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.
Below is a list of general terms for muscle actions and their definitions. I will also define these in the lecture. 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 |
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 from
the Health Sciences Library. These models will be available
in the library after the end of the week. The models are
located on the 3rd floor. In the case of latissimus dorsi
and pectoralis major, you will be tested on pictures taken from
the Acland's videos. Some muscles 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. The Acland's videos are particularly
useful for understanding the actions. 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.
| 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 |
| 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 |
Now go to the page Muscle Anatomy: Material to Study.