According To The Program, Which Animal Has A Higher Percentage Of Slow-twitch Muscle Fibers?
Proc Natl Acad Sci U S A. 2017 Jul eleven; 114(28): 7343–7348.
Anthropology
Chimpanzee super strength and human skeletal musculus development
Matthew C. O'Neill
aDepartment of Basic Medical Sciences, University of Arizona College of Medicine–Phoenix, Phoenix, AZ, 85004;
Brian R. Umberger
bDepartment of Kinesiology, University of Massachusetts, Amherst, MA, 01003;
Nicholas B. Holowka
cDepartment of Human Evolutionary Biological science, Harvard University, Cambridge, MA, 02138;
Susan G. Larson
dDepartment of Anatomical Sciences, Stony Beck University School of Medicine, Stony Brook, NY, 11794;
Peter J. Reiser
eDivision of Biosciences, The Ohio Land University Higher of Dentistry, Columbus, OH, 43210
Significance
Chimpanzee "super strength" has been widely reported since the 1920s although a disquisitional review of the available information suggests that the chimpanzee–human muscular performance differential is only ∼1.five times. Some hypothesize that this differential reflects underlying differences in muscle mechanics. Here, we present direct measurements of chimpanzee skeletal musculus properties in comparison with those of humans and other terrestrial mammals. Our results show that chimpanzee muscle exceeds human musculus in maximum dynamic force and power output past ∼one.35 times. This is primarily due to the chimpanzee'south higher fast-twitch fiber content, rather than infrequent maximum isometric force or maximum shortening velocities. We propose that muscular functioning capabilities declined during hominin evolution in response to selection for repetitive, low-cost contractile behavior.
Keywords: chimpanzee, human, muscle, myosin heavy concatenation, muscle modeling
Abstract
Since at to the lowest degree the 1920s, it has been reported that common chimpanzees (Pan troglodytes) differ from humans in being capable of exceptional feats of "super strength," both in the wild and in captive environments. A mix of anecdotal and more than controlled studies provides some support for this view; however, a critical review of available information suggests that chimpanzee mass-specific muscular performance is a more pocket-sized 1.5 times greater than humans on average. Hypotheses for the muscular basis of this functioning differential accept included greater isometric force-generating capabilities, faster maximum shortening velocities, and/or a departure in myosin heavy concatenation (MHC) isoform content in chimpanzee relative to homo skeletal musculus. Here, nosotros show that chimpanzee musculus is similar to human musculus in its single-fiber contractile properties, just exhibits a much higher fraction of MHC 2 isoforms. Dissimilar humans, chimpanzee musculus is composed of ∼67% fast-twitch fibers (MHC IIa+IId). Computer simulations of species-specific whole-muscle models indicate that maximum dynamic strength and power output is 1.35 times higher in a chimpanzee muscle than a human muscle of similar size. Thus, the superior mass-specific muscular functioning of chimpanzees does non stem from differences in isometric force-generating capabilities or maximum shortening velocities—as has long been suggested—but rather is due in part to differences in MHC isoform content and fiber length. We propose that the hominin lineage experienced a decline in maximum dynamic strength and power output during the past 7–eight million years in response to selection for repetitive, low-cost contractile behavior.
Modern humans—with some exceptions—are often characterized every bit a weak and unathletic species compared with our closest living relatives, the chimpanzees. Whereas chimpanzees are adept tree climbers and arborealists (one), our hominin ancestors gave up a reliance on the forest awning after the emergence of the genus Human (ii). Subsequent evolution in brain size (3) and noesis also as advancements in tools and other textile culture (iv, v) have reduced our strict dependence on muscular strength for survival and fitness.
Since at least the 1920s, both anecdotal reports and more controlled experiments accept indicated that the strength of a chimpanzee can exceed that of a man (6–12). This has led to the now long-standing proposal that chimpanzees are "super potent" compared with humans. A critical review of experiments (i.e., pulling and jumping tasks) carried out between 1923 and 2014 suggests that chimpanzee mass-specific muscular performance consistently exceeds that of humans, with a differential of virtually i.five times, on boilerplate (SI Appendix, SI Give-and-take). Hypotheses for the muscular footing of the chimpanzee–human performance differential take included higher isometric force-producing capabilities (half dozen–8, 11), faster maximum shortening velocities (7, xi), and/or a unlike distribution of myosin heavy chain (MHC) isoforms than human skeletal muscle (x, 11). However, to date there have been no direct measurements of these parameters in the skeletal muscle of chimpanzees. Yet, if one or more of these hypotheses are correct, it would bespeak a pregnant (and previously unappreciated) evolutionary shift in the force and/or ability-producing capabilities of skeletal muscle in either Pan or Human since these ii lineages diverged near 7–eight million years ago (Mya) (13).
