<span>Skeletal muscle contractions pull on tendons that are attached to bones. If contraction of the muscle causes the muscle to shorten, the bone, the body part will move.</span>
You do it before a lab so as to prevent contamination of the materials. You might have things on your hands that might contaminate the experiment and the research. You do it after because you don't want to have things from your experiments on your hands because that can be extremely dangerous.
