Kinematics and dynamics

"In physics, kinematics is the branch of mechanics concerned with the motions of objects without being concerned with the forces that cause the motion. In this latter respect it differs from dynamics, which is concerned with the forces that affect motion." (from Wikipedia: 'kinematics')

3D motion analysis needs information from both, kinematics and dynamics, depending on the type of analysis.

Kinematics

Kinematics can be measured with optical devices like cameras which determine the position of body landmarks over time. Cameras capture their field of view in discrete time intervals: Normal video cameras have 25 frames per second (PAL) or 30 frames per second (NTSC).


kinematic stick diagram of running
Kinematic stick diagram as used in motion analysis of running

High-speed cameras often capture 100, 200, 1000 or even 20,000 frames per second but are much more expensive than standard video equipment.

Alternatively analog sensors like goniometers (to measure angles) or accelerometers (to measure acceleration) can be used to get kinematic information.

Dynamics

Dynamics can be measured with analog devices like force platforms, insole measurements or other force or pressure sensors.
Force platforms typically measure three components, i.e. forces in all spatial directions.
Pressure sensors measure forces per area and are limited to forces in perpendicular direction.


dynamic measurement: vertical ground reaction forces in gait
Dynamic measurement: vertical ground reaction forces in gait

Analog-digital converters are used to digitize the analog output voltage in order to visualize the measured values. A/D converters also digitize the measured voltage in discrete time intervals but with a much higher frequency than standard optical devices. Often 300 to 1000 samples per second are used but measurements with 2000 or even 10,000 Hertz (Hz) can be done with the same standard equipment. A/D converters usually can digitize 16 or 32 channels at the same time.

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