Muscle Soreness: What is Happening?

     Progress has been made in exercise science, giving us a better idea of what causes post-exercise muscle soreness. It’s not an accumulation of lactic acid (as the Ottawa Senators once believed), and it is not a continuous muscle spasm. Exercise physiology has discovered that muscle soreness is caused by the micro tears of muscles during physical activity. The question is, why does the pain feel its worst 24 to 48 hours AFTER you stop exercising?

     You may have learned, from painful experience, muscle soreness has a connection to intensity level. Surprisingly, intensity is not the lone factor in “delayed-onset muscle soreness”, aka DOMS.

A study from Sweden compared three groups of volunteers on a 45 minute treadmill test:

-          One group ran on an uphill slope of four degrees
-          One group ran on an uphill slope of four degrees
-          One group ran on a downhill slope of eight degrees.

What group would you expect to be the most sore the following day?

    The group running downhill was the only group that experienced DOMS, even though the uphill group had the hardest time maintaining pace. The reason for this is because running downhill requires “eccentric” muscle contractions, which is when your muscle is trying to shorten but an external load is forcing it to lengthen. A few examples of an eccentric muscle contraction:

-          The braking action of your quadriceps (front upper leg) muscle as you run down a hill.
-          Lowering the weight during a bicep curl.

    During these eccentric contractions, your muscle filaments are stretched to their limits, and sometimes beyond. The positive result of this damage is that it effectively picks out the weakest links in your muscle and makes them stronger post-recovery.

    The pain in the first 24 to 48 hours of exercise is believed to be caused by the repair process, instead of the damage itself. To clear out the damaged tissue and begin the rebuilding process, the body sends out cells called neutrophils and macrophages. In the process, outer membranes of the nearby muscle cells become damaged, which allows fluid to rush into the cell and cause your muscles to swell. The damaged muscle also releases bradykinin, which after a 12 hour delay, causes an increase of “nerve growth factor” that lasts roughly 48 hours. Nerve growth factor makes nerve endings more sensitive. This makes every movement of these damaged muscles hypersensitive and painful.

    The good news is that, although it’s painful, this process allows muscle to come back stronger once it is repaired. Without this cycle, you would miss out on many benefits from training. As the weak muscle fibers become weeded out, you will be healthier, stronger, and a lot less likely to experience the same amount of pain during DOMS.

References:

Glyn Howatson and Ken van Someren, "The prevention and treatment of exercise-induced muscle                     damage, "Sports Medicine,2008, 38(6), 483-503.

Christer Malm et al., "Leukocytes, cytokines, growth factors and hormones in human skeletal muscle and         blood after uphill or downhill running, "Journal of Physiology, 2004, 556(3), 983-1000.

Shiori Murase et al., Bradykinin and nerve growth factor play pivotal roles in muscular mechanical                         hyperalgesia after exercise (delayed-onset muscle soreness), "Journal of Neuroscience, 2010,30(10)             3752-3761.

    Hutchinson, A. (2011). Which comes first, cardio or weights?: Fitness myths, training truths, and other surprising discoveries from the science of exercise (pp. 61-62). New York: HarperCollins.