Feeling sore after a workout is known as delayed onset muscle soreness, or DOMS. This occurs as a result of muscle breakdown during a workout, particularly on the eccentric portion or downward phase of movements. While DOMS isn't necessarily a sign of an effective workout, it does generally indicate that you've trained hard and if you feel DOMS in one arm and not the other, you need to take a look at your muscle balance and the way you train.
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Addressing Your Imbalances
If one arm is getting sore and not the other, it could be down to a muscle imbalance. It's likely that your weaker side is the one that's getting sore, as this is having to work harder to complete the same amount of work as the strong side, leading to more muscle breakdown. This imbalance could be caused either by your joint structure, or your day to day posture.
Delve Into Dumbbells
Ditching your barbell and machines in favor of dumbbells is an effective way to fix your imbalances and ensure both sides are worked evenly, according to strength coach Tom Venuto. If you're used to performing an upper-body workout consisting of bench presses, machine shoulder presses, barbell rows, chin-ups and barbell curls for instance, try switching to dumbbell chest presses, dumbbell shoulder presses, dumbbell rows, single-arm pull-downs and dumbbell curls.
Go Hard on the Weak Side
While training the weak side first so as not to exacerbate the imbalance may seem sensible, coach Ben Bruno advises the opposite. By working the strong side first, you push yourself to get more reps on the weak side, notes Bruno. This might require short rests on the weaker side, but you'll undoubtedly get stronger. Using dumbbell rows as an example, you'd perform as many reps as you could on the strong side -- say 12 -- then aim to match this on the left, even if it means performing eight, resting for five seconds, then doing a further four reps.
Fixing Your Form
You might find that you use poor form when training your weaker side, which actually leads to the stronger side feeling sore after the workout. On a biceps curl for example, you may perform the reps on your strong side with perfect form, which activates the biceps and leads to muscle breakdown and soreness. On the weaker side, however, the biceps may not be strong enough to perform the movement on their own, leading to you using more body momentum. To make sure this doesn't happen, keep an eye on your form in a mirror, or have a training partner or exercise instructor scrutinize your technique.