Muscle architecture and out‐force potential of the thoracic limb in the eastern mole (Scalopus aquaticus)

Moles have modified thoracic limbs with hypertrophied pectoral girdle muscles that allow them to apply remarkably high lateral out‐forces during the power stroke when burrowing. To further understand the high force capabilities of mole forelimbs, architectural properties of the thoracic limb muscles were quantified in the Eastern mole (Scalopus aquaticus). Architectural properties measured included muscle mass, moment arm, belly length, fascicle length, and pennation angle, and these were used to provide estimates of maximum isometric force, joint torque, and power. Measurements of muscle moment arms and limb lever lengths were additionally used to analyze the out‐force contributions of the major pectoral girdle muscles. Most muscles have relatively long fascicles and little‐to‐no pennation. The humeral abductor/rotators as a functional group are massive and are capable of relatively high force, power, and joint torque. Of this group, the bipennate m. teres major is the most massive and has the capacity to produce the highest force and joint torque to abduct and axially rotate the humerus. In general, the distal limb muscles are relatively small, but have the capacity for high force and mechanical work by fascicle shortening. The muscle architectural properties of the elbow extensors (e.g., m. triceps brachii) and carpal flexors (e.g., m. palmaris longus) are consistent with the function of these muscles to augment lateral out‐force application. The humeral abductor/rotators m. latissimus dorsi, m. teres major, m. pectoralis, and m. subscapularis are calculated to contribute 13.9 N to out‐force during the power stroke, and this force is applied in a ‘frontal’ plane causing abduction of the humerus about the sternoclavicular joint. Moles have several specializations of their digging apparatus that greatly enhance the application of out‐force, and these morphological features suggest convergence on limb form and burrowing function between New and Old World moles. J. Morphol. 274:1277–1287, 2013. © 2013 Wiley Periodicals, Inc.     

Comparison of fascicle behaviors between superficial and deeper muscles of triceps brachii during isometric contractions

PurposeWe assessed fascicle behaviors of the upper extremities during isometric contractions at different joint angles in this study.MethodsThirteen healthy men and women performed isometric elbow extension tasks at 50% and 75% of maximal voluntary contraction (MVC) at 60°, 90°, and 120° of elbow extension (full extension = 180°). Extended field-of-view B-mode ultrasonography was used to obtain sagittal plane panoramic images of the long head (TB-Long) and medial head (TB-Med) of the triceps brachii at rest and during contraction; fascicle length and pennation angle were measured.ResultsIn the TB-Long, significant fascicle shortening from rest was found during 50% and 75%MVC at 60° and during 75%MVC at 90° of extension. There was no significant fascicle shortening in the TB-Med muscle under any conditions. There was no significant pennation angle change from rest in either muscle. The pennation angle of the TB-Long was significantly greater than that of the TB-Med under all conditions.ConclusionsThese results suggest that fascicle shortening in the TB-Long muscle occurs in flexion; however, no change was found in the TB-Med. In the upper extremity muscle–tendon complex, the superficial and deeper muscles may have different force-transmission efficiency at flexed joint angles.     

The isometric functional capacity of muscles that cross the elbow

We hypothesized that muscles crossing the elbow have fundamental differences in their capacity for excursion, force generation, and moment generation due to differences in their architecture, moment arm, and the combination of their architecture and moment arm. Muscle fascicle length, sarcomere length, pennation angle, mass, and tendon displacement with elbow flexion were measured for the major elbow muscles in 10 upper extremity specimens. Optimal fascicle length, physiological cross-sectional area (PCSA), moment arm, operating range on the force-length curve, and moment-generating capacity were estimated from these data. Brachioradialis and pronator teres had the longest (17.7cm) and shortest (5.5cm) fascicles, respectively. Triceps brachii (combined heads) and brachioradialis had the greatest (14.9cm(2)) and smallest (1.2cm(2)) PCSAs, respectively. Despite a comparable fascicle length, long head of biceps brachii operates over a broader range of the force-length curve (length change=56% of optimal length, 12.8cm) than the long head of triceps brachii (length change=28% of optimal length, 12. 7cm) because of its larger moment arm (4.7cm vs. 2.3cm). Although brachioradialis has a small PCSA, it has a relatively large moment-generating capacity (6.8cm(3)) due to its large moment arm (average peak=7.7cm). These results emphasize the need to consider the interplay of architecture and moment arm when evaluating the functional capabilities of a muscle.     

