One of the most important things a fitness professional should understand is
the function of a specific muscle or muscle group. Every muscle performs a
joint action, in other words, the muscle either flexes a joint or extends it.  I see
far too many people doing exercises trying to target muscles that have nothing
to do with the movements that they are performing.
Below is a list of muscles and the joint actions that they perform.  I hope that
this will help you to begin a more effective workout program.
Remember that QUALITY is more important than QUANTITY. Any kind of training
that ignores technique will get you results much more slowly and can put you at
risk for injury.  It is far better to perform one rep the right way--using perfect
form-- than to do 10 the wrong way.  And finally, *more* weight, *less*
repetitions will get you far better results...doing 500 reps with a 2 lb weight is
just not going to do anything for you and you are wasting your time. (No,
ladies...you are not going to"bulk" by using a heavier weight!)
FUNCTIONS OF MAJOR MUSCLE GROUPS

Muscle:
Rectus Femoris (Quads)

Function:
Hip flexion (with illiapsoas) and knee extension (with vastus
group--vastus lateralis, intermedius and medialis)

Exercises:
Cycling, leg press machine, vertical jumping, stair climbing, jump roping,
resisted knee extensions (these are usually incorrectly called the 'leg
extensions' on the machines. They should be called 'knee extension'.)

Muscle:
Gluteus Maximus (Butt)

Function:
Hip extension and external rotation

Exercises:
Squats, cycling, jumping rope, stair climbing

Muscle:
Gluteus Medius & Minimus & Tensor fasciae latae (Outer thigh)

Function:
Hip Abduction

Exercises:
Side-lying leg raises

Muscle:
Adductor Magnus and Adductor Brevis & Longus (Inner thigh)

Function:
Adduction and external rotation and adduction, flexion & internal
rotation, respectively

Exercises:
Side-lying botton leg raises, manual resistance adduction exercises,
resisted adduction

Muscle:
Gastrocnemius (Calf)

Function:
Plantar flexion at ankle (as in pointing the toes)

Exercises:
Hill running, jump rope, calf raises (with free weights), stair climbing

Muscle:
Rectus Abdominus (the Abs)

Function:
Forward spinal flexion (though the focus here is 'joint action', it is
important to remember that the abs are responsible for torso
stabilization and good posture)

Exercises:
the old tried and true Crunches (with or without weights; go to the ABS
page for more info), bent-knee sit ups, though it is important to know
that when doing sit-ups, the hip flexors take over as you progress
beyond a 45 degree angle, so I personally believe that you get better
isolation of the abdominals with just crunches.

Muscle:
External & Internal Obliques (front sides of the torso)

Function:
torso rotation

Exercises:
Twisting crunches (leading with the shoulder towards the opposite knee)

Muscle:
Erector Spinae (back)

Function:
Trunk extension

Exercises:
Prone (face down) back extension exercises, dead lifts (NOT
recommended if you have a pre-existing back condition, or without the
supervision of a certified trainer)

Muscle:
Pectoralis Major (front of shoulders, upper chest area; specifically:
Clavicle, Sternum)

Function:
Flexion, adduction, internal rotation

Exercises:
Push-ups, pull-ups, incline & regular bench press, climbing rope,
throwing

Muscle:
Deltoid (shoulder)

Function:
Abduction (entire muscle), Flexion, internal rotation (anterior fibers),
Extension, external rotation (posterior fibers)

Exercises:
Lateral 'butterfly' exercises with free weights, over-head or 'military'
press with free weights

Muscle:
Latissimus Dorsi (back; specifically the area of lower six thoratic
vertibrae, all lumbar vertibrae, crests of illium & sacrum, lower four ribs)

Function:
Extension, adduction, internal rotation

Exercises:
Chin-ups, rope climbing, rowing, any exercise that invollves pulling the
arms downward against a resistive force as in 'lat' pulls on the machine

Muscle:
Trapezius (back; area of occipital bone, spines of cervical & thoratic
vertebrae)

Function:
Elevation of scapula (upper), Adduction of scapula (middle), Depression
of scapula (lower)

Exercises:
Upright rows, shoulder shrugs with resistance

Muscle:
Rhomboid Major (back; upper region)

Function:
Adduction & Elevation of scapula

Exercises:
Chin-ups, supported free weight bent-over rows (Make sure that you are
not bending at the hip only; your knees should be bent and your "butt"
out to distribute your weight more evenly, otherwise, you are putting
unnecessary stress on to your lower back!)

Muscle:
Biceps Brachii (upper arm, above the elbow)

Function:
Elbow flexion

Exercises:
"Curling" with free weights, chin ups (Use full range of motion when
curling. Keep your "tailbone" to the floor and make sure the only thing
that moves are the elbows flexing and extending!  Never use your back
for momentum.  It's obviously not good for the back, but it also won't do
your biceps any good if another part of the body is doing the work for
them!)

Muscles:
Triceps Brachii (opposite the biceps)

Function:
Extension at the elbow

Exercises:
Dips on parallel bars, push-ups (you can easily do dips on a Reebok
step by placing your hands each side the hips; keep your fingers curled
over the edge;  if this bothers your wrists, place hand held weights each
side the hips on the bench and hold on to the weights while doing the
dips.  You can also place some weights on to your lap to make it a little
harder. Try to use three or four risers on each side of the bench for
these.)
  
           When working your muscles, you
always work against a resistive force.  An
important factor to consider when working
with free weights is that your weights are
not  
the resistive force, but
gravity is!
Therefore, when using free weights, always
work the muscle against gravity.  For
example, doing "butterflies" to work the pecs
while in an upright position will not work them
effectively.  Gravity is pulling down while the
muscles are moving forward. So you would
have to change your position to a supine
position (face up) on the floor (or bench) so
that you are moving in the opposite direction
of the pull of gravity.
Muscle Groups and Their Joint Actions
FLEXION: movement that shortens the angle
between two bones.  Most flexion
movements are forward movements. Major
exception is knee flexion.

EXTENSION: movement that increases the
angle between two bones. Most extension
movements are backward movements.
Exception is knee extension.

LATERAL FLEXION: bending the spine to the
side.

ABDUCTION: moving away from the midline
of the body.

ADDUCTION: moving toward the midline of
the body.

ROTATION: medial or lateral movement
around an axis.

CIRCUMDUCTION: movement in which an
extremety describes a 360 degree circle.

SUPINATION: the lateral rotation of the
forearm, bringing the palm of the hand
upward. In this position, the radius and ulna
are parallel.

PRONATION: medial rotation of the forearm,
with the palm in a downward position so the
radius lies diagonally across the ulna.

DORSIFLEXION:  movement that brings the
top of the foot toward the shin.

PLANTARFLEXION: movement that brings the
sole of the foot downwards (as in pointing
the toes).)

DEPRESSION: downward movement of the
shoulder girdle.

ELEVATION: upward movement of the
shoulder girdle.

SCAPULAR ADDUCTION: backward
movement of the shoulder girdle with
scapulae pulled toward the midline.

SCAPULAR ABDUCTION: forward movement
of the shoulder girdle with the scapulae
pulled away from the midline.

PLANES:
HORIZONTAL PLANE: divides the body into
upper and lower portions.(An example of this
would be torso rotation.)

FRONTAL PLANE: divides the body into front
and back. (Abduction & Adduction occur in
this plane.)

SAGITTAL PLANE: divides the body into right
and left. (Flexion & Extension occur within
the sagittal plane.)