Trying to understand some basic biomechanics related to racket head speed and the stretch-shortening cycle - When muscles are consciously activated to shorten, electrical signals can be measured. http://en.wikipedia.org/wiki/Electromyography Example of EMG measurements of the tennis serve. http://coutinhocci.net/pdf/5.pdf When muscles are prestretched and allowed to shorten rapidly, as in the Hill Muscle Model, what is going on with the EMG signal, is it still present? An EMG signal seems to imply that the muscle is active and important for the motion? But if the prestretch component does not produce an EMG signal and is also faster then.....???....... I believe that it's understood in biomechanics that prestretched muscles can produce more force (than unstretched muscles) at high shortening velocities. To research this issue in biomechanics measurements of force as a function of muscle shortening velocity are made. 'Force-velocity' curves are used for display. This prestretch supplied force probably drops to zero when the muscle is shortened to less than its resting length, that is, when the muscle has no stretch. From another thread - An interesting question is - when the body is using pre-stretch to shorten muscles, how does it feel? Is there a lack of feeling when using pre-stretch - the 'relaxed arm' 'free energy' that one of the Bryan twins described? Does it feel as if you are not doing that much when using prestretch?