Three-dimensional analysis of the arrangement and length distribution of fascicles in the triceps muscle of Galea musteloides (Rodentia, Cavimorpha)

Non-uniformity of fascicle parameters (fascicle lengths and orientation) within one skeletal muscle is well known. These parameters have an effect on the physiological cross-sectional area and lengthening rate of the skeletal muscle. Using a binocular microscope with a table driver (q- and p-axes) and vertical drive (v-axis) as a tool for reconstruction of the spatial orientation of single muscle fascicles, we developed an approach for three-dimensional analysis of the arrangement and length distribution in the skeletal muscle of small mammals. Two subunits of the triceps brachii muscle of the Galea musteloides forelimb, triceps longum and triceps laterale, were quantified and compared. Our data show that in the triceps laterale the fascicles are significantly longer (10.23 mm, SD=1.19, n=41) than those in the triceps longum (6.58 mm, SD=2.88, n=39). In the triceps laterale, the fascicle orientation is more or less uniform, whereas, in the triceps longum, there are two areas with different orientation of fascicles: anterior and posterior ones. Different inner architecture of the subunits can be interpreted as an adaptation to the main locomotory function of the triceps muscle, namely production of propulsive force during limb transfer phase and keeping dynamic stability during stance phase. Comparison of our data on the fascicle length and geometry with our previous histochemical results on G. musteloides, shows that the anterior region of the triceps longum, which differs in the fascicle orientation, also contains a significantly larger percent of slow muscle fibres. It is hypothesised here that this small region is involved in keeping posture. Accepted: 16 May 2000     

三种啮齿类动物前肢挖掘效率分析

以甘肃鼢鼠(Myospalax cansus)、棕色田鼠(Lasiopodomys mandarinus)和小鼠(Mus musculus)为对象,对其尺骨、桡骨和肱三头肌结构进行了比较,并通过力学模型,对这三种生活类型鼠类前肢的挖掘效率进行分析。结果显示,甘肃鼢鼠肘关节位置大幅度前移,尺骨鹰嘴特化突出,形成更加省力的骨学杠杆基础,其中甘肃鼢鼠的鹰嘴尺骨比例达0.40;棕色田鼠和小鼠的鹰嘴尺骨比例分别约为0.19和0.18。此外,甘肃鼢鼠提供挖掘动力的肱三头肌近体端长头覆盖整个肩胛骨下缘,外侧头和内侧头覆盖桡神经沟到肱骨肘关节髁附近区域,远体端扁腱附着于尺骨鹰嘴,整块肌肉非常发达,棕色田鼠和小鼠均无此特化现象。说明甘肃鼢鼠前肢结构更加适应地下掘土生活,其挖掘效率远大于棕色田鼠和小鼠。     

A 3D model of muscle reveals the causes of nonuniform strains in the biceps brachii

Biomechanical models generally assume that muscle fascicles shorten uniformly. However, dynamic magnetic resonance (MR) images of the biceps brachii have recently shown nonuniform shortening along some muscle fascicles during low-load elbow flexion (J. Appl. Physiol. 92 (2002) 2381). The purpose of this study was to uncover the features of the biceps brachii architecture and material properties that could lead to nonuniform shortening. We created a three-dimensional finite-element model of the biceps brachii and compared the tissue strains predicted by the model with experimentally measured tissue strains. The finite-element model predicted strains that were within one standard deviation of the experimentally measured strains. Analysis of the model revealed that the variation in fascicle lengths within the muscle and the curvature of the fascicles were the primary factors contributing to nonuniform strains. Continuum representations of muscle, combined with in vivo image data, are needed to deepen our understanding of how complex geometric arrangements of muscle fibers affect muscle contraction mechanics.     

