The clinical application of biomechanics helps chiropractors in the diagnosis and treatment of their patients

Why does one person show signs of degenerative changes at an early age, while another demonstrates minimal change at an older age? The answer may be due in part to how body tissues absorb the forces placed on them and the body’s reaction. Normal bone alignment, adequate muscle length and tone, and the normal joint function in motion and at rest are important to understand and successfully treat the circumstances mentioned above.

Human biomechanics is defined as the study of motion and the effect forces have on those motions. The applicability and practicality of biomechanical concepts in clinical practice help doctors in specifying diagnosis and treatment for their patients.

Poetry in Motion
First, there needs to be agreement on the definition of terms commonly used in the study of biomechanics and an understanding of normal structure function and associated biomechanical inter-relationships as these relate to human tissue, which includes forces involved with injury and healing. Once biomechanical defects and the consequences of excessive wear and tear on the human body are identified, correction and treatment become instinctive.

The three cardinal planes of motion will be the reference system that defines motion. Each plane is perpendicular to the other two and divides the body into halves. The flat, vertical frontal (coronal) plane that divides the body into anterior and posterior halves. The sagital plane is a flat, vertical plane, and divides the body into right and left halves. The transverse plane is flat and horizontal dividing the body into upper and lower halves. All three planes pass through the body’s center of gravity located anterior to the S2 tubercle of the sacrum.¹ Motion in any of these planes means that a body segment is moving through a path parallel to one of the three cardinal planes.

Motion occurring within this reference system is called kinematics, of which there are three types:

1) translatory (linear)—occurs when all parts of the body undergo an equal linear displacement during a specified time. For example, this is the motion of the tibia during the anterior drawer test of the knee used to identify the integrity of the anterior cruciate ligament. The tibia is grasped and translated forward on the femur as anterior displacement of it is evaluated. Translation of a joint is an important feature in assessing joint stress and tissue stability.

2) rotatory (angular)—when the parts of a body move in a circular path around the same center, or axis of rotation. Rotatory motion is a primary function of the musculoskeletal system.

3) curvilinear—produced when rotatory and translatory motions combine. As a bony segment is rotated on its own axis and translated forward by rotation at another joint axis. For example, during the motion of bringing a cup to the mouth, the shoulder is translating the hand and forearm forward, while the elbow is rotating the segment. The result is a curved path of motion toward the mouth. Curvilinear motion is probably the most common form of motion obtainable by a joint.²

Kinetics is the study of forces that act on a rigid body and result in translational, rotational, or curvilinear motion. Common forces acting on the body are muscle contraction, gravity, and the force imparted by the doctor or therapist. When force is applied to a rigid body, it produces translation, rotation, or a combination of both. When a force creates rotation of a body about an axis, the result is torque, T, or moment—a product of the magnitude of the applied force and the distance that force lies from the axis of rotation.

When a muscle contracts, a linear tensile force is generated, which acts on the osseous components of the joint, producing the resultant rotary movement, which generates torque about the joint axis. The position of the muscle makes a difference in the amount of force it can generate. Muscles having greater distance to the axis will consequently have a larger lever arm producing greater amounts of torque, thereby making it a more efficient lever.

Get a Handle On
There are three classes of levers, which are defined as rigid bars that rotate about an axis. A first-class lever, commonly recognized as a seesaw, rarely occurs in the body. When it does occur, this lever is used to maintain balance or posture. For example, this is the condition of the intervertebral joints while sitting or standing, where the weight of the trunk is balanced by the erector spinae muscle forces acting on the vertebral axis.

A second-class lever provides a force advantage such that a large weight can be supported or moved by a smaller force, such as the wheelbarrow or crowbar. The only muscular example in the body is the pull of the brachioradialis and the wrist extensors to maintain elbow flexion. However, when anything is held in the hand, the system changes to a third-class lever, where the brachioradialis and wrist extensor muscle forces cannot maintain the body position.

The third-class lever system is more common. This lever is designed for speed of motion of the distal segment and for moving a small weight a long distance.³ The small amount of shortening of muscles, such as the brachialis, causes a large arc of motion of the hand. This leverage is found in most open chain motions of the extremities: a) the deltoid acting on the glenohumeral joint; b) the flexor digitorum profundus and superficialis at the interphalangeal joint; c) the biceps-brachialis at the elbow; d) the extensor carpi radialis at the wrist; and e) the tibialis anterior at the ankle joint. All three types of levers demonstrate that what is gained in speed or distance is lost in force and, conversely, what is gained in force is lost in speed.

The classification of levers has relevance to function and the way the body and its function are structured. Mechanical advantage refers to the efficiency of a lever, which occurs only when a small effort is needed to overcome a large amount of resistance.This is why it is easier to perform a biceps curl with the elbow bent at 90° than with a straight elbow. Also, this is why the tibialis anterior generates less tensile force than the flexor hallucis longus, but is able to produce greater torque about the subtalar joint axis due to its larger lever.

Rule of Thumb
Knowledge of the concave-convex rule can help in evaluating joints for normal biomechanics, deciding which direction to adjust, and determining the direction of the adjustment. The concave-convex rule states when a convex surface moves on a concave surface, the arthrokinematic motion of roll and slide occurs in opposite directions. When a concave joint surface moves on a convex surface then roll and slide occurs in the same direction. Impingement syndrome is a common problem of the shoulder girdle.⁴

Remember to examine the glenohumeral joint for its arthrokinematics. Have the patient sit with the arm abducted to 90° and relax the deltoid muscle while gently moving the glenohumeral joint inferiorly. If the joint does not glide inferiorly, then a gentle impulse directed over the glenohumeral joint will restore the normal motion, decrease the impingement, and restore the normal motion.

