What Causes Muscle Soreness?

I’ve experienced muscle soreness from exercising regularly since being an athlete from a young age. I also get a lot of feedback from friends and clients about how sore their muscles are after a great workout sesh I’ve recommended or class I’ve taught. (Sorry, not sorry!) 😜

Most of us have been there, but do you know exactly why your muscles are feeling so achey and sore after a great workout?

For years I had coaches and trainers speak to the presence and build up of lactic acid in your muscles that causes soreness. I dug into the research a bit further, and it seems like our thoughts on lactic acid might not be so accurate.


Let’s talk about what lactic acid actually is and does. Muscles produce lactic acid as a fuel, producing it from glucose (sugars) and burning it as fuel to obtain energy. But it turns out that lactic acid fuel disappears from your muscles within about an hour of exercising - so no build up. To get even a little more science-y on you, the understanding now is that muscle cells convert glucose or glycogen (stored sugars) to lactic acid and is then taken up and used as fuel by the mitochondria, or the energy factories, in your muscle cells. As the intensity of your training increases, the more mitochondrial mass builds in each muscle cell.

Ok, so what’s the answer then? Why are my glutes so sore after a kick-ass leg workout? Turns out muscles soreness, which is medically referred to as delayed onset muscle soreness (DOMS), is understood to be caused from microscopic tears in muscle tissue during eccentric muscle movement. Those tears cause a certain type of controlled trauma to our muscle fibers, causing the feelings of soreness.

Within 2-3 days and sometimes even 7-8 days post exercise, your body responds by repairing those muscle fibers, increasing muscle mass and improving strength through the process of skeletal muscle hypertrophy.

Muscles soreness = good. Getting stronger = hell yeah!