Bone strengthening activities produce a force on the bones that promotes bone growth and strength. Very often this force is produced by impact with the ground. Many bone strengthning activities can also be aerobic (running) and muscle strengthening (gymnastics). A strong skeletal system is the body's foundation. Having strong dense bones is essential to good posture, strength and balance. According to the CDC, bone strengthening activities are especially important for children and young adolescents because the greatest gains in bone mass occur during the year just before and during puberty.

Examples of Bone Strengthening Activities for Adolescents and Children

United States Department of Health and Human Services
Physical Activity Guidelines for Americans - 2018
https://health.gov/sites/default/files/2019-09/Physical_Activity_Guidelines_2nd_edition.pdf