Why do muscles get sore after a workout?

Muscle soreness after a workout is primarily caused by Delayed Onset Muscle Soreness (DOMS). This occurs when you perform exercise that is more intense or different from what your body is accustomed to, particularly movements that involve "eccentric" contractions—where the muscle lengthens while under tension, such as the downward phase of a bicep curl or running downhill.

Here is what happens inside your muscles:

• Microscopic Tears: The physical stress of the exercise creates tiny, microscopic tears in the muscle fibers and the surrounding connective tissue.
• Inflammatory Response: These micro-tears trigger an inflammatory process as your body works to repair the damage. Your immune system sends cells to the area to clear away debris and begin the rebuilding process.
• Fluid Accumulation: This inflammation leads to localized swelling and an accumulation of fluids around the tissue, which increases pressure on the nerve endings in the area, causing that familiar dull ache or tenderness.
• The "Repair" Cycle: Contrary to the old myth, this soreness isn't caused by a buildup of lactic acid; lactic acid is typically cleared from the body within an hour or two after your workout. Instead, the soreness is a sign that your muscles are undergoing a process of remodeling. As they repair those micro-tears, they actually become stronger and more resilient to handle the same workload in the future.

This is why soreness often feels worse 24 to 48 hours after a session—it takes time for the inflammation and cellular repair signals to peak. Gentle movement, hydration, and adequate sleep usually help the body manage this process.