My interpretations of some biomechanics probably with some misunderstanding so....... -
The use of video for motion analysis involves observing object locations, object velocities and - most interesting - object accelerations. The object accelerations are most closely associated with muscle shortening forces both deliberate with 'innervation' and passive using stretch energy. See the Hill Muscle Model.
Object accelerations that are very rapid and occur over short distances are difficult to measure in videos.
Forces that might be applied to a tennis racket, golf club, etc. can also be sensed using sensors for pressure or acceleration.
For example, sensor gloves
https://www.google.com/search?q=sensor+ … mp;bih=669
I saw a commercially available sensor glove.
In the Hill Muscle Model muscle shortening forces can be supplied by deliberately sending nerve impulses (measured by EMG signals) to the muscles. If the muscle has also been stretched beforehand in the stretch-shortening cycle, it shortens 'passively'. The passive shortening may be most interesting for athletics because it might maintain force at higher velocities of shortening. See biomechanics references on Force vs Velocity.
For deliberate active shortening, EMG signals can be measured as a way of sensing activation. However, I believe that there is no EMG signal associated with passive stretch muscle shortening. This point confused me as I interpreted EMG signals in research papers to be synonymous with muscle shortening force. But for the passive shortening forces I believe there is no EMG signal as a biomechanical researcher told me. (Different opinions?)
See discussion and links to muscle shortening
http://tt.tennis-warehouse.com/showthread.php?t=441023
See reply #5 on muscle shortening and interesting new research on Titin, the stretch protein molecule -
http://tt.tennis-warehouse.com/showthread.php?t=427364
A poster was interested in coordinating EMG signals with the video.
http://www.kinovea.org/en/forum/viewtopic.php?id=658
Let's say that a researcher was recording EMG signals vs time, sensor glove data vs time and finally the resulting speed of a tennis serve in MPH or KmPH. General data boxes to display in the video screen that would show for example, EMG signal over the time from one frame to the next, glove data over the same time, a digital display of serve MPH or other velocity measured by an instrument.
http://en.wikipedia.org/wiki/List_of_sensors
