A method for in-vivo kinematic analysis of the forearm

PURPOSE: To develop a method for in-vivo kinematic study of normal forearm rotation using computed tomographic (CT) images and a custom apparatus which allows for control of amount of forearm rotation. METHODS: The forearm of one asymptomatic volunteer was CT-scanned in five positions: neutral, 60 degrees pronation, maximal pronation, 60 degrees supination, and maximal supination. Surface registration of the pronated/supinated image datasets with the neutral position was performed. The resulting transformation matrices were decomposed into finite helical axis (FHA) parameters. Kinematics were expressed as motion of the radius relative to the ulna. RESULTS: The axes of the forearm passed through the volar region of the radial head at the proximal radioulnar joint (PRUJ), extending towards the dorsal region of the ulnar head at the distal radioulnar joint (DRUJ). Distinct FHAs were calculated for each forearm position analyzed relative to neutral rotation. Forearm pronation FHAs were different from forearm supination FHAs. CONCLUSIONS: Our experimental methodology is capable of describing the in-vivo kinematics of the forearm with good accuracy and reliability. Future in-vivo studies would need to be performed using a larger sample size to further validate our preliminary results. An ideal clinical application of this methodology would be in the comparative study of patients with forearm dysfunction.     

Design and implementation of an instrumented ulnar head prosthesis to measure loads in vitro

The development of a novel instrumented implant for ulnar head replacement is presented in this study. This implant was instrumented with strain gauges to quantify bending moments about the anatomic axes of the distal ulna, and subsequently the distal radioulnar joint (DRUJ) reaction force magnitude. The implant was surgically inserted in seven cadaveric upper extremities, which were subsequently mounted in a custom joint simulator. Simulated active unresisted pronation and supination motion trials were conducted using computer-controlled pneumatic actuators to simulate forearm musculature. Passive (unloaded) trials were also conducted. The reaction force across the DRUJ ranged from 2 to 10 N in magnitude during this unresisted motion. Increased bending moment magnitudes were measured when the forearm was positioned in supination compared to pronation. The magnitude of joint bending moments showed a consistent pattern with forearm position, regardless of simulated active or passive rotation, or supination and pronation motion trials. This result illustrates that the primary influence on joint load is likely the position and contact with the radial articulation. This study of DRUJ loading should be useful for biomechanical modeling, implant design considerations and improved knowledge of articular mechanics.     

A musculoskeletal model to estimate the relative changes in wrist strength due to interacting wrist and forearm postures

Wrist rotations about one wrist axis (e.g. flexion/extension) can affect the strength about another wrist axis (e.g. radial/ulnar deviation). This study used a musculoskeletal model of the distal upper extremity, and an optimization approach, to quantify the interaction effects of wrist flexion/extension (FE), radial/ulnar deviation (RUD) and forearm pronation/supination (PS) on wrist strength. Regression equations were developed to predict the relative changes in strength from the neutral posture, so that the changes in strength, due to complex and interacting wrist and forearm rotation postures, can be incorporated within future ergonomics assessments of wrist strength.     