Muscle fascicle and series elastic element length changes along the length of the human gastrocnemius during walking and running

Ultrasound imaging has recently been used to distinguish the length changes of muscle fascicles from those of the whole muscle tendon complex during real life movements. The complicated three-dimensional architecture of pennate muscles can however cause heterogeneity in the length changes along the length of a muscle. Here we use ultrasonography to examine muscle fascicle length and pennation angle changes at proximal, distal and midbelly sites of the human gastrocnemius medialis (GM) muscle during walking (4.5 km/h) and running (7.5 km/h) on a treadmill. The results of this study have shown that muscle fascicles perform the same actions along the length of the human GM muscle during locomotion. However the distal fascicles tend to shorten more and act at greater pennation angles than the more proximal fascicles. Muscle fascicles acted relatively isometrically during the stance phase during walking, however during running the fascicles shortened throughout the stance phase, which corresponded to an increase in the strain of the series elastic elements (SEEs) (consisting of the Achilles tendon and aponeurosis). Measurement of the fascicle length changes at the midbelly level provided a good approximation of the average fascicle length changes across the length of the muscle. The compliance of the SEE allows the muscle fascicles to shorten at a much slower speed, more concomitant with their optimal speed for maximal power output and efficiency, with high velocity shortening during take off in both walking and running achieved by recoil of the SEE.     

Architectural Properties of Sloth Forelimb Muscles (Pilosa: Bradypodidae)

Tree sloths have reduced skeletal muscle mass, and yet they are able to perform suspensory behaviors that require both strength and fatigue resistance to suspend their body mass for extended periods of time. The muscle architecture of sloths is hypothesized to be modified in ways that will enhance force production to compensate for this reduction in limb muscle mass. Our objective is to test this hypothesis by quantifying architecture properties in the forelimb musculature of the brown-throated three-toed sloth (Bradypus variegatus: N = 4). We evaluated architecture from 52 forelimb muscles by measuring muscle moment arm (r _m), muscle mass (MM), belly length (ML), fascicle length (L_F), pennation angle (θ), and physiological cross-sectional area (PCSA), and these metrics were used to estimate isometric force, joint torque, and power. Overall, the musculature becomes progressively more pennate from the extrinsic to intrinsic regions of the forelimb, and the flexors are more well developed than the extensors as predicted. However, most muscles are indicative of a mechanical design for fast joint rotational velocity instead of large joint torque (i.e., strength), although certain large, parallel-fibered shoulder (e.g., m. latissimus dorsi) and elbow (e.g., m. brachioradialis) flexors are capable of producing appreciable torques by having elongated moment arms. This type of functional tradeoff between joint rotational velocity and mechanical advantage is further exemplified by muscle gearing in Bradypus that pairs synergistic muscles with opposing L_F/r _m ratios in each functional group. These properties are suggested to facilitate the slow, controlled movements in sloths. In addition, the carpal/digital flexors have variable architectural properties, but their collective PCSA and joint torque indicates the capability for maintaining grip force and carpal stability while distributing load from the manus to the shoulder. The observed specializations provide a basis for understanding sustained suspension in sloths.     

Microvascular architecture of the pampiniform plexus-testicular artery system in the rat: a scanning electron microscope study of corrosion casts

Because the architectural and biochemical properties of skeletal muscle dictate its force, velocity, and displacement properties, the major extensors (triceps brachii) and flexors (biceps brachii, brachialis, and brachioradialis) of the elbow in a primate (cynomolgus, monkey) were studied. Functional cross-sectional areas (CSA) were calculated from muscle mass, mean fiber length (normalized to a 2.20 microns sarcomere length), and angle of fiber pinnation measurements from each muscle. Fiber-type distributions were determined and used as a gross index of the biochemical capacities of the muscle. The extensor group had a shorter mean fiber length (31 vs. 47 mm), a larger CSA (13 vs. 8 cm2), and a higher overall percentage of slow-twitch fibers (47 vs. 26%). Consequently, the elbow extensors had a relatively greater potential for force production and force maintenance than the flexors. In contrast, the flexors were designed to optimize their length-velocity potentials; i.e., they had relatively long fibers and a higher fast-twitch fiber composition than the extensors. These morphologic differences between antagonistic muscle groups should be considered when evaluating the motor control mechanisms regulating reciprocal movements about the elbow.     

