The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

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


1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Department of Community and Family Medicine, Duke University School of Medicine, 3Department of Evolutionary Anthropology, Duke University

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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 take-off. 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.

This study was supported by the Leakey Foundation, the Turkana Basin Institute, Stony Brook University, NIH (R24 HD050837-01) and NSF (BCS-0452160 and BCS-0094522).

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