Evolution of tetrapod locomotion

Fish-like ancestors of tetrapods did not need strong limb musculature because they inhabited waters and were practically imponderable. In the primitive tetrapods, principal function of the limbs was initially restricted to passive anchoring in the course of animal movements on the substrate by means of lateral bending of the body (undulation). However, progressive development of carrying function of tetrapod limbs lead to clearing the body off the substrate which reduced friction costs and made the tetrapods less dependent on the substrate properties. Along with this, the limbs became more important as the active locomotory organs. But at the beginning, this diminished locomotory speed as the momentum caused by undulation could no longer provide additional forward sliding. Locomotory function of the tetrapod limb could be carried out due to both retraction and pronation at the shoulder joint. Relatively short humerus of the primitive tetrapods made it indifferent which of these two particular actions lead to elongation of the steps. In most of the recent tetrapods with sprawling limbs (Urodela, Lacertilia Sphenodontia, Crocodilia), step elongation was carried out mainly by retraction at the shoulder joint. Contrary to this, in Tachyglossidae (Mammalia: Monotremata) retraction is absent while pronation at the shoulder joint becomes the most important component of step elongation. This made it possible to recognize two principal types, pronatory and retractory, of locomotion on the basis of the main movement in the phase of support. A mathematical model describing changes in step length during the phase of support in both of these types is elaborated. It takes into account relative sizes of stylopodium and zeugopodium, the angles of pronation and retraction at the shoulder joint, the angle of adduction at the elbow joint, and the angle of body undulation arc. It is shown on the basis of this model, varying of which of the above parameters is advantageous and which is disadvantageous in each of the locomotory types. In the pronatory locomotory type, adduction (lateral mobility) at the elbow joint is employed. It leads to special changes in morphology of the elbow joint due to which humeral condyle becomes spherical and promotes both adduction and rotation of the entire antebrachium. In the retractory locomotory type, amplification of pronation is to be limited in order to provide step elongation, so certain morphological adaptations occur in the elbow joint which prevent adduction at this joint. For step elongation, retraction at the shoulder joint is usually more advantageous than pronation, therefore historical emergence of the pronatory type could be considered as inadaptive. However, transversal horizontal axis of rotation at the shoulder joint appeared to be a prerequisite of the subsequent appearance of the most perfect locomotion in the therian mammals with their parasagittal limbs. Transition to the parasagittal limb construction was associated with adaptation to jumping asymmetric locomotion. It caused elongation of the shoulder bone downward which lead to widening of rotation cone of the humerus and, at the same time, to reduction of the coracoid portion of the glenoid fossa, the latter became horizontal rather than lateral. As a part of this process, the longitudinal axis of the scapula was displacing caudally with destruction of the suture-like articulation of the acromion process with the clavicle. The latter became articulated with the sternum directly or via much reduced interclavicle (or via procoracoid rudiment). This increases amortisatory function of the shoulder girdle during landing at the final stage of jump.     

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

The question of how tetrapod limbs evolved from fins is one of the great puzzles of evolutionary biology. While palaeontologists, developmental biologists, and geneticists have made great strides in explaining the origin and early evolution of limb skeletal structures, that of the muscles remains largely unknown. The main reason is the lack of consensus about appendicular muscle homology between the closest living relatives of early tetrapods: lobe‐finned fish and crown tetrapods. In the light of a recent study of these homologies, we re‐examined osteological correlates of muscle attachment in the pectoral girdle, humerus, radius, and ulna of early tetrapods and their close relatives. Twenty‐nine extinct and six extant sarcopterygians were included in a meta‐analysis using information from the literature and from original specimens, when possible. We analysed these osteological correlates using parsimony‐based character optimization in order to reconstruct muscle anatomy in ancestral lobe‐finned fish, tetrapodomorph fish, stem tetrapods, and crown tetrapods. Our synthesis revealed that many tetrapod shoulder muscles probably were already present in tetrapodomorph fish, while most of the more‐distal appendicular muscles either arose later from largely undifferentiated dorsal and ventral muscle masses or did not leave clear correlates of attachment in these taxa. Based on this review and meta‐analysis, we postulate a stepwise sequence of specific appendicular muscle acquisitions, splits, and fusions that led from the ancestral sarcopterygian pectoral fin to the ancestral tetrapod forelimb. This sequence largely agrees with previous hypotheses based on palaeontological and comparative work, but it is much more comprehensive in terms of both muscles and taxa. Combined with existing information about the skeletal system, our new synthesis helps to illuminate the genetic, developmental, morphological, functional, and ecological changes that were key components of the fins‐to‐limbs transition.     

Forelimb Kinematics of Rats Using XROMM,with Implications for Small Eutherians and Their Fossil Relatives

The earliest eutherian mammals were small-bodied locomotor generalists with a forelimb morphology that strongly resembles that of extant rats. Understanding the kinematics of the humerus, radius, and ulna of extant rats can inform and constrain hypotheses concerning typical posture and mobility in early eutherian forelimbs. The locomotion of Rattus norvegicus has been extensively studied, but the three-dimensional kinematics of the bones themselves remains under-explored. Here, for the first time, we use markerless XROMM (Scientific Rotoscoping) to explore the three-dimensional long bone movements in Rattus norvegicus during a normal, symmetrical gait (walking). Our data show a basic kinematic profile that agrees with previous studies on rats and other small therians: rats maintain a crouched forelimb posture throughout the step cycle, and the ulna is confined to flexion/extension in a parasagittal plane. However, our three-dimensional data illuminate long-axis rotation (LAR) movements for both the humerus and the radius for the first time. Medial LAR of the humerus throughout stance maintains an adducted elbow with a caudally-facing olecranon process, which in turn maintains a cranially-directed manus orientation (pronation). The radius also shows significant LAR correlated with manus pronation and supination. Moreover, we report that elbow flexion and manus orientation are correlated in R. norvegicus: as the elbow angle becomes more acute, manus supination increases. Our data also suggest that manus pronation and orientation in R. norvegicus rely on a divided system of labor between the ulna and radius. Given that the radius follows the flexion and extension trajectory of the ulna, it must rotate at the elbow (on the capitulum) so that during the stance phase its distal end lies medial to ulna, ensuring that the manus remains pronated while the forelimb is supporting the body. We suggest that forelimb posture and kinematics in Juramaia, Eomaia, and other basal eutherians were grossly similar to those of rats, and that humerus and radius LAR may have always played a significant role in forelimb and manus posture in small eutherian mammals.     

