Biomechanical Basis for Movement

Published on 16/03/2015 by admin

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Biomechanical Basis for Movement

Mitchell A. Collins and Michael Hales

Biomechanics is the study of the internal and external forces acting on the body and the effects these forces produce. The data obtained from biomechanical research provide a great deal of insight into human movement interactions in fields such as physical and occupational therapy, sports medicine, human factors, prosthetics, orthotics, and ergonomics.12,31 Clinical biomechanics, a subdiscipline, provides direct measures of human motion which influence our knowledge of injury mechanics, rehabilitation, treatment, and prevention. This information can directly affect how a physical therapist assistant (PTA) rehabilitates a patient, an orthopedist repairs a broken limb or ruptured ligament, an athletic trainer implements modalities for treatment, or a clinician evaluates an individual’s gait.19,21,24

The field of biomechanics combines the study of applied human anatomy with that of mechanical physics. These combined sciences allow for detailed descriptions of how and why the human body moves the way it does and why a person may or may not have sustained an injury. Understanding the neuromuscular and mechanical factors associated with human movement provides the PTA with the knowledge and skills necessary for administering rehabilitative techniques correctly and performing patient assistive lifting tasks safely. These biomechanical descriptions influence health professionals to refine their knowledge and, therefore, their approach to injury rehabilitation as well as to consider new and innovative techniques that may lead to improved rehabilitation processes. This information also provides insight into the mechanical causes of injuries, potentially leading to safer participation as individuals interact with the environment.

Biomechanics can provide the health professional with a better understanding and a broadened knowledge of the causes and degrees of severity associated with an injury. In order to improve upon a current rehabilitative technique, repair procedure or treatment process, the mechanics associated with the causation of the injury must be fully understood. To help the health professional accomplish this task, several factors need consideration when determining the cause and severity of an injury36:

The major role of a PTA revolves around both the causes and effects of human motion and therefore it is imperative to have a fundamental understanding of the mechanical basis for whole body or segmental movement. Since biomechanics is concerned with the effects of forces on the human body, biomechanical principles are involved when a force is present. The following chapter will offer an overview of the science of biomechanics with practical applications for the PTA.


To facilitate the process of describing human movement, standardized terminology has been adopted to identify body positions and directions of motion (Box 16-1). Movements are defined based on a reference or starting position often referred to as the anatomic position (Fig. 16-1, A), which is a standing position with arms at one’s side and palms facing forward. From the anatomic position, three imaginary cardinal planes bisect the body along three dimensions (Fig. 16-1, B). The transverse or horizontal plane segments the body into upper and lower parts; the frontal or coronal plane separates the body into front and back parts; and the sagittal or anteroposterior plane divides the body into right and left parts. It is important to note that the planes do not necessarily divide the body into equal parts. However, when the segments are equal, the mid-intersection point of the transverse, frontal, and sagittal planes is referred to as the center of mass (COM) or center of gravity (COG). Although human movement is not restricted to a single plane, most named movements (e.g., flexion and abduction) are described based on the three cardinal planes.

Movements in the transverse plane occur around the longitudinal axis, which runs superiorly–inferiorly while perpendicularly intersecting the transverse plane. These movements include medial and lateral rotation of the leg, thigh, and shoulder; supination and pronation of the forearm; and horizontal abduction and adduction of the shoulder. Movements in the frontal plane occur around the sagittal axis, which runs anteriorly–posteriorly while perpendicularly intersecting the frontal plane. These movements include abduction and adduction of the shoulder and hip, lateral flexion of the neck and trunk, elevation and depression of the shoulder girdle, and inversion and eversion of the foot. Movements in the sagittal plane occur around the frontal axis, which runs from left to right while perpendicularly intersecting the sagittal plane. These movements include flexion and extension of the knee, hip, trunk, elbow, shoulder, and neck; and dorsiflexion and plantar flexion of the foot as well as hyperextension movements. A key role of the PTA is to facilitate patient rehabilitation through the incorporation of various exercises using basic movement patterns. Therefore it is important to be familiar with the appropriate terminology for these movements (Fig. 16-2).


To facilitate the discussion of biomechanical principles, a working understanding of various rudimentary concepts is essential. The following are definitions of some common terms along with their appropriate unit of measure. Most of these terms will be discussed in more detail as various biomechanical concepts are introduced.

