The Foundation of Biomechanical Evaluation Following Injury

Clinical Monograph 9

By R. C. Schafer, DC, PhD, FICC

INTRODUCTION

The study of human biomechanics includes the mechanical principles involved, the physiologic considerations of muscle length-tension relations, and an understanding of the controlling neuromotor mechanisms and the sensory feedback apparatus, reflecting both locomotor activity and cerebral function. Applied biomechanics is the application of the practical principles of mechanics (the study of forces and their effects) to the body in movement and at rest.

The more biomechanics are understood, the better musculoskeletal disorders in sports and the workplace can be appreciated. The same can be said of physical work and recreational activities. The athlete is constantly attempting to improve performance by applying biomechanical principles to specific movements. The same is true for ergonomics in the workplace. From the viewpoint of the doctor, knowledge of the mechanisms involved in an injury is necessary to evaluate an injury accurately.

PERTINENT BIOMECHANICS

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.

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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

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