Re: is there a difference between a synergist muscle and a fixator?

Dear Chris,

thank you very much for your question regarding muscle terminology. In
fact, the terms synergist and fixator have very little to do with each
other. I think your confusion is caused by the fact that the definitions
you have read may sound similar but relate to different areas.

The terms synergist and antagonist relate to the action of single muscles
on the movement about one axis of one joint; while the term fixator relates
to the concerted action of several muscles (synergists and antagonists) on
all possible movements of a joint. 

Huh?

Let us start with the easier term: Two muscles are called synergists if
their contraction leads to a movement into the same direction about the
axis of a joint. Take the elbow: Two of  its parts  (the humero-ulnar and
humero-radial articulations) form a common joint moving about a single axis
allowing  flexion and extension (supination and pronation are produced by
the third part of the elbow joint, the proximal radioulnar articulation).
Two well-known muscles , the biceps and the brachioradialis muscles, are
synergistic for the flexion-extension movement, as the contraction of  both
leads to flexion.

Two muscles are called antagonists if their contraction produces movement
in opposite directions about the axis of a joint. In the case of the elbow
joint, the triceps (for extension) on the one hand and the biceps on the
other (for flexion) are good examples. 

Note that synergism and antagonism are not necessarily inherent to the
muscles themselves – they also depend on the position of the joint: When
the elbow joint is almost fully extended, the main axis of contraction of
the brachioradialis muscle crosses the axis of the elbow joint; beyond this
point (only a few degrees before full extension, though), contraction of
the brachioradialis leads to extension, not flexion. The brachioradialis
thus becomes a synergist to the triceps muscle of the elbow joint, and an
antagonist of the biceps. 

In more complex muscles, different parts of the same muscle can be
antagonistic to others – The deltoid muscle of the shoulder is a good
example: While the anterior parts of the deltoid effect inward rotation,
the posterior parts effect outward rotation. So for rotation, the anterior
and posterior parts of the deltoid muscle are antagonists. 

Fixator muscles, on the other hand, are all the muscles that serve to fix a
joint in a given position during the movement of other joints

Say you want to do some weightlifting, for example, to train your biceps.
You want to do your workout in a relaxed, standing-up position. You want to
hold a nice weight in your hand (the back of your hand facing backwards).
You want your upper arm to remain in a vertical position pointing
downwards. To train your biceps muscle, your forearm is supposed to move up
and down, swinging from a (nearly) downward pointing position to nearly
horizontal. What does it take to make this happen? 
 
Well, you need fixator muscles or rather you need a lot of muscles to act
as fixators: The anterior and posterior muscles of the leg have to fix your
ankles so you do not fall forward when the weight is lifted and carried
forward with the swinging movement of your forearm. Your knees, hip and
spinal column must stay extended so you do not collapse; finally, the
shoulder joint has to fixed in position so the upper does not make a
compensatory swing backwards every time you bend your forearm. 

So, a fixator is a muscle engaged in fixing a joint in given position to
make isolated movements in a different joint possible. In the case of the
shoulder joint during your weightlifting exercise, the rotator cuff, the
latissimus dorsi, and the major pectoral muscles all act together as
fixators of the shoulder joint. For effective fixation, the action of
agonistic and antagonistic muscles is usually recruited simultaneously. 

You may already know that to classify muscle action, the concept of a
punctum fixum versus the punctum mobile is used. In our case of the elbow
and the action of the biceps lifting a weight, the puncta  fixa are the
upper arm and the shoulder containing the two heads of the biceps muscle,
while the swinging forearm is the punctum mobile. So you may define a
fixator muscle as a muscle that serves to provide the punctum fixum for a
given movement, in this case, the immobilized shoulder joint. Of course,
the definitions of p. fixum and p. mobile are arbitrary – if you are doing
a pull-up instead of weightlifting, the shoulder joint is certainly mobile,
and the wrist serves as the punctum fixum. In that case, several muscles
spanning the wrist are the fixator muscles, combining their antagonist and
synergistic strengths to hold the wrist straight and the fingers bent. 
So synergist/antagonist relates to the action of isolated muscles about
single axes of isolated joints; while fixator relates to the concerted
action of muscles on a joint and its position/movement as a whole.

I wish you very good luck with your project!
Yours truly
Jens Peter Bork 

P. S. having found your question again on the Internet at
lists.iinet.net.au/pipermail/ catalist/2005-March/000729.html (Googling for
„fixator muscles“), I think I understand better why you feel the definition
of the two terms is circular. Take again the example on weightlifting: To
be able to lift weight more effectively it is of course helpful not only to
train the biceps but also its synergists, for example, the brachioradial
muscle, simply because two muscles will lift more weight than one. But you
will also have to train the fixators immobilizing the adjacent joints
(especially the shoulder) so you can do your exercise more effectively,
without unwarranted movement disturbing the exercise in the muscle you want
to develop. So you have to train them both, but they are not the same. 

References: 
It is hard to name a specific resource as a reference. I suggest you read
the general chapters on the skeleton and muscles in a good anatomy textbook. 

Some web-sites in the bodybuilding and sports education sector refer to the
concept of fixator muscles, although rather informally. Among them are 
www.bodybuilding-advice.co.uk/ designing_your_training.html
www.tennisserver.com/wildcards/wildcards_99_12.html

www.merckmedicus.com/pp/us/hcp/thcp_dorlands_content.jsp?pg=/
ppdocs/us/common/dorlands/dorland/dmd-m-043.htm has a formal (and much
shorter) definition of fixator muscles.