PLANES OF MOTION

Osteokinematics describes the motion of bones relative to the three cardinal (principal) planes of the body: sagittal, frontal, and horizontal. These planes of motion are depicted in the context of a person standing in the anatomic position as in Figure 1-4. The sagittal plane runs parallel to the sagittal suture of the skull, dividing the body into right and left sections; the frontal plane runs parallel to the coronal suture of the skull, dividing the body into front and back sections. The horizontal (or transverse) plane courses parallel to the horizon and divides the body into upper and lower sections. A sample of the terms used to describe the different osteokinematics is shown in Table 1-2. More specific terms are defined in the chapters that describe the various regions of the body.

AXIS OF ROTATION

Bones rotate around a joint in a plane that is perpendicular to an axis of rotation. The axis is typically located through the convex member of the joint. The shoulder, for example, allows movement in all three planes and therefore has three axes of rotation (Figure 1-5). Although the three orthogonal axes are depicted as stationary, in reality, as in all joints, each axis shifts slightly throughout the range of motion. The axis of rotation would remain stationary only if the convex member of a joint were a perfect sphere, articulating with a perfectly reciprocally shaped concave member. The convex members of most joints, like the humeral head at the shoulder, are imperfect spheres with changing surface curvatures. The issue of a migrating axis of rotation is discussed further in Chapter 2.

DEGREES OF FREEDOM

Degrees of freedom are the number of independent directions of movements allowed at a joint. A joint can have up to three degrees of angular freedom, corresponding to the three cardinal planes. As depicted in Figure 1-5, for example, the shoulder has three degrees of angular freedom, one for each plane. The wrist allows only two degrees of freedom (rotation within sagittal and frontal planes), and the elbow allows just one (within the sagittal plane).

Unless specified differently throughout this text, the term degrees of freedom indicates the number of permitted planes of angular motion at a joint. From a strict engineering perspective, however, degrees of freedom apply to translational (linear) as well as angular movements. All synovial joints in the body possess at least some translation, driven actively by muscle or passively because of the natural laxity within the structure of the joint. The slight passive translations that occur in most joints are referred to as accessory movements (or joint “play”) and are commonly defined in three linear directions. From the anatomic position, the spatial orientation and direction of accessory movements can be described relative to the three axes of rotation. In the relaxed glenohumeral joint, for example, the humerus can be passively translated slightly: anterior-posteriorly, medial-laterally, and superior-inferiorly (see short, straight arrows near proximal humerus in Figure 1-5). At many joints, the amount of translation is used clinically to test the health of the joint. Excessive translation of a bone relative to the joint may indicate ligamentous injury or abnormal laxity. In contrast, a significant reduction in translation (accessory movements) may indicate pathologic stiffness within the surrounding periarticular connective tissues. Abnormal translation within a joint typically affects the quality of the active movements, potentially causing increased intra-articular stress and microtrauma.

OSTEOKINEMATICS: A MATTER OF PERSPECTIVE

In general, the articulation of two or more bony or limb segments constitutes a joint. Movement at a joint can therefore be considered from two perspectives: (1) the proximal segment can rotate against the relatively fixed distal segment, and (2) the distal segment can rotate against the relatively fixed proximal segment. These two perspectives are shown for knee flexion in Figure 1-6. A term such as knee flexion, for example, describes only the relative motion between the thigh and leg. It does not describe which of the two segments is actually rotating. Often, to be clear, it is necessary to state the bone that is considered the primary rotating segment. As in Figure 1-6, for example, the terms tibial-on-femoral movement and femoral-on-tibial movement adequately describe the osteokinematics.

Most routine movements performed by the upper extremities involve distal-on-proximal segment kinematics. This reflects the need to bring objects held by the hand either toward or away from the body. The proximal segment of a joint in the upper extremity is usually stabilized by muscles, gravity, or its inertia, whereas the distal, relatively unconstrained, segment rotates.

Feeding oneself and throwing a ball are common examples of distal-on-proximal segment kinematics employed by the upper extremities. The upper extremities are certainly capable of performing proximal-on-distal segment kinematics, such as flexing and extending the elbows while one performs a pull-up.

