Every element present in nature is composed of chemical bonds between molecules. Chemically, every element that is subjected to physical stress (e.g. pressure or traction) deforms, sometimes more slightly, other times more significantly, stretching the chemical bonds between the molecules of the material in question. In the case of biological structures this 'stress' creates a slight electrical flow through the material. This electrical flow is called 'piezoelectric charge'. This charge, which can be 'read' by the connective tissue cells in its proximity, causes these cells to respond by increasing, decreasing or exchanging intercellular substances in that area.
Let's take the head of the femur for example. It is composed of trabeculae of spongy bone (particular lamellar formations of distinct layers of fibers and bone cells held together by collagen fibres). An analysis of the trabeculae of the bone shows that they are built to perfection, so as to be able to resist the forces transmitted from the pelvis to the femur. This conformation is a safety system even for the lightest and thinnest bones.
Inside, the bone is shaped to reflect not only the interrelationship and environmental needs of each species, but also the individual's fitness and activity level. If we dissected the femur of a subject with a certain posture and activity and compared it to that of another subject with a different posture and activity level, the two femurs would appear very different, we would see that each femoral head has slightly different trabeculae, precisely because it was 'designed' to better resist the forces that particular person exerts.
In this way, the connective tissue responds to everyone's request. Whatever need there is, a continuous effort or a sedentary lifestyle, the extracellular elements are modified along the way depending on the type of load applied to satisfy the request, but within the limits dictated by some parameters such as: nutrition, age, and protein synthesis.
Inside the bone and around it there is a sparse but active group of two types of osteocytes (bone tissue cells): osteoblasts and osteoclasts. Each of these is activated with simple and precise tasks: osteoblasts consolidate new bone; osteoclasts clean up old bone. Osteoblasts are allowed to consolidate new bone wherever they want as long as it is within the periosteum.
Osteoclasts can 'eat' bone anywhere except those parts that are undergoing piezoelectric charging. Letting nature operate in this way over time shows that a femoral head is specifically produced to resist individual forces passing through it, but also capable of changing, after a certain reaction time, to accommodate new forces as they are constantly applied.
This mechanism explains how dancers' feet become harder boned during an intense period of dancing: the increase in hours of load (dancing) determines an increase in forces that create greater piezoelectric charges which reduce the ability of osteoclasts to remove bone while simultaneously osteoblasts lay down more bone resulting in greater bone density.
All this explains why physical exercise is important in subjects with incipient osteoporosis: the forces created by the increased stress on the tissues serve to discourage and decrease the absorption of osteoclasts.
The reverse process however occurs in astronauts who, deprived of the force of gravity, are subject to greater activity of the osteoclasts on the osteoblasts so much so that they require total assistance upon their return for travel until their bones become less porous.