A horse of a different color?: Tensile strength and elasticity of sloth flexor tendons

Tendons must be able to withstand the tensile forces generated by muscles to provide support while avoiding failure. The properties of tendons in mammal limbs must therefore be appropriate to accommodate a range of locomotor habits and posture. Tendon collagen composition provides resistance to loading that contributes to tissue strength which could, however, be modified to not exclusively confer large strength and stiffness for elastic energy storage/recovery. For example, sloths are nearly obligate suspenders and cannot run, and due to their combined low metabolic rate, body temperature, and rate of digestion, they have an extreme need to conserve energy. It is possible that sloths have a tendon ‘suspensory apparatus’ functionally analogous to that in upright ungulates, thus allowing for largely passive support of their body weight below-branch, while concurrently minimizing muscle contractile energy expenditure. The digital flexor tendons from the fore- and hindlimbs of two-toed ( Choloepus hoffmanni ) and three-toed ( Bradypus variegatus ) sloths were loaded in tension until failure to test this hypothesis. Overall, tensile strength and elastic (Young’s) modulus of sloth tendons were low, and these material properties were remarkably similar to those of equine suspensory ‘ligaments’. The results also help explain previous findings in sloths showing relatively low levels of muscle activation in the digital flexors during postural suspension and suspensory walking.


Introduction
Tendons are specialized connective tissue structures that can withstand large tensile forces that are generated during locomotor activities (Bennett et al. 1986;Ker et al. 1988).Consequently, tendons need to be strong and stiff (and tough) enough to withstand such forces, as well as have a reasonable safety factor for body weight support (Shadwick, 1990).Strength is the maximum amount of tensile stress that a tendon can withstand before failure, while stiffness is the ability of tendon to resist deformation (i.e., strain) and toughness indicates the total amount of energy a tendon can absorb before rupture.Distal limb tendons in some terrestrial mammals are known to function as appropriately stiff elastic elements for joint position control and/or as efficient biological springs during locomotion to conserve energy (Alexander, 1984(Alexander, , 1988;;Alexander and Dimery, 1985).For example, muscle-tendon units (MTU) of ungulates and kangaroos and wallabies are extremely modified in this aspect to undergo large strain by having long, thin tendons capable of undergoing large strains for substantial elastic strain energy storage and recovery (Dimery and Alexander, 1985;Dimery et al., 1986;Biewener and Baudinette, 1995;Biewener, 1998;Butcher et al., 2009).
Strain in MTU may also occur with little-to-no muscle activation for static body weight support, thus the resisting force is largely passive tension during postural behavior (Lieber, 2002;Hodson-Tole et al., 1960).This type of MTU function is typical in the distal limbs of upright ungulates during grazing.Notably, horses have a weight-bearing suspensory apparatus (Hildebrand, 1960(Hildebrand, , 1987) ) which allows them to remain standing for long periods of time without fatigue (Hermanson and Cobb, 1992).As a part of their suspensory apparatus, composed mainly of the suspensory 'ligament' (i.e., m. interosseous medius) and collateral ligaments about metacarpo-metatarsophalangeal joints, the digital flexor tendons act in parallel with these ligamentous structures that are more immediate to the support of the digits during standing (Hildebrand, 1960;Dyce et al., 1996;Butcher et al., 2006), in addition to the tendons resisting substantial load during the mid-stance portion of locomotion.The specializations in digital flexor MTU structure-function of equids substantially reduce metabolic energy expenditure that would otherwise need to be supplied by muscle work (Biewener, 1998;Butcher et al., 2007), which is critical for both postural and locomotor activity of all mammals.Thus, it is possible that similar specializations are also present in the distal limbs of taxa not adapted for cursorial habit, including sloths, because they are capable of suspending below tree branches by pairs of limbs for extended periods of time, an ability that is functionally analogous to ungulates standing upright for many hours grazing.
Tree sloths (an uncommon model system) are some of the rare mammals that demonstrate nearly obligatory suspensory habits as part of their arboreal lifestyle (Hayssen, 2010(Hayssen, , 2011;;Granatosky et al., 2018b), and accordingly, they have exceptionally modified distal limb morphology as a representation of their suspensory adaptations (Mendel, 1981a(Mendel, , 1981b(Mendel, , 1985;;Nyakatura et al., 2010;Olson et al., 2018).Sloths do not use pronograde (above-branch) locomotion as do their primate counterparts that are also adapted for suspensory habits, and although sloths can perform both arboreal (suspensory) and terrestrial (crouched-crawl) locomotion, they are incapable of a running gait for either mode of transport (Mendel, 1981a(Mendel, , 1985)).Moreover, sloths do not utilize pendulum-like energy exchanges when suspensory walking (Nyakatura and Andrada, 2013), and with no necessity to bounce below-branch, their flexor tendons serve no functional role as biological springs like those in upright quadrupeds.
Therefore, energy conservation is extraordinarily critical to sloths, and akin to other mammals, they can be expected to have one or several mechanisms to offset the cost of force production during locomotor and postural behaviors.The need to conserve metabolic energy is further compounded by a combination of extremely low basal metabolism (Pauli et al., 2016) and low body temperatures (Cliffe et al., 2018), as well as an energy poor diet (Montgomery and Sunquist, 1978) and low rates of digestion (Cliffe et al., 2015).It is hypothesized that the distal limbs of sloths may effectively demonstrate a suspensory apparatus to supplement offset metabolic energy expenditure during periods of suspensory activity.
The objective of this study is to evaluate the material/mechanical properties of sloth deep digital flexor tendons.Based on previous data (Gorvet et al., 2020) indicating that small, fastcontracting fibers (and/or motor units) in the distal flexor musculature are activated at low levels during suspensory walking and posture, the low muscle activation observed in sloth digital flexors may provide a similar low level of force; a magnitude adequate to counteract strain in their flexor tendons (i.e., modulating stiffness), such that a large portion of the body weight in sloths is supported by tensile loading of their robust flexor tendons.Such a mechanism would save sloths energy during suspension (see Fig. 1) and additional evidence on the tendon strength and elasticity as key components of their tensile limb system could also help validate a previous hypothesis of entirely passive support provided by strong 'locking' flexor tendons (Mendel, Downloaded from https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa032/5940014 by guest on 29 October 2020 1981a, b, c).Last, the expected combination of large tensile strength and modest elasticity for sloth flexor tendons may also account for necessary selected advantages of sloths having an unusually low metabolism for placental mammals and the slow, deliberate movement patterns associated with their arboreal lifestyle.