In this study, we present direct measurements of single-fiber contractile backdrop and MHC isoform distributions of chimpanzee skeletal muscle to examination these hypotheses. We compare our chimpanzee data to like information from humans and a wide range of other terrestrial mammals. The unmarried-fiber and MHC datasets are then used to parameterize chimpanzee and homo musculus models. Computer simulation of these models under matched contractile conditions permit a controlled comparison of chimpanzee and human being muscular performance capabilities. Based on our results, nosotros suggest that the maximum dynamic forcefulness and power-producing capabilities of skeletal musculus has declined during the past 7–8 meg years of hominin evolution, likely due to choice for repetitive, low-price contractile behavior.
Results
Using an isolated musculus fiber grooming, we directly measured the maximum isometric strength and maximum shortening velocity of the skeletal muscle of the common chimpanzee (Pan troglodytes). Data were collected at xv °C from fibers containing pure MHC I, IIa, and IId isoforms. The maximum isometric force (P o) of chimpanzee skeletal muscle ranged from 96 kN⋅k−2 to 150 kN⋅m−two, and the maximum shortening velocity (V o) ranged from 0.64 to 4.96 Fifty⋅southward−i, depending on MHC type (Fig. 1 and SI Appendix, SI Methods and Tables S1 and S2). These results, which are taken to be representative of limb and trunk skeletal muscle in full general, point that chimpanzee muscle is like to humans and other terrestrial mammals in its single-fiber contractile backdrop (Fig. ane D and E ). Indeed, the P o and V o of chimpanzee muscle are not significantly unlike from humans (P > 0.05, one sample t test) and are mostly consistent with expectations based on torso size scaling (Fig. 1 F and G ). Given these cellular-level results, humans and chimpanzees can be expected to exhibit commonalities in the molecular properties that affect single-cobweb operation, such as actin–myosin kinetics.
Muscle contractile properties. (A) Chimpanzee single fibers were sampled from chiliad. vastus lateralis (VL) and m. gastrocnemius lateralis (GL). Insets evidence a chimpanzee single musculus fiber too as the identification of the fiber MHC isoform content using gel electrophoresis after P o and Five o measurements. (B) The main effect of MHC isoform content on single-fiber P o; n = 55; fault bars, SD; P value is the result of an ANOVA; F (2,52) = 21.xx. Paired comparisons point that the MHC I (n = 31), IIa (n = 15), and IId (n = 9) P o samples all differ significantly from each other (P < 0.05, Tukey'south honest significant difference tests). (C) The master event of MHC isoform content on single-fiber V o; n = 22; error confined, SD; P value is the effect of an ANOVA; F (2,19) = 97.sixteen. Paired comparisons indicated that the MHC I (n = 14), IIa (n = seven), and IId (n = 1 estimate, SI Appendix, SI Methods) V o samples all differed significantly from each other (P < 0.05, 1-sample t tests). (D and Eastward) The hateful P o and 5 o of chimpanzee (stars) muscle compared with human (circles) muscle; P values are the results of one-sample t tests. (F and G) The size scaling of P o and V o beyond mammals ranging in mass from 0.01 kg (mouse) to ii,500 kg (rhino) for MHC I, IIa, and IId. Dashed lines are pGLS regression lines of P o and Five o against body mass by MHC isoform.
