Fiber type composition of spinal extensors is geared toward facilitating rapid spinal extension in the leaper, Galago senegalensis

Leaping is considered a hindlimb-driven locomotor behavior, requiring enhanced maximum shortening velocity and excursion of some hindlimb muscles to facilitate acceleration during take-off. Some investigators note that spinal extension is also important during leaping, as it increases leap length by extending the spine from a flexed position at the beginning of takeoff. If spinal extension is important during leaping, we hypothesized that the extensors of Galago senegalensis (a habitual leaper) should contain a higher proportion of Type II (fast), compared to Type I (slow) fibers. We used immunohistochemistry to compare Type I and Type II fiber proportions in thoracic and lumbar segments of mm. iliocostalis, longissimus, and multifidus between G. senegalensis (n = 3); and Nycticebus coucang (n = 3), a slow-moving arboreal quadruped. Stained fibers were counted, and ratios were computed. G. senegalensis epaxial muscles comprise a higher proportion (58%) of Type II compared to Type I fibers (30%). Conversely, N. coucang epaxial muscles comprise a higher proportion of Type I (91%), compared to Type II (7%) fibers. Thus, G. senegalensis spinal extensors are capable of generating high contraction rates, thereby allowing rapid back extension during leaping. The higher proportion of Type I fibers in the N. coucang epaxial muscles is consistent with previous studies linking a predominance of slow-twitch fibers in some hindlimb muscles to posture and stabilization during slow-climbing. Collectively, these findings support previous studies linking vertebral morphology to trunk extension during leaping. Future studies should focus on more fine-grained analyses of fiber-types.