Acceleration--the rate of change of velocity (vector).

Accelerometer--a device that measures acceleration.

Accommodation--the decrease in biological response to an unchanging stimulus.

Angular acceleration--the rate of change of angular velocity (vector).

Angular displacement--the change in angular position (vector).

Angular momentum--the quantity of angular motion, calculated as the product of the moment of inertia times the angular velocity (vector).

Angular velocity--the rate of change of angular displacement (vector).

Angular impulse--the angular effect of a torque acting over time, the product of the torque and the time it acts (vector).

Anisotrophic--a body with different mechanical properties for loads in different directions.

Anthropometrics--the study of the physical properties of the human body.

Ballistic--fast, momentum assisted movement.

Bernoulli's principle--the pressure a fluid can exert decreases as the velocity of the fluid increases.

Center of mass/gravity--the point that represent the total weight/mass distribution of a body. The mass centriod is the point where the mass of the object is balanced in all directions.

Center of pressure--the location of the vertical ground reaction force vector. The center of pressure measured by a force platform represents the net forces in support and the COP may reside in regions of low local pressure.

Common mode rejection--a measure of the quality of a differential amplifier in rejecting common signals (noise).

Compression--a squeezing mechanical loading created by forces in opposite directions acting along a longitudinal axis.

Compliance--the ratio of change in length to change in applied force, or the inverse of stiffness(see stiffness). A material that is easily deformed has high compliance.

Concentric muscle action-- the condition were an activated muscle(s) creates a torque greater than the resistance torque (miometric).

Creep--the increase in length (strain) over time as a material is constantly loaded.

Degrees of freedom--the number of independent movements an object may make, and consequently the number of measurements necessary to document the kinematics of the object.

Direct dynamics--biomechanical simulation technique where kinematics of a biomechanical model are iteratively calculated from muscle activation (kinetic) inputs.

Direct linear transformation (DLT)--a short-range photogrammetry technique to create 3D coordinates (x, y, z) from the 2D coordinates (x, y) of two or more camera views.

Displacement--change in position in a particular direction (vector).

Double differential amplification--EMG technique to eliminate cross-talk.

Dynamics--the branch of mechanics studying the motion of bodies under acceleration.

Deccentric muscle action--the condition were an activated muscle(s) creates a torque less than the resistance torque (pliometric).

Elastic--the resistance of a body to deformation (see stiffness).

Elastic (strain) energy--the potential mechanical work that can be recovered from the restitution of an body that has been deformed by a force (see hysteresis).

Electromyography (EMG)--the amplification and recording of the electrical signal of active muscle.

Energy (mechanical)--the ability to do mechanical work (potential, strain, or kinetic energy are all scalar mechanical energies).

Excursion--the change in the length of a muscle as the joints are moved through their full range of motion.

External work--work done on a body by an external force.
Finite element model--advanced biomechanical model to study how forces act within a deformable body.

Firing rate--the number of times a motor unit is activated a second.

Force--an instantaneous push, pull or tendency to distort between two to bodies.

Force-length relationship--skeletal muscle mechanical property that shows how muscle force potential depends on muscle length.

Force platform--a complex force transducer that measures all three orthogonal forces and moments applied to the surface.

Force-time relationship--(see elecromechanical delay).

Force-velocity relationship--skeletal muscle mechanical property that shows how muscle force potential depends on muscle velocity.

Fourier series--a mathematical technique of summing sin and cos terms that can be used to represent the frequency content of a signal.

Frame (video)-- a complete video image.

Free body diagram--a technique for studying mechanics by creating a diagram that isolates the forces acting on a body.

Frequency domain/content--time varying signals can be modeled as sums of weighted (see Fourier series).

Frequency response--the range of frequencies faithfully reproduced by an instrument.

Global reference frame--measuring kinematics relative to an unmoving point on the earth.

Goniometer--a device to measuring angular position.

High pass filter--a signal processing technique that removes low frequency components of a signal.

Hill muscle model--a common three component model of muscle force consisting of a contractile component, series elastic component, and parallel elastic component.

Hysteresis--the energy loss in the elastic recoil of a material.

Impulse--the mechanical effect of a force acting over time (vector). J = F?t

Impulse-momentum relationship--principle that the change in momentum of an object is equal to the net impulse applied. Original language of Newton's second law and is equivalent to the instantaneous version: F = ma.

Integrated EMG (IEMG)--the area under a rectified EMG signal. Correctly the time integral reported in units amplitude?time (mV?s). Unfortunately, old EMG equipment and some report IEMG that is not really integrated, but filtered or smoothed EMG values (mV) that is essentially a liner envelope detector.

Internal work--work done on body segments by internal forces (muscles, ligaments, bones).

In situ--Latin for "in place" or structures isolated by dissection.

