Skeletal Muscle Actions

Anna Melby

16 Skeletal Muscle Actions

Muscle Terminology

Muscles create force and movement by pulling on their ends (contraction). The attachments of the muscles dictate the actions that the muscle perform. Consider the movement of forearm flexion, where the arm bends at the elbow. The muscle that performs the movement is called the agonist. So if contraction of biceps brachii (the agonist in this example) causes the arm to bend at the elbow (flexion of forearm), then contraction of an antagonist muscle is required to straighten out the arm (extension of forearm). Triceps brachii is the antagonist to biceps brachii and produces the opposing movement because it is attached at the posterior of the elbow, while biceps brachii is attached to the front of the radius at the radial tuberosity.

Muscles are attached to bones at features such as tuberosities, but they may also be attached to sheets of connective tissue called fascia. Muscle attachments are classified as either origins or insertions.

The origin is the proximal attachment. It tends to be stationary, and is often quite broad. In some cases muscles have more than one origin.
The insertion is the distal attachment. It tends to be the mobile end of the muscle. Compared to the origin, the insertion is usually a more narrow, focused attachment.

Muscle Actions

Below is a list of general terms for muscle actions and their definitions. I will also define these by acting them out in class. The actions are in pairs because of the way that muscles work: muscle can only contract (shorten). So when the contraction of one muscle or set of muscles moves a limb to a new position, contraction of a different muscle with different attachment points is needed to restore the limb to its original position. Thus the two actions in each pair represent the corresponding action of an agonist and its antagonist.

NOTE: To truly specify a muscle’s action, you must account for which joint the muscle crosses (or effectively which part of the limb it moves). For example, the action of triceps brachii is not just “extension” but “extension of the forearm”.

Action	Description
flexion	movement in the sagittal plane, makes a smaller angle around joint
extension	makes a larger angle around a joint and restores a flexed limb to anatomical position; hyperextension is when limb moves beyond anatomical position

abduction	angular movement in the frontal plane that moves the limb away from the body
adduction	moves the limb toward body; with adduction you “add” back the limb

medial rotation	turning limb around long axis in the direction toward the midline of the body; also called internal rotation
lateral rotation	turning limb around long axis in the direction away from the midline of the body; also called external rotation

pronation	specific to the forearm; equivalent to medial rotation of the forearm
supination	equivalent to lateral rotation of the forearm

plantar flexion	specific to the ankle; occurs in sagittal plane; movement made when pointing the toes; in walking or biking this action pushes the ball of the foot against a surface and lifts the heel
dorsiflexion	bringing the top of the foot towards the shin

inversion	specific to the foot; turning the foot inward toward the midline, occurs in the frontal plane
eversion	turning the foot outward away from the midline

List of Muscles and Their Actions

You are responsible for being able to identify the muscles in purple bold-face that are listed in the two tables below. You will be tested on pictures of anatomical models of the arm and leg. Labeled pictures of the arm and leg models are in the next chapters.

For the muscles that move the arm at the shoulder, you will be tested on pictures taken from the required Acland’s video. Some muscles in the leg are included in the list but will not be on the test because they are deep muscles that are difficult to see on the models.

You should also know the actions of the muscles highlighted in purple bold-face below. For any indicated muscle, you should be able to name an action that the muscle performs. Or, if shown a cartoon of an action, you should be able to identify a muscle that performs that action. Note that the list below does not necessarily include all the actions for each muscle. The optional Acland’s videos are particularly useful for understanding the muscle actions.

