I have been drawing muscles as contracting by shortening for you. That’s a bit of an oversimplification. What is certainly true is that whenever a muscle contracts, it does so by shortening of sarcomeres (or at least by trying to shorten its sarcomeres).
Have you ever tried to pick something up that was really heavy, only to find out that you couldn’t lift it? Or, how about, have you ever pulled on something, like a door handle, but didn’t open it? In these situations, you contracted muscles, but they did not shorten.
There are two major terms for types of contractions: isotonic and isometric. The isotonic contractions are ones when your muscles actually shorten. The isometric contractions are ones when your muscles don’t shorten. Your book has the following figure it uses to try to distinguish these two types:
Consider what these terms “isotonic” and “isometric” mean by picking them apart. “Iso-” always means “the same.” So you can have a same-tonic or same-metric contraction. Now let’s consider the suffixes.
Muscle always has tone. What is muscle tone? Our muscles are not limp, but there are some connected cross-bridges that keep them somewhat taut.
So, isotonic means that a muscle has the “same tone” throughout its movement. In other words, the muscle maintains tone and never gets saggy while shortening. If you think about this, it should make sense. If I were to bend your arm for you, so that your forearm flexed, the muscle in the diagram above would be shortened by an external force (me) rather than by electrical activation from the nervous system. If I do the flexing for you, then, as your muscle was externally shortened, it might go saggy. However, if you voluntary contract this muscle, causing the shortening yourself (through nervous system activation), the muscle will never get saggy. It will maintain its tone throughout your movement. Got it? Same tone?
Whenever you see the term “metric,” you should think of meters in the metric system. Meters are length measurements. Therefore, isometric means “same length.” If you contract your muscle but don’t let it shorten, the muscle maintains its same length.
The only confusing point here is how does the contraction occur if the muscle doesn’t change length? Well, it doesn’t change length from attachment site to attachment site, but it does pull at its tendons, stretching the tendons a little bit while the muscle tissue pulls in a bit. This is what happens when you flex your arm and show off your muscles. If you hold your arm flexed and then increase your contraction force (but don’t bend your arm any more), you will see a bulge in your upper arm that is due to the isometric contraction you just caused. The reason that it is still considered isometric is because this contraction force did not produce any movement (additional flexion or extension), but your limb remained fixed in position.