The lower extremities routinely perform both proximal-on-distal and distal-on-proximal segment kinematics. These kinematics reflect, in part, the two primary phases of walking: the stance phase, when the limb is planted on the ground under the load of body weight, and the swing phase, when the limb is advancing forward. Many other activities, in addition to walking, use both kinematic strategies. Flexing the knee in preparation to kick a ball, for example, is a type of distal-on-proximal segment kinematics (see Figure 1-6, A). Descending into a squat position, in contrast, is an example of proximal-on-distal segment kinematics (see Figure 1-6, B). In the latter example, a relatively large demand is placed on the quadriceps muscle of the knee to control the gradual descent of the body.

The terms open and closed kinematic chains are frequently used in the physical rehabilitation literature and clinics to describe the concept of relative segment kinematics.9,20,25 A kinematic chain refers to a series of articulated segmented links, such as the connected pelvis, thigh, leg, and foot of the lower extremity. The terms “open” and “closed” are typically used to indicate whether the distal end of an extremity is fixed to the earth or some other immovable object. An open kinematic chain describes a situation in which the distal segment of a kinematic chain, such as the foot in the lower limb, is not fixed to the earth or other immovable object. The distal segment therefore is free to move (see Figure 1-6, A). A closed kinematic chain describes a situation in which the distal segment of the kinematic chain is fixed to the earth or another immovable object. In this case the proximal segment is free to move (see Figure 1-6, B). These terms are employed extensively to describe methods of applying resistive exercise to muscles, especially to the joints of the lower limb.

Although very convenient terminology, the terms open and closed kinematic chains are often ambiguous. From a strict engineering perspective, the terms apply more to the kinematic interdependence of a series of connected rigid links, which is not exactly the same as the previous definitions given here. From this engineering perspective, the chain is “closed” if both ends are fixed to a common object, much like a closed circuit. In this case, movement of any one link requires a kinematic adjustment of one or more of the other links within the chain. “Opening” the chain by disconnecting one end from its fixed attachment interrupts this kinematic interdependence. This more precise terminology does not apply universally across all health-related and engineering disciplines. Performing a one-legged partial squat, for example, is often referred to clinically as the movement of a closed kinematic chain. It could be argued, however, that this is a movement of an open kinematic chain because the contralateral leg is not fixed to ground (i.e., the circuit formed by the total body is open). To avoid confusion, this text uses the terms open and closed kinematic chains sparingly, and the preference is to explicitly state which segment (proximal or distal) is considered fixed and which is considered free.

TYPICAL JOINT MORPHOLOGY

Arthrokinematics describes the motion that occurs between the articular surfaces of joints. As described further in Chapter 2, the shapes of the articular surfaces of joints range from flat to curved. Most joint surfaces, however, are at least slightly curved, with one surface being relatively convex and one relatively concave (Figure 1-7). The convex-concave relationship of most articulations improves their congruency (fit), increases the surface area for dissipating contact forces, and helps guide the motion between the bones.

FUNDAMENTAL MOVEMENTS BETWEEN JOINT SURFACES

Three fundamental movements exist between curved joint surfaces: roll, slide, and, spin.²⁷ These movements occur as a convex surface moves on a concave surface, and vice versa (Figure 1-8). Although other terms are used, these are useful for visualizing the relative movements that occur within a joint. The terms are formally defined in Table 1-3.

PREDICTING AN ARTHROKINEMATIC PATTERN BASED ON JOINT MORPHOLOGY

As previously stated, most articular surfaces of bones are either convex or concave. Depending on which bone is moving, a convex surface may rotate on a concave surface or vice versa (compare Figure 1-11, A with Figure 1-11, B). Each scenario presents a different roll-and-slide arthrokinematic pattern. As depicted in Figures 1-11, A and 1-9, A for the shoulder, during a convex-on-concave movement, the convex surface rolls and slides in opposite directions. As previously described, the contradirectional slide offsets much of the translation tendency inherent to the rolling convex surface. During a concave-on-convex movement, as depicted in Figure 1-11, B, the concave surface rolls and slides in similar directions. These two principles are very useful for visualizing the arthrokinematics during a movement. In addition, the principles serve as a basis for some manual therapy techniques. External forces may be applied by the clinician that assist or guide the natural arthrokinematics at the joint. For example, in certain circumstances, glenohumeral abduction can be facilitated by applying an inferior-directed force at the proximal humerus, simultaneously with an active-abduction effort. The arthrokinematic principles are based on the knowledge of the joint surface morphology.

