Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content + A Download Section PDF Listen + Abduction Movement of a segment in the frontal plane away from the midline of the body. + Acceleration The rate at which velocity changes. + Adduction Movement of a segment in the frontal plane toward the midline of the body. + Afferent neuron Neuron that carries impulse toward the central nervous system; sensory neuron. + Agonist Muscle primarily responsible for motion; prime mover. + Angle of attack The angle between the long axis of a projected object and the direction of airflow. + Angle of incidence The angle at which an object strikes a hard surface. + Angle of projection The angle between the horizontal and the initial velocity vector of a projectile. + Angle of pull The angle between the mechanical axis of bone and the line of pull of a muscle. + Angle of reflection The angle at which an object will leave a hard surface after striking it. + Angular acceleration The rate at which angular velocity changes. + Angular displacement Any change in angle. + Angular impulse A torque applied over some period of time. + Angular momentum The quantity of angular motion a lever possesses, equal to the product of the moment of inertia and the angular velocity. + Angular velocity The rate at which angular displacement takes place. + Anteroposterior (AP) axis The axis that passes horizontally from front to back, perpendicular to the frontal plane. + Archimedesâ principle Law governing buoyancy, which states that a body immersed in water is buoyed up by a force equal to the weight of the fluid displaced by the body. + B Download Section PDF Listen + Backward tilt Pelvic motion in which the posterior surface moves somewhat backward and downward. + Base of support Any part of the body in contact with the supporting surface and the intervening area. + Bernoulliâs principle Law governing fluid forces, which states that when flow velocity is high, fluid pressure is low, and when flow velocity is low, fluid pressure is high. + Bilateral axis The axis that passes horizontally from side to side, perpendicular to the sagittal plane. + Biomechanics The study of the mechanics of biological systems. + Boundary layer A layer of fluid that is immediately adjacent to the surface of an object. + Buoyancy An upward force that acts to support a body immersed in water. + C Download Section PDF Listen + Cardinal plane A primary plane, one that passes through the center of gravity. + Center of buoyancy That point in the body at which the upward force of buoyancy acts. + Center of gravity The âbalance pointâ of the body; the center of mass; the intersection of the three cardinal planes. + Centrifugal force The âcenter-fleeingâ force acting on an object undergoing circular motion; a reaction force to centripetal force. + Centripetal force The âcenter-seekingâ force constraining an object to a circular path. + Coefficient of elasticity A number that represents the ability of a material to resist deformation and to return to its original state. + Coefficient of friction A number that represents the resistance to rolling or sliding motion that exists between any two surfaces. + Compression Force that acts to press or compact. + Concentric muscle contraction Contraction in which the muscle fibers shorten. + Concurrent forces Forces acting at the same time and point of application but at different angles. + Concurrent muscle action Action in which a biarticular muscle loses tension at one attachment while increasing tension at the other, producing no net change in muscle tension. + Conservation of angular momentum In the absence of any angular impulse, angular momentum remains constant. + Conservation of momentum In the absence of an impulse, momentum remains constant. + Countercurrent muscle action One biarticular muscle shortens at both joints while the antagonist lengthens at both joints. + Curvilinear motion Motion that follows a curved path. + D Download Section PDF Listen + Deceleration The rate at which velocity decreases. + Density The amount of mass per unit of volume. + Diagonal planes Infinite number of planes of motion lying on the diagonal between the cardinal planes. + Displacement A vector quantity that reflects any change in position. + Distance A scalar quantity that reflects the amount of space moved. + Drag A fluid force that acts to resist motion. + Dynamics The study of objects or systems subject to acceleration. + E Download Section PDF Listen + Eccentric contraction A lengthening muscle contraction. + Eccentric force An âoff-centerâ force, one that is not in line with the center of gravity or the axis and therefore tends to cause rotation. + Efferent neurons Neurons that carry impulses away from the central nervous system; motor neurons. + Effort arm (EA) The perpendicular distance between the applied effort force and the axis of a lever. + Elasticity The ability to resist deformation and to return to the original shape. + Energy The ability to do work, based on either position or motion. + Equilibrium A state of balance. + Extension The return from flexion. + F Download Section PDF Listen + First-class lever One in which the effort and the resistance are on opposite sides of the axis. + Flexion A reduction in joint angle. + Force A push or a pull. + Force couple Parallel forces on either side of the axis acting in opposite directions. + Form drag Fluid resistance to forward motion based on the cross-sectional area of a body. + Forward tilt Movement of the pelvic