We measured the distribution of MHC isoforms within 35 pelvis and hind-limb muscles of chimpanzees and constitute a balanced distribution of MHC I, IIa, and IId on average (Fig. ii and SI Appendix, Table S3). This is in marked contrast to humans, who showroom a meaning bias toward MHC I fibers in these same muscles and throughout the limbs and body overall (14, 15). Although the genetic basis of skeletal musculus MHC isoform specification is an active area of research (e.g., ref. 16), the magnitude of the chimpanzee–homo contrast in MHC I fibers appears to exceed the more modest shifts that may be induced through intense able-bodied training (∼ten–fifteen%) (17, 18). Furthermore, label of fiber-type distributions in the muscles of lemurs, galagos, and macaques suggests that a predominance of MHC II (IIa + IId) isoforms (i.due east., fast fibers) is common among primates, likewise every bit other terrestrial mammals (SI Appendix, SI Methods and Table S5). Indeed, the boring loris (Nycticebus coucang) is the only other mammal measured to appointment with a predominance of slow fibers beyond its skeletal muscles. Thus, we advise that the loftier percentage of MHC I fibers in man skeletal muscle is a derived trait within the hominin lineage, rather than a characteristic of African apes or other nonhuman primates in full general.
MHC isoform distributions and average fiber length of chimpanzee and man skeletal muscles. (A) Chimpanzees exhibit a counterbalanced distribution of the three MHC isoforms beyond 35 skeletal muscles (SI Appendix, Table S3). P value is the event of an ANOVA [F (two,111) = i.339, P = 0.197]. (B) For the same muscles, humans exhibit a significant bias toward dull-twitch fibers in their skeletal muscle with measurements ranging from (i) 69.2 ± 11.7% (xiv) [t (72) = 14.04, P < 0.0001, t examination] to (ii) 52.6 ± 7.9% (15) [t (73) = 9.29, P < 0.0001, t test]. This is in contrast to 31.5 ± 11.four% in chimpanzees. (C) Chimpanzee musculus fibers also constitute a greater percentage of their total musculus–tendon unit length than do human muscle fibers (i.e., [L o/(L o + L south)]⋅100, C: 59.0 ± 0.21; H: 44.0 ± 0.25) (23, 24) [t (84) = 2.87, P = 0.0052, t test].
A salient architectural divergence between chimpanzee and man skeletal muscle is that chimpanzees accept longer muscle fibers (both in absolute and relative length) (19). Longer musculus fibers have a broader force–length relation that may enhance the dynamic strength, work, and power capabilities of a muscle–tendon unit (20). Therefore, to estimate the net interacting furnishings of P o, 5 o, MHC distribution, and muscle fiber length on maximum dynamic muscle force and power output in vivo, nosotros designed Hill-type "chimpanzee muscle" and "homo muscle" models that reflected the parameter differences measured herein and elsewhere (fourteen, 15, nineteen). Using computer simulations, we determined the maximum dynamic force and power-producing capabilities of these models at the whole-muscle level.
Simulation of a single-flare-up maximal wrinkle against a heavy, inertial load predicted that chimpanzee muscle would have a ane.35 times higher maximum dynamic forcefulness [chimpanzee (C): 125.6 kN⋅m−2; man (H): 93.0 kN⋅thousand−2] and power (C: 220.7 W⋅kg−1; H: 163.eight W⋅kg−one) output than homo musculus (Fig. iii). Similarly, simulation of a series of cyclical contractions predicted a 1.34 times higher maximum power output from chimpanzee muscle when the control variables governing muscle excitation and contractile frequency were optimized (C: 172.9 W⋅kg−i; H: 129.2 Westward⋅kg−1). These results suggest that the larger fraction of MHC II fibers and the longer muscle fiber lengths feature of chimpanzee skeletal muscle will increase their dynamic force and power-producing capabilities overall. If a chimpanzee-like or macaque-like phenotype characterized the skeletal muscles of the last mutual ancestor of chimpanzees and humans, then the maximum dynamic force and ability-producing capabilities of hominin skeletal muscles have declined over the past 7–eight million years (13).
Muscle model simulations. Single-burst maximal accelerations of an inertial load (first cavalcade) and controlled cyclical contractions (second and third columns) were false with our chimpanzee muscle and man muscle models. The design of each simulation appliance is shown at the column summit in schematic grade with a musculus model affixed in situ. Dashed line is optimal cobweb length (L o). The chimpanzee muscle model generated higher maximum dynamic forcefulness and power outputs than the human muscle model under matched simulation conditions.
