Force is any influence causing an object to undergo change. The evidence shows that our bodies, particularly our bones, tolerate some forces much better than others. This especially matters when facing a heavy resistance, which magnifies the effect of a force. Understanding how forces apply to our bones gives yet another reason to favor compound over isolation exercises.
Types of Force
Tension consists of two pulling forces. A person pulling on each end of a rope demonstrates this force. It makes an object longer and thinner along the line of force. This force usually occurs with the pull of a muscle on its tendons. Tension can harm us through strains and sprains, so appear more relevant to discussing the muscles, ligaments, and tendons versus the bones. Some bones have developed to resist them though. The fibula of the leg handles tension well.
Compression consists of two pushing forces. Pressing your hands together demonstrates this force. It deforms an object to make it shorter and thicker along the line of force. Specific bones handle large compressive forces, with examples including the tibia of the leg and the femur of the thigh.
Shear consists of two equal and opposite forces acting parallel that tend to displace an object between the lines of force. Molecules slip past each other as opposed to compression that pushes molecules together. Many bones cannot handle shear well.
Bending places maximum tension on the convex surface of a bent object and maximum compression on the concave surface of a bent object. Bones resists bending well in the areas meant to handle this force.
Torsion is a twist that affects one end with the other end fixed. Twisting can be very harmful on the lower back. The spine is meant to resist torsion, not to initiate it. Torsion applies shearing forces within the structure itself.
Bones
Long bones extend longer than wide and provide the main levers that allow muscles to create movement. Some include the humerus, radius, ulna, femur, tibia, and fibula. They are beam-shaped to create strength for minimizing loads imposed through bending.
A long bone acts strongest when stressed by forces acting along the long axis of the bone, as occurs during compression. Several joints must rotate to move in a linear path to allow bones to bear the load along the long axis. This occurs during compound movements. Bone suffers when handling forces applied transversely across its surface. These bones, and all bone in general, function strongest in compression and weakest in shear.
Short bones resemble long bones though with smaller sizes and more flexibility. These bones assist in shock absorption and the transmission of forces. The sesamoid bone, a specific type of short bone, embeds in a tendon or joint capsule to alter the angle of insertion for the muscle. This also diminishes friction created by the muscle. The patella at the knee joint serves as a good example. Other sesamoid bones include the phalanges, metacarpals, and metatarsals. Short bones also fail to act properly if loaded transversely.
Other bones serve to protect vital areas, facilitate movement, serve as attachment sites, dissipate loads, minimize friction, and reinforce areas subjected to a force. They adapted to perform roles that came about from free weight environments opposing gravity that our bodies faced since our earliest ancestors. When we load a motion heavily that these bones did not prepare to encounter, injury can result.
Avoid Shear, Allow Compression
Fool me once, shame on you. Fool me twice, shame on me.
– Randall Terry
Isolation tends to focus on shear and compound focuses on compression. Excessive ranges of motion of any exercise can also promote shear. Subjecting your body to excessive shear will increase the risk of injury. You cannot force yourself to overcome these limits. Can you train to survive jumping off bridges? Use compound over isolation exercises for heavy lifting.