As a species, our bones have gone through many changes over time. A recent report published in the Proceedings of the National Academy of Sciences suggests that as humans transitioned from hunter-gatherer culture to farming our food, our bones became weaker.

Today in the U.S., approximately 50% of women and 25% of men over 50 will break a bone due to osteoporosis, a condition where bones become weak and less dense. If the broken bone is in the hip or spine, there is about a 20% chance that the individual will die within one year. For those who survive, quality of life declines significantly and many lose their independence or end up in long-term care. The good news is that osteoporosis can be prevented, and we can start to focus on prevention years before reaching middle age by developing lifestyle strategies to decrease our risk.

Bones become stronger by resisting stressors, such as the repeated pounding on the foot and leg bones when we run or the effort involved in lifting heavy weights. Regular weight-bearing and impact exercises provide a planned and structured way to put stress on the bones. Over time, bone responds to exercise-related stress by becoming stronger and more dense. Stronger bones are more resistant to breaks.

The body also constantly strives for efficiency—to use as little energy as possible—to minimize the amount of work required to maintain function in the body’s systems. When stress doesn’t exist—like when we stop our exercise routine—the body kicks into efficiency mode. It stops using energy to maintain bone strength and the bones begin to lose their density.

The amount of exercise needed to strengthen bones will vary person to person. The key factor is to regularly challenge the body with an amount of physical stress that exceeds what is typical for that individual on a daily basis. For people who don’t move much, simply walking and performing light resistance training may be enough. More active individuals may benefit from high-impact activities such as jogging or stair climbing and moderate to high levels of resistance training.

To learn more about how exercise strengthens bones, visit the National Osteoporosis Foundation website.

Kim Henige, EdD, CSCS, ACSM EP-C, is an associate professor and undergraduate program coordinator in the department of kinesiology at California State University, Northridge.