In vitro--Latin for "in glass" or tissues removed from the body but preserved.

In vivo--Latin for "in the living" or during natural movement.

Isometric muscle action--the condition were an activated muscle(s) create a torque equal to the resistance torque.

Jerk--the third derivative of displacement with respect to time.

Joint center--an approximation of the instantaneous center of rotation of a joint.

Joule--the unit of mechanical energy and work.

Kinematics--the measurement of the motion of an object relative to some frame of reference.

Kinematic chain--a linkage of rigid bodies. An engineering term used to simplify the degrees of freedom needed to document the mechanical behavior of the system.

Load-- a force or moment applied to a material.

Load cell--a force measuring device.

Load-deformation curve--the mechanical behavior of a material can be documented by the instantaneous measurement of the deformation and load applied to a material.

Local reference frame-- measuring kinematics relative to an moving point on nearby rigid body (joint, segment, or center of mass).

Low pass filter--a signal processing technique that removes high frequency components of a signal.

Markers--high-contrast reflective materials attached to subjects to facilitate the location of segments or joint centers for digitizing.

Moment (moment of force, torque)--the rotary effect of a force.

Moment arm--the leverage of a force for creating a moment. The perpendicular distance from the axis of rotation to the line of action of the force.

Moment of inertia--the resistance to rotation (angular acceleration) of a body.

Momentum--the quantity of motion of an object calculated by the produce of mass and velocity (vector).

Motor unit--a motor-neuron and the muscle fibres it innervates.

Newton--the SI unit of force. One Newton is equal to 0.22 pounds.

Pascal--the SI unit of pressure or stress (force per unit area).

Pennation--the angle of the muscle fibre bundles relative to the tendon.

Potentiometer--a device for measuring rotation.

Power (mechanical)--the rate of doing mechanical work. Peak mechanical power represents the greatest mechanical effect, the ideal combination of force and velocity.

Power can be calculated as W/t or F?V.

Radius of gyration--a convenient way to summarize an object's moment of inertia, defined as the distance from the axis of rotation half the object's mass would have to be to equal the object's moment of inertia.

Recruitment--the activation of motor units of muscles by the central nervous system.

Resonance--the frequency of vibration that matches the physical properties of the body so the amplitudes of the vibration grow rather than decay over time.

Scaling--converting image measurements to actual size.

Scalar--simple quantity completely defined by a single number (magnitude).

Shear--mechanical loading in opposite directions and at right angles to the surface of a material.

Shutter speed--limiting the time that a photographic/video image is made (e.g. 1/1000 of a second) to prevent blurring of moving objects.

Simulation--use of a biomechanical model to predict motion given input conditions in order to study the factors that affect motion (see direct dynamics).

Smoothing parameter--a index of the amount of smoothing allowed in splines. The larger the smoothing parameter the more smoothing (allowable deviation between the raw and fitted curve).

Snap--the fourth derivative of displacement with respect to time.

Statics--the branch of mechanics studying bodies at rest or uniform motion.

Strain--the amount of deformation of a material by an applied force, usually expressed as a percentage change in dimensions.

Strain gage--a small array that is bonded to materials and senses the small changes in size (strain) as the material is loaded. Usually used to measure force or acceleration.

Strength (muscular)--the maximum force or torque produced by a muscle group in an isometric action at a specific joint angle. Research has found several domains of strength expression depending on the time, velocity, and resistance involved.

Strength (mechanics)--the total work or peak force required to break a material.

Stiffness--the elasticity of a material, measured as the slope of the stress/strain or load-deformation curve in the elastic region (Young's modulus of elasticity) of a loaded material.

Stress (mechanical)--The force per unit area in a material.

Stress-relaxation--the decrease in stress in a material with time when subjected to a constant force.

Stress-strain curve--(see load-deformation).

Stretch-shortening cycle (SSC)--a common coordination strategy where agonists for a movement are eccentrically loaded in a countermovement, immediately before the concentric action and motion in the intended direction. SSC result in larger initial force and greater concentric work than purely concentric actions (reversible muscle action).

Telemetry--a technique to send biomechanical signals to recording devices without wires, using a FM radio transmitter and receiver.

Tension-- a pulling apart (making longer) mechanical loading created by forces in opposite directions acting along a longitudinal axis of a material.

Time constant--typically an averaging/smoothing value in EMG processing, the larger the time constant the larger the time interval averaged over, meaning more smoothing

Vector--a complex quantity requiring the description of size and direction.

Weight--vertical resistance due to gravitational force.

Work (mechanical)--work is done when a force moves an object in the direction of the force and is calculated by the product of force and displacement.

Work-energy relationship--principle of physics that the work done on a body is equal to the net change in energy.

Yield point--point on the load-deformation curve where a material continues to deform without increasing load.