Why do you get sore muscles after a workout? Is it because you forgot to stretch previous before exercise? Maybe the overall was to rigorous and had you out of breath? Is it because you have not worked out in a while? Well, all of these may be the reason for sore muscles. But there are a couple theories that could explain the post physical trauma and pain days after you hard work out.
Sore muscles can be explain by one overall concept : Delayed onset muscle soreness (DOMS) This is the muscles soreness that occurs post-workout. Delayed onset muscle soreness can be explained by two theories : the mechanical trauma theory and local ischemic theory.
Explaining sore muscles via the mechanical trauma is the direct structural damage to the anatomy of your muscles (specifically muscle fibers) from constant contractions. As the tissue is damage, Calcium (the nutrient that aids in muscle contraction) disrupts normality, so the tissue starts to die. Your immune system reacts to heal the damaged muscles, therefore swelling and inflaming that specific area. As a result sore muscles come from the painful sensation of delayed onset muscle soreness.
Another possible theory of having sore muscles is the local ischemic theory. Just like the first theory, it is also initiated by atraumatic activities that rip muscle fibers, and cause swelling of the muscle tissue. This swelling leads to an increase of tissue pressure, which prevents blood flow this area (ischemia). No blood flow, no healing, therefore induces pain. Your muscles may also spasm (which are involuntary muscle contractions that come randomly, and cause pain). This pain may be the ultimate reasons behind your sore muscles.