed that bone mineral density increases significantly over the first two years of collegiate training in male and female basketball, hockey, and soccer players, in part due to resistance training performed. Another study observed increases around 3 percent in bone mineral density in athletes, with the greatest increases in female basketball and male hockey players, though all groups were significant.

Endurance Athletes Get Bone Benefits From Strength Training

Training strategies for increasing skeletal strength are necessary for all athletes. Surveys show as much as 21 percent of all athletes suffer from stress fractures. Endurance athletes are at higher risk of stress fractures than those participating in strength/power sports, and females are particularly vulnerable due to the tendency to fall into the Female Athlete Triad (low bone density, low body fat, amenorrhea), which leads to higher rates of fracture and bone disorders than in males. Resistance exercise is effective for increasing bone density in at-risk athletes. For instance, in male collegiate cyclists and distance runners, resistance training increased bone density.

What Type of Protocol Is Best For Increasing BMD?

With regards to the training parameters that lead to the greatest improvement in bone density, high intensities that load the skeleton are most effective. One study found that the back squat is the best exercise for increasing BMD density in the hip and that heavier loads have the greatest osteogenic effect, loading the spine and hip with the highest ground reaction forces.

Researchers tested trained athletes for their maximal squat to parallel (90° of knee flexion). Then they tested the bone building impact as measured by ground reaction forces (GRF) during the squat using loads of 80 percent, 100 percent, and 120 percent the 1RM. The two lighter loads had athletes squat to parallel, whereas the supramaximal load of 120 percent was a partial squat to 65°. Results showed that the back squats performed at 120 percent produced the highest GRF. The researchers recommend using supramaximal loads for increasing bone strength.

The impact of longer term maximal load training on bone mineral density is evident in two case studies of competitive female senior powerlifters who had been lifting for over 30 years with the majority of training loads above body weight (Walters). One woman was 48 years old and she typically performed 70% of her training lifts (bench press, squat, and deadlift) at near the maximum weight possible. A second woman was 54 years old, trained the same lifts, and did approximately 75% near her 1RM. Bone scans showed that both women had BMD that was significantly higher than the norms for gender and age. They were also far above the average for women ages 20 to 29 who are considered to be at the peak of BMD.

The take away is that to maximize bone strength, do the following things:

Perform weight bearing exercise on a daily basis, such as running, stair climbing or jumping.

Consider wearing a weighted vest when doing aerobic exercise because this will load the spine and have a bone building effect.

Perform a strength training program that prioritizes heavy lifting using compound exercises that load the spine and periodically incorporates supramaximal load training into your protocols.