Discussion
The 1.35 times differential predicted here seems small compared with popular accounts of "super strength" in chimpanzees. Even so, a critical review of the controlled dynamic force- and ability-limiting experiments (vi–11) that accept attempted to quantify this performance differential indicates that, on a mass-specific basis, chimpanzees outperform humans in pulling and jumping tasks by most ane.5 times on average (SI Appendix, SI Word). Although our simulations exercise not reproduce the earlier experimental designs in detail, the close approximation of our results to the 1.5 times average suggests that musculus mechanics—MHC isoform content, in detail—accounts for much, but not necessarily all, of the measured chimpanzee–human performance differential. Musculus "static force," divers as maximum isometric strength-producing capabilities (P o), is not significantly unlike between these two species and therefore does non contribute to their performance differential (6–eight, 10). Of class, linking muscle mechanics to whole-body performance tasks is difficult due to the complexities that arise from the many muscle–tendon units with differing excitations interim across joints with variable leverages. For example, in maximal pulling, chimpanzee functioning may do good from the larger moment arms of some of their fore (upper) limb musculature (xix). More detailed musculoskeletal modeling and integrated experimental-simulation work would be required to determine the contributions of these and other possible factors to the remaining 1.fifteen differential on average. This would also allow a more precise assessment of the contribution of muscle mechanics to the task-specific details that underlie the full range of measured chimpanzee–man performance differentials (i.east., i.xx–two.05 times across studies, SI Appendix, SI Discussion).
It has also been hypothesized that humans have greater cortical and/or spinal inhibition of maximal musculus recruitment than chimpanzees, thereby limiting their muscular functioning capabilities in comparison (12). If true, this would further increment the chimpanzee–human differential; even so, nosotros are unaware of whatsoever data that directly back up this "inhibition hypothesis." Instead, experimental studies betoken that humans are capable of consummate (or near consummate) voluntary activation of their musculature when assigned a maximal operation task (e.g., ref. 21). Thus, the expectation that both species are capable of optimizing their neuromuscular control strategies in response to the mechanical demands of a given task appears to exist more consistent with bachelor information.
Our integrated experiment-simulation results bespeak that the skeletal muscle of chimpanzees is better suited for maximum dynamic force and power output than that of humans, mayhap reflecting the chimpanzee's greater reliance on tree climbing and suspension for survival and fettle. Furthermore, we propose that the higher fraction of MHC I fibers and shorter muscle fiber lengths in human being skeletal muscle are adaptive for repetitive, low-cost contractile behavior. MHC I fibers have high mitochondrial volume densities and capillary-fiber contact length (17), which facilitate O2 diffusion; curt fibers can reduce the cost of isometric force output due to a reduction in the musculus volume to cross-sectional surface area ratio (22). A high fraction of MHC I fibers can also reduce fatigue by limiting the muscle's reliance on glycogen and other intracellular substrates during contractions (17), which permits more than frequent muscle activations per day in the aerobic range. Indeed, the large musculus masses (both in absolute and relative terms) in human hind (lower) limbs (19, 23, 24) further enhance our aerobic range during bipedalism (25). Taken together, our results provide an explanation of how chimpanzees can outperform humans in some muscle-driven tasks (e.1000., maximal pulling and jumping) (vi–11), but non in others (e.m., metabolic cost of walking) (22). Our results may too business relationship for the exceptionally high metabolite concentrations found in human muscle compared with chimpanzee musculus (xi).
These information propose that, although intrinsic musculus contractile properties appear to be conserved, the hominin lineage was characterized past an increase in MHC I isoform content and a decrease in muscle fiber length (xix), both of which can impact a range of musculus-driven tasks. The timing of shifts in MHC content and fiber length within the hominin lineage is hard to establish; however, we anticipate these shifts to have been concurrent with major transitions in locomotor behavior, increases in daily travel distances, increases in home range size, or some combination thereof. The early on hominin Ardipithecus ramidus is probable like to chimpanzees and other African apes in its body mass to hind-limb length (26), suggesting that this species still allocated a smaller fraction of its total muscle mass to its hind limbs than humans. The earliest australopithecines (i.e., Australopithecus anamensis and Australopithecus afarensis) showroom changes in the pelvis and hind-limb skeleton from Ar. ramidus (27, 28), suggesting an adaptive shift toward greater overground locomotion about four Mya. Even so, the proximate ecological trigger for this transition remains elusive (29). By about 1.eight Mya, Homo erectus had likely reduced tree climbing to contemporary hunter-gatherer levels while increasing daily travel distances and home range size, mayhap aimed at enhancing diet quality through the inclusion of meat or other low-abundance foods (26, 30, 31).