Nonuniform shortening in the biceps brachii during elbow flexion.

This study tested the common assumption that skeletal muscle shortens uniformly in the direction of its fascicles during low-load contraction. Cine phase contrast magnetic resonance imaging was used to characterize shortening of the biceps brachii muscle in 12 subjects during repeated elbow flexion against 5 and 15% maximum voluntary contraction (MVC) loads. Mean shortening was relatively constant along the anterior boundary of the muscle and averaged 21% for both loading conditions. In contrast, mean shortening was nonuniform along the centerline of the muscle during active elbow flexion. Centerline shortening in the distal region of the biceps brachii (7.3% for 5% MVC and 3.7% for 15% MVC) was significantly less (P < 0.001) than shortening in the muscle midportion (26.3% for 5% MVC and 28.2% for 15% MVC). Nonuniform shortening along the centerline was likely due to the presence of an internal aponeurosis that spanned the distal third of the longitudinal axis of the biceps brachii. However, muscle shortening was also nonuniform proximal to the centerline aponeurosis. Because muscle fascicles follow the anterior contour and centerline of the biceps brachii, our results suggest that shortening is uniform along anterior muscle fascicles and nonuniform along centerline fascicles.     

Behavior of human muscle fascicles during shortening and lengthening contractions in vivo.

The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.     

Muscle force per cross-sectional area is inversely related with pennation angle in strength trained athletes

The present study aimed to examine the effect of pennation angle on the force per cross-sectional area for elbow extensor muscles in strength-trained athletes. A total of 52 male bodybuilders (n = 32) and Olympic weightlifters (n = 20) did maximal isometric elbow extension on an isokinetic dynamometer. Muscle cross-sectional area (CSA) and muscle-fiber pennation angle (PA) of the triceps brachii muscles were measured by ultrasonography. Bodybuilders had significantly greater isometric elbow extension force (F), CSA and PA than weightlifters. The ratio of force to CSA (F/CSA) of bodybuilders was significantly lower than that of weightlifters. A significant positive correlation was observed between CSA and PA in both groups (r = 0.832, P < 0.001, and r = 0.682, P < 0.001, for bodybuilders and weightlifters, respectively). The F/CSA was negatively correlated to PA both for bodybuilders (r = -0.408, P < 0.05) and weightlifters (r = -0.465, P < 0.05). Thus present study indicates that the larger pennation angle is associated with the lower force relative to muscle CSA in strength-trained athletes.     

Muscle architecture of the upper limb in the orangutan

We dissected the left upper limb of a female orangutan and systematically recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA), in order to quantitatively clarify the unique muscle architecture of the upper limb of the orangutan. Comparisons of the musculature of the dissected orangutan with corresponding published chimpanzee data demonstrated that in the orangutan, the elbow flexors, notably M. brachioradialis, tend to exhibit greater PCSAs. Moreover, the digital II-V flexors in the forearm, such as M. flexor digitorum superficialis and M. flexor digitorum profundus, tend to have smaller PCSA as a result of their relatively longer fascicles. Thus, in the orangutan, the elbow flexors demonstrate a higher potential for force production, whereas the forearm muscles allow a greater range of wrist joint mobility. The differences in the force-generating capacity in the upper limb muscles of the two species might reflect functional specialization of muscle architecture in the upper limb of the orangutan for living in arboreal environments.     

