Picture this: you’ve been sitting on the couch, watching replays of the Tokyo Olympics, for the past three hours. “I should go for a jog,” you think to yourself as you watch the best athletes in the world hurl themselves into the air, cut through water like sharks, and travel faster on two feet than you’re allowed to drive your car in your neighborhood. You peel yourself off the couch, lace up those dusty shoes from the back of the closet, and head off down the street. Half an hour later, you’re feeling sweaty and accomplished, but the next morning, your legs feel like sore bags of wet cement. While it happens to everyone, even the Olympians on your screen, the reasons for the soreness and fatigue that come from a hard workout are not well understood or agreed upon. Let’s take a closer look at what might be happening in those quads.

Movement requires muscle contraction, or the shortening of bundles of muscle cells called myofibrils. To make those contractions happen, proteins in the myofibrils called myosins generate force by pulling against long structural proteins called actin. As these myosins cycle through attaching to, pulling on, and releasing the actin filaments like tiny ratchets, they shorten the myofibrils, ultimately resulting in the bending of your knee during a sprint or the lifting of your forearm during a bicep curl. That pulling and releasing cycle is fueled by ATP, the body’s energy currency.

ATP for muscle contraction can be made in two ways. In a resting muscle cell with plenty of oxygen, ATP is made through aerobic breakdown of glucose, or sugar. This process is highly efficient, making about 30 ATP molecules for every molecule of glucose broken down. However, when oxygen is in short supply, like during hard exercise, the muscles rely on anaerobic glucose breakdown, otherwise known as the process of glycolysis. Glycolysis generates many fewer ATP molecules and also produces a byproduct called lactic acid. A buildup of lactic acid in the body during a workout is called lactic acidosis and is responsible for the muscle burning, nausea, and exhaustion you feel when you go too hard doing cardio.

However, contrary to popular belief, lactic acid buildup in muscles is probably not the cause of that pesky pain the day after exercise, or delayed onset muscle soreness (DOMS). Lactic acid concentration in runners’ blood is not correlated with their level of soreness, and that concentration returns to its pre-exercise baseline within about an hour of finishing a workout. Further, lactic acid rapidly gets turned into lactate in the blood, and lactate is actually a key player in metabolism, thanks to its ability to be used as fuel by the mitochondria in muscle cells. So although lactic acid has been vilified in the sports world for years, it’s not to blame for DOMS and may actually play a part in helping muscles work more efficiently.

While the precise cause of DOMS is still debated, most exercise physiologists actually point to microtrauma: teeny tears in the muscle caused by exercise, which cause inflammation and the tenderness that makes getting out of bed the next day hard. The good news? As those tears heal, the muscle strengthens, making you more resistant to DOMS if you keep up the exercise regularly. 

So the next time the Olympics inspire you to go out for a run, you can thank ATP, myosin, and lactate for making your legs move!

Peer Editor: Jillian Battista

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