Kinematics

Kinematics is a branch of biomechanics that describes the motion of a body without regard to the forces that produce the motion. In biomechanics, the word body is used rather loosely to describe the entire body, particular segments such as an individual bone, or an area of the body such as the arm. In general, two types of motion exist: translation and rotation.

Translation occurs when all parts of a “body” move in the same direction as every other part. This can occur in a straight line (rectilinear motion), for example, sliding a book across a table, or in a curved line (curvilinear motion), such as the arc of a ball being tossed to a friend. Figure 1-1 illustrates the curvilinear motion that occurs during walking, reflecting the normal up-and-down translation of the head as the entire body moves forward.

Rotation describes the arc of movement of a “body” about an axis of rotation. The axis of rotation is the “pivot point” about which the rotation of the body occurs. Figure 1-2 illustrates rotation of the forearm around the axis of rotation of the elbow.

Movement of the entire human body is generally described as a translation of the body’s center of mass, or center of gravity (Figure 1-3). An activity such as walking results from forward translation of the body’s center of mass, thus the entire body. It is interesting to note, however, that movement or translation of the entire body is powered by muscles that rotate the limbs. This concept is illustrated in Figure 1-4, which shows an individual running (anterior translation of the center of mass) as a result of muscles rotating the legs around the axis of rotation of each hip. It is important to note that the functional movement of nearly all joints in the body occurs through rotation.

Regardless of the type of body movement, a movement can be classified as either active or passive. Active movements are generated by stimulated or “active” muscle, for example, when an individual flexes his or her arm overhead, this is considered an active movement. Passive movements, on the other hand, are generated by sources other than muscular activation, such as gravity, the resistance of a stretched ligament, or a push from another person. For example, when a clinician provides the force to move an individual’s limb through various ranges of motion, this is considered a passive movement—thus the common clinical term passive range of motion.

Terminology

The study of kinesiology requires the use of specific terminology to describe movement, position, and location of anatomic features. Many of these terms are illustrated in Figure 1-5.

• Anterior: Toward the front of the body

• Posterior: Toward the back of the body

• Midline: An imaginary line that courses vertically through the center of the body

• Medial: Toward the midline of the body

• Lateral: Away from the midline of the body

• Superior: Above, or toward the head

• Inferior: Below, or toward the feet

• Proximal: Closer to, or toward the torso

• Distal: Away from the torso

• Cephalad: Toward the head

• Caudal: Toward the feet (or “tail”)

• Superficial: Toward the surface (skin) of the body

• Deep: Toward the inside (core) of the body

• Origin: The proximal attachment of a muscle or ligament

• Insertion: The distal attachment of a muscle or ligament

• Prone: Describes the position of an individual lying face down

• Supine: Describes the position of an individual lying face up

Osteokinematics

Planes of Motion

Osteokinematics describes the motion of bones relative to the three cardinal planes of the body: sagittal, frontal, and horizontal (Figure 1-6) (Box 1-1).

Box 1-1

Common Osteokinematic Terms

Sagittal Plane	Frontal Plane	Horizontal Plane
• Flexion and extension • Dorsiflexion and plantar flexion	• Abduction and adduction • Lateral flexion • Ulnar and radial deviation • Eversion and inversion	• Internal (medial) and external (lateral) rotation
		• Axial rotation
• Forward and backward bending

From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, St Louis, 2002, Mosby, Table 1-2. Many of the terms are specific to a particular region of the body. The thumb, for example, uses different terminology.

• Sagittal plane: Divides the body into left and right halves. Typically, flexion and extension movements occur in the sagittal plane.

• Frontal plane: Divides the body into front and back sections. Nearly all abduction and adduction motions occur in the frontal plane.

• Horizontal (transverse) plane: Divides the body into upper and lower sections. Nearly all rotational movements such as internal and external rotation of the shoulder or hip and rotation of the trunk occur in the horizontal plane.

Anatomic Position

The anatomic position, illustrated in Figure 1-6, serves as a standard reference for anatomic descriptions, axis of rotation, and planes of motion. For example, the action of a muscle is based on the assumption that it contracts with the body in the anatomic position.

Axis of Rotation

The axis of rotation of a joint may be considered the pivot point about which joint motion occurs. Consequently, the axis of rotation is always perpendicular to the plane of motion. Traditionally, movements of the body are described as occurring about three separate axes of rotation: anterior-posterior, medial-lateral, and vertical—sometimes referred to as the longitudinal axis (Figure 1-7).

The anterior-posterior axis of rotation is oriented in an anterior-posterior direction through the convex member of the joint and allows movement to occur in the frontal plane, for instance, abduction and adduction of the hip.

The medial-lateral axis of rotation is oriented in a medial-lateral direction through the convex member of the joint. The medial-lateral axis of rotation allows motion to occur in the sagittal plane, for instance, flexion or extension of the elbow.

The vertical (longitudinal) axis of rotation is oriented vertically when in the anatomic position. However, if motion occurs out of the anatomic position, it is often described as occurring about the longitudinal axis; this axis courses through the shaft of the bone. Motion about the vertical or longitudinal axis of rotation occurs in the horizontal (or transverse) plane. Typically, these are called rotational movements and are seen in rotation of the trunk when twisting side-to-side or in internal and external rotation of the shoulder. A summary of these axes can be found in Table 1-1.

Table 1-1

Axes of Rotation and Associated Movements

Axis of Rotation	Plane of Motion	Examples of Movement
Anterior-posterior	Frontal	Hip abduction-adduction
		Shoulder abduction-adduction
Medial-lateral	Sagittal	Elbow flexion-extension
		Knee flexion-extension
Vertical or longitudinal	Horizontal	Shoulder internal-external rotation
		Rotation of the trunk

Degrees of Freedom

Degrees of freedom refers to the number of planes of motion allowed at a joint. A joint can have 1, 2, or 3 degrees of angular freedom, corresponding to the three cardinal planes (see the earlier section on terminology). As depicted in Figure 1-7, for example, the shoulder has 3 degrees of freedom, meaning the shoulder can move freely in all three planes. The wrist, on the other hand, allows motion in two planes, so it is considered to have 2 degrees of freedom. Joints such as the elbow (humeroulnar joint) allow motion in only one plane and therefore are considered to have just 1 degree of freedom.

Fundamental Movements

For movements of the body, specific terminology is used to help describe the motion at a joint or region of the body.

Flexion and Extension

The motions of flexion and extension occur in the sagittal plane about a medial-lateral axis of rotation (Figure 1-8). Generally, flexion describes the motion of one bone as it approaches the flexor surface of the other bone. Extension is considered a movement opposite that of flexion; it is an approximation of the extensor surfaces of two bones.

Abduction and Adduction

Abduction describes movement of a body segment in the frontal plane, away from the midline, whereas adduction describes a frontal plane movement toward the midline (Figure 1-9).

Exceptions to this definition occur in the hands and feet; these are described in the joint-specific chapters.

Rotation

Rotation describes the movement of a bony segment (or segments) as it spins about its longitudinal axis of rotation. For example, turning the head or turning the trunk side-to-side are considered rotational movements (Figure 1-10, A). Motions of the extremities can be further classified into internal and external rotation.

Internal rotation describes the motion of a bony segment that results in the anterior surface of the bone rotating toward the midline. External rotation involves rotation of the anterior surface of a bone rotating away from the midline (Figure 1-10, B).

Circumduction

Circumduction describes a circular motion through two planes; therefore joints must have at least 2 degrees of freedom if they are to circumduct. A general rule is that if a joint allows a circle to be “drawn in the air,” the joint can circumduct (Figure 1-11).

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