Exercise has an important role both in the prevention and treatment of osteoporosis.  Understanding the mechanisms by which exercise helps bone density has important implications as to the most beneficial exercise pattern. 

The relationship between exercise and bone health relates to mechanisms:

• Increasing weight bearing loading of bone
• Increasing muscle release of a signaling myokine, irisin.

Increased loading causes bone to undergo more remodeling increasing bone volume and density.  Low BMI or body mass index is a known risk factor for osteoporosis as less daily loading of bone occurs.  Physical inactivity also chronically reduces bone loading and increases the risk for bone density loss.  Daily exercise of 45 minutes increases bone loading which increases bone density.

Newer research has helped to understand yet an additional benefit to loading in helping bone density.  Exercise triggers the release of irisin from muscle.  Irisin has stimulatory effect both on muscle and on bone.  While small amounts of irisin are found in several body tissues, the majority is produced muscle during exercise.  Exercise increases the circulating levels of irisin 70%. 

Irisin stimulates regeneration/growth of both muscle and bone.

Bone undergoes constant remodeling to repair microcracks that develop with time.  The arrows in the diagram show areas of microcracks that require remodeling. Remodeling involves osteoclasts removing bone in the areas with microcracks and osteoblasts filling new bone back in.  About 10% of bone is replaced with remodeling each year with the entire skeleton being remodeled each decade.  With age and estrogen decline, osteoclasts tend to get more active, and osteoblasts become less active.  The result is bone density loss with each remodeling cycle.

Drug treatments for osteoporosis work only on one side of the remodeling process.  Bisphosphonates inhibit osteoclasts slowing bone removal.  While this may help bone density initially, it eventually causes increased bone fragility as bone remodeling is slowed, the number of unrepaired microcracks increase. 

Irisin works both to inhibit the increased osteoclast removal of bone but also increases bone production activity of osteoblasts.  In essence, it restores healthy bone remodeling. At the same time irisin also stimulates muscle slowing the muscle loss seen progressively with age.  More pronounced muscle loss called sarcopenia, often occurs with oseteoporosis.  Sarcopenia increases fall risk which is a major cause of fracture in osteoporosis.

The optimal exercise program would be one both increasing bone loading and increasing muscle activity to increase irisin.  Riding a recumbant exercise bike does generate muscle activity but contributes only as smaller amount of loading to the hips and especially the spine.

An example of a more suitable exercise may be 45 minutes of brisk walking daily with a weight vest.  While the walking requires muscle use and loading in the hips, it may not provide enough loading to the lower and mid-spine where osteoporostic fractures are common.  As a weight vest loads weight on the shoulders close to the top of the spine it adds load to the areas of the spine at risk.

The ideal timing of an exercise program to reduce osteoporosis is as prevention before bone loss occurs.  Peak bone density is realized about age 20 -25 and remains stable until age 35-40.  At that point bone density declines 1-2% each year until 50-55 years when it accelerates to 3-5% loss per year.  Once osteoporosis is present, exercise is not enough as a sole treatment but it is an important part of more comprehensive treatment program.

New study has quantified the benefit of exercise. In a study of 3014 women 75-80 years old, those with the highest physical activity had a 30% lower risk of any fracture, and a 54% lower risk of hip fracture compared to the lowest physical activity group.  Exercise matters!

Johannson et aal.  HIGH PHYSICAL ACTIVITY IS ASSOCIATED WITH GREATER CORTICAL BONE SIZE, BETTER PHYSICAL FUNCTION, AND WITH LOWER RISK OF INCIDENT FRACTURES INDEPENDENTLY OF CLINICAL RISK FACTORS IN OLDER WOMEN FORM THE SUPERB STUDY. Journal of Bone and Mineral Research, 2024;39(9):1284–1295.