Why do my muscles get sore after exercise?

You know the feeling. You wake up the day after you’ve started a new workout routine and you are stiff and sore all over. What happened?

Damage, that’s what. And you can thank your immune system for the pain…

Muscle soreness myths

But first let’s just talk about what is NOT causing the pain: Lactic acid. I’ll talk more about Lactic acid in a future post, but right now I want you to know one thing: the burning you feel in your muscles when you exercises IS caused by lactic acid (scientifically – it’s a build up of hydrogen ions which slightly lowers the pH of you muscle’s chemistry). But then you stop and the burning goes away. About 45 seconds later, the lactic acid is almost all gone from the muscle. So let’s be clear, the Lactic Acid DOES NOT cause the pain you experience the next day.

What really causes muscle soreness

4.365 Workout
The morning after the night before (of exercise)
Remember when we talked about how eccentric contractions cause damage to the muscle and then the muscle adapts by creating more sarcomeres? If you don’t please go back and read “Your muscles are out of breath but you’re getting stronger”. Once this damage has been done, your immune system kicks in. (as it does with any infection or other kind of tissue damage). Immune cells and chemicals flood the area where the damage has occurred, with all a manner of biological repair machinery – and in the process it destroys some healthy tissue. Your immune system isn’t perfect.

The pain, which usually takes 24 hours to really get started, is from the immune reaction to its own imperfect healing process. The extent of the damage caused by the exercise will dictate the degree and length of pain you experience. Some people might have a day of general soreness to the exercised area and about 48 hours later they’re ready to damage the muscle again. Take note, if you overdo your workouts you might have a week of really uncomfortable, achy soreness.

After damage, muscle growth begins

The interesting thing about this damage/repair cycle is that only one exposure is enough to create a noticeable adaptation. Let me explain: imagine you decide to do three rounds of those eccentric push-ups I explained in a previous post. Say you were able to do 5 really good slow ones in each round. Then 24 hours later you’d probably feel some pretty good soreness in your chest, triceps, and your abdomen. Now, if you repeated the exact same process one week later you would not have the same level of soreness as you did the first time you did the exercise. You will probably still be sore but not to the extent you were the first time. If you want to feel that same level of soreness gain, you’d have to add in another push up or two per round. Or do a 4th round. You get the picture right? After you’ve adapted, you have to make the stress slightly harder to create the same amount of damage.

I don’t advise that you try and go harder every time you exercise. I just want to explain what is happening to your muscles, over time, in an understandable way.

So let’s sum up:

  1. Lactic acid causes a burning sensation at the time of the exercise.
  2. Muscle damage occurs.
  3. Then your immune response causes the delayed muscle soreness the next day.

Heal, and repeat.


Next up: The truth about Muscle ‘Toning’! – Stay tuned by following Guru on Twitter or Facebook

Read all of Matthew’s articles on health and fitness here.


Article by Matthew Linsdell

February 15, 2012

Matt is a certified personal trainer and has a degree in Environmental Science. He calls himself an evidence-based trainer, because training is a field which is littered with well-disguised pseudoscience – his emphasis is always on teaching the biology behind exercise. He lives at the edge of the beautiful and expansive Gatineau Park in Quebec and works across the water in Ottawa, Ontario. If he’s not out walking his two pit bulls, you’ll find him doing press ups with insanely large weights on his back. Follow Matt on Twitter at @smartfitmatt.

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8 thoughts on “Why do my muscles get sore after exercise?”

  1. Great reading
    Thanks so much
    I cycle a lot
    A lot for a 56 y/o
    I recently endured a 170 k ride
    I enjoyed the ride & suffered the normal pain the next day
    But the pain continued in the hamys,& lower back for a further 10 days
    I have been diagnosed with extremely low testosterone .
    How much do you think this played a roll in my recovery
    And how much protien should I consume after the event ?
    Thanking you from down under

    1. Hey Rollsy, I really can’t comment regarding how low testosterone will affect recovery. That is a complex issue and goes beyond my expertise. But I will say that 170km is a long distance. I don’t know how adapted to rides of that duration/distance you are but unless you had ridden several rides close to that distance in the months leading up to the 170km ride then you are bound to have many days of soreness.

      Cycling is thought of as a “low impact” activity. Many people think that because it is low impact you can do as much as you want without fear of over training. But low impact is not “no impact”. Cycling a distance like that WILL cause damage to all manner of tissues and the immune response will kick in big time. The tissues in your body that were least adapted (lower back and hamstrings) will feel the most sore for the longest because that is where the most damage was done.

      Another aspect for this prolonged soreness is your age. As we age it takes longer to recover from exercise induced damage. Assuming you were the same level of fitness, you could have done the same activity when you were 20 years old and only suffered 3 or 4 days of soreness.

      One other point to consider is your position on your bike. You might want to consider a shorter stem to take the strain off your lower back or even a different seat position. I used to work in a bike shop in Adelaide and the owner had us size people to their bike the same way every time. This was ridiculous because we all have different rations of bone length.

      Experiment with the configuration of your bike. Seek assistance with this but don’t assume that just because you ask someone who owns a bike shop to help you that you are getting the best advice.


      Try exposing yourself to the stresses of a long distance ride more gradually over a longer period of time with plenty of rest in between.

      As for the protein aspect this is also an area which over steps my expertise. Consult a registered dietitian. Or you might want to consult one of the evidence based guides for protein consumption in a nutrition textbook. I especially like anything written by William D. McArdle. The current estimation is 0.8 grams of protein per kilogram of body weight and 1.2 grams of protein per kilogram of body weight for athletes engaged in training and competition. This many not be good advice for an individual with low testosterone or for someone in your age bracket. So do not take this advice as a prescription. I just wanted to illustrate that the human animal can only process small-ish amounts in a given 24 hour period. Be weary of any advice you receive touting large amount of protein or supplementation. I want to stress that I am not qualified to give meaningful advice regarding nutrition.

      1. Hey Matthew
        Thanks heaps bro
        Your comments were awesome and much appreciated
        I did a lot of prep rides building up to the 170 ks
        but maybe not enough
        I just wanted to proove that im not too old [giggle]
        Thanks again mate, ill keep you posted
        Regards Rollsy

        1. No Prob Rollsy, Thanks for reading my article and asking questions.
          I’ve got a bunch more on the site that might interest you also.
          Questions are always welcome.
          And your 50s are the prime cycling years. Just always wear your helmet. I want to know you’re gonna make 57.

  2. Hi Matthew,
    I exercise almost evryday, and do combat training as an additional exercise, i usually do get sore muscles after combat training, is it ok not to do any workout the next day? As i cant really bend properly.


    1. Ging,
      It is VERY ok to not to exercise the next day. ESPECIALLY if you are sore. I say this to people constantly. You need to recover from exercise. And the harder the exercise the more recovery time you’ll need. To get so addicted to exercise that you start over training. You will be worse off in the long run. Training smart is know how much to train and how much to rest. If you start adding in rest days you’ll probably find you get stronger or faster. Rest is as important as the exercise stress itself.

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