Joints of the Human Body

Human joints can be classified into three groups based on the range of motion permitted at the joint. An immovable joint is called synarthrosis in anatomy. These are the joints found between the bones of the skull and between teeth and the surrounding bone of the jaw. In the skull, the edges of the bones are interlocked and bound together by dense connective tissue. These joints are called sutures.

The second group of joints, such as the distal articulation between tibia and fibula, allow for slight movements. Such a joint is called am-phiarthrosis. The bones forming these joints do not have to be touching each other but they are connected tightly by ligaments. The articulations between adjacent vertebrae form this type of a joint. In this case the bones are separated by pads of fibrocartilage. The slight movements allowed between adjacent vertebrae permit the vertebral column to bend forward and backward and to the sides as well as rotate to some extent about its longitudinal axis.

The joints that allow considerable motion of the articulating bones are called freely moving joints (diarthrosis or synovial joints). These joints are typically found at the end of the long bones, such as those of the leg and arm. under normal conditions, the bony surfaces do not contact one another because articulating surfaces are covered by cartilage and because there is a layer of fluid (synovial fluid) between the opposing surfaces. The matrix of the cartilage contains water and is squeezed out in com-pressive loading, creating a lubrication layer on the surface of the interface. This thin layer of fluid reduces the frictional forces and help distribute the compressive stress more uniformly along the surfaces of the articulating bones. The hip, knee, and ankle joints are all examples of synovial joints.

A joint is called a monoaxial joint when rotation is allowed only on one axis. An example of a monoaxial joint is the one that attaches the two vertebrae which are most proximal to the skull. This joint allows rotation of the head to the left or to the right. Because it acts like a pivot, it is called a pivot joint (Fig. 1.8a). Another example of pivot joint is the articulation between the forearm bones, the radius and ulna, at the elbow. These bones have the capacity to rotate relative to each other along the long axis of the forearm.

The elbow and the knee are called hinge joints because they permit flexion and extension in the sagittal plane (Fig. 1.8b). In terms of their physical function, these joints are much like door hinges. The elbow and the knee joints are monoaxial joints. As shown in Fig. 1.7, in the knee joint the rounded surface of the distal end of the femur and the flatter surface of the tibia do not fit together. Collateral ligaments on either side hold the bones together while allowing the knee to bend. But because these ligaments run along the axis of the leg, they cannot prevent small movements of one bone on the other. Angled ligaments found within the capsule of the knee joint, the anterior cruciate ligament and posterior cruciate ligament, also connect the femur to the tibia. These ligaments are crucial for the stability of the knee. Also contributing to stability are the two crescent-shaped wedges of fibrous cartilage (the menisci) that lie in the gap between the articulating surfaces of the femur and the tibia. The menisci are not freestanding but are held together by ligaments. They help distribute the contact force between the femur and tibia over the surface of articulation.

The ankle joint permits sole elevation and sole depression. The articulation between the lower end of the tibia and the talus of the foot is responsible for bending the ankle toe up and toe down. The ankle allows rotations in other directions too, thanks to the movement of the small bones of the ankle relative to each other. The complexity of the articulating surfaces and the abundance of ligaments of this joint can be a challenge even to the leading experts of the biology of movement. "I do not fully understand the complicated array of ligaments that hold the ankle bones together (and I do not think any one else does, either) so I will not try to explain them," writes R. McNeill Alexander in his illuminating book, The Human Machine.

Joints The BodyBall And Socket Joints The Human Body

Figure 1.8a-c. Engineering representations of the synovial joints of the human body. The figure shows the pivot joint between axis and atlas, the two vertebrae closest to the head (a), the elbow (b), and the shoulder joint (c). The pivot joint between atlas and axis allows the head to rotate to the left and to the right. A hinge joint allows relative rotation in the plane of two articulating long bones. The knee and elbow are examples of hinge joints. A ball-and-socket joint allows rotation in three directions. This type of joint has the most degrees of freedom in movement. Examples are the shoulder joint and the pelvic girdle.

Figure 1.8a-c. Engineering representations of the synovial joints of the human body. The figure shows the pivot joint between axis and atlas, the two vertebrae closest to the head (a), the elbow (b), and the shoulder joint (c). The pivot joint between atlas and axis allows the head to rotate to the left and to the right. A hinge joint allows relative rotation in the plane of two articulating long bones. The knee and elbow are examples of hinge joints. A ball-and-socket joint allows rotation in three directions. This type of joint has the most degrees of freedom in movement. Examples are the shoulder joint and the pelvic girdle.

Ball-and-socket joints are articulations in which the round head of one bone rests within a cup-shaped depression in another (Fig. 1.8c). These joints are called multiaxial because they permit rotation on more than one plane or axis. Examples of these joints include the shoulder joint and the hip joint. The shoulder joint is formed by the head of the humerus and the small, shallow pear-shaped cavity of the scapula. This joint allows the greatest range of motion of any joint in the body, mainly because the cavity of the scapula is shallow in depth and also the articular capsule enclosing it is remarkably loose. Perhaps because of the greater degrees of freedom, the shoulder joint is also the most frequently dislocated joint.

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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