Muscle co-activation in neurological conditions

Co-activation of agonist and antagonist muscles in healthy people is considered a normal strategy that aids movement efficiency during the performance of functional activities. In some cases, however, it is an abnormal strategy that aids stability and/or precision but not efficiency of movement. Co-activation varies not only between agonist and antagonist pairs, but within a pair from time to time, and depending on a range of factors. Muscle co-activation strategies are used when uncertainty in relation to execution of a specific task occurs, during the learning of a new skill or as compensation. Movement dysfunction in neurological conditions may coincide with abnormal levels of co-activation. It is unclear at what level co-activation assists or impedes functional ability, and to what extent reduced agonist excitation and antagonist co-activation combine to cause significant weakness. A major limitation in understanding the mechanisms of co-activation in neurologically impaired individuals is the wide range of methods that are used to determine levels of antagonist activation and hence co-activation levels. The muscle action, joint position, contraction type and testing conditions may influence the antagonist EMG recorded. This review investigates the methods routinely used to quantify co-activation, and relevant findings, particularly with respect to neurologically impaired individuals. Inconsistencies in methods used limit comparison between studies, but it is clear that whilst co-activation may occur alongside clinical features in neurological conditions, it is often viewed as a task-related strategy used to cope with the complexity of a task. This review highlights whether the use of co-activation strategies should be considered useful contributions to successful rehabilitation.