Muscles move our bodies. They help us get from one place to another. It’s difficult to think of many times during the day where muscles within us aren’t working. Even while we’re sleeping there are muscles that keep us breathing and help to digest food.
At the simplest level, muscles contract and relax. It is the coordination between different muscle groups contracting and relaxing, which moves us. A good example of this is in your arm. As you flex your arm (curling your hand toward your shoulder), the bicep contracts as your tricep lengthens. It is very important to note that during this movement the tricep is not active. In other words the tricep is not doing any work, nor expending energy. It is passively lengthening.
There are 2 types of contractions that a muscle will undergo. Calling them contractions is somewhat misleading though. It would be better to say there is 2 ways to load a muscle through movement. However, traditionally these ways have been called concentric contractions and eccentric contractions.
In concentric contractions, muscles shorten and generate force. In the arm curl example, this would be the movement where your hand is brought to your shoulder. Your bicep is undergoing a concentric contraction while your tricep is passively and non-actively lengthening.
An eccentric contraction occurs as you load a muscle in the direction opposite to its concentric contraction. A great example of this is when you lower your hand away from your shoulder with a weight in hand. The bicep will be lengthening but is still the muscle responsible for the movement of the weight. For all purposes, the tricep is passively (non-actively) shortening in this instance.
Now armed with this knowledge of the different types of muscle contractions, you can begin to understand the finer details of muscle physiology, and the sliding filament theory and excitation-coupling. Sliding filament theory and excitation-coupling are two of the foundational concepts required to best comprehend how muscles work. And those are the topics of my next blog entries.