Chapter 11 Muscular System- The Motors of the Body
Distinguishing characteristic of muscle is its ability to contract.The development of tension within a muscle causing it to pull on its attachments
Types of Muscular Contractions
Concentric (shorten)- when a muscle is active and its attachments draw closer together
Eccentric (lengthens)- when a muscle is active and its attachments are drawn farther apart
Isometric (no change)- when a muscle is active and its attachments do not move relative to each other
The 4 Roles of Muscle
Agonist, definition of
What motion is occurring? What is causing it? If a muscle caused it, then that is the agonist.
Muscles capable of creating a torque in the same direction as the joint action referred to.
Muscles that are active concentrically are agonists to the action occurring at the joint they cross.
Antagonist, definition of
Muscles capable of creating a torque opposite the joint action referred to or opposite the other muscle referred to.
Term antagonist can be used in reference to a joint action. Muscles active eccentrically are antagonists to the action occurring at the joint they cross.
Antagonist may also be used in reference to another muscle. The torque the antagonist creates opposes the torque created by the muscle referred to.
Stabilizer, definition of
Muscles that are active isometrically to keep a limb from moving when the reference muscle contracts.
When a muscle is active, it will tend to move both bones to which it is attached.
The isometric action of the stabilizing muscles keeps unwanted movement at the joint from occurring.
Neutralizer, definition of
"Eliminates the other movements, joint rotations"
Muscle that creates a torque to oppose an undesired action of another muscle.
The torque created by many muscles (Ex. biceps brachii) components in several planes.
If only one joint movement is desired (Ex. friction), the neutralizer acts to oppose the unwanted movements (Ex. forearm supination or shoulder flexion)
3 Factors that Affect the Magnitude of Muscle Contraction Force
Physiological Cross-Sectional Area
Physiological Cross-Sectional Area
Muscles behave similarly to rubber bands- increasing the number of fibers side by side and parallel to each other increases the strength of the muscle.
PCSA of the muscle perpendicular to the muscle fibers and line of pull of a muscle gives an indication of the maximal tensile force a longitudinal muscle can produce
PCSA continued (2)...
The maximum tensile force a pennate muscle can produce cannot be estimated by the cross-sectional area perpendicular to the muscle fibers and line of pull of a muscle.
The muscle fibers and line of pull of a pennate muscle are not in the same direction
A cross section taken perpendicular to the line of pull of a pennate muscle would not include all the fibers of the muscle.
PCSA continued (3)...
Longitudinal versus Pennate
Pennate muscles allows more fibers (force) compared to longitudinal muscles.
Less energy to do so
Muscle fibers are short
Longitudinal allows big ranges of motion
PCSA continued (4)
Longitudinal Versus Pennate (2)
An equivalent volume of pennate muscle will generate more tensile force than a longitudinal muscle.
However, there is a drawback to pennate muscles- the shorter fibers of the pennate muscle and their orientation relative to the angle of pull, limit the distance over which a pennate muscle can shorten.
(2 of 3 factors that affect the magnitude of muscle contraction force)
Maximum muscle force is also dependent on the length of the muscle.
Total muscle tension developed depends on- active tension developed by the contractile elements, plus the passive tension developed when the muscle is stretched beyond its resting length.
(3 of 3 factors that affect the magnitude of muscle contraction force)
Maximum tensile force developed by a muscle is dependent on the velocity of shortening, as well as its length.
The Relationship between muscle length and tension.
Need to stretch the muscles
This curve shows that you can't perform both maximum contraction and full range of motion.
Contraction Velocity Figure
Concentric, Eccentric and Isometric Activity
Negative velocities of shortening represent eccentric contractions.
A muscle contracting eccentrically or isometrically is capable of producing more force than a muscle contracting concentrically.