girdle in the sagittal plane so that the symphysis pubis moves forward and downward. + Free-body diagram A schematic representation of all the forces and levers that make up a defined system. + Friction A resistance to rolling or sliding, based on the nature of the two interacting surfaces. + Frontal plane Vertical plane passing through the body from side to side, dividing it into anterior and posterior halves. + Fulcrum An axis; the point about which rotation occurs. + G Download Section PDF Listen + Gravity An acceleration toward the center of the earth. + Ground reaction force Equal and opposite reaction produced by the supporting surface. + H Download Section PDF Listen + Horizontal abduction Movement of a segment in the transverse plane away from the midline of the body. + Horizontal adduction Movement of a segment in the transverse plane toward the midline of the body. + I Download Section PDF Listen + Impulse The product of a force and the amount of time over which it is applied. + Inertia A resistance to motion based on the mass of an object. + K Download Section PDF Listen + Kinematics The âmeasurement of motionâ; those variables that describe motion with respect to time. + Kinetic energy Energy based on motion; a product of mass and velocity. + Kinetics The study of the forces that act to produce motion. + L Download Section PDF Listen + Laminar flow Smooth, unbroken fluid flow. + Law of reaction The law of motion that states that for every action there will be an equal and opposite reaction. + Lever A rigid bar that is fixed at a single point, about which it may be made to rotate. + Lift A fluid force that acts in response to the pressure differential produced by differing flow velocities around an aerodynamic shape; acts perpendicular to fluid flow. + Line of gravity A line that extends from the center of gravity of an object toward the center of the earth. + Linear forces Forces applied in the same direction and along the same line of action. + Linear motion Motion in a line, in which all parts of the object move in the same direction and at the same speed. + M Download Section PDF Listen + Magnus effect An explanation of the lift forces that cause a spinning object to curve in the direction of the spin. + Mass The quantity of matter an object contains. + Mechanical advantage The ratio of effort arm to resistance arm, or the ratio of resistance to effort in a lever. + Mechanical axis of bone A line drawn from the proximal joint center to distal joint center. + Moment arm The perpendicular distance from any force to an axis in a lever system. + Moment of inertia The quantity of a rotating mass and its distribution around the axis of rotation. + Momentum The amount of motion an object possesses; the product of mass and velocity. + Motor unit A single motor neuron and all the muscle fibers it innervates. + P Download Section PDF Listen + Parallel forces Forces that act parallel to each other. + Potential energy Energy based on the position of an object, usually its height from the surface. + Power The rate at which work is done. + Principle of levers For a lever to be in equilibrium, the clockwise torques must equal the counterclockwise torques; E x EA = R x RA. + Projectile An object that is given some initial velocity and then is released. + R Download Section PDF Listen + Radian A unit of angular measure, 57.3Â°. + Reciprocating motion Repetitive motion. + Relative motion Motion with respect to some reference object. + Resistance arm (RA) Perpendicular distance between the resistance force vector and the axis of a lever. + Resultant The combined effect of two or more vector quantities. + Rotary component That vector component of force which acts perpendicular to a lever. + S Download Section PDF Listen + Sagittal plane Vertical plane passing through the body from front to back, dividing it into right and left halves. + Scalar A single quantity, measuring only amount. + Second-class lever One in which the resistance is closer to the axis than is the effort. + Specific gravity Ratio between the density of an object and that of water. + Speed Scalar quantity quantifying the rate of motion. + Stability The ability to remain in or return to a state of equilibrium. + Stabilizing component That vector component of force which acts parallel to a lever. + Statics The study of the mechanics of systems or objects in equilibrium. + Strain The amount of deformation experienced by an object when stressed. + Stress The force applied per unit of area. + Surface drag Resistance to forward motion produced by the interaction of a fluid with a surface. + T Download Section PDF Listen + Tension A form of stress that involves pulling forces. + Third-class lever One in which the effort is closer to the axis than is the resistance. + Torque The tendency for rotation; turning effect. + Torsion A form of stress that involves twisting. + Transverse plane Horizontal plane that passes through the body, dividing it into upper and lower halves. + Turbulence Chaotic fluid motion at the trailing edge of a moving object. + V Download Section PDF Listen + Vector A double quantity; one that specifies both magnitude and direction. + Velocity A vector quantity describing the rate of displacement. + Vertical axis The axis that is perpendicular to the ground. + W Download Section PDF Listen + Weight The product of the mass of an object and the acceleration that is due to gravity. + Work The product of a force and the distance over which that force produces motion.