Upper Limb

Muscle	Action	Notes
*Muscles that move the arm at the shoulder*
pectoralis major	flexion, adduction, and medial rotation of arm at shoulder	pectoralis major is the large muscle visible on the surface of the chest
deltoid	abduction of arm at shoulder; medial and lateral rotation of arm at shoulder; flexion and extension of arm at shoulder	parts of deltoid can be activated separately: anterior fibers produce flexion or medial rotation, posterior fibers produce extension or lateral rotation; contraction of all the fibers together cause abduction of the arm
latissimus dorsi	extension, adduction, and medial rotation of arm at shoulder	“lata” means wide and “latissimus” means the widest: latisimus dorsi is the large sheet-like muscle that dominates the surface of the lower back
*Muscles that move the forearm*
biceps brachii	flexion of forearm at elbow; supination of forearm	2 origins on scapula; insertion at radial tuberosity
brachialis	flexion of forearm at elbow	lies underneath biceps brachii; visible in posterior view of arm
brachioradialis	flexion of forearm at elbow	strap-like muscle whose tendon runs toward the thumb
triceps brachii	extension of forearm at elbow	3 origins (scapula and humerus); insertion on olecranon process
pronator teres	pronation of forearm	can be seen proximally between brachioradialis and flexor carpi radialis
*Muscles that move the wrist and digits*
extensor group
extensor carpi ulnaris	extension of wrist	medially located on posterior forearm
extensor digitorum	extension of fingers; extension of wrist	look for tendons to the fingers; centrally located on posterior forearm
extensor carpi radialis	extension of wrist	actually two muscles: longus and brevis; runs parallel and adjacent to brachioradialis
flexor group
flexor carpi ulnaris	flexion of wrist	most medial tendon on anterior side of wrist
palmaris longus	flexion of wrist	variably absent; when present, tendon visible in surface anatomy
flexor carpi radialis	flexion of wrist	runs diagonally across anterior forearm

Lower Limb

Muscle	Action	Notes
*Muscles that move the thigh at the hip*
tensor fasciae latae	abduction of thigh; tenses fascia lata	anterolateral location on hip; inserts into iliotibial band
gluteal group
gluteus maximus	extension of thigh	large muscle forming the bulk of the buttocks, important for extension while climbing stairs or getting up from a sitting position
gluteus medius	abduction of thigh	action important in walking to keep pelvis from sagging
adductor group
adductor longus	adduction of thigh	visible from the anterior in a location that is medial to sartorius
adductor magnus	adduction of thigh	deep to adductor longus but visible in posterior upper thigh
gracilis	adduction of thigh	thin strap-like muscle on the medial surface of thigh
*Muscles that move the leg*
sartorius	flexion and lateral rotation of thigh	thin strap-like muscle that angles across anterior thigh
hamstring group
biceps femoris	extension of thigh; flexion of lower leg	two origins (heads): one origin at ischial tuberosity, one origin on femur; inserts laterally on bones of lower leg
semimembranosus	extension of thigh; flexion of lower leg	origin on ischial tuberosity; inserts medially on tibia
semitendinosus	extension of thigh; flexion of lower leg	origin on ischial tuberosity; superficial to semimembranosus
quadriceps group
rectus femoris	flexion of thigh; extension of lower leg	insertion at tibial tuberosity (patellar tendon)
vastus lateralis	extension of lower leg	insertion at tibial tuberosity (patellar tendon)
vastus medialis	extension of lower leg	insertion at tibial tuberosity (patellar tendon)
vastus intermedius	extension of lower leg	deep muscle; included to show 4 muscles in quadriceps group (“quadriceps” = 4 heads)
*Muscles that move the foot*
gastrocnemius	plantar flexion; weak flexion at knee	two heads visible on surface of upper calf; insertion on calcaneus (Achilles tendon)
soleus	plantar flexion	large muscle deep to gastrocnemius but only visible at sides; insertion on calcaneus (Achilles tendon)
tibialis anterior	dorsiflexion; inversion of foot	located on medial anterior surface of tibia, just lateral to hard ridge of tibia
extensor digitorum longus	extends the toes; dorsiflexion	look for tendons fanning out to the toes
fibularis longus	eversion of foot; plantar flexion	located laterally over the fibula (another name for this muscle is peroneus longus; either name is acceptable on a quiz section test