Or, three reasons to exercise better:
1) It hurts.
2) It REALLY hurts.
3) It REALLY hurts for a week.
Okay, so you get the message. You may also have gathered that I did some exercise recently and it hurt. You’d be right.
Common knowledge seems to blame lactic acid, but this does not seem to be the case, especially for the prolonged pain that can last up to a week, known as delayed onset muscle soreness, or DOMS (Cheung et al., 2003).
A number of mechanisms have been proposed, but none of them alone can explain why pain that starts so soon after exercise begins and can last for so long afterwards.
First though, a crash course in muscle anatomy. Fig. 1 shows the microstructure of a muscle:
The filaments (bottom right) are strands of protein which constantly form and re-form cross-links with one another. These filaments are attached at either end to a z-disc, each of which anchors hundreds of these filaments to form a sarcomere about 0.002 millimetres long (Lodish et al., 2000). Sarcomeres are arranged end-to-end to form a myofibril (top right) and form alternating light and dark bands under a microscope (striations, bottom left). Many myofibrils bundle together to make a muscle fibre (top middle) and many muscle fibres bundle together to make a muscle. Each of these layers is surrounded by tough and slightly elastic connective tissue.
When a muscle contracts the cross-links made by the filaments form and reform in synchrony causing the filaments to slide over one another and bringing z-discs closer together. Since many thousands of sarcomeres end-to-end make a myofibril, the cumulative effect is the shorten the entire muscle. Muscle contraction itself is initiated by motor neurone nerve fibres which cause channels to open in the sarcoplasmic reticulum (yellow tubules, centre) to release calcium into the cells which activates the filaments to move in-sync.
Delayed Onset Muscle Soreness (or, Why am I still hurting!?)
The short-term effects of intense exercise include tiny tears in the membranes of muscle cells which isn’t too bad, and appears to be linked to muscle repair and growth through exercise. Over-exercising can physically tear the filaments from their z-discs and strain the sheath of connective tissue that covers the muscle and forms the tendon connecting muscle to bone, leading to longer-lasting damage.
Damage to the cell membranes and connective tissue also damages the sarcoplasmic reticulum meaning that calcium is not contained properly. When left to accumulate in the cell calcium can activate enzyme which break down some of the proteins which form the z-discs, causing an inflammation response from the immune system within a few hours.
The damage to the cell membranes and connective tissues allows small proteins into the bloodstream that recruit immune cells to the muscle. These cells start to produce histamine which causes that burning feeling the following day, and is exactly the same chemical that causes a burning feeling in a nettle sting, insect bite or hay fever. This activity typically peaks 48 hours after exercise and gradually decreases, all the while producing chemicals that stimulate nerve endings that sense acid and heat burns.
While all this is happening, the immune cells are also busy dismantling (literally swallowing up) and recycling muscle cells that are too damaged to be repaired. Any badly damaged muscle cells that aren’t being swallowed by the phagocytes (cell-eaters) are dying in a much less controlled manner and releasing all sorts of irritant chemicals into the rest of the muscle. It is this, combined with localised bruising from all the muscle damage that activates our pain-sensing nerve endings, or nociceptors (literally, harm sensors).
Pain increases when you move because increasing the pressure in the muscle, on top of the bruising and inflammation already present, will cause a spike in activation of the nerve endings that sense pain in response to pressure, such as being poked really hard.
It’s worth emphasising again that this sequence of events is the most likely explanation for DOMS, since none of the individual theories that have been proposed can explain why the pain comes on so quickly but can last for so long afterwards.
Can I ever make it stop?
Short of rest, and lots of protein and fat to restore the muscle and the connective tissue there’s not much to do on the repair front. To reduce the risk of it happening again, make sure you do a full, proper warm up that has you sweating by the time you’ve finished. This should include plenty of stretching of the main muscle groups you’ll be using, but be careful not too over-stretch: ‘dynamic range of motion stretching should probably be used to increase flexibility without decreasing stability of the joint over the full range of motion you’ll be using in that exercise period (Robinson & Wilson, 1999).
Super. If you want to add insult to injury, feel free to mock me in the comments for damaging myself playing Ultimate Frisbee.
P.S. I think it’s time I admitted to the world that I am indeed partial to a bit of Ultimate Frisbee, and it would also be a good time to mention that yes, it is a real sport with tactics, rules, two separate scoring/points systems and the potential to make all your muscles hurt like mad for an entire week. The muscle pain part is probably due to the fact that my only forms of physical activity since last October have been a 10 minute cycle to and from campus every day, a weekly swim of less than an hour and scooting around the room on a wheely office chair. Not exactly ideal preparation for three hours of 15 metre sprints, as Ultimate Frisbee tends to be if you play it for three hours straight.
Cheung, K., Hume, P., & Maxwell, L. (2003). Delayed onset muscle soreness: treatment strategies and performance factors. Sports Medicine, 33, 145–164. Freely available here.
Lodish, H. et al., 2000. Muscle: A Specialized Contractile Machine. In Molecular Cell Biology. New York: W. H. Freeman. Freely available here (NCBI).
Roberts, J. M., & Wilson, K. (1999). Effect of stretching duration on active and passive range of motion in the lower extremity. British Journal of Sports Medicine, 33, 259–63. Freely available here.