Coming off our last discussion of 3 types of muscle contraction: eccentric, isometric, and concentric, this article will now focus on the functional relationships between different muscle groups. When engaging in a particular fitness training exercise, the movement will primarily target a specific group of muscles that are performing most of the work = the agonist muscle group.
For instance, a standing dumbbell curl targets the primary muscle group of elbow flexors: biceps brachii, brachialis, and brachioradialis. The angle of forearm rotation through the movement dictates the balance of force between these 3 elbow flexors, but all 3 are contracting to flex the elbow and are thus the agonist muscles performing the dumbbell curl. But they are not the only muscles contracting. The triceps muscle is also contracting throughout the range of movement, even though it is an elbow extensor, not an elbow flexor.
The person can only control the speed and extent of his biceps contraction by counterbalancing the elbow flexion force with some degree of elbow extension force, using the antagonist muscle group, the triceps. The importance of having antagonist muscle function is evident when examining nerve paralysis patients who, lacking control of an antagonist muscle, literally end up flinging the limb in the direction of the agonist muscle group without the counterbalancing effect.
In addition to the agonist (primary target muscle group) and antagonist muscle groups, numerous other muscles must contract to perform the motion correctly, the central stabilizing muscles. These are muscles closer to the midline of the body that create a stable foundation against which the agonist and antagonist muscles push or pull. For instance, during the standing dumbbell curl, the shoulder stabilizers must contract so that the upper arm remains stable and only the elbow hinges back and forth. To keep the body as a whole stable and avoid pitching forward as the dumbbell is raised, the trunk muscles of the upper and lower back, the abdominals, pelvic stabilizers, quadriceps, hamstrings, calves, and even small muscles in the feet all must work together.
The brain coordinates the timing and extent of contraction between all the different muscle groups to produce the net effect of the functional movement. The more muscle group coordination involved, the more the fitness exercise simulates real world functional performance of the body, which brings us to the next topic of how the body interacts with different types of fitness equipment. We will cover body resistance training, free weights, functional trainers, solid support equipment, and hinged machines.