A rigid body model of the forearm

In this article the forearm, with its complex, continuous motion of masses during pronation/supination, was approximated by a rigid body model consisting of a radial segment rotating around an ulnar segment. The method used to obtain the model parameters is based on three-dimensional voxel data that include velocity information. We propose a criterion that allows the voxels to be attributed to either of the two segments. It is based on the notion that the rotational kinetic energy determined from the voxel data equals the kinetic energy of the rigid body model. To obtain a three-dimensional smoothing we further propose a parameterization of the shape of both segments. These shapes can then be used to determine the dynamic integrals of the segments, i.e. mass, center of mass, and inertia. Using this approach we determined all model parameters for a human forearm from three series of MRI scans in a supinated, a pronated, and an intermediate position. In the appendix, a procedure is described that allows the dynamic quantities to be scaled homogeneously without recalculation of the integrals. Thus, this article provides all essential parameters required for three-dimensional dynamic simulations of general movements of the forearm.     

Measurements of wrist and forearm positions and movements: effect of, and compensation for, goniometer crosstalk.

Flexible biaxial goniometers are extensively used for measuring wrist positions and movements. However, they display an inherent crosstalk error. The aim was to evaluate the effect, of this error, on summary measures used for characterizing manual work. A goniometer and a torsiometer were combined into one device. An algorithm that effectively compensated for crosstalk was developed. Recordings from 25 women, performing five worktasks, were analyzed, both with and without compensation for crosstalk. The errors in the 10th, 50th and 90th percentiles of the flexion/extension distributions were small, on average <1 degrees. The ulnar/radial deviation distributions were weakly dependent on forearm position. The flexion/extension velocity measures were, for the 50th and 90th percentiles, as well as the mean velocity, consistently underestimated by, on average, 3.9%. For ulnar/radial deviation, the velocity errors were less consistent. Mean power frequency, which is a measure of repetitiveness, was insensitive (error <1%) to crosstalk. The forearm supination/pronation angular distributions were wider, and the velocities higher, than for the wrists. Considering wrist/hand exposure in epidemiologic studies, as well as for establishing and surveillance of exposure limits for prevention of work-related upper extremity musculoskeletal disorders, the crosstalk error can, when considering other errors and sources to variation, be disregarded.     

The role of the forelimb in prey capture in the Late Triassic reptile Megalancosaurus (Diapsida,Drepanosauromorpha)

The pectoral girdle and forelimb of the Late Triassic drepanosauromorph reptile Megalancosaurus are redescribed and their function reinterpreted. The whole skeleton of this diapsid is highly specialised for arboreal life, and also the peculiarities of the shoulder girdle and forelimb were interpreted as adaptations for a limb-based locomotion using gap-bridging to move from one support to another, as in chameleons. Re-examination of the pectoral girdle and forelimb revealed the presence of clavicles fused into a furcula-like structure, a saddle-shaped glenoid and a tight connection between the radius and ulna that strengthened the forearm but hindered pronation and supination movements at that joint. The new information plus a reconstruction of the pectoral and forelimb musculature suggests that the forelimb was also specialised for grasping and raking in addition to climbing and thus prey capture may have been an important function for the forelimb. The new functional interpretation fits well with the overall body architecture of Megalancosaurus’ skeleton, suggesting that this reptile was an ambush predator that may have assumed a stable tripodal position, secured by the hooked tail and hind limbs, freeing its forelimbs to catch prey by sudden extension of the arm and firm grasping with the pincer-like digits.     

Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates

The origin of terrestrial tetrapods was a key event in vertebrate evolution, yet how and when it occurred remains obscure, due to scarce fossil evidence. Here, we show that the study of palaeopathologies, such as broken and healed bones, can help elucidate poorly understood behavioural transitions such as this. Using high-resolution finite element analysis, we demonstrate that the oldest known broken tetrapod bone, a radius of the primitive stem tetrapod Ossinodus pueri from the mid-Viséan (333 million years ago) of Australia, fractured under a high-force, impact-type loading scenario. The nature of the fracture suggests that it most plausibly occurred during a fall on land. Augmenting this are new osteological observations, including a preferred directionality to the trabecular architecture of cancellous bone. Together, these results suggest that Ossinodus, one of the first large (>2m length) tetrapods, spent a significant proportion of its life on land. Our findings have important implications for understanding the temporal, biogeographical and physiological contexts under which terrestriality in vertebrates evolved. They push the date for the origin of terrestrial tetrapods further back into the Carboniferous by at least two million years. Moreover, they raise the possibility that terrestriality in vertebrates first evolved in large tetrapods in Gondwana rather than in small European forms, warranting a re-evaluation of this important evolutionary event.     

Co-contraction of the pronator teres and extensor carpi radialis during wrist extension movements in humans.

In order to elucidate the functional significance of excitatory spinal reflex arcs (facilitation) between musculus (M.) pronator teres (PT) and M. extensor carpi radialis (ECR, longus: ECRL, brevis: ECRB) in humans, activities of the muscles were studied with electromyography (EMG) and electrical neuromuscular stimulation (ENS). In EMG study, activities of PT, ECRL, ECRB, and M. flexor carpi radialis during repetitive static (isometric) wrist extension and a series of a dynamic motion of wrist flexion/extension in the prone, semiprone, and supine positions of the forearm were recorded in 12 healthy human subjects. In the prone, semiprone, and supine positions, PT and ECR showed parallel activities during the static extension in all, eight, and eight subjects, respectively, and at the extension phase during the dynamic motion in all, eight and five subjects, respectively. These findings suggest that co-contraction of PT and ECR occurs during wrist extension movements at least with the prone forearm. The facilitation must be active during the co-contraction. In ENS study, ENS to PT was examined in 11 out of the 12 and that to ECRL was in the 12 subjects. Before ENS, the forearm was in the prone, semiprone, and supine positions. In all the subjects, ENS to PT induced a motion of forearm pronation to the maximum pronation. ENS to ECRL induced motions of wrist extension to the maximum extension and abduction (radial flexion) to 5-20 degrees of abduction regardless of the positions of the forearm. Moreover, it induced 30-80 degrees supination of the forearm from the prone position. Consequently, combined ENS to PT and ECRL resulted in motions of the extension and abduction while keeping the maximum pronation. These findings suggest that the co-contraction of PT and ECR during wrist extension movements occurs to prevent supinating the forearm. Forearm supination from the prone position should be added to one of the actions of ECRL.     

A new kinematic model of pro- and supination of the human forearm

We introduce a new kinematic model describing the motion of the human forearm bones, ulna and radius, during forearm rotation. During this motion between the two forearm extrem-positions, referred to as supination (palm up) and pronation (palm down), effects occur, that cannot be explained by the the established kinematic model of R. Fick from 1904. Especially, the motion of the ulna is not properly reproduced by Fick's model. During forearm rotation an evasive motion of the ulna is observed by various authors, using magnetic resonance imaging MRI) technology. Our new kinematic model also simulates this evasive motion. Furthermore, the model is enlarged to include angulations of the forearm bones. Using these results the influence of forearm fractures on the range of forearm motion can be predicted. This knowledge can be used by surgeons to choose the optimal therapy in re-establishing free forearm mobility.     

Comparative skeletogenesis of the forearm of the little brown bat (Myotis lucifugus) and the Norway rat (Rattus norvegicus).

This study describes, quantifies, and compares the growth and development of the volant forelimb morphology of Myotis lucifugus with that of the terrestrial forelimb morphology of Rattus norvegicus. In M. lucifugus there is 1) accelerated growth in forearm length after parturition, 2) cessation in growth of the midshaft diameter of the ulna just after the onset of osteogenesis, 3) proximal fusion of the radius and ulna, which results in the radius occupying 97% of the articular surface of the elbow joint in adults, 4) fusion between the cartilaginous distal epiphyses of the radius and ulna which results in formation of a radioulnar bridge that becomes fully ossified in adults, and 5) incomplete ossification of the ulna with a section of the diaphysis becoming ligamentous. None of these events occurs during development in R.norvegicus.     

