The Neural Origins of DOMS

Post-workout muscle soreness isn't just in your muscles.

Monday, August 12, 2013, 12:00 am

Some things we know:

- Hard exercise can leave you sore the next day, in particular if it involves (a) unfamiliar movements, and (b) eccentric muscle contractions. This is "delayed onset muscle soreness," or DOMS.


- The second time you do an unfamiliar workout, you'll be much less sore afterwards compared to the first time. And you'll be even less sore after the third time, and so on. This is called the "repeated bout effect."

It's easy to understand this intuitively by thinking about muscle fibers: unfamiliar eccentric contractions damage them; your body then makes repairs so that next time you do the same workout there's less damage. There's some truth in this picture, but DOMS is actually a much more complicated phenomenon that's still not fully understood. For one thing, there's also a "central" component to this soreness -- an element that involves neural signals via the spine rather than physical damage in the muscle.

Researchers in Denmark just published a neat study in the European Journal of Applied Physiology that explores different origins of pain and the repeated bout effect. They had a group of volunteers to do two sessions of DOMS-inducing eccentric exercise a week apart. They measured pain sensitivity before, immediately after, and a day later, using two different techniques:

- "Pressure pain threshold" uses a device called an algometer, which is basically a blunt needle that applies steadily increasing pressure until the subject says "enough." This gives you a good measure of the local pain sensitivity at a specific point in the muscle.

- "Nociceptive withdrawal reflex" is the spinal reflex that causes you to, say, pull your hand off a hot stove before you even realize what's happening. It involves nerve signals that travel directly up your spine to your motor cortex to produce a protective action, no thought required. The researchers applied steadily increasing jolts of electricity to the arch of the subjects' feet, while using EMG electrodes to monitor nerve activity traveling up the leg. When the jolt of electricity got high enough, they were able to see a burst of nerve activity corresponding to this nociceptive withdrawal reflex. Cool stuff!

The point of the study was to see how these two measures of pain sensitivity were affected by DOMS and the repeated bout effect. As expected, the pressure pain threshold was lower the day after the first exercise bout, meaning the muscle was more sensitive to pain. After the second bout a week later, there was no signficant change -- the classic repeated bout effect.

Similarly, the nociceptive withdrawal reflex was triggered at a much lower electric current the day after the first exercise bout. In other words, DOMS is partly the result of your nervous system getting, well, nervous -- it's hypersensitive to any potentially painful or damaging stimulus. But the picture was different a week later: there was no significant change in the reflex after the second exercise bout. If anything, the threshold actually increased, meaning it became less sensitive to pain. In other words, the nervous system reacted very strongly to the first damaging bout, but didn't raise the alarm when it encountered the same situation a week later.

So what does this mean? This underlines a point that previous studies have demonstrated: you don't actually have to damage your muscles and get super-sore to induce the repeated bout effect, because it's as much about nerve signals as it is about muscle fibers. When you're starting a new training stimulus, there's a tendency to dive straight in to "get the soreness over with." But if you keep things relatively easy on the first session or two -- lift only 60-80% of what you'd normally be capable of, for example -- you'll get some protection from DOMS in future full-strength workouts without having to waste time with extra recovery from being a hero in the first workout of a new season.

