The cost of inactivity is high for the human body, systems start to improperly function, in regards to bones, the absence of regular physical activity leads to loss of bone mineral density, calcium, phosphorus is lost from the bone. This results in bones being brittle and weak and if we do not address it, can lead to poor health and longevity. If there are injuries it will take longer to heal if the bone is weak and cause more problems. This is why so many falls are a huge risk for the older population. If they do not have adequate bone strength and density, a fall can lead to an injury that is hard to come back from. So in order to take care of ourselves and be less prone to injury we need to take the steps to grow strong bones.
From the day you are born your body is growing and adapting to the world around you so as we grow as kids i'm sure you heard your mom or dad say drink your milk its good for your bones. Well the premise is accurate but not totally the best way to grow bones (see milk nutrition and do we really need it?). What your parents were getting at is that the consumption of calcium is beneficial for bone growth and they are correct. Calcium is a big component that makes up bone and so we need to be getting it in our bodies to lay new layers of bone down. As we are young this helps as we are in growth spurts and growth phases, leading to maturing into an adult human. Build a strong base and the organism will thrive! As kids we are introduced to many activities, sports, jungle gyms, playgrounds, games, hop scotch, skipping, tag etc. All these things place a demand on the body, to be able to handle the forces we need to jump over a log to chase our friend or to hang onto the monkey bars our body needs to adapt. As we are adapting we are creating stronger bones just like we create stronger muscles and gain strength, the axial skeletal system stays strong so it can keep us upright and living life.
As we get older we start to find sports or activities we like and we gravitate toward them and focus our attention and partial time on them to improve our abilities. These improvements will put more demand on the body and continue the growth cycle of bones along with the rest of the body. When graduating high school young adults earn their freedom, now they are on their own in the world and the structures of sports, and physical education in school is gone, some go to college some go elsewhere. With the reduction of the above the sedentary lifestyle of us humans starts to kick in and the systems that we worked hard to adapt will revert back. It is costly to keep excess things on the body if it will not use it, so it will slowly go down to the demand that you need it for and stay there. This sedentary lifestyle will lead to bone mineral loss leading to poor bone health if not addressed.
To address the issue of the sedentary lifestyle and bone loss is to put the bone under constant stress to create a change. Physical activity yields stressors on the bones, this can be done through resistance training and certain aerobic exercise modalities. Mechanical loading activates osteoblasts that migrate to the stressed area of the bone, and lay down collagen molecules to form the bone matrix and that will become mineralized and a new bone is formed. This bone formation leads to stronger, more resilient bones. The mechanical load is important for bone formation, too little stress and the bone will not see an adaptation. This is where lower intensity exercises such as walking, swimming, light jogging do not show significant increases in bone, the intensity needs to be high such as sprinting where you are putting more force into the ground to run fast. Minimal essential strain is the threshold stimulus that initiates new bone growth and that is the goal we want to target when we are performing resistance exercises. The stress needed on the bones puts a high importance on progressive overload to increase bone size and strength and that is increasing the resistance/weight over a period of time.
Gaining the effect of the increase of bone mass has to do with exercise selection, performing single joint movements on machines are not encouraged due to the machine providing structural support to the system. We want to place a full demand on the whole axial skeleton, thus performing full body exercises are the best way to do this, exercises like squats, deadlifts, standing shoulder press, lunges, rows, dumbbell presses etc. The more we create freedom of movement the more beneficial the loading of the system will be.
The sedentary lifestyle is what we are up against and the only way to combat it is physical training. The best stress is provided from resistance training, so a program with aerobic work mixed with strength training is the optimal program for longevity, health, and quality of life.
https://pubmed.ncbi.nlm.nih.gov/9927006/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6279907/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4121709/