Mass (m) is the amount of matter an object possesses within its physical boundaries; generally, the denser the material that comprises the object, the greater the mass. For example, muscle tissue is denser than fat tissue; therefore, two persons of equal size or volume may differ in mass if one is more muscular than the other.

Inertia is the resistance an object offers to a change in its state of motion (velocity) or direction of motion and is directly related to its mass. The greater the mass of the object, the more resistance it offers to any attempt at changing its velocity or direction of motion.

Force (F) is a push or pull acting on an object. A force will have both direction and magnitude, and it is commonly expressed in newtons (N). Forces applied to objects, if sufficient to overcome their inertia, will cause them to accelerate in direct proportion to the magnitude of the force.

Friction is created when two objects are in direct contact with one another and a force acts to impede motion of the objects. Frictional force can be increased or decreased by adding substances between the two surfaces, such as the installation of tennis balls on the rear support for walkers. Joint damage (osteoarthritis) caused by chronic exposure to high frictional forces can lead to arthroplasty.

COM or COG is the point within which the weight and mass of an object is equally distributed or balanced in all directions. The COM is important because when a force is applied to an object, the movement pattern will vary based on the relation of the point of force application to the COM.

Kinetic energy (KE) is energy by virtue of an object’s motion. The units for KE are typically expressed as a joule (J), however, one may also see units of newton-meters (N-m), which is an equivalent unit (i.e., 1 N-m = 1 J). An injury mechanism is predominantly due to the transfer of KE to the body arising from different sources under a variety of conditions: from blunt trauma (impact of object’s colliding with the body), penetrating trauma, acceleration/deceleration motion (rapidly moving forward and backward), and crushing weight (high compression forces).

Potential energy is energy generated by virtue of the position or shape of an object. Potential energy may be affected by how much elastic energy is generated by either stretching or compressing the object (e.g., cartilage, tendon, connective tissue) such that, if the distorting force is removed, the object will recoil to its resting length. The units for potential energy are typically expressed as a J; however, one may also use units of N-m.

Torque (T) is the product of the force and the perpendicular distance from the line of action to the axis point, also called lever arm length. Torque is considered a rotary force, but more specifically a measure of the ability of a force to cause rotation. Consequently, torque can be increased or decreased easily by altering the length of the moment arm of the force. A force couple is formed in situations where there are two torques that are equal in magnitude but opposite in direction. The resultant action of a force couple is rotation without any translation. Torque is typically expressed in units of N-m.

Work (W) is the product of force and the distance the object moves. If no displacement of the object occurs, even though force may have been applied to the object, no work was done. The unit of measure is a N-m or a J.

Power (P) is the rate of performing work, and can be expressed algebraically as the product of force and displacement over time. If work is accomplished very quickly (i.e., in a very short amount of time) then a higher magnitude of power is generated as compared to the same amount of work being done over a longer interval of time. Given this explanation, power is work divided by time. Power is expressed in watts (w), and 1 w is equal to 1 J of work per second.

Pressure (p) is a measure of the distribution of a force over a given area (force/area), and it is expressed in newtons per meter2 (N/m2). An example of the concept of pressure in a rehabilitation setting is the development of decubitus ulcers that commonly occur among diabetics.3,32,38 Innovations in shoe design help dissipate the forces applied to the foot over a larger area (e.g., reduced pressure) during locomotion, thus minimizing soft-tissue damage and the incidence of ulceration.6,10,35

Momentum is the product of mass and velocity used to determine the outcome of collisions between two objects of mass as well as to determine the ease with which one can stop or change the direction of travel when velocity is present. A motionless object has no momentum. The unit of measure is kilogram × meters/second (kg × m/sec).

Impulse is the product of the force magnitude and the force application time interval expressed in newton × seconds. The direct relationship between an applied force and the change in momentum it creates is known as the impulse-momentum principle. Consider a high force applied to the musculoskeletal system over a very short duration, as is often the case in force related injuries.


