10


PART I FOUNDATION CONCEPTS KINEMATICS


Kinematics is a branch of biomechanics that describes human movement in terms of position, velocity, and acceleration. Movement can occur as linear or angular (rotational) motion or as a combination of the two.


LINEAR MOTION


Linear motion, in which all parts of the object move in the same direction and parallel to each other, is also known as translation, or translatory motion. Linear motion can be described as movement in either a straight line or a curved line and is measured in units of meters or feet. When describing linear motion of the body as a whole unit, a clinician may document that a person walked 50 ft (15.24 m) on a level surface.


Straight Line Motion


Figure 1.7 A sagittal view of the femur, tibia, and fibula. The axis of motion moves slightly as the knee moves during flexion and extension.


as having three degrees of angular freedom. In contrast to these joints, the elbow moves only in fl exion and exten- sion, having one degree of angular freedom, as the joint moves in only one plane of motion. The metacarpopha- langeal joints of the hand have two degrees of freedom, as fl exion and extension and abduction and adduction motions can occur at these joints.


Clinical Connection 1.1


Loss of even one degree of freedom in a joint from either paralysis or loss of range of motion can result in dysfunction of the entire extremity. Consider a person who loses the ability to fl ex the shoulder. Even though her hand may be able to hold a hair- brush and her elbow can fl ex through full range, she may be unable to raise the brush up to brush her hair.


Straight line linear (rectilinear) motion depicts point- to-point straight movement. Typically, the movement is in a two-dimensional or three-dimensional system in the planes established by the Cartesian coordinate system (Fig. 1.9). The system describes the position of any point in three-dimensional space. The movement occurs on the x, y, or z ordinance. Linear movement can take place in an anterior/posterior, medial/lateral, or vertical direction. An example of linear movement is the path of an object as it drops vertically from a height. In the body, limb and trunk movements do not typically occur in a purely linear fashion. Gliding linear motions occur between the surfaces of synovial joints as the limb moves in a rotation arc of movement. This passive linear motion is called accessory movement (joint play) and describes the amount slight joint motion that occurs between most joint surfaces. Figure 1.10 illustrates the linear gliding motion between the carpal bones in the wrist. (This gliding motion is addressed in greater detail in the arthrokinematics discussion in Chapter 2 and for each joint specifi cally in the individual chapters related to specifi c joints.)


Curved Line Motion


Curved line (curvilinear) motion is movement from one point to another in an arched path. An example of curvilinear motion is the vertical displacement of the center of the body as it moves upward and down- ward during walking. Curvilinear motion of the center of mass of the body and linear motion of the entire body that occurs during walking are shown in Figure 1.11.


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