When a voluntary, or skeletal, muscle receives the signal to contract, ionic changes inside of the muscle cell lead to actin filaments sliding across myosin filaments. The contraction of these filaments is what allows muscle to generate the force that is needed to do work. There are numerous times when the contraction of actomyosin complexes does not lead to the shortening of the muscle fibers. Muscle can undergo several different types of contractions.
Isometric Contractions
In this type of contraction the muscle generates the needed force without changing its length. This happens because other forces are acting to keep the muscle from shortening.
For example, when someone carries a package on their outstretched arms, the load is forcing downward with both its weight and the force of gravity. The arm and hand muscles supply just enough counterforce to keep the package from falling. The muscles of the upper arm (biceps and triceps for instance) are staying the same length yet they are still actively contracting. Another example of this is when people do isometric exercises, making their muscles work against each other without any muscles changing their lengths at all.
Concentric Contractions
This is the type of contraction that is most familiar. Actomyosin inside of the muscle contracts and the muscle shortens in response while the force is generated. An example of this would be someone doing curls with a barbell. As the curl is begun, the biceps muscle generates force and shortens, lifting the barbell. When the muscle relaxes its length again increases.
Eccentric Contractions
Eccentric contractions are probably a little difficult to visualize. For this type of contraction force is being generated while the muscle is actively lengthening. But this type of contraction is actually quite commonplace. When the package that was being carried in the above example is at the spot where it needs to be put down, eccentric contractions play an important role. In order to place the package down slowly and with care, the biceps muscle actually generates force while it is slowly lengthening due to the weight of the package.
The fact that the vast majority of our skeletal muscles act via their attachment to tendons at either end and usually traverse a movable joint doesn’t always mean that they must get shorter when they do work. In many circumstances the muscle needs to do work but does not need to shorten its length to accomplish the needed task. In fact, for many muscles eccentric contractions are capable of generating more force than is generated when the muscle actively shortens during concentric contractions.
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