Muscular SystemPart A

Prepared by Vince Austin and W. Rose.

Figures from Marieb & Hoehn, 7th and 8th eds.

Portions copyright Pearson Education

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Interactions of Skeletal Muscles

Skeletal muscles work together or in opposition

Muscles only pull (never push)

As muscles shorten, the insertion generally moves toward the origin

Whatever a muscle (or group of muscles) does, another muscle (or group) “undoes”

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Muscle Classification: Functional Groups

Prime movers – provide the major force for producing a specific movement

Antagonists – oppose or reverse a particular movement

Synergists

Add force to a movement

Reduce undesirable or unnecessary movement

Fixators – synergists that immobilize a bone or muscle’s origin

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Naming Skeletal Muscles

By location: associated bone or body region

By shape: e.g., deltoid muscle (deltoid = triangle)

By relative size – e.g., maximus, minimus, longus

By fiber direction: e.g., rectus (fibers run straight), transversus, oblique

By number of origins (heads): e.g., biceps, triceps

By site of attachment: point of origin or insertion

By action: e.g., flexor, extensor

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Fascicle Arrangment Varies

Parallel – fascicles run parallel to muscle long axis (e.g., sartorius)

Fusiform – spindle-shaped muscles (e.g., biceps brachii)

Pennate (feather-like) – short fascicles attach obliquely to central tendon running lengthwise (e.g., rectus femoris)

Convergent – fascicles converge from a broad origin to a single tendon insertion (e.g., pectoralis major)

Circular – fascicles are arranged in concentric rings (e.g., orbicularis oris)

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Arrangement of Fascicles

Figure 10.1

Lever SystemsLever: rigid bar that rotates about a …

Fulcrum: fixed point or hinge point

Effort: force applied to lever

Load: resistance to the effort, usually at a different point along the lever

A & P equivalents: – lever = bone (one or more)

– fulcrum = joint

– effort = force applied by muscle

– load = “external” force which the effort acts against

• Name and description of the muscle – be alert to information given in the name

• Origin and insertion – there is always a joint (or two – biarticular muscles) between the origin and insertion

• Action – best learned by acting out a muscle’s movement on one’s own body

Muscles: Name, Origin, Insertion, Action

Muscle force and torques

For a muscle to carry a load without giving way, the torque on the joint from the muscle must be equal and opposite to the torque on the joint from the load.

Torque is twisting force, i.e. force that causes angular motion.

Torque = Force x Distance*

where Distance is from the point where force is applied to the “axis” or center of rotation

* I am making the assumption that the force is at right angles to “axis of pull”.

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Lever Systems

Figure 10.2a

In figure above: shovel=lever, right hand=fulcrum, load=weight of dirt, effort=upward force applied by left hand

Lever Systems

Figure 10.2b

Muscle force and torques

Joint Torque due to muscle =(Muscle force) x (Distance from joint to muscle insertion)

Joint Torque due to load = (Load force) x (Distance from joint to load).

Muscle force and torquesExample:Load =100 N. Distance from joint to load = 20 cm.Distance from joint to muscle insertion = 4 cm.What force must the muscle provide, to support the load?Answer: Using the equation above, we see that

(Muscle force) x 4 cm=

100 N x 20 cm.Therefore: Muscle force = 2000 N-cm / 4 cm = 500 N. The muscle force must be bigger than the load. This is not uncommon.* I am making the assumption that the forces are at right angles to the limb axis.

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Major Skeletal Muscles: Anterior View

The 40 superficial muscles here are divided into 10 regional areas of the body

Figure 10.4b

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Major Skeletal Muscles: Posterior View

The 27 superficial muscles here are divided into seven regional areas of the body

Figure 10.5b