In Situ Fiber Length Estimates from Sectioned Muscle Bellies are not Systematically Shorter Compared with Fiber Length Estimates Obtained Using Chemical Digestion

Abstract

Architectural characteristics of skeletal muscles are important determinants of whole muscle function. Fiber length (Lf) and physiologic cross‐sectional area (PCSA) are correlated with skeletal muscle excursion/contraction velocity and force, respectively. Thus, fiber architecture provides important insights into muscle function and performance. A variety of methods are available for estimating Lf and PCSA, including traditional gross anatomical dissection and chemical digestion, imaging methods such as ultrasound and magnetic resonance imaging, and newer applications of iodine‐based contrast using computed tomography. A considerable body of work on muscle fiber architecture has been accomplished using chemical digestion as well as cadaver dissection in which in situ Lf measurements are sampled from muscle bellies that have been sectioned parallel to probable lines of action. Recently, the validity of findings based on in situ measurements sampled from sectioned muscle bellies has been questioned on the grounds that sectioning muscles theoretically results in shorter fascicle measurements compared to chemical digestion. To date, no study has compared measurements obtained from these two methods.We compared Lf estimates obtained using both data collection methods to determine if the in situ measurement technique systematically results in shorter fibers compared with those obtained using chemical digestion. We sectioned masseter and temporalis muscles and obtained in situ Lf estimates for a maximum of 14 species of strepsirrhine primates (n=38 specimens). Most of our samples ranged between n=2 and n=6 per species, but we were limited to n=1 for four species. We compared our results with previously published data based on chemical dissection methods for strepsirrhines (Perry et al., 2011; samples per species ranged between n=1 and n=3; n=20 specimens).Our results showed that in 5/14 species (36%) chemical digestion‐based Lf estimates for the masseter fell within or below our ranges for in situ Lf measurements; in 4/14 species (29%), masseter Lf estimates fell above our ranges. Masseter Lf estimates using chemical digestion were shorter than our single in situ Lf estimates in 3/14 species (21%) and greater in 2/14 species (14%). For the temporalis, in 5/10 species (50%), chemical digestion‐based Lf estimates fell within or below our ranges; in 2/10 species (20%) temporalis Lf estimates fell above our range. Temporalis Lf estimates using chemical digestion were shorter than our single in situ Lf estimates in 2/10 species and greater in 1/10 species. Thus, chemical digestion‐based Lf estimates either fell within our ranges or were actually shorter in 8/14 species for the masseter and in 7/10 species for the temporalis, compared with our in situ measurements. We observe similar results when we compare our in situ masseter and temporalis Lf estimates for platyrrhines with those previously published based on chemical digestion (Hartstone‐Rose et al., 2018).We conclude that there is no empirical support for the contention that in situ Lf estimates derived from sectioned muscle bellies result in systematically shorter fibers compared with estimates based on chemical digestion.Support or Funding InformationNSF (BCS 0452160, 0962677) and NIH (R24 HD050837‐1).