Isotonic and Isometric contractions
To move an object, referred to as load, the sarcomeres in the muscle fibers of the skeletal muscle must shorten. The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. However, muscle tension also is generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.
In isotonic contractions, as the length of the muscle changes and tension is produced a load is moved. There are two types of isotonic contractions: concentric and eccentric. A concentric contraction involves the muscle shortening to move a load. Think concentric circles get smaller and smaller as you move toward the center, so a concentric contraction the sarcomeres get shorter and shorter. An example of this is the biceps brachii muscle contracting when a hand weight is brought upward with increasing muscle tension. As the biceps brachii contract, the angle of the elbow joint decreases as the forearm is brought toward the body. Here, the biceps brachii contracts as sarcomeres in its muscle fibers are shortening and cross-bridges form. You can feel the shortening happen. Place your right hand on your left biceps brachii as the left arm is fully extended. Now flex your arm, keeping your right hand in place. You can feel the muscle get shorter and begin to bunch up under your hand. This is the concentric isotonic contraction. An eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens. In this case, the hand weight is lowered in a slow and controlled manner as tension is released from the biceps brachii, the angle of the elbow joint increases. Eccentric contractions are also used for movement and balance of the body. Just like before, you can feel the elongation happen. Place your right hand on your left biceps brachii as the left arm is flexed. Now extend your arm, keeping your right hand in place. You can feel the muscle get longer and begin to relax under your hand while still maintaining enough force to hold the weight. This is the eccentric isotonic contraction. Both of these movements involve the muscle changing length and a load being moved as a result.
An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint. Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the angle of the elbow joint. In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability. However, holding your head in an upright position occurs not because the muscles cannot move the head, but because the goal is to remain stationary and not produce movement. This is the same principle behind the idea of the yoga workout; muscles produce tension and are being exercised but the goal is to remain stationary and resist movement. Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes.
Figure 4. During isotonic contractions, muscle length changes to move a load. The concentric contraction shortens the muscle to move the load. The eccentric contraction lengthens the muscle to move the load, while maintaining enough force to hold the load. During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate.
