However, problems arise when this useful framework escapes its original context. What begins as sensible benchmarking can become a generalised message that asymmetry is inherently ‘bad’ and must be corrected. And once that happens, the presence of an asymmetry can distract the practitioner from what the difference actually means.

There are also situations where asymmetry is not a problem to be corrected, but an expected consequence of how we move. Consider people with a below-knee amputation in one leg: they show lower quadriceps muscle activation in the amputated leg than in the intact leg, when the same strength test is performed on each side (3). These differences likely reflect adaptations built up over years of asymmetrical movement, during which the intact limb has had to bear a larger share of the mechanical work. Similar adaptations can develop in athletes who repeatedly favor one limb, such as throwers and racket-sport players. In tennis players, for example, higher EMG activity has been measured in the internal rotator muscles of the dominant shoulder than the non-dominant shoulder, when the same exercise is performed on each side (4).

Even when a task appears more symmetrical, asymmetry can sometimes represent an effective individual strategy. For example, when vertically jumping off both legs, evidence suggests that moderate asymmetry in muscle activation between the two limbs can be associated with better performance, whilst excessive activation of the non-dominant limb seems detrimental (5). But even with results like these across a group of people, it’s important to remember that not everyone will solve the same movement problem in the same way. Differences in physical activity, previous injury, anatomy, strength, and experience can all influence how muscular work is distributed between the limbs. An EMG assessment may help reveal these differences, providing information about how an individual is achieving an outcome that would otherwise remain hidden.

This is one of the reasons EMG can be so valuable. It is not simply a method of producing numbers that need to be brought closer together. It can provide a window into how movement is being produced. Two people may achieve the same external outcome through different patterns of muscle activation, and those patterns may tell us something important about how each person is responding to the demands of the task.

But EMG cannot, by itself, tell us whether an asymmetry needs fixing. That judgement requires context: the individual's anatomy, the demands of the task, their training and injury history, how the pattern changes over time, and whether it appears to influence the outcome (health or performance) we care about.

Asymmetry, therefore, is not a verdict. It is information. Sometimes it may signal dysfunction. Sometimes it may reflect normal variation or adaptation. And sometimes it may be part of what allows a person to move optimally. The value of EMG is not simply that it can reveal the difference, but that it gives the practitioner another piece of information with which to understand what that difference might mean.