Muscle contractions carried out by the central nervous system, occur when the brain sends a signal to the motor neuron, via the nervous system to activate the muscles. The motor neurons innervates the muscle fibers by generating tension, via the cross-bridging of muscle fibers, called actin and myosin. Even though the word 'contraction' implies shortening, in this case we are actually referring to the tension generated by the muscle fibers in response to the motor neurons.
Muscle contractions occurring in the body can be classified into four categories: isotonic, eccentric, isometric, and isokinetic. An isometric contraction is that type of contraction in which the muscle is activated, however, instead of shortening or lengthening, its length remains constant. There is no movement at the joint, when the contraction takes place.
Example of Isometric Contractions
Carrying a Heavy Object
The best and most common example used to explain isometric contraction is carrying an object in front of you. While carrying the object and walking, the weight of the object being carried will be acting downwards, while your hands will be acting in the opposite direction. The upward pressure exerted by the hands equals the downward pressure exerted by the object, thus, enabling you to carry the object to the required destination. At this stage, the biceps in your arms are isometrically contracting, and preventing your arms from raising or lowering their position.
Gripping a Tennis Racket
Another example of such a contraction is when we grip a tennis racket. During this action, the joints of the hand do not move, however, the muscle contracts, thereby, providing enough strength and force to the hand to hold on to the racket. The force produced as a result of this contraction, actually depends on the length of muscle involved at the point of contraction. The maximum isometric force that a muscle can produce will depend on each muscle's optimum length.
Isometric Contraction Exercises
Isometric exercises are strength-training exercises used by bodybuilders. In these exercises, the joint angle and muscle length remain unchanged. Moreover, the best part about these exercises is that they can be used for general, as well as rehabilitation strengthening, without stressing out the joints of the body. These exercises help us develop the static strength required to pull or push heavy objects. Moreover, since they can be easily carried out without the need for expensive exercise equipment, these exercises are even more favored. However, the popular notion that isometric exercises increase muscular endurance is not true! Isometric exercises increase static strength, only in the angle at which the exercise is carried out.
Isometric contractions are part of our daily activities, although we rarely seem to recognize them. Right from holding a bag of groceries to holding a pile of books, etc., all involve such contractions. So, the next time you lift an object in front of you, you know which forces are behind it!