Crouched posture and high fulcrum, a principle in the locomotion of small mammals: The example of the rock hyrax (Procavia capensis) (Mammalia: Hyracoidea)

The cineradiographic study of the locomotion of the rock hyrax (Procavia capensis) and the functional interpretation of its locomotory system, reveals that the main action of proximal segments is combined with flexed position and low movements of limb joints. This observation can be applied to the locomotion of other small mammals. In the forelimb, scapular rotation and translation account for more than 60% of step length. Effective shoulder joint movements are mostly restricted to less than 20°, and elbow movements range mainly between 20°-50°. The detachment of the shoulder girdle of therian mammals from the axial skeleton, and development of a supraspinous fossa, are correlated with movements at a high scapular fulcrum. Movements at such a high fulcrum are in interdependency with a crouched posture. Only flexed limbs can act as shock absorbers and prevent vertical changes in the center of gravity. Basic differences in forelimb movements exist between larger primates (humeral retraction) and smaller mammals (scapula retraction). In the hyrax, propulsion is due mainly to hip joint movements in symmetrical gaits, but sagittal lumbar spine movements play the dominant role at in-phase gaits. Joint and muscular anatomy, especially of the shoulder region, are discussed in view of the kinematic data.     

Experimental investigation of the scaphoid strain during wrist motion

The scaphoid is the most frequently fractured of the carpal bones [Taleisnik, J., The Wrist, Churchill Livingstone, New York (1985)]. This project was undertaken to qualitatively evaluate the strain in the scaphoid during wrist motion using a newly developed strain gage method. Strain gage resettes were mounted within the scaphoid and the range of motion of the hand was monitored using a custom designed electrogoniometer and data acquisition system. Ten specimens were utilized for this study. Results indicated that supination/pronation (S/P) of the forearm did not affect the strain in the scaphoid. A map of the strain in the waist of the scaphoid, as a function of flexion/extension (F/E) and radial/ulnar deviation (R/U), was generated. The contour plot of scaphoid strain vs range of motion (ROM) shows a valley where strains are low. Minimum scaphoid strain was found near neutral F/E and 15° of ulnar deviation.     

Surgical strategy for improving forearm and hand function in late obstetric brachial plexus palsy

For the purpose of treatment, obstetric brachial plexus palsy can be subdivided into two distinct phases: initial obstetric brachial plexus palsy, and late obstetric brachial plexus palsy. In the latter, nerve surgery is no longer practical, and treatment often requires palliative surgery to improve function of the shoulder, elbow, forearm, and hand. Late obstetric brachial plexus palsy in the forearm and hand includes weakness or absence of wrist or metacarpophalangeal or interphalangeal joint extension; weakness or absence of finger flexion; forearm supination, or less commonly pronation contracture; ulnar deviation of the wrist; dislocation of the radial or ulnar head; thumb instability; or sensory disturbance of the hand. Palliative reconstruction for these forearm and hand manifestations is more difficult than for the shoulder or elbow because of the lack of powerful regional muscles for transfer. This report reviews the authors' experience performing more than 100 surgical procedures in 54 patients over a 9-year period (between 1988 and 1997) with a minimum of 2 years' follow-up. Surgical treatment is highly individualized, but the optimal age for forearm and hand reconstruction is usually later than for shoulder and elbow reconstruction because of the requirement for a preoperative exercise program. Multiple procedures for forearm and hand function were often performed on any given patient. Frequently, these were done simultaneously with reconstructive procedures for improving shoulder and/or elbow function. Traditional tendon transfer techniques do not provide satisfactory reconstruction for those deformities. Many of the authors' patients required more complex techniques such as nerve transfer and functioning free-muscle transplantation to augment traditional techniques of tendon and/or bone management. Sensory disturbance of the forearm and hand in late obstetric brachial plexus palsy seems a minor problem and further sensory reconstruction is unnecessary.     

Comparison of measurement accuracy between two wrist goniometer systems during pronation and supination.

Pronation and supination have been shown to affect wrist goniometer measurement accuracy. The purpose of this study was to compare differences in measurement accuracy between a commonly used biaxial, single transducer wrist goniometer (System A) and a biaxial, two-transducer wrist goniometer (System B) over a wide range of pronation and supination (P/S) positions. Eight subjects moved their wrist between -40 and 40 degrees of flexion/extension (F/E) and -10 and 20 degrees of radial/ulnar (R/U) deviation in four different P/S positions: 90 degrees pronation; 45 degrees pronation; 0 degrees neutral and 45 degrees supination. System A was prone to more R/U crosstalk than System B and the amount of crosstalk was dependent on the P/S position. F/E crosstalk was present with both goniometer systems and was also shown to be dependent on P/S. When moving from pronation to supination, both systems experienced a similar extension offset error; however R/U offset errors were roughly equal in magnitude but opposite in direction. The calibration position will affect wrist angle measurements and the magnitude and direction of measurement errors. To minimize offset errors, the goniometer systems should be calibrated in the P/S posture most likely to be encountered during measurement. Differences in goniometer design and application accounted for the performance differences.     

