Training one limb reduces soreness in the other limb

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This one surprised me. It’s a new study from the University of Exeter, just published online in the European Journal of Applied Physiology, about DOMS (delayed onset muscle soreness), the exact causes of which have been hotly debated for years (including a section in Cardio or Weights, I might add).

Here’s what they did: 15 volunteers did a hard biceps workout with one arm, emphasizing eccentric rather than concentric contractions in order to produce more post-workout soreness. Two weeks later, they did the same workout; half of them (okay, 7) did the workout with the same arm as before, while the other half did the workout with the opposite arm. An hour after each workout, and then again 24 and 48 hours later, the researchers measured a series of parameters related to soreness, including loss of strength, perceived soreness, and resting arm angle.

As expected, due to what’s known as the “repeated bout effect,” the amount of post-workout soreness was less after the second workout than after the first workout. What’s weird is that it was less even when the subjects did the workout with the opposite arm!

Apparently, this effect has been observed in one previous study, though the results weren’t quite as clear. And it fits with other results showing that training your right arm (for example) can lead to strength gains in your left arm. In that case, it’s not that the muscles in your left arm get bigger — instead, it’s neural adaptations. As your brain learns to send “contract!” signals more effectively to your right arm, it does so symmetrically, so some of the benefits transfer to your left arm. Something similar appears to happening with the post-workout soreness:

Data from the present study, therefore, provide limited evidence that the neural adaptations that provide protection from EIMD [exercise-induced muscle damage]  following a second bout of exercise are likely to be centrally [i.e. in the brain] mediated.

A clue as to how this might work comes from the EMG data they took of muscle activity during the workouts. Eccentric muscle contractions preferentially recruit fast-twitch muscle fibres, which thus sustain greater damage than slow-twitch fibres. As a result, during the second of two exercise bouts you automatically use a higher proportion of slow-twitch fibres — and that shows up as a change in the average frequency of muscle activity measured by EMG, which decreases by 20-30% because there are more slow twitch contributions. In the new experiment, this is exactly what the researchers found: the muscle frequency decreased in the second bout, no matter which arm they used. The brain seems to have learned from bitter experience that it should recruit fewer fast-twitch fibres, and it applies that lesson to both arms.

So what’s the practical value of this? If you have an injury that immobilizes one arm or leg, this suggests that training the opposite limb, with a focus on eccentric contractions, can help protect the bad limb from muscle damage once you start rehabilitation exercises. And hey, it’s also just a pretty cool piece of trivia.