Statics and Dynamics

Statics is the branch of mechanics concerned with the analysis of loads (force, torque/moment) on physical systems in static equilibrium. In biomechanics, statics is the study of the body under conditions where no accelerations or velocity changes are occurring. When acceleration of the body occurs, as is required if a person is to change positions, static conditions would no longer be present. Static conditions are common when considering the immobilization of a joint or when an individual is in traction to immobilize a body segment. Quite often, a PTA will incorporate proprioceptive neuromuscular facilitation (PNF) stretching exercises to help a patient regain normal range of motion (ROM) to a joint postinjury. During PNF stretching a static (isometric) contraction is performed by a patient and health practitioner.

Dynamics is the study of a body segment experiencing accelerations. As a result, body segments are increasing and decreasing in velocity as a particular skill is performed. This requires varying levels of force to produce these accelerations. Depending upon the intensity of the exercise, these forces and accelerations may range from very small to very large in magnitude. The legs in walking and running, and the arms in wheelchair propulsion are examples of dynamic segments used in performing human activities. A quantified movement analysis can clarify which muscles should be active during a posture or movement in the context of several external forces acting on the body.

Linear and Angular Motion

Linear motion is the point-to-point, straight-line movement of a body in space. The motion is generally measured in either a two-dimensional or three-dimensional system depending on the complexity of the activity being monitored. These measures are made in the geometric planes established by the Cartesian coordinate system, which are oriented to the human body (local) and/or Earth (global) such that anteroposterior, vertical, and mediolateral measures of motion are described linearly. Forces applied by or on the body in these respective directions lead to accelerations or velocity changes of these body points. Linear forces may be applied by muscles, gravity, the ground, or any number of other animate or inanimate objects.

Angular motion is the measurement of rotation about an axis of a rigid lever and is quantified through the use of a polar coordinate system. This is generally represented in the human body by body segments; an example is the upper arm rotating about a joint (axis of rotation) such as the shoulder. By tracking over time how the lever, as established by its endpoints (for the upper arm these would be represented by a line connecting the shoulder and elbow), rotates around its proximal joint (the shoulder), one can determine angular positional changes of the lever, rotational velocities of the lever about the joint, and increases/decreases in rotational velocity. These angular measures describe the quality of motion generated angularly by an individual performing an activity of daily living (ADL), occupational activity, or exercise. This is important because there is a direct link between the quality of angular motion of body segments or levers and the potential for overuse injuries to the subsequent joints. For example, the faster the forearm/racket combination is rotating or extending about the elbow at the instant before impact in a tennis serve, the faster the racket is moving linearly at the moment of impact with the ball. The faster the racket head is moving linearly at impact, the greater the momentum or force imparted to the tennis ball by the athlete. Conversely, because of the high action force and moment, the greater the reaction force and moment imposed on the joints, which could lead to inflammation of the lateral epicondyle (tennis elbow) if poor service mechanics are demonstrated.22 The rotation of the forearm about the elbow is due in part to the contraction of the muscles (triceps group), which causes acceleration in the direction of elbow extension. The linear force of the triceps tendon pulling on the ulna generates a torque or rotational force. The greater the torque produced by the muscles, the greater the angular acceleration generated, leading to changes in angular velocity that lead to changes in angular position.

Kinematics and Kinetics

Kinematics is the description of human motion in terms of position, velocity, and acceleration. These three variables describe the quality of the motion resulting from forces produced by the muscular system or forces external to the body, such as gravity, other persons, and inanimate objects (ground, implements, and so on). However, the study of kinematics is not concerned with force measurements, therefore the magnitude or type of force responsible for generating these human motions is disregarded.

An understanding of kinematic principles is extremely important to PTAs. Analysis of motion can facilitate the determination of the etiology of injury, extent of damage, and assessment of the effectiveness of treatment. Historically, researchers have studied injuries in sport settings, but the same applications are pertinent to nonathletic settings, such as gait analysis of individuals with lower extremity joint injury or joint replacement.7,9,11,16,27,28 Thus it is imperative for PTAs to be knowledgeable about normal movement patterns from a kinematic perspective to facilitate the recognition of abnormal movements of rehabilitating patients.

Kinetics is concerned with the forces responsible for maintaining equilibrium and the sources of motion generating the kinematic qualities described earlier. The various forces applied on or by a system can be quantified to determine why a body sustains an injury. This information can lead to very detailed analysis of movement mechanics and injury potential, because these forces lead directly to accelerations that cause increases and/or decreases in velocity, which in turn lead to changes in body position over time.