An analysis of bailer movements responsible for gill ventilation in the crab,cancer novae‐zelandiae

The movements of the bailer during normal ventilation can be resolved into two components, a cycle of pronation and supination being superimposed on a cycle of protraction and retraction. Pronation leads protraction with a phase angle of about 90° in a normal cycle. Pronation is accompanied by flexion of the bailer.

The skeletal anatomy of the bailer is such as to restrict movements of the bailer to those described above. Further the pronated and supinated positions of the limb represent the two stable positions of a skeletal click mechanism, the operation of which may help to resolve the functional duality of the promotor and remotor muscles.

This functional duality arises because the muscles are positioned so as to produce either protraction or supination of the limb. Other muscles in the limb are monofunctional. The bulk of muscle tissue responsible for protraction and supination seems to be greater than that responsible for pronation and retraction.

The sequence of muscular activity during the ventilation cycle follows that expected for a sequence, of protraction, supination, retraction and pronation. Overlap in the periods of activity of the bifunctional muscles and muscles responsible for pronation may also help to resolve the functional duality of the former.

The amplitude of bailer excursion (protraction‐retraction) is not greatly affected by changes in frequency. An advance in the onset of activity in some muscles at higher ventilation ? frequencies suggests that the system is tailored to produce a constant beat amplitude at all frequencies.

Pauses in ventilation occur with the bailer in the retracted position, and it is maintained in this position by tonic activity in the appropriate muscle. During normal ventilation the relative contraction duration of this muscle is positively correlated with cycle period, so that pauses apparently represent a prolongation of the normal retracted phase. The relative contraction durations of some other muscles are negatively correlated with cycle period. The different signs of these correlations may be related to the type of endogenous oscillator present in the central nervous system.     

Functional differences between anatomical regions of the anconeus muscle in humans

This study sought to resolve a longstanding debate of the function of anconeus. Intramuscular and surface electromyography electrodes recorded muscle activity from two regions of anconeus and from typical elbow flexion and extension muscles. Eleven participants performed pronation–supination around the medial and lateral axes of the forearm, elbow flexion–extension in pronation, supination and neutral positions of the forearm, and gripping. Maximal voluntary contractions (MVC) and submaximal (10% MVC) force-matching tasks were completed. Activity varied between longitudinal (AL) and transverse (AT) segments of anconeus. Although both muscle regions were active across multiple directions (including opposing directions), AL was more active during pronation than supination, whereas AT showed no such difference. During pronation, activity of AL and AT was greatest about the lateral forearm axis. AT was more active during elbow extension with the forearm in pronation, whereas AL did not differ between pronated and neutral forearm alignment. These findings are consistent with the proposal that AL makes a contribution to control of abduction of the ulna during forearm pronation. Different effects of forearm position on AL and AT activity during elbow extension may be explained by the anatomical differences between the regions. These data suggest anconeus performs multiple functions at the elbow and forearm and this varies between anatomically distinct regions of the muscle.     

Radius of Paranthropus robustus from member 1, Swartkrans formation, South Africa

Recently recovered hominid postcrania from Member 1, Swartkrans Formation include the proximal and distal ends of a right radius attributed to a single individual of Paranthropus robustus. These fossils are essentially similar to Australopithecus afarensis, A. africanus, and P. boisei homologues. The head manifests an ape-like circumferentia articularis, and the distal end has prominent medial, dorsal, and lateral tubercles and a well developed brachioradialis crest, features also commonly exhibited by extant great apes. The volar set of the P. robustus radiocarpal joint, like that of Australopithecus homologues, more closely resembles the neutral condition exhibited by Homo than the greater flexion evinced by living apes. Compared with fossil and recent specimens of Homo, the configuration of the P. robustus radial head suggests enhanced stability against medial displacement during pronation and supination; the strong crest for the attachment of brachioradialis may attest to enhanced forearm flexor capability. In addition, this crest and the prominent dorsal tubercles may indicate enhanced hand extensor and, therefore, hand flexor capabilities. The differences in radial morphology between Paranthropus and Homo may relate to significant behavioral differences between these two synchronic taxa.