Contrary to some long-standing hypotheses (6–11), development has not altered the basic force, velocity, or ability-producing capabilities of skeletal muscle cells to induce the marked differences between chimpanzees and humans in walking, running, climbing, and throwing capabilities (22, xxx, 31). This is a significant, but previously untested assumption. Instead, natural option appears to have altered more global characteristics of musculus tissue, such as MHC distributions and muscle fiber lengths. Our integrated experiment-simulation results point that these changes have led to a general reduction in maximum dynamic force and power-producing capabilities; nevertheless, they have enhanced metabolic characteristics and endurance capacities of human muscle. Today, intensive athletic grooming can mitigate some of our inherent limitations in maximal musculus performance, but primarily through force enhancement via skeletal musculus hypertrophy (eastward.g., ref. 32). More by and large, although higher levels of anatomical organization, such every bit the size and shape of musculus, tendon, and os take been the main targets of evolutionary processes, hominin muscle dynamic force and power-producing capabilities have also been contradistinct since the Pan and Human lineages diverged vii–viii one thousand thousand years ago.
Materials and Methods
Animals.
Muscle fibers were sampled from three young male common (P. troglodytes) chimpanzees (age: 5.5 ± 0.2 y; mass: 26.5 ± vi.seven kg). All animals were housed in an American Association of Creature Laboratory Intendance International accredited infinite with facilities that immune them to engage in normal social and locomotor behaviors. They had a dedicated animate being handler and a staff of technicians who played and interacted with them daily. All of our procedures followed the guidelines of the Stony Brook Academy Institutional Beast Care and Utilize Committee.
Muscle Fiber Preparation.
Small samples of skeletal musculus (about 1 cm × 0.v cm × 0.v cm) were removed from the muscle (m.) vastus lateralis and m. gastrocnemius lateralis of the correct hind limb of each animal while nether general anesthesia. The samples were immediately immersed in cold relaxing solution with 50% (vol/vol) glycerol for overnight transport to The Ohio State Academy. All of the solutions used for the storage of bundles and measurements of single-fiber contractile properties followed previous experiments (33). Upon arrival, musculus bundles were dissected in a dish containing cold relaxing solution and were stored in fresh relaxing solution with l% glycerol at −twenty °C. Single fibers were subsequently isolated by dissection from the bundles for force and velocity measurements. Single, isolated fibers were mounted in a temperature-controlled experimental bedchamber, maintained at 15 °C. The cobweb was connected at i end to a servo-controlled torque motor (model 322C, Aurora Scientific) and, at the other terminate, to an isometric strength transducer (model 403, Aurora Scientific). The motor and transducer were attached to three-manner positioners. The length of the mounted fiber was adjusted by moving the motor or transducer to set the resting striation spacing (i.e., sarcomere length). Striation spacing was adamant using a microscope digital camera and prototype analysis software. Cobweb width and depth were measured and fiber cantankerous-sectional area (CSA) was calculated, bold an ellipsoidal cross-section.
Recording Force and Velocity.
The fiber was activated past transferring it to a well in the experimental chamber containing a calcium ion (Ca)-activating solution. The force/pCa (−log[Ca]) relationship was adamant in an initial prepare of fibers to determine the maximally activating solution for subsequent force and velocity measurements. The sarcomere length for these measurements was gear up to 2.40–2.50 μm. The strength/pCa relationship indicated that pCa four.0 was maximally activating. The active strength generated in pCa 4.0 solution was normalized by cobweb CSA to summate the P o. The 5 o was measured using the "slack test" (34). Each fiber was activated during a series of exposures to a pCa 4.0 solution. A known amount of slack was quickly (inside ∼two ms) introduced into the fiber during each exposure past movement of the motor arm after steady isometric force was attained and the time required to take up the slack was measured from the strength tape (35). Each fiber was immediately stored at −twoscore °C until assay of MHC isoform limerick to determine the cobweb type.
MHC Isoforms.