Materials and Methods
Tensile testing was performed on m. flexor digitorum profundus (FDP) tendons from subadult/adult C. hoffmanni (N=6, 5.47±1.14kg) and B. variegatus (N=5, 3.36±1.37kg).As a comparator data set, we tested the deep flexor tendons from the forelimbs of vervet monkeys, Chlorocebus aethiops (N=3, 4.20±2.00kg), a semi-arboreal primate, using the following protocol.Tendons were loaded in tension using a uniaxial Instron machine (Instron, Norwood, MA, USA) and photographs of the loading chain are shown in Figure 2. Two custom grips, one on the machine base (aluminum clamp block) and the other on the load cell attached to the crosshead (screw-tensioned clamp) were used to hold the tendon test specimen in place.Prior to enclosing the tendons in the grips, each end of the tendon was secured between four 2.5 cm x 2.5 cm pieces of 100 grit adhesive-backed sandpaper, such that two pieces were adhered to each other leaving the grit sides exposed to both the tendon and grip.Tendons were further secured to the sandpaper by using veterinary grade tissue glue (3M) and then photographed on a 1cm x 1cm measurement grid.Once the tissue adhesive cured (quick drying), one end of the tendon was clamped in the top grip followed by the clamping between the base grip when the tendon test sample was vertically aligned within the testing frame (Fig. 2).Tendon tissue was kept hydrated with PBS during this entire process.Initial tendon length (L 0 ) between the grips was standardized to 15-20mm (specifically, for sloth tendons) and set using digital calipers.Also, measurements of tendon width and depth were measured at three locations (top grip, middle tendon, and bottom grip) along L 0 with the test tendon clamped in place.These measurements were used to input specimen dimensions into Instron software for reference, as well as for the calculation of elastic (Young's) Modulus.Still photographs in two dimensions were also taken prior to testing to upload into SolidWorks® software (v.25: Dassault Systèmes, Waltham, MA, USA) for accurate digital measurement of resting tendon dimensions for more calculation of material properties.All tendons were loaded to failure in tension at a crosshead displacement (strain rate: 10 mm min -1 ; ~1% strain per min: Javidi et al., 2019).Force and extension were continuously recorded during testing and output from the load cells (250 N load cell) to Instron data acquisition software (v.2.25: Instron).After each test, tendons were removed from the testing rig and once again photographed on the measurement grid to assess any patterns of differential strain throughout tendon length leading to its permanent deformation and/or rupture.All recorded data were exported as .csvfiles to MS Excel (Microsoft Corp., Redmond, WA, USA) for computation of material properties.Stress (in MPa) and strain (%) were calculated from the records of force and extension using digitally calculated values of tendon cross-sectional area (CSA) and L 0 , respectively, and plotted as a normalized stress-strain curve (Fig. 3).The following material properties were measured/calculated from the linear elastic regions of either raw force-extension or normalized stress-strain data: elastic (Young's) Modulus (E), ultimate tensile strength, stiffness (in N/mm), extensibility (in mm), toughness (in J/mm 3 ), elastic strain energy (in mJ), and safety factors.
MANOVA performed in SPSS (version 20.0: IBM, Armonk, NY, USA) was used to test for differences in tendon material properties for the a priori factors species (Choloepus vs. Bradypus) and limbs (forelimb vs. hindlimb).A non-significant finding for both factors then allowed for a similar MANOVA test among forelimb tendon properties between sloths and vervet monkeys.ANOVA (in SPSS) was used to test for differences in elastic (Young's) modulus and ultimate tensile strength among limb tendons based on the following locomotor habit (substrate preference) categorizations: terrestrial, semi-arboreal, and arboreal.Data for these tests included rat calcaneal tendon (terrestrial) from the literature (LaCroix et al., 2013), in addition to those values from vervet monkeys (semi-arboreal) and sloths (arboreal) sampled in the present study.