Morphometry of Macaca mulatta forelimb. I. Shoulder and elbow muscles and segment inertial parameters

The present study examined the morphometric properties of the forelimb, including the inertial properties of the body segments and the morphometric parameters of 21 muscles spanning the shoulder and/or elbow joints of six Macaca mulatta and three M. fascicularis. Five muscle parameters are presented: optimal fascicle length (L(0)(M)), tendon slack length (L(S)(T)), physiological cross-sectional area (PCSA), pennation angle (alpha(0)), and muscle mass (m). Linear regressions indicate that muscle mass, and to a lesser extent PCSA, correlated with total body weight. Segment mass, center-of-mass, and the moment of inertia of the upper arm, forearm, and hand are also presented. Our data indicate that for some segments, radius of gyration (rho) predicts segment moment of inertia better than linear regressions based on total body weight. Key differences between the monkey and human forelimb are highlighted.     

In vivo estimation of contraction velocity of human vastus lateralis muscle during "isokinetic" action.

To determine the shortening velocities of fascicles of the vastus lateralis muscle (VL) during isokinetic knee extension, six male subjects were requested to extend the knee with maximal effort at angular velocities of 30 and 150 degrees /s. By using an ultrasonic apparatus, longitudinal images of the VL were produced every 30 ms during knee extension, and the fascicle length and angle of pennation were obtained from these images. The shortening fascicle length with extension of the knee (from 98 to 13 degrees of knee angle; full extension = 0 degrees ) was greater (43 mm) at 30 degrees /s than at 150 degrees /s (35 mm). Even when the angular velocity remained constant during the isokinetic range of motion, the fascicle velocity was found to change from 39 to 77 mm/s at 150 degrees /s and from 6 to 19 mm/s at 30 degrees /s. The force exerted by a fascicle changed with the length of the fascicle at changing angular velocities. The peak values of fascicle force and velocity were observed at approximately 90 mm of fascicle length. In conclusion, even if the angular velocity of knee extension is kept constant, the shortening velocity of a fascicle is dependent on the force applied to the muscle-tendon complex, and the phenomenon is considered to be caused mainly by the elongation of the elastic element (tendinous tissue).     

Passive and dynamic muscle architecture during transverse loading for gastrocnemius medialis in man

External forces from our environment impose transverse loads on our muscles. Studies in rats have shown that transverse loads result in a decrease in the longitudinal muscle force. Changes in muscle architecture during contraction may contribute to the observed force decrease. The aim of this study was to quantify changes in pennation angle, fascicle dimensions, and muscle thickness during contraction under external transverse load.Electrical stimuli were elicited to evoke maximal force twitches in the right calf muscles of humans. Trials were conducted with transverse loads of 2, 4.5, and 10 kg. An ultrasound probe was placed on the medial gastrocnemius in line with the transverse load to quantify muscle characteristics during muscle twitches.Maximum twitch force decreased with increased transverse muscle loading. The 2, 4.5, and 10 kg of transverse load showed a 9, 13, and 16% decrease in longitudinal force, respectively. Within the field of view of the ultrasound images, and thus directly beneath the external load, loading of the muscle resulted in a decrease in the muscle thickness and pennation angle, with higher loads causing greater decreases. During twitches the muscle transiently increased in thickness and pennation angle, as did fascicle thickness. Higher transverse loads showed a reduced increase in muscle thickness. Smaller increases in pennation angle and fascicle thickness strain also occurred with higher transverse loads.This study shows that increased transverse loading caused a decrease in ankle moment, muscle thickness, and pennation angle, as well as transverse deformation of the fascicles.     

