Hello!

I've made an observation while recording emg values that I cannot match with
information I gathered from publications:
While the subject is standing he's lifting and lowering his thigh (shank is
pointing down) quite slowly (roughly 2 sec per direction) and the signal is
recorded from the m. iliopsoas.
When I compare the rms (or low-pass filtered) values at the same angle
(between trunk and thigh) from
a) the upward movement and
b) from the downward movement

the values from b) are always larger then the values from a) (factor 1.1 to
1.5 roughly).
Also when I add a case
c) upward movement again (right after b), no recovering for the muscle)
the values from b) are still larger than those recorded during b).

Why is this the case?

After reading some publications I came to the following conclusions:
- muscle fatigue cannot be the reason, since I can repeat a) and b) right
after one another again and again and get the same results: values from b)
are larger than those recorded during a)/c)
- force-length relationship cannot be important here, since the length of the
muscle is the same for the same angle no matter if the thigh is lifted or put
down (or is it not?? any effects I didn't consider?)
- force-time relationship: the movement is so slow that it can be neglected
also (the elastic element will be stretched in almost the same manner whether
the movement is a) or b))

- force-velocity relationship: first I thought that this might explain it, but
if I make no mistake in my reasoning, my observations show it just the other
way round:
The muscle is shortening during a), so the normalized velocity of the muscle
is v < 0.
Hill,Zajac,Hatze,Happee and Delp all propose functions for the
force-velocity-factor that give results < 1.0 for v < 0 and results >= 1.0
for v >= 0.

Wouldn't this mean, that the muscle needs to be stimulated more during a) (v <
0) to produce the same force as during b) (because the force-velocity-factor
is smaller for v < 0)?
But if it needed to be stimulated more, then I would record higher rms
values...

Either I've made a mistake in my reasonings or there's another effect I didn't
consider.
Can anyone shed some light on this matter? :-)

Any help is highly appreciated!
Best regards,
Christian

--
Dipl.-Ing. Christian Fleischer
Technical University of Berlin
Computer Engineering and Microelectronics
Sekr. EN 10 - Einsteinufer 17 - 10587 Berlin - Germany
Fon:+49 (0)30 314-73114 - mailto:fleischer@cs.tu-berlin.de
Fax:+49 (0)30 314-21116

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