PDA

View Full Version : RESPONDS TO: emg & muscle: force-length,force-velocity relationship



cfleischer68
02-23-2004, 08:36 PM
Hello!

I've received many answers to my question and I think it is a good thing to
collect the answers an repost them for everyone to read, so here it comes.

First of all, the question, followed by another explanation that came to my
mind a few days ago:

=== THE QUESTION ================================================== =======

'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? :-)

=== ANOTHER THOUGHT OF MINE ============================================

The low-pass filter I used to smoothen the rectified
EMG-values had too a small low-cut frequency ( read more about this?
Search for references to Stegeman and Blok, and Stegeman and
Roeleveld
Good luck again,
At
-------------------------------------------------------------------------------------

Do you also have EMG recordings of the opposing muscles?
Subjects might be co-activating the muscles to control the downward
movement. In the upward movement gravity is stabilizing the movement while
in the downward movement it destabilizes the movement. In order to make the
system controllable it might be favourable to co-activate more. I agree with
you idea that force-length and force-velocity charateristics are probably
not the cause of your findings.

If you do have emg recordings, please let me know if I'm right!

Greeting Luc

-------------------------------------------------------------------------------------



Both iiacus and psoas are rather deep muscles. I don't know much about EMG
application, but I suppose that in order to measure their activity
separately from other hip muscles one would need to use a fine wire
electrode inserted into the muscle.

If you were using surface electrodes then I might suggest that the
contraction of different muscles during the raising and lowering movements
would alter the distance between the muscle producing the signal and the
measuring electrode.

Probably this is not of much help. However, I would be interested to find
out what other EMG experiments you are carrying out here in Berlin. Maybe we
might have some common interests.

best wishes

Mark Thompson

-------------------------------------------------------------------------------------
Could other hip flexors (e.g., rectus femoris, sartorius, tensor fascia
latae) be contributing force to the upward movement, but not the
downward movement?

Susan Larson

-------------------------------------------------------------------------------------

Looking at the left hand side of the force-velocity curve (eccentric portion)
you will see the muscle force continues to increase past the isometric
(zero-velocity) muscle force, and the eccentric muscle force increases to a
plateau that is somewhat higher than that of the isometric muscle force. EMG
signals have also been reported to be larger during eccentric muscle action
than compared to concentric. I cant recall the exact Herzog reference, but
Dr. Walter Herzog has written papers on this. From what I can gather from the
description of your movement, the (b) portion of the movement is when the
iliopsoas is eccentrically activated, lowering the thigh against gravity.
Therefore, the increased EMG may be explained through the eccentric muscle
action.

Regards,

Kathleen Costa, ABD
-------------------------------------------------------------------------------------

I enjoyed your posting. Nice to see someone express their logic flow so
concisely.

Regarding the Force-Velocity relationship. I think perhaps you need to
reconsider the sign of contraction velocity. A concentric (shortening)
contraction should be a positive velocity, and negative for the eccentric
(lengthening) contraction.
In addition to Hill et al's work, you might be interested in the following
classic article:
Bigland, B. and Lippold, O.C.J. The relation between force, velocity
and integrated electrical activity in human muscles. Journal of
Physiology (London), 1954, 123: 214-224.

Take care,
Mike

-------------------------------------------------------------------------------------

After I replied to your first posting, I quickly reviewed Bigland and
Lippold's paper, and there may be something there that could cause some
distress on your part. They show that I-EMG values for the same absolute
velocity were less in the eccentric condition, than in the concentric. I
believe this is one of the more interesting aspects of muscle activation ...
that Eccentric force production can be higher, while requiring less neural
activation. You will find in your review of the literature that the
musculotendinous system can produce passive tension when stretched. Pretty
interesting function.

Have a great weekend.

Mike

-------------------------------------------------------------------------------------

This may be na´ve, but couldn't the difference be attributable to the fact
that the person is performing an eccentric contraction (higher values) during
thigh lowering and a concentric contraction (lower values) during thigh
raising. I believe that some have stated that more fast-twitch/high tension
fibers are recruited during eccentric contractions and, thus, greater EMG
values. Perhaps I'm misunderstanding your data?

Kimberly B. Harbst, P.T., Ph.D.

-------------------------------------------------------------------------------------

See the work of Peter Huijing such as:1-3

1. Huijing, P.: Muscular force transmission: a unified, dual or
multiple system? A review and some explorative experimental results.
Arch Physiol Biochem, 107(4): 292-311, 1999.
2. Huijing, P. A., and Baan, G. C.: Extramuscular myofascial force
transmission within the rat anterior tibial compartment: proximo-distal
differences in muscle force. Acta Physiol Scand, 173(3): 297-311, 2001.
3. Huijing, P. A., and Baan, G. C.: Myofascial force transmission
causes interaction between adjacent muscles and connective tissue:
effects of blunt dissection and compartmental fasciotomy on length force
characteristics of rat extensor digitorum longus muscle. Arch Physiol
Biochem, 109(2): 97-109, 2001.


Muscles are interconnected and do not work independently. Also: EMGs
show only active contraction and do not indicate muscle tone. The muscle
may be isotonic and exert considerable force with no EMG indication of
any activity.

Stephen M. Levin

-------------------------------------------------------------------------------------
Could it be because lowering the thigh is a lengthening (eccentric)
contraction?


raju
-------------------------------------------------------------------------------------

The interconnectedness of muscles is covered in the articles by
Huijing. You might want to get a hold of Hubb Maas' PhD thesis in book
form, /Myofascial Force Transmission/, that puts it all together.
Contact him through his email: hmass@fbw.vu.nl .

Muscle is never flaccid; there is always a certain amount of tension or
'muscle tone' in the system. This tension varies with posture, activity,
excitement and other factors. For instance, the muscle tone is higher
when standing then lying down but, once standing, the EMG s are the
same. The acumulated isotonic tension may be considerable, but you
would never know it from looking at the EMGs. Check the O2 consumption
and you would see a difference. EMGs only show increasing tension. When
a muscle is isotonic, EMGs are flat even though there is tension of the
muscle. A flat EMG does not mean there is no tension, just that there is
no increasing tension. That tension, (muscle tone), can do 'work', for
instance, it keeps you standing. Think of the spokes of a bicycle wheel.
Once you set the tension, the tension remains constant no matter what
the load on the bicycle. I have never seen this discussed in the
literature, but the facts are indisputable.

P.S. Your English is a lot better than my German!

Best regards,

Steve

-------------------------------------------------------------------------------------
>
> 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.
How do you do this? It is commonly assumed that it is not
possible to record surface EMG from psoas. You need some 20
cm (!) long needles to perform this.

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...
>

Yes this is entirely correct. Usually you see it this way. I can see
two confounders.
1) You may have recorded from the wrong muscle. Maybe rectus
femoris? This is a hip flexor, but also a knee extensor, so when the
knee is free hanging it should give no signal. But this is
theoretically, it may well be different. Sartorius is also possible.
2) Sometimes you see high rhythmic (8-10 Hz) bursts in eccentric
contractions, resulting in 'incorrect' high 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? :-)
>

-------------------------------------------------------------------------------------

Thanks again!
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

-----------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
Please consider posting your message to the Biomch-L Web-based
Discussion Forum: http://movement-analysis.com/biomch_l
-----------------------------------------------------------------