Results
Means ± SD of measured and calculated material properties are shown in Figure 4. Overall, tensile strength was lower than expected and as was elasticity, while tendon stiffness was moderate.Despite some mean variation between species, especially in tendon safety factors (Table 1), tendon properties did not vary (P = 0.950) between two-toed (C.hoffmanni) and threetoed (B.variegatus) sloths or between fore-and hindlimb pairs (P = 0.907).Comparisons among the material properties of flexor tendons from semi-arboreal vervet monkeys (C.aethiops) versus those from sloth limbs showed that only extensibility (P = 0.030) and elastic strain energy (P = 0.041) differed significantly, with greater values for extensibility in sloths and the inverse for elastic strain energy storage capacity in monkeys.Significant differences in tensile strength and Downloaded from https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa032/5940014 by guest on 29 October 2020 elastic (Young's) modulus were also found in relation to locomotor habit and/or substrate preference as shown in Figure 5. Overall, tensile strength (P = 0.006) and elastic modulus (P = 0.001) were greater in rat (calcaneal) tendons than in sloth flexor tendons (Fig. 5a), whereas only tendon elastic modulus (P = 0.024) was greater in rats over that in vervet monkeys (Fig. 5b).

Discussion
To our know knowledge, the present study is the first to determine material properties of flexor tendons in an obligate suspensory (tensile) limb system.Despite differences in adult body mass (Grand, 1978), distal limb form (Mendel, 1981a, b), and frequency of suspensory locomotion and posture (Sunquist and Montgomery, 1973;Urbani and Bosque, 2007), tendon strength, elasticity, and stiffness are the same for C. hoffmanni and B. variegatus.This major finding may not be surprising due to the remarkable breadth of evolutionary convergence for suspensory habits observed between two-toed and three-toed forms (Gaudin, 2004;Nyakatura, 2012;Montanez-Rivera et al., 2018;Spainhower et al., 2018).In particular, peak vertical substrate reaction forces (SRF) do not differ between the fore-and hindlimbs during suspensory walking in Choloepus (Granatosky and Schmitt, 2017), thus both limb pairs play an equal role for body weight support (Granatosky et al., 2018b) necessitating that limb tendon properties remain consistent.Future work with Bradypus aims to demonstrate similar locomotor mechanics to determine if there is a universal lack of body weight shifting during suspensory locomotion in sloths.Moreover, we initially expected that safety factors might be elevated in the flexor tendons of Choloepus, which tends to spend more time in postural suspension than Bradypus, as well as having greater active time incorporating longer bouts of suspensory walking (Urbani and Bosque, 2007).These behavioral differences, which correspond with the ecological preferences of C. hoffmanni (Hayssen, 2011), suggest that lower safety factors for the tendons of two-toed sloths may result from them operating at longer lengths during suspension.Nonetheless, elevated safety factors in B. variegatus coupled with its less active lifestyle bears no statistical difference, further signifying functional convergence in sloths.
Tendon safety factors estimated for sloths are higher than those typically found for the tendons from upright mammals (Alexander, 1981).The species of two-and three-toed sloths studied have an average body mass of nearly 4-6 kg as adults (Grand, 1978;Spainhower et al., 2018).At worst, when supported by only one limb in suspension, the flexor tendons can sustain 2.5 to 4.2 times the body weight of sloths.While these values are within the typical range of safety factors for mammalian tendons, sloths rarely employ single limb postural support and walk with a lateral sequence diagonal couplet gait (Mendel, 1985;Nyakatura et al., 2010;Gorvet et al., 2020) where three limb support is most common.When supported by three limbs, safety factors increase substantially (Table 1), ranging upwards of 7-10 for sloths, although we caution that our estimates are calculated with the assumption that the tendons alone are supporting the body mass (i.e., functional load) with no assistance from muscle activation.Nevertheless, despite lacking large tensile strength as we originally expected, the flexor tendons of sloths appear to be 'overbuilt' for the usual loads that they must resist, consistent with an important role in suspension.It is also likely that the robustness of sloth tendons belies the large strength and stiffness properties typical of thick tendons (Bennett et al., 1988).The relative composition of proteoglycans (ground substance) versus Type I collagen, as well as the orientation of the collagen fibers in the flexor tendons of Choloepus and Bradypus are not known.Future studies of tendon histology seek to overcome this limitation to our present interpretation.In particular, such evaluations should test the hypothesis that the relationship between tissue composition and function in limbs tendons is a possible plesiomorphic trait in sloths.

