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John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.
Did you know that your muscles are contracting even when you're trying to rest? Skeletal muscles are contractile organs composed of several motor units. A motor unit includes all the muscle fibers innervated by a single motor neuron, and at least some motor units are always contracting - again, even when we're at rest. The contraction may not be enough to produce movement, but they keep the muscle at a resting level of tension or tone.
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Muscle tone is the resting tension in the skeletal muscle, and it helps to stabilize the position of the bones and the joints. This lesson will describe the nature of isometric contractions, in which the muscle does not change length, and isotonic contraction, in which the muscle length changes without additional tension or force development.
Isotonic literally means 'same tension,' where 'iso'- means 'the same' and 'tonic' refers to 'tension or force' - in this case, in the muscle. In an isotonic contraction, tension develops to a point and then remains constant while the muscle changes its length. In other words, motor units are activated to develop the necessary tension in the muscle. We utilize isotonic contractions when we move our limbs. Let's consider an example.
Let's consider a man attempting to curl 25 kilograms. This movement requires the use of the biceps brachii, a.k.a. our guns. Before the muscle can shorten and thus move the 25 kilogram load, it must first develop slightly more than 25 kilograms of tension. Once enough motor units are activated to produce tension slightly greater than the load, the muscle can shorten. If you try this yourself, you will notice tension developing in your muscle even before you move the weight. Once muscle tension slightly exceeds the load, muscular contraction can then be used to shorten the muscle and move the load. It's important to note that force does not change in the muscle while it's shortening - hence the term isotonic contraction.
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It's also important to note there are two types of isotonic contractions. Concentric contraction occurs when the muscle shortens, while eccentric contraction occurs when the muscle lengthens. Let me explain. Let's re-examine our weight lifter. The muscle shortens when the arm flexes at the elbow in the act of curling - therefore, this is a concentric contraction.
Does the muscle contract during extension while doing a curling exercise? As it turns out, the answer is yes. As the man extends his arm, that same muscle, the biceps brachii, lengthens and maintains tension slightly greater than the load - in this case, slightly greater than 25 kilograms. This tension is maintained by eccentric contraction. Eccentric contraction helps to control the rate of movement.
Let's move on to isometric contractions. Isometric literally means 'same length,' where again 'iso'- means 'the same' and here 'metric' refers to length - now, we're talking about the length of the muscle. During an isometric contraction, the muscle as a whole does not change its length, and the tension developed in the muscle never exceeds the load. This is what happens when we attempt to lift more than we can.
Let's take a look at our weight lifter one more time. Let's imagine his attention is diverted by a person of interest walking by. While he's looking away, a mischievous friend replaces the 25 kilogram weight with a 50 kilogram weight. Now, the weight lifter attempts to make an impression by curling the weight, not knowing that it has doubled in size. Much to his dismay, he's unable to move the weight. Why not? Simply put, he was not able to develop enough tension in the muscle to slightly exceed the load - in other words, 50 kilograms was too much. Even with all of the motor units activated, he could not develop enough tension.
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Let's imagine that we could measure the tension developed in the muscle during this isometric contraction. As you can see on the image above, the tension increases to 40 kilograms and then remains constant the whole time our friend is trying to move the weight. As the muscle does not change its length, it is referred to as an isometric contraction, and again, he can't curl the weight.
Our daily activities utilize both isometric and isotonic contractions. For example, our postural muscles are undergoing isometric contractions to keep your head up while you watch this lesson. Some of those same muscles undergo isotonic contraction in order to move your head to the left or to the right.
In summary, skeletal muscle contractions are either isometric, in which the muscle does not change its length, or isotonic, in which the muscle changes its length without increasing tension. Each muscle is composed of several motor units, and these motor units include multiple muscle fibers innervated by a single motor neuron. When attempting to move a load, motor units are activated to develop tension slightly greater than the force of the load. Once the load is exceeded, contraction can then be used to shorten the muscle and move the load. If a load exceeds the ability of the total motor units within a muscle, shortening simply cannot occur.
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