A preliminary comparison of spinal extensor muscle fiber architecture in Galago senegalensis and Nycticebus coucang

Leaping is generally considered a hindlimb-driven locomotor behavior that requires enhanced maximum shortening velocity and excursion of muscles to facilitate acceleration during take-off. However, some investigators have noted that spinal extension is also an important component of leaping, as this movement increases the leap length by extending the spine from a flexed position at the beginning of the take-off phase. We compared absolute and relative measures of fiber architecture (mass, pinnation angle, fiber length [Lf], and physiological crosssectional area [PCSA]) of selected spinal extensors (thoracic and lumbar segments of mm. iliocostalis, longissimus, multifidus) between one Galago senegalensis, a habitual leaper, and one Nycticebus coucang, a slow-moving arboreal quadruped. We hypothesized that since G. senegalensis engages in rapid spinal extension during leaping, it should exhibit extensors that are relatively long-fibered and thus well-suited for generating relatively high shortening velocities and excursions compared to those of N. coucang. Relative to the thoraco-lumbar spine length, G. senegalensis has longer, more parallel-fibered extensors compared to N. coucang. As Lf is proportional to maximum shortening velocity/excursion, the relatively long fibers of the spinal extensors indicate that G. senegalensis has the capacity to generate relatively high shortening velocity and greater excursion compared to N. coucang, which would facilitate the rapid back extension during leaping. As an architectural trade-off between maximizing muscle excursion/contraction velocity and force, G. senegalensis also exhibits relatively smaller muscle PCSAs. These results highlight the potential trade-off between maximizing muscle force- and velocity/excursion, and add an important dimension to the study of leaping behavior.