Ligament, Tendon & Joint Mechanics


Ligaments provide the body with stability, connecting 1 bone to another. They consist of soft-tissue and hold hard, dense bones together, forcing them to be resistant to stress factors. Ligaments are composed of fibroblast cells which consist of collagen fibers mixed with some elastic fibers. Like many other structures of the body a ligament’s ability to heal when injured is widely dependent on its blood supply. Ligaments of the knee, like the MCL or medial collateral ligament, can heal without surgery on its own with proper conservative treatment. This includes rest, cold therapy, supports, bracing and therapeutic exercise.

Conversely, another ligament of the knee, the ACL or anterior crucial ligament, does not have the ability to heal on its own accord and will likely require surgical reconstruction when torn. The ACL is typically torn due to sudden or extreme movements during activity and in high-intensity sports like football, basketball and skiing. The most commonly injured ligament, however, is contained within the ankle, called the talofibular ligament. The talofibular ligament can be torn during a simple ankle sprain and symptoms of a tear will be significant.

A tendon is a structure that attaches a muscle to a bone. While similar in design to a ligament, it is built to withstand pulling and tensile stress factors. Tendons are composed of 10% elastin, the flexible tissue of the human body, so they can stretch up to 10% of their resting length. Once a tendon exceeds the 10% threshold the cells begin to rupture and tear apart. If a tendon is conditioned through exercise and activity it will rupture less easily, conversely, it will rupture easier when unaccustomed to stress. Age is also factor in the resiliency of tendons. At 30, the flexibility and strength of tendons diminishes by 20%. Degradation can be mitigated and even offset with proper conditioning.

Tensile stress is the most common type of stress tendons are exposed to. When a tendon is stretched from both ends it is subjected to tensile stress. If the pulling motion that is stretching the tendon continues it will create tiny tears or microtears along the fibers. With sufficient rest microtears will heal and the tendons will remain strong. However, if those tears accumulate without an opportunity to heal, grow and recover it may lead to serious injury or a developing condition. This is the case with Achilles tendinitis, a nagging injury that can be disabling to a runner as it will cause symptoms of significant pain and soreness at the back of the ankle. An increase in exercise duration instead of a gradually build up or a change in the running surface from dirt to concrete, which sharply increases impact to the foot and heel, will make little tears much larger and more difficult to resolve. Injuries of this magnitude require time to heal and the site of the torn tissues will continue to expand until eventually walking becomes limping; like when you get out of bed in the morning or after long periods of inactivity.

A joint capsule is tissue that surrounds a joint and is where 2 bones come in contact with one another. Like ligaments, joint capsules consist of fibroblasts in a collection of collagen and elastin fibers. The joint capsule can withstand tension exerted in numerous directions and provides some flexibility and stretch. If a joint capsule is too tight motion will be restricted, but if it’s too stretched bones of that joint will slip out of place resulting in either a partial or full dislocation.

Joints are where bones come together, allowing one to move or remain still, but weight-bearing. Joints are referred to as articulations and are classified in several different categories. Joints contained within the shoulder and knee are called synovial joints. They are characterized by fluid-containing joint cavities that separate bones and contain hyaline cartilage, a joint capsule, and a synovial membrane that produces synovial fluid, the body’s natural lubricant. Joints can also contain other components, including ligaments, articular discs or menisci and bursae.

Joints have an architectural simplicity to them. Some, like the interphalangeal joints that form your fingers are hinges that move along 1 plane. Joints of the hip are ball-and-socket and can move through 3 planes. In general, however, the greater the freedom of motion the less inherent stability the joint provides.

The knee, for example, functions as a fulcrum that transfers energy to and from the leg for kicking and locomotion, but also provides a sturdy base of support for standing still. This design balances stability with range of motion by employing a hinge known as a condyoid. The end of the femur or thigh bone is shaped like a cam with a notch at the center. The ligaments from the end of the femur crisscross within the notch and attach on either side, tying the bone securely to the tibia or lower leg. This gives the knee a range of motion of 150 degrees forward and backward as well as 15 degrees of side-to-side rotation. This allows one to sit on the ground cross-legged or cut across a field, but not so much that one would wobble when trying to stand still.

Joints are well suited for specific functions, but they are equally unsuited for many others. The shoulder, for example, with its great range of motion, 16,000 degrees, must sacrifice stability for mobility. The shoulder cannot handle heavy or repeatedly stressful loads. Should the shoulder be subjected to large arcs of motion repeatedly under stress it can overwhelm the stabilizing soft tissue. Baseball pitchers, tennis players and competitive swims have a high incidence of shoulder problems. Conversely, the knee, manages compressive forces well, handling up to 2,000 pounds of up-and-down pressure. Torque, however, the twisting force where the lower leg turns one way and the upper leg another can tear knee ligaments apart, as many pro and recreation football, basketball and skiers can testify.