Within- and Between-session Reliability for the Quantification of the Thigh Muscles Co-activation Index during Isometric Contractions

Co-activation is the simultaneous activation of agonist and antagonist muscle groups around a joint, which contributes to joint stability and movement efficiency [1]. Co-activation becomes very important during ambulation and balance particularly around the knee. While certain levels of co-activation are normal in healthy populations, increased or reduced levels of co-activation may be associated with neuromuscular problems [2]. However, the wide range of methodological approaches for the quantification of co-activation makes comparison across studies and populations difficult. Most of the techniques that quantify coactivation index (CI) can be grouped within four categories, with normalization of each muscle with its respective maximum contraction to be considered the most advanced [2]. Nonetheless, there are still inconsistencies in smoothing techniques for the EMG signal. Root mean square (RMS) is the most commonly used smoothing approach [3] but the choice of RMS parameters may affect the reliability and meaning of results within and between sessions. Therefore, the purpose of the present study was to determine withinand between-session reliability of different methodological approaches for the quantification of the CI of the thigh extensor and flexor muscles of the knee during maximum voluntary isometric contractions (MVIC).