Similar to ungulates, which have relatively long, thin tendons and suspensory 'ligaments'
for their large size, as well as elongated distal limb length, the digital flexor tendons of Choloepus (Mendel, 1981b, c) and Bradypus (Olson et al., 2018) are equally impressive and also paired with elongate skeletal elements of the feet.Moreover, sloths have long, re-curved claws and the combination of a hook-like shape to their feet and robust tendon (volar) tunnels have additionally been argued to be features acquired independently for suspensory habits (Mendel, 1981a, b).Based on the observed morphology, it was previously proposed (Mendel, 1985) that the body weight of sloths was supported by passive tension in the digital flexor tendons that 'locked' in a fixed position to confirm grip on the substrate by the feet/claws.Support for this functional hypothesis depends on the relative stiffness versus compliance of the tendons, and by having both low tendon tensile strength and elasticity, some form of suspensory modification to sloth digital flexor tendons is indicated.However, it is not clear how these tendons would 'lock' into place or how they would be latched and released by their well-developed volar tunnels.
Moreover, sloth digital flexor tendons may be too compliant for strictly passive support, often displaying strains approximating 20-25% in the linear elastic region.This finding suggests a  (Grand, 1978).
Grip on the substrate is critical to the ability of sloths to remain in suspension and distal flexor muscles are in series with flexor tendons for joint position control (Biewener and Roberts, 2000).It is the integrated function of an MTU that determines a role in support and/or energy savings and we speculate that the compliance observed in sloth flexor tendons may allow fascicles of m. flexor digitorum profundus (FDP) to modulate tendon stiffness by undergoing minimal length changes during tensile loading of the MTU.Patterns of low muscle activation observed for the FDP in B. variegatus during suspensory hanging (Gorvet et al., 2020) match this expectation and conceivably serve to lower muscle contractile energy expenditure.By this mechanism, the FDP would produce limited force but enough to counterbalance tensile loading of its four tendon slips to sustain flexion of the digits/claws, all while minimizing muscle activation, thus allowing the digital flexors of sloths to act as important anti-gravity muscles (Fig. 1).EMG recordings from the digital flexors and forelimb extensors of horses support our interpretations by also showing minimal activation during standing (Jansen et al., 1998;Butcher et al., 2009;Harrison et al., 2012).The action of postural muscles to stabilize joints saves horses appreciable amounts of metabolic energy (Biewener, 1998).In addition, the m.interosseous medius provides largely passive support to the metacarpophalangeal joints in horses for standing (Hildebrand, 1960); however, activation of its extremely short muscle fibers stiffens this highly modified flexor muscle in accordance with dynamic body weight support needs of the animal (Soffler and Hermanson, 2006).Again, we anticipate analogous functional modulations in the distal limbs of sloths.
Beyond the material properties of the flexor tendons, the structure/function of the muscle bellies of sloth digital flexors is similar to those participating in the suspensory apparatus of upright ungulates (Hildebrand, 1960;Butcher et al., 2007Butcher et al., , 2010)).Specifically, the digital flexor complex (FDP and m. flexor digitorum superficialis) is correspondingly the most massive (intrinsic) muscle functional group in the forelimb of B. variegatus and each muscle belly has pennate fascicles with appreciable PCSA (Olson et al., 2018) as seen in horse forelimbs (Hermanson and Cobb, 1992;Butcher et al., 2010).Other observations indicate that FDP in sloths may have appreciable tendinous fibers throughout its cross-section (Spainhower et al., 2018), which suggests that active contractile muscle mass could be sacrificed for enhanced This type of tissue exchange may also contribute to the reduced skeletal muscle mass in sloths (Grand, 1978).Overall, functional tradeoffs between tendon compliance and stiffness appears to be an energetically economical means to support body weight while still providing the ability of joint position control for a strong grip on the substrate.That is, sloths may maintain the minimal amount tendon tissue necessary for suspensory support (vertical SRF = 0.7 BM: Granatosky and Schmitt, 2017) and rely on muscle force when greater tendon stiffness is needed.

