Weightlifting is an activity that builds up your muscles as you resist an opposing force or weight. If you use enough weight to increase your muscle size, you will typically feel a “burn” as your muscles reach their working capacity. You can also experience soreness when your muscles break down and repair themselves in the period following your workout.
When you lift heavy weights, your body gets the energy to support your activity by breaking down a substance called glucose. When glucose breaks down, it forms another substance called pyruvate. As you continue working your muscles, pyruvate turns into another substance called lactate, or lactic acid, which lets you keep lifting for a little longer. However, lactic acid also builds up rapidly in your muscles and triggers the “burn” commonly associated with weightlifting. This sensation acts a signal from your muscles to stop working and rest your body.
Building Muscle Tissue
Weightlifting triggers small-scale damage in the tissues of your affected muscles, according to Young sub Kwan and Len Kravitz of the University of New Mexico in the article "How Do Muscles Grow?" In turn, damage to your muscle fibers triggers activity in nearby cells called satellite cells. These satellite cells fuse to each other, as well as to your injured tissue. The cells that fuse to your tissues rejuvenate your existing muscle by filling in for damaged cellular structures. The cells that fuse to each other form new protein structures called myofibrils, which in turn add extra muscle tissue and muscle mass.
Delayed Muscle Soreness
The after-exercise soreness associated with weightlifting or other heavy activities probably comes from the damage to your muscle tissue, as well as the release of various breakdown substances into your nearby tissue, professor Stephen M. Roth explains in “Scientific American.” Typically, this soreness arises within 24 hours following exercise and reaches its peak within the next couple of days; for this reason, it is frequently known as delayed onset muscle soreness, or DOMS. Most of the soreness associated with DOMS occurs when you lower a lifted weight and subject your muscles to a form of contraction called an eccentric contraction.
You typically have to lift enough weight during any given exercise to tire your muscles after roughly 12 repetitions. If you perform more than one set of your exercise, you must rest long enough to reduce your lactic acid levels and restore your muscles’ working capacity. To give your muscles the chance to rebuild and grow after you lift, you shouldn't work the same part of your body two or more days in a row. DOMS-related pain is usually mild. If you feel sharp pain, or experience pain or swelling in your associated joints, you’re probably using too much weight or performing too many exercise repetitions. Consult your doctor and a certified trainer for more information.