CLOSE-PACKED AND LOOSE-PACKED POSITIONS AT A JOINT

The pair of articular surfaces within most joints “fits” best in only one position, usually in or near the very end range of a motion. This position of maximal congruency is referred to as the joint’s close-packed position.²⁷ In this position, most ligaments and parts of the capsule are pulled taut, providing an element of natural stability to the joint. Accessory movements are typically minimal in a joint’s close-packed position.

For many joints in the lower extremity, the close-packed position is associated with a habitual function. At the knee, for example, the close-packed position is full extension—a position that is typically approached while standing. The combined effect of the maximum joint congruity and stretched ligaments helps to provide transarticular stability to the knee.

All positions other than a joint’s close-packed position are referred to as the joint’s loose-packed positions. In these positions, the ligaments and capsule are relatively slackened, allowing an increase in accessory movements. The joint is generally least congruent near its midrange. In the lower extremity, the loose-packed positions of the major joints are biased toward flexion. These positions are generally not used during standing, but frequently are preferred by the patient during long periods of immobilization, such as extended bed rest.

IMPACT OF FORCES ON THE MUSCULOSKELETAL SYSTEM: INTRODUCTORY CONCEPTS AND TERMINOLOGY

A force that acts on the body is often referred to generically as a load.²³ Forces or loads that move, fixate, or otherwise stabilize the body also have the potential to deform and injure the body.^23,24 The loads most frequently applied to the musculoskeletal system are illustrated in Figure 1-12. (See the glossary at the end of this chapter for formal definitions.) Healthy tissues are typically able to partially resist changes in their structure and shape. The force that stretches a healthy ligament, for example, is met by an intrinsic tension generated within the elongated (stretched) tissue. Any tissue weakened by disease, trauma, or prolonged disuse may not be able to adequately resist the application of the loads depicted in Figure 1-12. The proximal femur weakened by osteoporosis, for example, may fracture from the impact of a fall secondary to compression or torsion (twisting), shearing, or bending of the neck of the femur. Fracture may also occur in a severely osteoporotic hip after a very strong muscle contraction.

The ability of periarticular connective tissues to accept and disperse loads is an important topic of research within physical rehabilitation, manual therapy, and orthopedic medicine.14,18 Clinicians are very interested in how variables such as aging, trauma, altered activity or weight-bearing levels, or prolonged immobilization affect the load-accepting functions of periarticular connective tissues. One method of measuring the ability of a connective tissue to tolerate a load is to plot the force required to deform an excised tissue.^8,22 This type of experiment is typically performed using animal or human cadaver specimens. Figure 1-13 shows a hypothetic graph of the tension generated by a generic ligament (or tendon) that has been stretched to a point of mechanical failure. The vertical (Y) axis of the graph is labeled stress, a term that denotes the internal resistance generated as the ligament resists deformation, divided by its cross-sectional area. (The units of stress are similar to pressure: N/mm²). The horizontal (X) axis is labeled strain, which in this case is the percent increase in a tissue’s stretched length relative to its original, preexperimental length.²⁴ (A similar procedure may be performed by compressing rather than stretching an excised slice of cartilage or bone, for example, and then plotting the amount of stress produced within the tissue.²⁹) Note in Figure 1-13 that under a relatively slight strain (stretch), the ligament produces only a small amount of stress (tension). This nonlinear or “toe” region of the graph reflects the fact that the collagen fibers within the tissue are initially wavy or crimped and must be drawn taut before significant tension is measured.¹⁸ Further elongation, however, shows a linear relationship between stress and strain. The ratio of the stress (Y) caused by an applied strain (X) in the ligament is a measure of its stiffness