Have you ever wondered why your legs felt heavy and sore at the height of a hard workout? This feeling may be tied to fatigue caused by consecutive workouts and not a lot of rest. However, the common reason for this feeling stems back to the body’s work while we exercise.
As we exercise, our muscles are breaking down carbohydrates into sugars that can be used for energy production. When our bodies demand energy during exercise, our muscles work to break down sugars in the presence of oxygen. If the body’s energy demand requires more oxygen than what is available, the muscles switch over and use anaerobic metabolism to break down carbohydrates and supply more energy to the body.
Anaerobic metabolism is when our bodies create energy without oxygen, while aerobic metabolism is when our bodies make energy by using oxygen to break down proteins, carbohydrates, or fats. Our bodies usually switch back and forth between these processes if we are doing a workout with high-intensity segments, but generally, our body uses aerobic metabolism for an activity that requires the production of energy over a sustained amount of time (1).
BY-PRODUCT OF ANAEROBIC METABOLISM
When our muscles do use anaerobic metabolism, they create lactic acid as a by-product. Often, people directly associate lactic acid with muscle fatigue and soreness. However, lactic acid does not directly cause fatigue or tiredness; rather, it is the hydrogen ions that are created, when lactic acid is broken down, that does cause some pain during a hard workout (2). When the body breaks down lactic acid, it creates hydrogen ions and lactate.
The hydrogen ions may create acidity in your muscles during the workout, but they are not the reason you feel sore the days after your workout. It might have been a reason, but our bodies quickly clear up lactate in our muscles when we stop exercising. If it were the reason for soreness, there would be little reason to explain why people feel sore after long runs or other long workouts where aerobic metabolism is predominantly used. In result, it is correct to say that you may have built up lactate in your muscles during a hard workout. However, that is not the reason that you may be tired and soon to be sore. On the contrary, the lactate that builds up in our muscles decrease the effects of depolarization, which creates fatigue (3).
If lactate build-up is not the reason behind soreness, what is?
Delayed onset muscle soreness (DOMS) is a sign of minor damage to your muscles (4). It tells you that your body is getting used to new physical demands and, therefore, getting stronger. When we workout, microscopic tears form on our muscles (5). In this state, the damaged muscles cause us to feel sore a couple days after a workout. Even though our muscles are being torn slightly, this damage is not bad. When muscles rebuild themselves, they are able to handle the same physical demands better (6).
Soreness may be a sign of future growth, so next time you feel sore after a hard workout, smile because you may be becoming stronger.
References and Footnotes
- Bumgardner, Wendy. “Anaerobic Metabolism vs. Aerobic Metabolism in Exercise.” Verywell Fit, Verywellfit, www.verywellfit.com/anaerobic-metabolism-3432629
- Miller, Joe. “Muscle Fatigue & Soreness from Lactic Acid.” LIVESTRONG.COM, Leaf Group, 11 Sept. 2017, www.livestrong.com/article/376532-muscle-fatigue-soreness-from-lactic-acid/ .
- Fitzgerald, Matt. “Six Lies You Were Taught About Lactic Acid.” Competitor.com, 18 Jan. 2016, running.competitor.com/2014/01/training/six-lies-you-were-taught-about-lactic-acid_29432.
- There are a lot of other potential causes of muscle soreness and not all are known. This is just a common cause.
- Crown Copyright. “Why Do I Feel Pain after Exercise?” NHS Choices, NHS, 30 Nov. 2017, www.nhs.uk/live-well/exercise/pain-after-exercise/ .
- Lowry, Vicky. “Work Out Now, Ache Later: How Your Muscles Pay You Back.” The New York Times, The New York Times, 16 Nov. 2004, www.nytimes.com/2004/11/16/health/nutrition/work-out-now-ache-later-how-your-muscles-pay-you-back.html .