Long muscle-tendon-units are known to contribute to spinal stiffness and hysteresis in-vivo, yet their contribution as a passive structure in-vitro is less well defined. Twelve full length caprine spines including the head, pelvis and all spinal muscles were tested during displacement in Flexion-Extension (FE) and coupled Lateral and Rotational (LR) motion in a material testing machine. Hysteresis and modified stiffness (modST), i.e. mean force divided by the total displacement, were calculated. This was repeated following removal of dorsal muscles (longissimus dorsi and gluteus) and ventral muscles (iliopsoas), in a random order. ModST and hysteresis in the different dissection stages were identified. Correlations between modST, hysteresis, body mass, spine length and longissimus muscle thickness were calculated. Removal of dorsal musculature reduced extension modST significantly by 23%, and flexion modST by 40%. Ventral muscle removal reduced extension modST by 1% and flexion modSt by 13%. Hysteresis was reduced by 27% after dorsal and 2% after ventral muscle removal in FE hysteresis. Out of 105 correlation coefficients, five values were significantly correlated (p<0.05, range r(2): 0.61-0.71) and 10 values were highly significantly correlated (p<0.01, range r(2): 0.75-0.97). Strongest correlations were between hysteresis and modST for the same movement direction and dissection state. The results of this study demonstrate that the presence of muscles stiffens the spine with dorsal muscles showing more effect. This supports the concept that muscle volume even in a non-contractile state provides a stabilising function to the spine.
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