Variability of trunk muscle co-activation during repeated thoracolumbar flexion in 4-point kneeling

The aim of this study was to evaluate intra and interparticipant variability of trunk muscle co-activation during thoracolumbar flexion in 4-point kneeling. Kinematic data were collected using 10 high speed cameras to identify reflective marker movement, positioned over the 11th thoracic vertebra (T11) in 12 healthy participants. Surface electromyography (EMG) electrodes were attached to external oblique (EO) and internal oblique (IO) muscles (left and right sides). EMG signals were transmitted via telemetry and synchronised with kinematic data collected during 4-point kneeling whilst thoracolumbar flexion was performed (10 trials). EMG data after processing were expressed as a percentage of the Maximally Observed EMG (%MOE). Significant differences between muscles for the smallest maximal %MOE, mean %MOE and time difference between EMG maximum and 90% of T11 zdisplacement were calculated. Intra-participant muscle
variability was defined as the difference from the mean (DM) across the trials and correlations between muscles were performed. Correlations of the line-of-best-fit from the DM were calculated across participants. The slope of the line of best fit identified co-activation of the left OE and OI, and the left and right sides of OI across all participants, as identified by significant positive correlations. Within-participant variation in muscle coactivation was high. OI right and OI left showed the most frequent significant correlations in variation from the mean. IO and EO show synchronous co-activation during thoracolumbar flexion across healthy participants, but intra-individual variability of these muscles is considerable.