Reliability of the knee muscle co-contraction index during gait in young adults with and without knee injury history

Despite the frequent use of the electromyography-based muscle co-contraction index (CCI) to examine muscular control of the knee joint in young adults with and without knee injury history, the reliability of the CCI in this population is unknown. The purpose of this study was to quantify within-day and between-day reliability of the knee muscle CCI during gait in young adults with and without knee injury history. Twenty young adults (10 males, 10 females) with and without history of intra-articular knee injury performed repeated gait analyses on two different days. Surface electromyography of periarticular knee muscles was performed to determine CCIs for medial and lateral knee extensor - flexor pairs. Absolute (Bland-Altman ratio limits of agreement) and relative (ICCs) reliability were determined between two sessions on the same day as well as on different days. Within-day reliability was good to excellent for most analyzed co-contraction outcomes (ICCs > 0.9) and was deemed acceptable in the context of clinically relevant changes in co-contraction in response to interventions. Between two separate days, the CCI showed poor reliability with measurement errors of up to 300% and was consequently not recommended as a tool to monitor long-term changes or group differences in knee muscular control.