This is an excerpt from Dynamic Human Anatomy 2nd Edition With Web Study Guide by William C. Whiting.
One of the most fundamental and important goals of movement analysis is identifying which specific muscles are active in producing and controlling movement at a particular joint. In chapter 4, we presented specific muscles and their concentric actions. However, we know that muscles can act in three modes: isometric, concentric, and eccentric. The task at hand, therefore, is to determine for a given joint movement (1) the specific muscles involved in controlling the movement and (2) the type of muscle action.
The following muscle control formula provides a step-by-step procedure for determining the involved muscles and their action for any joint movement. This formula may appear a bit cumbersome and complex at first glance. However, with practice, you should be able to get through it quickly. Eventually (with enough practice), the process will become automatic and instinctive, and you will be able to analyze movements without consciously going through each step in the formula. It helps, though, to use the formula until you develop these movement analysis instincts.
Muscle Control Formula
We begin the muscle control formula with a statement of the problem: Given a specific joint movement (or position), identify the name of the movement (or position), the plane of movement, the effect of the external force acting on the system, the type of muscle action (i.e., shortening or concentric, lengthening or eccentric, or isometric), and the muscles involved (i.e., which muscle or muscles are actively involved in producing or controlling the movement or in maintaining a position).
Now, we move on to the formula itself, which involves six steps:
- Step 1: Identify the joint movement (e.g., flexion, abduction) or position.
- Step 2: Identify the effect of the external force (e.g., gravity) on the joint movement or position by asking the following question: What movement would the external force produce in the absence of muscle action (i.e., if there were no active muscles)?
Step 3: Identify the type of muscle action (concentric, eccentric, isometric) based on the answers to step 1 (#1) and step 2 (#2) as follows:
- If #1 and #2 are in opposite directions, then the muscles are actively shortening in a concentric action. Speed of movement is not a factor.
- If #1 and #2 are in the same direction, then ask yourself, "What is the speed of movement?"
- If the movement is faster than what the external force would produce by itself, then the muscles are actively shortening in a concentric action.
- If the movement is slower than what the external force would produce by itself, then the muscles are actively lengthening in an eccentric action.
- If no movement is occurring, yet the external force would produce movement if acting by itself, then the muscles are performing an isometric action.
- Movements across gravity (i.e., parallel to the ground) are produced by a concentric action. When gravity cannot influence the joint movement in question, shortening (concentric) action is needed to pull the bone against its own inertia. The speed of movement is not a factor.
By this point, we have identified the type of muscle action. The next steps identify which muscles control the movement.
- Step 4: Identify the plane of movement (frontal, sagittal, transverse) and the axis of rotation (i.e., line about which the joint is rotating). The purpose of this step is to identify which side of the joint the muscles controlling the movement cross (e.g., flexors cross one side of a joint, while extensors cross the opposite side).
- Step 5:Ask yourself, "On which side of the joint axis are muscles lengthening and on which side are they shortening during the movement?"
- Step 6: Combine the information from steps 3 and 5 to determine which muscles must be producing or controlling the movement (or position). For example, if a concentric (shortening) action is required (from step 3) and the muscles on the anterior side of the joint are shortening (from step 5), then the anterior muscles must be actively producing the movement. The information in chapter 4 allows us to name the specific muscles.