The MHC isoform(due south) expressed in each fiber was identified using SDS/PAGE. The composition, grooming, staining, and densitometric scanning of the gels were identical to those described previously (36). The amount of each MHC isoform in fibers that expressed more one isoform was calculated as a pct of the total amount of MHC. The 3 MHC isoforms that were detected on protein gels were identified by mass spectrometry. MHC gel bands were excised from a Coomassie Blue-stained gel and submitted to the Campus Chemical Instrument Center at The Ohio State Academy to obtain identification by liquid chromatography–tandem mass spectrometry. The slowest migrating band was identified every bit fast-blazon MHC-IId, with a MOWSE score of 27,153, based on 741 peptide matches. The ring with intermediate electrophoretic mobility was identified as fast-type MHC-IIa, with a MOWSE score of 23,010, based on 732 peptide matches. The fastest migrating band was identified as slow-type MHC-I, with a MOWSE score of 23,070, based on 694 peptide matches.
The MHC distribution was further determined in samples (10–xx mg) of 35 pelvis and hind-limb muscles from three common chimpanzee specimens not involved in the musculus contractile property measurements. Before each chimpanzee's expiry (of natural causes), it had been living at a zoo or research establishment. The cadaveric remains of each chimpanzee were kept fresh frozen at −20 °C after decease until dissection. For each chimpanzee pelvis and/or hind limb, the pare and fascia were removed; individual muscles were identified and dissected free from their attachments. Post-obit the work of others (14), a 2-cm × 0.v-cm × 0.5-cm sample was then taken from each muscle and used to make up one's mind MHC isoform composition in the same fashion every bit for the unmarried fibers (SI Appendix, Table S3).
Muscle Modeling.
Hill-type musculus models were used to investigate the combined furnishings of P o, V o, MHC isoform distributions, and fiber length on maximum dynamic forcefulness and power output at the whole-muscle level. The chimpanzee and human models each included species-specific force–velocity, forcefulness–length, and activation–deactivation relations, scaled from the generic Thelen2003Muscle (37) model implemented in the OpenSim musculoskeletal modeling environment (38). P o and Five max parameters were assigned the measured P o and V o values scaled to normal operating temperature (∼35 °C) (SI Appendix, SI Methods and Tabular array S4). The curvature of the force–velocity relationship (A rel) and activation–deactivation time constants (τhuman action, τdeact) were calculated based on the average fraction of MHC II (fast-twitch) fibers determined for each species (C: 67%, H: 40%) (39). The distribution of fast-twitch fibers in humans represents an average of contempo MHC measurements (i.e., MHC IIa+IIx) (fourteen) and older immunohistochemistry measurements [i.e., fast oxidative glycolytic (FOG) + fast glycolytic (FG)] (xv). To isolate the effects of muscle on operation, the simulations were conducted using rigid tendons with fiber pennation angles set to 0°. This ensured that the fiber strain matched the muscle–tendon unit of measurement strain in the simulations.
A salient architectural difference between chimpanzee and human being skeletal musculus is that chimpanzees possess longer muscle fibers on average (19). Chimpanzee (23) and human (24) musculoskeletal models were used to calculate the boilerplate relative fiber length [i.east., L o/(L o + 50 south)] for each species and make up one's mind the magnitude of the interspecific divergence. The two musculoskeletal models include all of the same muscles, except that the homo model lacks an chiliad. obturator externus and an m. gluteus maximus superficialis, which is distinct from the k. gluteus maximus in all humans (23). Based on the pelvis and hind-limb muscle–tendon units in the ii musculoskeletal models, relative muscle fiber length was 0.59 ± 0.21 in chimpanzees and 0.44 ± 0.25 in humans. This indicates that chimpanzee muscle fibers constitute an ∼15% longer fraction of musculus–tendon unit of measurement length on average. We incorporated this difference into our chimpanzee model by increasing relative fiber length from an initial value of 0.43 to a value of 0.58 (SI Appendix, Tabular array S4).
Muscle Model Simulations.
The functioning of each muscle model was simulated under two conditions. For the showtime condition, the muscle was mounted between a fixed clamp and a mobile, 500-kg inertial load on a frictionless surface (Fig. 3). The inertial load had i degree of freedom, which allowed the muscle to shorten from its optimal length (50 o) thereby accelerating the mass from rest. This simple design approximates the dynamics of a single-burst muscle-powered dispatch, as in a maximal pulling or jumping task. For each muscle model and simulation, the muscle was maximally excited and the dynamic simulation was run for 0.four ms. The duration of the simulation was sufficient to elicit the maximum dynamic force and power output of each muscle model.
