Synergies; functional relationships among muscles

Agonists
Antagonists

Helping synergists
True Synergists

Stabilizers


AGONIST

according to your text: "A contracting muscle (or muscle group) that is considered to be the principal muscle producing a joint motion or maintaining a posture ... (Smith, Weiss, & Lehmkuhl, 1996, p.131)."

Our definition: ONE MEMBER OF A GROUP OF MUSCLES WHOSE LINE OF APPLICATION PRODUCES A GIVEN MOMENT AT A GIVEN JOINT.

Viewed in a particular plane, the muscles whose lines of application are on the same side of a joint axis are agonists.

For example: Muscles whose lines of application lie on the anterior side of the knee joint's lateral axis are agonists for extension.

Muscles whose lines of application lie on the opposite, posterior side are agonists for flexion.


STABILIZATION

We typically focus our analysis on the moving bone or segment, and on the muscle forces which produce movement in that segment. We must remember that these muscles exert equal forces on the "non-moving" or stable bone. For instance, when the vastus lateralis exerts force on the tibial tuberosity to move the tibia, it exerts an equal force on its attachment to the femur. However, the femur is stable because some force prevents its moving.

Any force can provide this stabilization. In most cases of human movement, stabilizing forces are gravitational; the stable segment, in this case the femur and the rest of the body, are literally too heavy for the muscle to move.

The stabilizing force could also be muscular. For instance, when you perform a sit-up, the abdominal muscles pull on the pelvis with the same force that they pull on the ribcage. The pelvis doesn't tilt posteriorly because some force tilts it anteriorly. This force might come from activity in the hip flexors.

If so, then what force stabilizes the lower extremities to which the hip flexors attach? Gravity might stabilize the LE, that is, the LE might be heavy enough to stay put. However, if the abdominals pull very forcefully on the pelvis, and the hip flexors in turn pull very forcefully on the LE, someone may have to sit on the legs to stabilize them.

Here's another example: "When I lie supine, and flex one hip, I feel no action in my abdominals.  Hoever, I notice though that I push into the ground with my "resting" leg. When I attempt to flex both hips, my abdominals act vigourously.  What is the purpose of the abdominals?"

The muscle activity that you feel, aside from that in the hip flexors, are likely attempts at stabilization of the pelvis, which is a site of attachment for the hip flexors. When you flex both hips, the abdominals prevent the hip flexors from tilting the pelvis anteriorly. Similarly, the pushing that you perceive in the contralateral leg during the "single leg raise" comes from the contralateral hip extensors, which also prevent anterior pelvic tilt.

ELEMENTS OF DEFINITION OF A HELPING SYNERGY

  1. MULTI-AXIAL JOINT
  2. 2 (OR MORE) MUSCLES WITH COMMON ACTION IN ONE PLANE
  3. AND OPPOSING ACTIONS IN OTHER PLANE(S)
NOTE THAT HELPING SYNERGISTS ARE NOT ANTAGONISTIC.

An example: Muscle A flexes, internally rotates, and abducts the hip

Muscle B extends, externally rotates, and abducts the hip

Acting together, in a synergy, the two muscles can abduct the hip while producing little or no movement in other planes.


ELEMENTS OF DEFINITION OF A TRUE SYNERGY:

  1. A MULTI-ARTICULAR MUSCLE WHICH CAUSES MOVEMENT AT EACH JOINT IT CROSSES

  2. A SECOND MUSCLE WHICH OPPOSES THE MULTI-ARTICULAR MUSCLE'S ACTION AT AT LEAST ONE JOINT (BUT NOT AT EVERY JOINT.)

EXAMPLE OF A TRUE SYNERGY - HAMSTRINGS

  1. HAMSTRINGS ARE MULTI-ARTICULAR (CROSS MORE THAN ONE JOINT). HAMSTRINGS PRODUCE HIP EXTENSION AND KNEE FLEXION

  2. FOR HAMSTRINGS TO PRODUCE KNEE FLEXION WITHOUT HIP EXTENSION,
    A (ONE-JOINT) HIP FLEXOR MUST ACT WITH THE HAMSTRINGS IN A TRUE SYNERGY.

  3. FOR HAMSTRINGS TO PRODUCE HIP EXTENSION WITHOUT KNEE FLEXION: A (ONE-JOINT) KNEE EXTENSOR MUST ACT WITH THE HAMSTRINGS IN A TRUE SYNERGY.

QUESTION:

My definition of a true synergist is one that involves a muscle which inhibits all of another muscles actions except one (the one that is the desirable action) but doesn't aid in the desirable function either.

If my definition is correct, I can't seem to see a true synergist acting in the foot. Can you give me an example of one at the foot and tell me how it acts?

ANSWER:

Your definition of a true synergy makes sense to me: one of the muscles must act on several joints. The other "inhibits" or counteracts its actions on at least one but not all of those joints.

The example we used in lecture involves the extensor digitorum longus (EDL), a muscle which crosses the ankle, subtalar joint (STJ), and, in fact, every joint all the way to the DIP joint.

The EDL will act on each of those joints, dorsiflexing the ankle, pronating the subtalar joint, then extending the MP, PIP, and DIP joints.

If we want to extend the DIP joint, the EDL is the only muscle we have to do this. However, if we use the EDL by itself, it will also move the ankle, STJ, etc. If we want to extend the toes at the DIP joint and not move every other joint, we need to activate another muscle, a synergist, along with the EDL.

This synergist should counter the EDL at both the ankle and STJ. Since the EDL dorsiflexes the ankle and pronates the STJ, the syngergist should plantar flex the ankle and supinate the STJ. A muscle which does this is the tibialis posterior. Thus, the EDL and the tibialis posterior act in a true synergy to extend the DIP joints without moving the ankle and STJ.

(Note that the flexor digitorum longus also plantar flexes the ankle, and supinates the STJ. It would not work for this action, however, because it also crosses the PIP and DIP joints. In fact, it would be an antagonist for the EDL.)


ANTAGONISTS

Muscles with opposite actions at a joint are antagonists. Viewed in a particular plane of movement, the muscles whose lines of application are on opposite sides of a joint axis are antagonists.

We shouldn't think of muscles as being organized in agonist-antagonist pairs. We won't find it useful, for instance, to try to name an antagonist for every single muscle. Instead, given a functional movement, we should think of muscles in terms of functional groups which have opposite actions.

Antagonists generally relax when agonist acts. Cocontraction or, to use a term I prefer, coactivation, occurs when muscles on both sides of a joint axis are active. We often coactivate muscles when we perform unlearned or novel movements. We also coactivate muscles when we perform helping or true synergies.


Reference: Smith, L.K., Weiss, E.L., & Lehmkuhl, L.D. (1996). Brunnstrom's clinical kinesiology. (5th ed.). Philadelphia: F.A. Davis.

Last updated 9-22-00 © Dave Thompson PT
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