Isotonic Contraction
The muscles in our body comprises more than 70% water and nearly 22% of muscle tissue is protein, containing a millions of strands of actin, a thin protein and myosin, a thick protein. Muscles basically contract when the actin and myosin protein strands slide over each other, making the length of the cell to shorten. Muscle contraction is controlled by the central nervous system of our body and takes place due to conscious effort initiating in the brain. Actually, the brain transmits signals in the form of action potentials, through the nervous system to the motor neuron which innervates many muscle fibers. In the case of some reflexes, the signal to contract or condense may originate in the spinal cord through a feedback loop with the gray matter. Involuntary muscles including the heart or smooth muscles in the gut and vascular system, condense due to unconscious brain activity or stimuli proceeding in the body to the muscle itself. However, there are three more types of muscle contractions besides isotonic contractions, they are: eccentric, isometric and isokinetic contractions.
Isotonic Contraction
Isotonic contraction is a type of muscle contraction in which the tension remains unchanged and the distance between the muscle's origin and insertion becomes lessened. They are the most common and many exercises and physical activities involve this type of contraction. In fact, all lifting exercises involve isotonic contractions.
A simple example is flexing or bending the bicep muscle. Here, the isotonic contraction can be analyzed, by standing with one arm straight and the palm of the hand facing up. Then, roughly measure the length from the start of the biceps muscle to the end where it meets the shoulder. Now, curl the hand towards the shoulder, and the biceps muscle will shorten due to contraction. When you reach the end point, take another rough measurement of the biceps again, you will notice that it will be much shorter than the previous measurement. Here are some more examples:
- Lifting objects above the head, front shoulder (anterior deltoid) shortens
- Lifting object up from lying position, chest muscle shortens
- Lifting body up from squat position, quadriceps muscle shortens as legs extend
- Doing a sit-up, throwing a ball and even swinging a bat, the muscles shorten
Concentric Isotonic Contraction
In a concentric isotonic contraction, the muscle tension increases to meet the resistance, then remains the same as the muscle shortens. Here, the external force on the muscle is less than the force the muscle generates, causing a shortening contraction. It occurs frequently in our daily and sporting activities and its effect is visible during classic biceps curl.
Eccentric Isotonic Contraction
Eccentric isotonic contraction is completely opposite of concentric and takes place when the muscle lengthens as it contracts. In this, the absolute tensions achieved can be very high relative to the muscle's maximum tetanic tension, hence increasing muscle capacity to bear heavier weights that normal. This is less common and normally involves the control or deceleration of a movement being originated by the eccentric muscles. For example, kicking a football, wherein the Quadriceps muscle contracts concentrically to straighten the knee and the hamstrings contract eccentrically to decelerate or slow down the motion of the lower limb. This type of contraction exerts a lot of strain through the muscle and can cause muscle injuries.
Isotonic Muscle Training
Isotonic muscle training involves contractions where tension is uniform throughout the range of motion. It basically involves the contraction and shortening of a muscle to allow movement and is done using exercise equipment like dumbbells, barbells or elastic resistance bands. This muscle training technique uses both eccentric and concentric movements. When the weight is lifted, the movement is considered as concentric, and when the weight is returned to the initial position, the movement is known as an eccentric movement.
Isotonic muscle training benefits by strengthening and toning the muscles throughout the entire range of movement, and also improves the joint mobility. However, its only disadvantage is that it can make muscle sore due to stress and the muscles acquire strength at the weakest point of the action, rather than evenly throughout.
