Forces Transmitted by Muscles-Tendons-Ligaments-Bones

Movement occurs through the coordinated working of bones, tendons and ligaments. All three pertinent parts of the human body work together in response to neurological signals, and if there is any disease or condition that interrupts the nerves' signals or if there is any injury to any of these structures, movement can be hindered. In order to gather a fuller understanding lets recount what tendons, bones and ligaments are. Tendons are tough bands of connective tissue found in the joints. They connect muscles to bones. Each muscle has tendons attached at each end. Tendons are designed to only stretch a small amount. Their job is to transmit force between the bones and the muscles. For example, when the biceps muscle on the front top of the arm contracts, the tendon attached to the biceps muscle and elbow bone helps the muscle to pull on the elbow bones so the joint can bend. Ligaments are made of the same material as tendons. Ligaments connect the bones to each other, and are designed to help stabilize the joints and provide a structure for the bones. Since they have limited stretching ability, they limit how far a joint moves to help protect against injury. As the elbow joint bends, the ligaments stabilize the elbow bones so the arm can move with control.

Muscles in the body contract or shorten when they receive nerve signals initiated by the brain. There are three types of muscles including, skeletal muscles, which can be voluntarily controlled, involuntary smooth muscles, such as those that control breathing, digestion and other functions, and involuntary cardiac muscles, which control the function of the heart. Skeletal muscles travel across the length of joints and stretch between the bones.

Forces transmitted by muscles can be illustrated using Internal Load Modeling. Muscles, ligaments and tendons are treated as ropes, thus there is no 3D modeling for their forces. Internal Load Modeling does not take into account any interaction with surrounding muscles and bony structures. For each body segment of interest, the following quantities are estimated: the position vector and orientation matrix relative to both the laboratory frame (g)  and the anatomical frame (a), plus the local position vector of the intersegmental loads which is represented by reduction point K.

The left most diagram below depicts forces acting outward on the knee structure, and to the right shows the forces acting downward on the lower half of the leg.

Kinematic quantities and Inertia parameters are taken into consideration when using Internal Load Modeling as well as the forces shown above. For each body segment of interest, the following quantities are estimated in addition to the ones stated above... Mass is taken into consideration represented by m and K is no longer an arbitrary point.  The local position vector is represented by the notation CM (note in the equation CM is broken up into sub vectors) and the principal axes of inertia by I. The orientation matrix a is also considered as well as moments of inertia. All such things are coupled together to estimate Intersegmental Force (the force between two segments,) as represented in the equation below.