flexor digitorum latency

[baptiste]

Hello.
I am not sure to be at the right place but I have a biomechanics question. I am a visual psychophysicist and I have been looking for a number for a few years without being able to find it in the literature: I would like to know how long it takes for the fingers to press a key once the impulse received. I know that this latency has been measured for the extensor digitorum (where observers had to stop pressing a key to answer), but I haven't been able to find it for the flexor digitorum. So I ended up measuring it myself with surface EMG and found fairly similar latencies as for the extensor (what a surprise...), i.e. between 40 and 50ms. But still, I would like to know if that number has been measured by someone else.

Would you know if this number has been measured and where to find it? If not, do you know why no one has measured it before? Is it only because the flexor digitorum is more difficult to record than the extensor? Are there reasons not to trust this estimation (artifacts etc)?

Thanks a lot.

Regards,
Baptiste

[Edmund Cramp]

Re: flexor digitorum latency

"Are there reasons not to trust this estimation"

One factor that may influence the results is the latency of the equipment used to measure the EMG signals. The delay introduced by the measurement system can be anywhere from a couple of milliseconds to greater than 40-50ms. Are you confident that you took the EMG system signal latency into account when you made your measurements?

[baptiste]

Re: flexor digitorum latency

Thanks for the reply.

Are you confident that you took the EMG system signal latency into account when you made your measurements?

No, how would I measure that? I used a Biosemi setup with active electrodes. I assume there is virtually no delay on the pushbuttons (which are plugged to the amplifier) so if there is a latency on the EMG system then I would, in facts, underestimate the muscle contraction latency. Am I correct?

[Edmund Cramp]

Re: flexor digitorum latency

That's is correct, the system latency would need to be added to the measured latency to get an accurate measurement. You may find the system latency in the documentation, if not I'd suggest simply contacting the manufacturer and asking them for the details if you can't find it documented. It's easy to confirm the numbers if your system has analog outputs but much harder to measure with digital systems.

[Todd Shewman]

Re: flexor digitorum latency

See Ed's responses and the manufacture may give this to you in sample number delay as opposed to to ms. Which may leave you with a bit of math but helpful just the same.