Why do our muscles ache?

Normally, when our muscles contract they shorten and bulge,

much like a bodybuilder’s biceps. But if the muscle happens to
be stretched as it contracts it can cause microscopic damage.
The quadriceps muscle group located on the front of the thigh
is involved in extending the knee joint, and usually contracts and
shortens to straighten the leg. However, when walking down a steep
slope, the quadriceps contract to support your body weight as you step
forward, but as the knee bends, the muscles are pulled in the opposite
direction. This tension results in tiny tears in the muscle and this is
the reason that downhill running causes so much delayed-onset pain.
A muscle is made up of billions of stacked sarcomeres, containing
molecular ratchets that pull against one another to generate
mechanical force. If the muscle is taut as it tries to contract, the
sarcomeres get pulled out of line, causing microscopic damage. The
muscle gets inflamed and fills with fluid, causing stiffness and pain.