How much force was applied? (magnitude)

 Where on the body was the force applied? (location)

 Where was the force directed? (direction)

 How long was the force applied? (duration)

 How often was the force applied? (frequency)

 Does the force application vary in magnitude? (variability)

 How quickly is the force applied? (rate)

REFERENCE TERMINOLOGY

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).

BASIC CONCEPTS

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 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.