Characteristics of sarcomere length operating ranges in the rat lumbar spine extensor muscles: comparison to human

Proper coordination and function of muscles is required to move and stabilize the spine. However, due to difficulty in sampling, few empirical data exist regarding the specific structural and architec-tural characteristics of spine muscles in humans. The use of animal models is an alternative; but comparisons between animal and human spine muscles are lacking. Therefore, ten adult male Sprague-Dawley rats were euthanized, skinned and immersion-fixed in formalin; six rats in a neu-tral spine position and four rats in a fully flexed spine position. Longissimus, iliocostalis and multifi-dus muscles were excised, individual fascicles were dissected and sarcomere lengths were measured via laser diffraction. Results demonstrat-ed that mean sarcomere lengths in the neutral po-sition were 2.29 μm, 2.44 μm and 2.44 μm for the multifidus, longissimus and iliocostalis, respecti-vely. In the fully flexed posture, sarcomeres were lengthened to 2.79 μm, 2.92 μm and 2.91 μm, res-pectively. In the neutral spine posture, these mus-cles were closer to optimal length compared to those previously measured in humans. In the fullyflexed posture, these muscles exhibited similar relative lengths (on the descending limb of the for-ce-length relationship) compared to those pre-viously modeled in humans. In summary, rat spine extensor muscles have longer sarcomeres, relative to optimal length, in the neutral posture compared to human spine extensor muscles. However, in the fully flexed spine position, both rat and human spi-ne muscles act on the descending limb of the force-length relationship, all at similar relative lengths. These data provide a context to guide the use of rat spine muscles as a model to understand struc-tural and functional characteristics in the human.