Minimal Essential Strain (MES)
There is a minimal threshold (called the minimal essential strain (MES)) that must be reached and repeated often enough to signal to the osteoblasts to travel to the area of strain and lay down collagen to increase the strength of the bone. By overloading the body with intensities greater relative to normal daily activities, the MES can be reached or exceeded. Physical activities need to be weight-bearing – having the additional stress of your body weight involved – to provide the most effective stimulus for bone formation.
The MES of a bone is thought to be a level of strain approximately one tenth of the force required to fracture a bone and acts as a defence mechanism – if a large force is regularly applied to a bone, the area of bone strained needs to be large enough to disperse the forces and prevent damage following loading (hence the laying down of more collagen). Increasing the diameter of the bone by laying down new bone at the periosteum allows the force to be spread over a larger surface area, therefore decreasing the amount of force per unit area across the bone surface. After bone growth, the same force that previously exceeded the MES will now be below the MES threshold and will not represent a stimulus to the MES.
The total mass of a muscle reflects the force that it can generate to the bone to which it attaches. Larger and better-trained muscles exert greater forces and therefore bone and other connective tissue must increase their mass and strength accordingly to provide a sufficient support structure for the bigger muscles. An increase or decrease in muscle strength or mass through training or periods of inactivity results in an equivalent increase or decrease in connective tissue and bone.
Activities that stimulate muscle hypertrophy and strength gains also appear to stimulate the growth of bone and associated connective tissue. Different parts of the skeleton should be stressed to gain maximum benefits, especially when trying to offset the commencement or progression of osteoporosis (reduced bone mass and mineral density to critically low levels) in regions of the skeleton most commonly affected. With osteoporosis, forces that would normally be absorbed by the skeleton now result in fractures. Sites of fractures that are most devastating are the hips and spine. In order to minimise the risk it is suggested that individuals should maximise their peak bone mineral density in early adulthood, when they are most able to perform the intense physical activities necessary to increase bone mineral density.