Consider a joint limited to 60° of flexion. Assume that a passive motion is manually applied to the bone containing the concave (female) surface. If a force is applied on the distal aspect of this segment and the joint motion is firmly restricted, the effect can be a levering of the female surface in the direction opposite to that desired in normal arthrokinematics. Under these conditions, the segment can only move in the same direction as the required rolling. This situation is preferred because normal arthrokinematics are produced by the therapeutic effort. This concept is necessary to maintain when stretching a joint in a person who may be osteopenic or osteoporotic. Proper application of force could allow for therapeutic stretching while decreasing the chances of fracturing the joint with excessive force.

Anatomic asymmetries must be examined and ultimately corrected over the course of therapeutic intervention. The body has evolved in an organized pattern of form determining function. If joints are allowed to function the way they were created, there would be fewer arthritic and degenerative conditions. On the other hand, if the joints and tissues are stressed beyond their adaptive capabilities and made to adapt to excessive loads and forces, the tissues break down. The body does not permit this destructive process and resorts to its many adaptation mechanisms by changing muscle length, causing osseous adaptations, and compensating to maintain homeostasis.

Work the Q-Angle
The vertebral column can increase its thoracic kyphosis in response to an increase to lumbar lordosis. Osteophytes might develop over a degenerative segment of the spine in an attempt to provide stability at that level. These methods help the body adapt to excessive and abnormal loads, but eventually, as the abnormal loads continue and stress persists, internal stabilization methods fail with resultant degenerative tissue damage, disability, pain, and altered function. For example, there could be asymmetry caused by an increase in the quadriceps or Q-angle. The Q-angle is formed by the intersection of lines from the anterior superior iliac spine and the tibial tubercle as it passes through the mediolateral superior-inferior bisection of the patella. This angle influences direction of the quadricep pull and affects the tracking mechanism of the patella. The angle is measured with the patient in the functional position of standing. If the Q-angle increases, patients are at risk for developing anterior knee pain. As the patella is repeatedly pulled laterally over the lateral femoral condyle, the posterior aspect of the patella begins to deteriorate from the excessive friction. Over time, this friction causes irritation, swelling, and anterior knee pain.

The patient often has difficulty walking stairs and feels knee stiffness after sitting. As the friction continues over time, the synovium begins to wear down, and degenerative changes take place. These degenerative changes can be seen on x-rays and lead to the diagnosis of chondromalacia patella. The question is how can we prevent the Q-angle from increasing in the beginning? The cause of an increased Q-angle is often related to an increase in body weight causing a collapsed posture, muscle weakness, and pronation of the feet, which causes rotation of the tibia and an increase in genu valgus. The patient often develops a short leg on the pronated foot side resulting in pelvic unleveling. Naturally, the position of the fifth lumbar vertebrae is altered, leading to stress of the disc and dysfunction at that level. The changes continue up the kinetic chain, as does the body’s attempt to compensate for these alterations.

Weighing the Imbalance
The effect on the body of muscle imbalance has been receiving much attention from chiropractors lately. It is well known that tight hip flexor muscles are accompanied by tight low back muscles with weak, stretched abdominal and hamstring muscles. This pattern is considered a contributing factor in the etiology of back pain. The resulting hyperlordosis is a factor in the development of many low back syndromes. There are a number of ways to assess muscle shortening using orthopedic tests like the Thomas test, which assesses the length of the hip flexors, or the sit-and-reach test to measure the length of the gastroc-soleus group, hamstrings, and low back extensor muscles. Goniometric, inclinometer, and tape measurements are commonly used to assess muscle length and joint range of motion.

Overtight muscles also cause excessive stress at the various joints. For example, tight hamstrings contribute to anterior knee pain causing compression of the patella on the femur. Short leg syndrome due to foot pronation causes imbalances of the hip adductors and abductors as compensation for the leg length imbalance. Over time, these changes in muscle length can become difficult, if not impossible, to correct with muscle lengthening therapies. Often, these require aggressive rehabilitation and chiropractic therapy for remediation. Occasionally, when degeneration has occurred untreated over a long enough period of time and conservative therapies fail, surgical intervention becomes necessary.

Shinsplints are another example of altered biomechanics. The etiology is a posterior tibial tendonitis secondary to abnormal subtalar joint pronation. Excessive pronation overuses the tibialis posterior muscle as it attempts to support the medial longitudinal arch. Treatment includes rest to allow for healing, ice and modalities to decrease inflammation, manipulation of the foot and ankle, stretching of the calf muscles, gradual resistive exercises to the anterior muscle group if weakness is noted, and functional orthotics to prevent excessive pronation of the subtalar joint.¹ CP

Manuel A. Duarte, DC, MS, DABCO, DACBSP, CSCS is professor, senior staff clinician, and clinic director at National University of Health Sciences, Lombard, Ill. He is also a postgraduate faculty member and lectures on manual treatment procedures, rehabilitation, orthopedics, and sports medicine. Duarte can be reached via email: mduarte@nuhs.edu.

References
1. Hunt CG. Clinics in Physical Therapy: Physical Therapy of the Foot and Ankle. New York: Churchill Livingstone; 1995.
2. Norkin C. Joint Structure and Function. Philadelphia: FA Davis; 1983.
3. Lehmkuhl LD, Smith LK. Brunnstrom’s Clinical Kinesiology. Philadelphia: FA Davis; 1983.
4. Soderberg LG. Kinesiology, Application to Pathological Motion. Baltimore: Williams and Wilkins; 1986.