Segmental variation in the activity and function of the equine longissimus dorsi muscle during walk and trot

Abstract

Muscle function depends in part on the interplay between its activity and its length within the stretch-shortening cycle. The longissimus dorsi is a large epaxial muscle running along the thoracic and lumbar regions of the equine back. Due to its anatomical positioning, the longissimus dorsi has the capability of contributing to many functions: developing bending moments in the dorsoventral and lateral (coupled to axial rotation) directions and also providing stiffness to limit motion in these directions. We hypothesize that the exact function of the longissimus dorsi will vary along the back and between gaits as the relation between activity and motion of the back changes. Electromyograms (EMG) were recorded at walk (inclined and level) and trot (on the level) on a treadmill from the longissimus dorsi at muscle segments T14, T16, T18 and L2. Back motion was additionally measured using a fibre-optic goniometer. Co-contractions of the muscle between its left and right sides were quantified using correlation analysis. A greater dominance of unilateral activity was found at more cranial segments and for level walking, suggesting a greater role of the longissimus dorsi in developing lateral bending moments. Timing of the EMG varied between muscle segments relative to the gait cycle, the locomotor condition tested and the flexion–extension cycle of the back. This supports the hypothesis that the function of the longissimus dorsi changes along the back and between gaits.

Function of the epaxial muscles in walking, trotting and galloping dogs: implications for the evolution of epaxial muscle function in tetrapods

“…bilateral activity) or asymmetry (i.e. unilateral activity) between the two sides of the body, a net extensor or net lateral bending/torsional moment can be inferred, respectively (Wakeling et al, 2007). A net extensor moment is expected if sagittal forces dominate (e.g.…”

“…When the sample size was 4 or fewer, the timing was different when the P-value was less than 0.10 (Table2). To infer whether a net extensor moment (bilateral activity) or a net lateral bending/torsional moment (unilateral activity) dominated (Wakeling et al, 2007), we compared the activation at the ipsilateral side with the activity that can be assumed to occur at the opposite side of the body at a respective vertebral level. Note that the inferred net-moment may not result in a movement in the particular body plane.…”

“…For example, activity of the m. longissimus dorsi in one dog was recorded at three vertebral levels (Tokuriki, 1973a;Tokuriki, 1973b;Tokuriki, 1974). One study investigated cats at different gaits, but did not also quantify muscle recruitment (English, 1980); and three studies recorded the activity of the m. longissimus dorsi in walking or trotting horses but did not study asymmetrical gaits (Licka et al, 2004;Wakeling et al, 2007;Zaneb et al, 2009). No study, however, has systematically investigated epaxial muscle activation in the same species at different gaits and different vertebral levels.…”

Activity of extrinsic limb muscles in dogs at walk, trot and gallop

“…Although the activity of the external oblique muscle at the thoracic site that we recorded is appropriate for this anti-sagging function, previous experiments designed to increase sagging by loading the mid-trunk of trotting dogs showed no significant increase in the activity of the m. obliquus externus at an abdominal recording site (Fife et al, 2001). The difference in amplitude between the two bursts in each locomotor cycle indicates that the external oblique may have other functions (Wakeling et al, 2007;Schilling and Carrier, 2010). The larger burst of activity during ipsilateral stance is appropriate to stabilize the ribs against activity of the m. serratus ventralis thoracis associated with support against gravity (Carrier et al, 2006), and possibly stabilization of the trunk against torques induced by the extrinsic limb muscles (Fife et al, 2001).…”

Muscle activity and hydrodynamic function of pelvic fins in trout (Oncorhynchus mykiss)

“…Locomotory muscles in mammals can be recruited compartmentally along their long axis (Wakeling et al, 2007;Wakeling, 2009) which makes it reasonable to think the AdP muscle of the pelvic fin is being recruited compartmentally along its width. Only the lateral component of the AdP is active during steady swimming.…”