Re-evaluation of mammalian tendon properties
Figure 6 shows values of ultimate tensile strength and elastic (Young's) modulus (E) for mammalian tendons taken from the literature and grouped by locomotor habits (e.g., saltatorial, cursorial, generalist, etc.).Contrary to the persisting dogma that emphasizes high strength and elasticity in distal limb tendons for elastic strain energy storage and recovery (Alexander, 1984;Bennett et al., 1986;Biewener, 1998), sloth digital flexor tendons are characterized by both low strength and elasticity and these data group completely opposite to that of tendons with cursorial and saltatorial specialization.Moreover, similar data for equine suspensory 'ligaments' (SL) match well with the material properties measured from sloth flexor tendons, albeit the values for E that we extrapolated for the SL (Jansen et al., 1998) are minimal.While methods of data acquisition might have been different from some studies for which values were taken from the literature, we are confident in our analysis and propose that this combination of tendon properties observed from sloths represents a simplification for suspensorial habits, thus providing the most direct support to our hypothesis that sloth FDP tendons have the ability to function analogous to an ungulate suspensory apparatus for energy conservation.
Recognizing that distal limb tendons from mammals with diverse locomotor behaviors can be modified (or 'simplified') in their material properties is not necessarily novel (Alexander and Dimery, 1985;Dimery and Alexander, 1985;Dimery et al., 1986;Bennett et al., 1986;Ker et al., 1988Biewener, 1998;Biewener and Roberts, 2000), but the extent to which functional adaptation influences tendon performance has been overlooked until recently.Javidi et al. (2019) showed that large tensile strength and elastic modulus are decoupled in the long, thin hindlimb (calcaneal and plantaris) tendons of kangaroo rats, which perform ricochetal jumping without reliance on elastic energy storage/recovery.Strength and elastic modulus of tendons do not have to correlate as they are observed for numerous other mammals that are typically studied, Downloaded from https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa032/5940014 by guest on 29 October 2020 including rats, humans, horses, and various domesticated species (LaCroix et al., 2013).And while exceptional tendon performance in kangaroo rat limb warrants further study for understanding mechanisms or microstructure that drive differential adaptation in tendon mechanical properties, sloth tendons are just as extraordinary, but for their lack of tendon strength and elasticity despite their robustness.Again, our broad interpretation of tendon structure and function is limited by the comparative lack of studies on tendon histology and tissue composition.Moreover, flexor vs. extensor tendons (e.g., calcaneal, quadriceps, and digital flexor tendons) with predicted major roles in limb loading patterns are too often the target of functional studies.It also may be insightful to study the material properties of tendons with a lesser role in weight-bearing support to address questions about universal investment in and maintenance of tendon tissue.
Across mammals, tendon strength and elasticity are broadly reflective of generalized substrate use.Thus, our comparative analysis shows that, overall, material properties are part of a continuum independent of body size (Fig. 6) and areas of overlap between the differing locomotor habits could be interpreted as functional trade-offs in tendon performance.For example, there are modifications in tendon material properties among arboreal, semi-arboreal, and generalized mammals where elastic modulus differs significantly between the selected taxa, whereas tensile strength is only different for the extremes of arboreal and generalized habits (Fig. 5).A comparison between sloths and rats emphasizes obligate (anti-pronograde) suspension in an arboreal mammal that requires greater tendon compliance versus greater strength and stiffness needed for translation of joint power during pronograde terrestrial locomotion.These results may also point to differences in functional roles between fore-and hindlimb pairs in suspensorial vs. terrestrial taxa.
Interestingly, data from the forelimb digital flexor tendons of vervet monkeys coincide with those from two-and three-toed sloths warranting their placement in our suspensorial data grouping.The species C. aethiops is semi-arboreal and is limited in its use of suspensory modes of locomotion and posture.However, whereas primates often bear more weight on their hindlimbs than forelimbs during terrestrial and pronograde arboreal locomotion (Schmitt and Hanna, 2004;Granatosky et al., 2016a, b), they shift body weight to their forelimbs as the main supports for suspension (Granatosky et al., 2018a) and this compensatory behavior may account for the similarity in primate and sloth flexor tendon properties.We speculate that there may be Available evidence suggests that there are indeed multiple similarities in forelimb bone shape (Patel et al., 2013) and microstructural properties such as bone density (Patel and Carlson, 2008) and trabecular architecture (Amson et al. 2017) between sloths and suspensory primates associated with functional adaptations for suspensory habits.Our future research aims to further understand modifications of tensile limb systems in primates and if the presence of a tendon suspensory apparatus is strictly adapted for near obligatory suspension or do tendon properties represent additional compromises (or simplifications) relating to various modes of arboreal locomotion.This work will also test the hypothesis that compressive versus tensile bone strength in mammals (Biewener, 1982;Currey, 1970Currey, , 1987Currey, , 1990;;Erickson et al., 2002) is more plastic than previously considered, and the forelimb skeletal elements from sloths and arboreal primates will be our main models.

