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ashortland99
06-14-2005, 07:34 AM
Hi Chris and other subscribers,

1. Is there, in fact, such a thing as electro-mechanical delay?

In isolated in-vivo preparations of large muscles (lattisimus dorsi) stimulated by supramaximal frequency and recruitment there is very little time difference between the onset of stimulation and the the onset of force (10-20 ms). The delay between the onset of supramaximal stimulation and 90% of peak force in latissimus dorsi may vary between 90ms and 300ms depending on the fibre type of the muscle (experimental data). These values will depend on the architecture (fibre length) of the muscles involved.

2. If so, how should it be defined?

In isolated preparations this would be a relatively easy question to answer. In the body, there will be a number of confounding factors that affect the temporal relationship between action potential and twitch (activation history, preload, etc), but probably if you could recruit all the motor units with one pulse and then measure the time between this pulse and the peak of the mechanical reponse, this would be a fair estimate.

3. What is it for common muscles?

15-30 ms depending!

The use of a 3Hz filter is really accounting for the superposition/summation of twitches individual motor units (MUs) to form a mechanical output. The problem here is that all MUs are not recruited simultaneously in voluntary movement, with slower units being recruited first (Henneman principle).

Interestingly, if you instantaneously deactivate a fully activated muscle the decay in the force produced is much slower than than the increase in force upon activation (you're very slow to unwind, but you're easy to wind up!).

All the best,

Adam

Adam Shortland PhD, MIPEM, SRCS
One Small Step Gait Laboratory,
Guy's Hospital
London
UK