Normally, when our muscles contract they shorten and bulge, much like a bodybuilder’s biceps. However, 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, say, 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.
At the microscopic level, 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 becomes inflamed and fills with fluid, causing stiffness and activating pain receptors – hence that achy feeling you get after unfamiliar exercise.
Pain – The soreness associated with exercise is the result of repetitive stretching of contracted muscles.
Straightening – As the arm straightens out, the biceps are stretched, but the weight is still pulling down on the hand, so the muscles remain partly contracted to support it.
Bending – Normally when the biceps muscle group contracts it shortens, pulling the forearm towards the shoulder.
Stretching – As the muscle tries to contract, the weight pulls in the opposite direction, causing microscopic tears within the muscle cells.