Viewed in a particular plane, the muscles whose lines of application are on the same side of a joint axis are agonists.
Muscles whose lines of application lie on the opposite, posterior side are agonists for flexion.
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.
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?"
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.
A (ONE-JOINT) HIP FLEXOR MUST ACT WITH THE HAMSTRINGS IN A TRUE SYNERGY.
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?
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.)
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.