Contractile behavior of the forelimb digital flexors during steady-state locomotion in horses (Equus caballus): an initial test of muscle architectural hypotheses about in vivo function

The forelimb digital flexors of the horse display remarkable diversity in muscle architecture despite each muscle-tendon unit having a similar mechanical advantage across the fetlock joint. We focus on two distinct muscles of the digital flexor system: short compartment deep digital flexor (DDF(sc)) and the superficial digital flexor (SDF). The objectives were to investigate force-length behavior and work performance of these two muscles in vivo during locomotion, and to determine how muscle architecture contributes to in vivo function in this system. We directly recorded muscle force (via tendon strain gauges) and muscle fascicle length (via sonomicrometry crystals) as horses walked (1.7 m s(-1)), trotted (4.1 m s(-1)) and cantered (7.0 m s(-1)) on a motorized treadmill. Over the range of gaits and speeds, DDF(sc) fascicles shortened while producing relatively low force, generating modest positive net work. In contrast, SDF fascicles initially shortened, then lengthened while producing high force, resulting in substantial negative net work. These findings suggest the long fibered, unipennate DDF(sc) supplements mechanical work during running, whereas the short fibered, multipennate SDF is specialized for economical high force and enhanced elastic energy storage. Apparent in vivo functions match well with the distinct architectural features of each muscle.     

Mechanical similarity across ontogeny of digging muscles in an Australian marsupial (Isoodon fusciventer)

Many mammals dig, either during foraging to access subsurface food resources, or in creating burrows for shelter. Digging requires large forces produced by muscles and transmitted to the soil via the skeletal system; thus fossorial mammals tend to have characteristic modifications of the musculoskeletal system that reflect their digging ability. Bandicoots (Marsupialia: Peramelidae) scratch-dig mainly to source food, searching for subterranean food items including invertebrates, seeds, and fungi. They have musculoskeletal features for digging, including shortened, robust forelimb bones, large muscles, and enlarged muscle attachment areas. Here, we compared changes in the ontogenetic development of muscles associated with digging in the Quenda (Isoodon fusciventer). We measured muscle mass (m _m), pennation angle, and fiber length (FL) to calculate physiological cross-sectional area (PCSA; a proxy of maximum isometric force) as well as estimate the maximum isometric force (Fmax) for 34 individuals ranging in body size from 124 to 2,390 g. Males grow larger than females in this bandicoot species, however, we found negligible sex differences in mass-specific m _m, PCSA or FL for our sample. Majority of the forelimb muscles PCSA showed a positive allometric relationship with total body mass, while m _m and FL in the majority of forelimb muscles showed isometry. Mechanical similarity was tested, and two thirds of forelimb muscles maximum isometric forces (Fmax) scaled with isometry; therefore the forelimb is primarily mechanical similar throughout ontogeny. PCSA showed a significant difference between scaling slopes between main movers in the power stroke, and main movers of the recovery stroke of scratch-digging. This suggests that some forelimb muscles grow with positive allometry, specially these associated with the power stroke of digging. Intraspecific variation in PCSA is rarely considered in the literature, and thus this is an important study quantifying changes in muscle architectural properties with growth in a mammalian model of scratch-digging.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

Functional Morphology of the Forelimb of the Nine-Banded Armadillo (<Emphasis Type="Italic">Dasypus novemcinctus</Emphasis>): Comparative Perspectives on the Myology of Dasypodidae

The nine-banded armadillo, Dasypus novemcinctus, is a member of the family Dasypodidae, which contains all extant species of armadillos and represents the most diverse group of xenarthran mammals by their speciation, form, and range of scratch-digging ability. This study aims to identify muscle traits that reflect specialization for fossorial habit by observing forelimb structure in D. novemcinctus and comparing it among armadillos using available myological data. A number of informative traits were observed in D. novemcinctus and among Dasypodidae, including the absence of m. rhomboideus profundus, the variable presence of a m. articularis humeri and m. coracobrachialis, two heads of m. triceps brachii with scapular origin, and a lack of muscle mass devoted to antebrachial supination. Muscle mass and myosin heavy chain (MHC) isoform content were also quantified from our forelimb dissections. New osteological indices are additionally calculated and reported for D. novemcinctus. Collectively, the findings emphasize muscle mass and power output for limb retraction and specialization of the distal limb for sustained purchase of soil by strong pronation and carpal/digital flexion. Moreover, the myological traits assessed here provide a valuable resource for interpretation of muscle architecture specializations among digging mammals and future reassessment of armadillo phylogeny.