From a pure musculoskeletal standpoint, the human body is a mechanical device. All mechanical devices are subject to wear during use that reflects their history of destructive forces. Unique to living tissue is its ability to heal, adapt, and strengthen, which provides a dialogue between catabolic and anabolic forces. While machines convert thermal or chemical energy into mechanical energy, muscle tissue transforms nutrients directly into mechanical energy without a thermal intermediary. Body energy enables it to overcome resistance to motion, to produce a physical effect, and to accomplish work.


Practical Concepts

The body's kinetic energy is reflected in its velocity, and its potential energy is reflected in its position. Work is the result of a force acting through a distance. Power relates to the time element and the work accomplished. There is a close association in the same unit of time between the work accomplished by a weight lifter and that of a sprinter.

Muscle contraction (work) reflects the consumption of mechanical energy. Some of this energy is unproductively used to overcome internal friction and loading, and some is stored for later use within elastic (contractile) tissues. The effect of muscle contraction essentially depends on: (1) the unique fiber arrangement determining the relationship of force that the muscle can produce and the distance over which it can contract, (2) the angle of pull, and (3) the muscle's location relative to the joint axis.

The resistance offered to musculoskeletal forces may arise from gravity, friction, stationary structures, elasticity of structures, or manual resistance. The effectiveness of resistance or load is determined by the angle of the line of resistance applied and the distance of the load from the axis of the lever system involved. Gravity is the most common load on the body and provides a line of force in a constant direction.

Although forces of all types may cause subluxations, dislocations, fractures, strains and sprains, and so forth, the biomechanics involved determine the type and extent of the injury produced depending on the applications of force and its resistance. Thus, different types of force may cause bending fractures, stress fractures, or compression fractures. When the examiner understands how an injury was caused, the tissues involved are more readily located and the injury extent is more quickly evaluated.


Biomechanical Forces on Joints

Joint structure is the product of the quality and quantity of the chemical constituents of bone and associated tissues to cope with the action of external and internal forces. Joint stress is defined as the force exerted. Pressure always results in compression stress, and a pull produces tensile stress that is an action directly opposed to compression. Common tensile and compression stresses (axial forces) operate along the axis of a body part without altering it.

A force directed against a structure at an angle to its axis permitting one part to slide over the other produces a shearing stress. Both parts may be movable with the parts sliding in opposite directions or one part fixed. A spinal curvature in any direction involves a constant state of abnormal tension and compression of bones, cartilage, and muscles. Spinal bending involves the dual actions of tension, compression, and torsion.

Stress is greatest on the short arm of first-class levers (eg, elbow, knee). Understanding the biomechanical principles involved helps us to prevent injury and restore functional integrity. While our lever-like extremities transmit forces and motion at a distance, they also favor musculoskeletal injuries by amplifying forces (usually external, occasionally internal) acting on the body's biomechanical system.

Another clinical consideration is that an applied force greater than structural resistance will fracture a bone or dislocate a joint. For example, while it requires from about 1,500-3,000 lbs of static weight to fracture the neck of the femur, a weight of only 20 lbs dropped on it from a few feet will have the same result. In weight lifting during a dead lift of 200 lbs by a 170-lb person, a 2000-lb force is exerted on the lumbosacral disc. If a deep-squat lift is done exactly as defined in competitive weight lifting, severe stress on inappropriate tissues is inevitable. Thus, the athlete whose goal is solely to strengthen his leg extensors by lifting weights should seek one of the many alternatives to the deep-squat position (eg, partial squat, leg-press machine, Klein bench).

Newton's basic laws of mechanical physics are the foundation of logical raining. Despite what degree of force is induced on a part, there is always a counteracting stress because for every action there must be a reaction. A downward pressure will be equal an opposing upward thrust. A force pulling right will be equal to a pull toward the left, expressed in terms of centripetal and centrifugal force. A twisting force in one direction must be followed by an equal twisting force in the opposite direction. A force allowing a part to slide downward must be resisted by an adequate upward force. And a force tending to bend a structure along its axis must be resisted by a force equal to prevent such bending.

The various body motions are not the sole result of muscular action alone; they are also the effect of the structure, balance, and position of the various bones forming the joints acted upon. This cooperative action of muscles and bones is the result of leverage, and levers operate according to mechanical laws.

What is the limit of human physical potential? Each year we see athletic performance draw closer to the structural limits of human capacity. Toward the goal of ideal fitness, Pollock/Wilmore emphasize that attention must be given to maintaining optimal function of the musculoskeletal system. "Prevention of poor posture, lower back complaints, fat-free tissue loss, and osteoporosis depends on the incorporation of a comprehensive strength and flexibility training program into the daily workout." The reader will see the obvious parallelism between the conclusions of these scientists and the chiropractic approach.


Implications of Closed-Packed Joint Positions

Alternating compression and distraction within a joint has a distinct influence on articular surface nutrition and lubrication. These alternating motions also comprise the basis of proprioceptive neuromuscular facilitation techniques.

Some joint movements are accompanied by compression, others by distraction, and others by compression and distraction depending on the range and angle of motion. The term closed-packed position refers to a specific joint position where the articular surfaces are at their maximum point of congruency. Opposing articular surfaces may be either in a state of approximation (compression) such as when moving toward the closed-packed position or separation (distraction) when moving away from the closed-packed position.

The closed-packed positions for many joints are shown in Table 1. The clinician's knowledge of the closed-packed position of each joint should include what movements involve compression and which involve distraction. This should be determined because most subluxations, dislocations, and fractures occur when a joint is in the closed-packed position. Most sprains, however, occur when the joint is in a loose-packed position because the force is imposed more on the supporting periarticular structures of the joint than on intra-articular structures.


