Main force directions of trunk muscles: A pilot study in healthy male subjects.

Muscles work most effectively along their anatomically defined action vector(s) which has implications in training and therapeutics. Action vectors can easily be identified in extremity muscles and smaller muscles of the trunk, but are less clear in larger trunk muscles. Trunk muscle exercises and diagnostics have traditionally relied on tasks in the sagittal plane - a practice that is being reconsidered. Therefore, this study aimed at identifying main force directions (MFDs) of major trunk muscles expressed in terms of deviation from the sagittal plane. 20 healthy male subjects underwent graded isometric submaximal static load applications on their trunk by application of simultaneous and incremental tilting and rotating from vertical to horizontal at rotational angles of 45° starting from 0° (forward tilting) around 360° with only the lower body secured. Surface EMG (SEMG) from six trunk muscles on each body side was recorded. The MFD of each trunk muscle was estimated by considering SEMG amplitudes of all rotational angles, separately for all tilt angles, and was expressed as angular deviation from sagittal plane. The calculated MFDs of trunk muscles deviated from sagittal plane to differing extents. Mean MFD angle was smallest (more parallel to sagittal plane) for rectus abdominis muscle (±14°), becoming more lateral for external oblique (OE, ±32°) and internal oblique abdominal muscles (OI, ±47°). As tilt angle increased, MFD angles increased for OE, but decreased for OI. Iliocostalis muscle showed an almost laterally directed MFD with systematic dependency on body side (-90° for left and +75° for right side). Both paravertebral muscles (longissimus and multifidus muscles) showed almost identical MFD angles of about ±145° and varied the least with tilt angle. All trunk muscles' MFDs deviate from sagittal plane and, in addition to flexing and extending, have both bending and/or rotational capabilities. MFDs of oblique abdominal muscles are systematically altered by tilt angle in accordance with their more divergent fiber directionality. The results provide a basis for specifically targeted diagnostics and training of trunk muscles.