Selective activation of the external oblique musculature during axial torque production

OBJECTIVE: To investigate whether different geographical regions of the external oblique musculature can activate at different levels and, if they do, to quantify the magnitude of these differences as a function of postural parameters during twisting exertions. DESIGN: Repeated measures design using electromyography on healthy subjects. BACKGROUND: The majority of the models currently used to assess spinal loading have represented the trunk musculature using single force vectors connecting a muscle's point of origin to its point of insertion. However, for muscles with large areas of origin and/or insertion (such as the external obliques), this single vector modelling approach misrepresents the multiple vector reality which, in turn, underestimates the complex loads these muscles can develop. METHODS: Nine subjects performed sub-maximal isometric axial twisting exertions (20, 40 and 60% of maximum voluntary contraction) while assuming six different postures defined by three levels of axial rotation (-20 degrees, 0 degrees and 20 degrees ) and two levels of sagittal flexion (0 degrees and 20 degrees ). As the subjects performed these isometric exertions, the integrated electromyographic activity was sampled using surface electrodes at five different locations over the right and the left external oblique muscles. RESULTS: The results showed significant (p<0.05) regional differences in the activation profiles and these activation profiles changed as a function of trunk posture. CONCLUSIONS: The external oblique musculature is capable of differential activation and the activation profiles of the different regions are affected by the posture of the torso. RELEVANCE: These findings suggest that the external oblique muscle is capable of selective activation of different regions along its cross-section and should, therefore, be modelled using multiple vectors. The result can have a direct bearing on the calculated spine loading, especially lateral and anterior/posterior shear forces.