Table 1.   Closed-Packed Joint Positions

Joint	Closed-Packed Position
Temporomandibular	When the heads of the condyles are at their most retruded position.
Glenohumeral	When horizontal adduction, abduction, and external rotation are fully achieved.
Acromioclavicular	During elevation and horizontal adduction of the arm; combining upward scapular rotation and narrowing of the scapula-clavicle angle (as seen from above).
Elbow	Full extension.
Wrist (as a whole)	Full dorsiflexion and radial deviation.
Trapeziometacarpal	Opposition.
Metacarpophalangeal	Full flexion.
Interphalangeal	Full Extension.
Hip	Full internal rotation, extension, and abduction.
Knee	Full extension and lateral rotation.
Ankle mortise	Full dorsiflexion.
Subtalar	Full eversion.
Forefoot (as a whole)	Wide weight-bearing position, where the forefoot is supinated relative to the heel and the longitudinal arch flattens.

* Modified from Kessler/Hartling.


Most long-bone joints are in the ovoid class in which the cross-sectional surface curves to make a smoothly changing radius. As an opposing articular surface moves along an ovoid surface, the apposing surfaces do not closely fit (impure swing) except at one particular point, which every joint has, where congruency is relatively close. This is the closed-packed position. It is at this point that movement normally halts.

An impure swing during joint motion requires conjugate rotation. This type of rotation produces a twisting action on the capsule and major ligaments of the joint that, in turn, causes the joint surfaces to approximate until the closed-packed position is reached. Falls on an outstretched hand, for example, throw almost every joint of the upper extremity into a closed-packed position. Two exceptions are the metacarpophalangeal and acromioclavicular joints. If the force exceeds structural strength enough, either a joint must dislocate or a bone must fracture.


Pain Produced by Faulty Biomechanics

Owing to the constancy involved, postural and mechanical faults may exhibit severe pain from what appears to be mild postural defects and exhibit little or no pain in obvious cases of severe postural deficit. Minor postural deficits are often associated with considerable joint stiffness, and a very faulty posture may be seen in a very flexible subject whose body positions change readily. It is also observed that cumulative effects of constant or repeated small stresses over a long duration can give rise to the same difficulties as severe sudden stress.

No clear picture can be drawn of pain associated with postural faults. In some cases only acute symptoms may appear; some cases have an acute onset that progresses into chronic symptoms. Some cases exhibit chronic symptoms that exhibit acute phases, and others remain in a chronic condition. Regardless of the clinical picture, it must be kept in mind that no matter where the stimulus may arise, the sensation of pain is conducted only by those nerve fibers affected by the mechanical or chemical factors involved.

Two important factors must be considered in problems of faulty body mechanics -structural tension and nerve pressure or irritation:

1.   Pain may be slight or excruciating depending on the severity of tension within structures containing nerve endings sensitive to stress such as found in overstretching muscles, tendons, ligaments, or capsules, especially those of and adjacent to joints. Piriformis, gluteus muscle attachments to the iliac crests, tensor fascia lata, and the lumbago syndromes are examples of nerve irritation associated with abnormal muscle, fascia, and tendon tautness and stress.

2.   Pain may also result from nerve pressure or irritation, or pressure on nerve roots, trunks, branches, or endings from some adjacent structure such as bone, cartilage, fascia, scar tissue, taut muscles, or swelling from congestion, edema, or a mass. Examples of nerve root pressure pain are osseous encroachments of a subluxation or tunnel syndrome, facet syndrome, enlarged capsular ligament, or protruded intervertebral disc. In a root lesion, pain radiates to the periphery, is usually deep seated, and is directly related to muscle tension enhanced by any movement that would cause stretch or contraction of the involved muscle(s).

An injured person rarely resembles the textbook stereotype. No two people react in an identical manner to actual or potential loss of body balance. All vary somewhat in the accommodation process according to one's structural and functional needs, the momentary potential for redistributing body mass, and the visual efficiency necessary to guide correct adjustments. Isolated muscle weakness should be suspected especially in situations of head or pelvic tilt, trunk imbalance, scoliosis, and uneven gait or limp.


Etiology

Tolerance.   Poor weight-bearing because of disease, noxious reflexes, or just habit results in constant structural malalignment allowing a disproportionate amount of weight and muscle pull to be carried by some parts and not others. This alters the normal locomotion apparatus and functions of the internal organs as well. While these changes may develop insidiously, the resulting static abnormalities progress to pathologic changes in the body during standing, sitting, lying, and motion. They have a distinct effect on physical performance. They are tolerated for a short time, but sooner or later, serious, often subtle, maladjustments result when the body's resources for compensation become exhausted. These factors total to predispose an individual to injury or hinder performance.

Endurance.   An important factor in health care is that, with good postural body mechanics, balance is maintained with the least amount of muscular effort, thus encouraging longer endurance, with less strain on any one part. Locomotion can be made without wasted time or energy. Muscle pull in sustaining an erect carriage is more direct, thus avoiding strain. A natural balance is maintained between the iliopsoas group and the hip extensors, and a similar condition exists at the knee and ankle joints.

Effort.     Energy requirements vary considerably with different postures. The rigid "military" posture requires about 20% more energy than the relaxed standing posture. In the rigid posture, blood pressure rises because of the muscle effort required. A completely relaxed standing position requires little more energy than that required for the sitting position.

Regional Effects.     Postural faults can lead to a number of regional disorders. For example, a round-shouldered posture alters the glenohumeral articulating mechanism by depressing the overhanging acromion in front and rotating the dependent arm internally. Both of these conditions encourage cuff entrapment and attrition. Exaggerated cervical or lumbar lordosis decreases the size of the intervertebral foramina, frequently resulting in chronic radiculitis and degenerative changes. An exaggerated thoracic kyphosis decreases rib excursion and alters the functional motion of the shoulder girdle.