For the 2d condition, an experimental ergometer was modeled every bit a muscle mounted between a fixed clamp and mobile clamp on a frictionless surface (Fig. iii). The mobile clench had one degree of liberty that could be slid to shorten or lengthen the muscle from its L o. This design was intended to mimic the in vitro grooming used for determining the maximum power-producing capabilities of musculus in cyclical contractions (forty). For each musculus model, the mobile clamp was prescribed a sinusoidal motion that shortened and lengthened the musculus by 0.2 L o, centered on 50 o, over three full cycles. This strain aamplitude is similar to straight in vivo measurements of muscle fiber strain during a range of high-power output activities (east.g., refs. 41–43). We used a faux annealing algorithm (44) to optimize a set of control variables that maximized the average net power of the last full strain cycle. Specifically, the command variables included the muscle excitation on (t on) and off (t off) times, also every bit the musculus strain bicycle frequency (f).
For each muscle model, the optimization problem was configured and solved using the OpenSim-MATLAB awarding programming interface (38). Maximum force and power outputs were expressed in kN⋅g−2 and West⋅kg−1, respectively. The muscle area (m2) and mass (kg) divisors were computed using the new chimpanzee P o values scaled to normal operating temperature so weighted past MHC isoform distribution for each species (C: 262 kN⋅one thousand−2; H: 248 kN⋅m−ii) (SI Appendix, SI Methods and Table S4). In all simulations, maximum musculus force was fixed at 2255.four N, which approximates a homo one thousand. vastus lateralis (24).
Statistics.
Measurements of the single-muscle-fiber contractile backdrop (i.eastward., P o, V o) of chimpanzees were evaluated for the event of MHC isoform composition using an analysis of variance (ANOVA) with pairwise comparisons between MHC isoforms. The chimpanzee single-muscle-fiber properties were then compared with like data for humans and other mammals culled from published studies of single muscle fibers at 15 °C. The homo dataset includes a range of populations and ages (SI Appendix, Table S1), with human subject area data from our laboratory falling well within this range (SI Appendix, SI Methods and Fig. S2); the mammalian dataset ranges in size from mice (0.025 kg) to rhinos (2,500 kg) (SI Appendix, Table S2). 1-sample t tests were used to compare the chimpanzee P o and V o ways to the human hateful values from the literature. The scaling relationships of logx P o and Five o with log10 body mass were evaluated using phylogenetic generalized to the lowest degree squares (pGLS). The phylogenetic structure of the analyzed species was taken from published mammalian trees (45–47). The pGLS parameters were estimated using the Comparative Analyses of Phylogenetics and Development in R package (48) with the parameter lambda (λ) fixed at 0 (star phylogeny) and ane (Brownian motion) and empirically estimated using a maximum-likelihood (ML) approach. The three different model fits (i.due east., λ = 0, 1, ML) were compared using a likelihood-ratio exam. In all cases, the log-likelihood ratio test indicated that λ = 0 (star phylogeny) provided a statistically equivalent fit to the other two models and therefore was our preferred model for size scaling of all mammalian single-muscle-fiber contractile properties.
The distribution of MHC isoforms within 35 pelvis and hind-limb muscles of chimpanzees (SI Appendix, Tabular array S3) was compared using an ANOVA. Independent sample t tests were used to deport pairwise comparisons between the average fraction of MHC I or tedious-twitch fibers in chimpanzees and humans, as well as betwixt the relative muscle fiber length in chimpanzees and humans. For the MHC I content and relative cobweb-length comparisons, the aforementioned 35 muscles were compiled for the chimpanzee and human samples, with the exceptions that m. gluteus minimus MHC content was not reported in Tirrell et al. (xiv) and m. obturator externus fiber length was not included in Arnold et al. (24). Exclusion of these muscles from the chimpanzee samples did non modify the statistical significance of the t-test results.
Supplementary Textile
Supplementary File
Acknowledgments
Nosotros thank Dr. T. Zimmerman, J. Rooney, G. Lasek, East. Moyer, and Fifty. Yohe for help with data drove; B. Demes and D. Schmitt for providing chimpanzee cadaveric material; C. Rose for muscle model illustrations; South. Zack for helpful input on mammalian phylogenetics; and Grand. Miller and J. Stern for comments on earlier versions of the manuscript. Funding was provided past the National Scientific discipline Foundation (BCS-0935321 to M.C.O. and South.K.Fifty. and BCS-0935327 to B.R.U.).
Footnotes
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