Steps for Completing a Muscular Analysis
It is often necessary to identify the muscles that are developing tension in order to cause or control movement of a body part. Knowledge of this information is important in preparing conditioning programs to improve performance as well as in diagnosing and determining reasons for injury or abnormal movement patterns. The following steps should be taken for each joint of interest:
1. Make a list of all the actions that occur at each joint in the movement. In order to do this, it is best to break the movement into phases where only one movement per plane is occurring (e.g., only shoulder flexion is occurring, not shoulder flexion and extension).
2. Determine what type of muscle action is occurring during each phase at each joint. To do this, complete the following steps:
a) Determine the motive and resistive forces/torques.
Gravity (weight) and muscle are usually the only forces that you must consider. You need to consider the speed of movement when determining this question. During slow, controlled movement of a body part away from the pull of gravity, muscle is usually the motive force and gravity (weight) is the resistive force. During slow, controlled movement toward gravity, gravity is usually the motive force and muscle is the resistive force. During ballistic movements (very rapid), muscle is usually the motive force at the beginning of the movement and the resistive force at the end of the movement. Remember that movement may also be passive in which there is no force development in the muscle (i.e., muscle is neither a motive nor a resistive force).
b) Once the motive and resistive forces are known, then type of muscle action can be determined.
When muscle is the motive force, then the muscle action is concentric and the functional muscle group has the same action as the observed joint action. In other words, the agonists are developing force while shortening (concentrically) to cause movement. The antagonists must relax to allow movement to occur.
When muscle is the resistive force, then the muscle action is eccentric and the functional muscle group has the opposite action as the observed joint action. In other words, the antagonists are developing force while lengthening (eccentrically) to control or slow down movement movement. The agonists must relax to allow movement to occur.
3. Make a list of all the the muscles that might belong to the functional muscle group developing force (agonists or antagonists) [i.e., all muscles that might be involved in causing (concentric) or controlling (eccentric) each joint action].
4. Prepare a list of all the additional joint actions that each muscle listed in step 3 might have. From this list you may find that there are joint actions caused by the agonists that need to be neutralized. This may occur in two ways:
1) It is possible that some or all of the undesired actions may be neutralized within the list. In other words, two (ant)agonists may offset the undesired actions of each other; a muscle may be an agonist and a neutralizer during concentric contraction, or an antagonist and a neutralizer during eccentric contraction.
2) If not all of the undesired actions are neutralized, additional muscles must contract as neutralizers.
In slow, unresisted movements the nervous system may choose not to recruit the agonist muscle with the undesired joint action since the additional force output is not needed. However, when a heavy load is being moved, this option is not available because all motor units that produce the desired joint action are needed. [NOTE: A muscle can not relax (as identified in step 2) and develop force to neutralize.]
Some specific examples of neutralization are provided below:
a) When the scapula or pelvis must be stabilized to provide a firm base from which muscles that move the femur and humerus can pull. EX: Many muscles that move the humerus attach on the scapula. Because the scapula is lighter than the humerus, it tends to move when those muscles contract. Therefore, shoulder girdle muscles must contract to stabilize the scapula.
b) When a two-joint muscle causes (or allows) movement at one joint while no movement at the second joint occurs. Because a muscle cannot determine which segment should be moving, a two-joint muscle that contracts would tend to cause (or allow) movement at all joints that it crosses. If movement is not occurring at both joints, then stabilization must be occurring to prevent movement at the joint not moving.
d) When the humerus is elevated. Any time the arm is elevated (flexed, hyperextended, or abducted), the rotator cuff muscles must contract to keep the humeral head from moving upward into the acromion process. In other words, the rotator cuffs must stabilize the humerus.
5. Identify bones/joints that must be stabilized and list the muscles that must be employed as stabilizers. Stabilization of bones (body parts) most often occurs in one of the following situations:
a) When there is no motion occurring at a joint. If there is no motion but force is being developed in a muscle group associated with that joint, then the muscle(s) is acting to stabilize the bone or segment.
b) When another force (muscle, weight, etc) attempts to translate a bone (shear or tensile translation) in a manner that might cause joint injury. Sometimes the agonists perform this function simultaneously while also causing the desired movement.
Remember that joint actions are the outcomes of a series of events. The brain or spinal cord sends nerve impulses to the muscle to stimulate muscle contraction. The action that results at the joint depends on not only the ability of the neuromuscular system to cause muscle contraction, but also on the combined effect of all the loads placed on the body segment (external forces as well as other muscular forces). Passive connective tissues also contribute to force output. When movement does not occur as desired, a problem may be occurring in any one of these areas.
Adapted from Barham, J.N., & Wooten, E.P. (1973). Structural kinesiology. New York: The Macmillan Company, pp. 67-68.