Running is a form of eccentric exercise, meaning that the muscle is forced to lengthen while trying to contract.
Prolonged or unaccustomed eccentric exercise often leads to pain, tenderness, and stiffness in the muscles hours or even days later. This is known as delayed onset muscle soreness and is very common.
The sensation of discomfort usually develops approximately 24 hours after exercise, peaks at about two days, and then gradually subsides. During the 24 to 48 hours post-exercise period, muscle swelling and stiffness usually result in a reduced range of motion and also muscle weakness.
Because the onset of muscle soreness is delayed, it cannot be attributed to the metabolic end products of exercise.
In fact it is caused by localized damage such as microscopic tears to the membranes and protein filaments of muscle fibers. One hypothesis is that the damaged muscle cells die because they are subject to excessive calcium inflow. Another is that exercise-generated free radicals attack the cell membranes, and this attack leads to their death.
In addition, there is increased blood flow to the muscles, which causes the tissue to swell. Such swelling increases pressure on the neighboring structures. The nerves in the muscle sense this and send pain messages to your brain as you move the morning after you have exercised.
Delayed onset muscle soreness, or DOMS, is a result of an excessive amount of tearing in the muscle.
In order to improve performance when we exercise we need to progressively challenge our muscles with the amount of work we expect them to perform. This progressive overloading, usually achieved by increasing the resistance they experience such as by lifting heavier weights or by running extra distances on successive days, causes tears in the muscle’s microfibers.
And, in a gradual overload/repair cycle, we experience moderate soreness up to a day later.
DOMS is caused when the expected load dramatically increases, causing a greater number of tears, rather than an increase in the magnitude of each tear. In this situation it takes longer for scar tissue to form because it grows in perpendicular fashion across the repair sites.
Once the new tissue is in place, we experience the soreness that comes with DOMS as we reactivate and stretch this new, less pliable muscle, until its strength and flexibility are restored.