Data Availability.
Tendon loading data are available from the corresponding author upon reasonable request.All other data generated or analyzed during this study are included in this published article in adherence with disclosure policy of the journal.Biewener, 1998, LaCroix et al., 2013;human and rat calcaneal tendons: LaCroix et al., 2013; kangaroo rat gastrocnemius/plantaris tendons: Javidi et al., 2019).Sloth and monkey data are from this study.Ellipses show functional categorizations of tendon properties based on locomotor habits (substrate use).Data grouped by the rectangle near the origin are placed in the proposed "suspensorial" functional grouping by emphasizing low-to-modest tensile strength Downloaded from https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa032/5940014 by guest on 29 October 2020 Tensile strength of sloth flexor tendons Mossor et al.
Downloaded from https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa032/5940014 by guest on 29 October 2020 Tensile strength of sloth flexor tendons Mossor et al. 13 additional undiscovered functional trade-offs in the strength/stiffness and elastic properties of both tendon and bone tissue across arboreal mammals that routinely load their limbs in tension.

Figure 1 .
Figure 1.Proposed integrated model of system function for counteracting tensile forces acting on sloth limbs during suspension.Sloth image redrawn and modified from that originally published in Granatosky and Schmitt (2017).

Figure 2 .
Figure 2. A-D) Photographs of the Instron load chain for material property testing on sloth flexor tendons.Shown sequentially is a robust digital flexor tendon gripped for initial measurement using upper and lower custom grips, undergoing a tensile test, and ultimately under maximum load near failure.

Figure 3 .
Figure 3. Representative normalized stress-strain curve of sloth digital flexor tendons.Diagrammed are measurements of material properties taken from the elastic loading region: ultimate tensile strength, elastic (Young's) modulus (E), toughness, and extensibility.

Figure 4 .
Figure 4. A-F) Bar charts of tendon material properties for the fore-and hindlimbs of C. hoffmanni and B. variegatus.Shown are mean ± SD.Forelimbs (blue or red) are indicated by solid bars while hindlimbs (orange or yellow) are dashed bars.