MUSCLE CONTRACTION
When one thinks of muscle contraction, he or she may visualize a familiar weight training exercise:
maybe an arm curl for the biceps, a leg extension for the quadriceps, a lat pull-down for the back, or other
activity. In most of these instances, the muscle or muscle group shortens and lengthens to complete the
exercise. This form of traditional strength training is known as isotonic training. In isotonic movement,
muscle contraction results in free, dynamic movement at a joint in the body. A muscle or muscle group
shortens, which is known as a concentric contraction, and the muscle or muscle group also lengthens,
which is known as an eccentric contraction.
Example #1: Dumbbell Curl, Isotonic Training
Start: from a standing position with arms at Finish: the person straightens the elbows,
the sides, the person brings the dumbbells
returning the arms to the sides. This action
upward by bending the elbows. This action lengthens the biceps, which is an eccentric
shortens the biceps (muscle in the upper
contraction.
arm), which is a concentric contraction.
Both of the images above are in the public domain, contributed by Everkinetic
Example #2: Bench Press, Isotonic Training
Start: from a supine (lying down on one’s
Finish: the person straightens the elbows,
back) position with arms straight above
returning the arms above the head. This
one’s head, the person lowers a barbell
action shortens the chest (and triceps)
toward the chest. This action lengthens the muscles, which is a concentric contraction.
chest muscle group (it also lengthens the
triceps in the back of the upper arm), which
is an eccentric contraction.
Both of the images above are in the public domain, contributed by Everkinetic
Although isotonic training is the most common form of muscle contraction seen in a workout class or
fitness center, isometric training is also common. In isometric training, the muscle exerts force but does
not shorten or lengthen, but instead remains at the same length. Large gains in muscular strength and
muscular endurance can occur with isometric training, but most of the fitness gains are specific to the
angle at which the exercise is performed.
Example #1: Side plank, isometric training
Start and Finish: The individual places a forearm on the floor as well as either the knees
or feet, supporting the position by using transversus abdominis muscles in the torso.
The above image is copyrighted by Rob Wood, though can be used for non-commercial
purposes.
Example #2: Flexed arm hang, isometric training
Start and Finish: The individual uses a chair to position him- or herself behind a chinup bar. The hands grasp the bar, the head is above the bar, and the chair is taken away.
The person holds him- or herself in the start position for as long as possible.
The above image is in the public domain, due to its being taken by someone working
for the federal government (U.S. Navy) while on duty.
A third form of muscular strength and endurance training is isokinetic training. Like isotonic training,
isokinetic training incorporates muscle shortening and lengthening to allow free, dynamic movement at a
bodily joint. In isokinetic training, however, specialized machinery regulates the movement so that,
regardless of the force exerted, the speed of the movement is maintained. In normal joint motion, muscles
involved in movement are stronger at certain angles and weaker at others. In isokinetic exercise, the
resistance of the machine adjusts as needed throughout the movement to ensure that the speed remains
constant. Because isokinetic equipment accommodates the user, it is often used in rehabilitative settings.