Muscle moment arms and function of the siamang forelimb during brachiation

Moment arms have an important modulating impact on muscle function, as they represent the capacity of the muscle to convert muscle action into limb movements. In the current paper, we provide muscle moment arm data of the forelimb of four siamangs, collected by detailed dissections on unfixed cadavers. The aim of this study is to assess the role of different forelimb muscles during brachiation. Moment arm data are compared with similar published data of non‐brachiating primates such as macaques, chimpanzees and humans. Our data show that shoulder adductors and endorotators and the elbow flexors are built for force generation, whereas the shoulder abductors, flexors and exorotators are best suited to gain speed and to change direction. Compared to non‐brachiating species, both elbow and wrist flexors are particularly noticeable in terms of moment of force‐generating capacity. However, the moment of force‐generating capacity of the elbow extensor is not negligible, which indicates that the triceps also plays an active role, especially at the end of the support phase. Except for the elbow flexors, all muscles reach their maximum moment of force‐generating capacity during the support phase of brachiation. When brachiating on a more complex setup, the siamang will flex the elbows to angles that induce maximum moment arms as well.     

Mechanical constraints on the functional morphology of the gibbon hind limb

Gibbons utilize a number of locomotor modes in the wild, including bipedalism, leaping and, most of all, brachiation. Each locomotor mode puts specific constraints on the morphology of the animal; in some cases these may be complementary, whereas in others they may conflict. Despite several studies of the locomotor biomechanics of gibbons, very little is known about the musculoskeletal architecture of the limbs. In this study, we present quantitative anatomical data of the hind limb for four species of gibbon ( Hylobates lar , H. moloch , H. pileatus and Symphalangus syndactylus ). Muscle mass and fascicle lengths were obtained from all of the major hind limb muscles and the physiological cross-sectional area was calculated and scaled to remove the effect of body size. The results clearly indicate that, for all of the species studied, the major hip, knee and ankle extensors are short-fascicled and pennate. The major hip and knee flexors, however, are long-fascicled, parallel muscles with relatively small physiological cross-sectional areas. We hypothesize that the short-fascicled muscles could be coupled with a power-amplifying mechanism and are predominantly useful in leaping. The long-fascicled knee and hip flexors are adapted for a wide range of joint postures and can play a role in flexing the legs during brachiation.     

The forearm and hand musculature of semi-terrestrial rhesus macaques (Macaca mulatta) and arboreal gibbons (fam.Hylobatidae). Part II. Quantitative analysis

Nonhuman primates have a highly diverse locomotor repertoire defined by an equally diverse hand use. Based on how primates use their hands during locomotion, we can distinguish between terrestrial and arboreal taxa. The ‘arboreal’ hand is likely adapted towards high wrist mobility and grasping, whereas the ‘terrestrial’ hand will show adaptations to loading. While the morphology of the forearm and hand bones have been studied extensively, functional adaptations in the forearm and hand musculature to locomotor behaviour have been documented only scarcely. In this paper, we investigate the forelimb musculature of the highly arboreal gibbons (including Hylobates lar,Hylobates pileatus,Nomascus leucogenys,Nomascus concolor and Symphalangus syndactylus) and compare this with the musculature of the semi-terrestrial rhesus macaques (Macaca mulatta). Anatomical data from previous dissections on knuckle-walking bonobos (Pan paniscus) and bipedal humans (Homo sapiens) are also included to further integrate the analyses in the scope of catarrhine hand adaptation. This study indicates that the overall configuration of the arm and hand musculature of these primates is very similar but there are some apparent differences in relative size which can be linked to differences in forelimb function and which might be related to their specific locomotor behaviour. In macaques, there is a large development of wrist deviators, wrist and digital flexors, and m. triceps brachii, as these muscles are important during the different phases of palmi- and digitigrade quadrupedal walking to stabilize the wrist and elbow. In addition, their m. flexor carpi ulnaris is the most important contributor to the total force-generating capacity of the wrist flexors and deviators, and is needed to counteract the adducting torque at the elbow joint during quadrupedal walking. Gibbons show a relatively high force-generating capacity in their forearm rotators, wrist and digital flexors, which are important muscles in brachiation to actively regulate forward movement of the body. The results also stress the importance of the digital flexors in bonobos, during climbing and clambering, and in humans, which is likely linked to our advanced manipulation skills.     

Muscle activity during forelimb stepping in decerebrate cats.

In decerebrate cats with the lower thoracic cord transected, electromyographic activities were analyzed in up to 41 forelimb muscles, almost all muscles involved in forelimb stepping (intrinsic hand muscles were not included). From the active period in the step cycle, muscles were classified into three groups: extensors, of which activity is like that of elbow extensors; flexors, activity like that of elbow flexors; others, including dorsiflexors of the wrist, pronators, and supinator. The results were well consistent with those from conscious animals as well as efferent pattern of fictive locomotion in elbow and distal muscles. Nevertheless, in some proximal muscles discrepancies were noted, suggesting their changeability depending on environmental conditions. Recording from almost all muscles allowed to estimate rhythmic change of the overall output of the forelimb central pattern generator.     

Properties of primary motor cortex output to forelimb muscles in rhesus macaques

Stimulus-triggered averaging (StTA) of electromyographic (EMG) activity from 24 simultaneously recorded forelimb muscles was used to investigate properties of primary motor cortex (M1) output in the macaque monkey. Two monkeys were trained to perform a reach-to-grasp task requiring multijoint coordination of the forelimb. EMG activity was recorded from 24 forelimb muscles including 5 shoulder, 7 elbow, 5 wrist, 5 digit, and 2 intrinsic hand muscles. Microstimulation (15 microA at 15 Hz) was delivered throughout the movement task. From 297 stimulation sites in M1, a total of 2,079 poststimulus effects (PStE) were obtained including 1,398 poststimulus facilitation (PStF) effects and 681 poststimulus suppression (PStS) effects. Of the PStF effects, 60% were in distal and 40% in proximal muscles; 43% were of extensors and 47% flexors. For PStS, the corresponding numbers were 55 and 45% and 36 and 55%, respectively. M1 output effects showed extensive cofacilitation of proximal and distal muscles (96 sites, 42%) including 47 sites that facilitated at least one shoulder, elbow, and distal muscle, 45 sites that facilitated an elbow muscle and a distal muscle, and 22 sites that facilitated at least one muscle at all joints. The muscle synergies represented by outputs from these sites may serve an important role in the production of coordinated, multijoint movements. M1 output effects showed many similarities with red nucleus output although red nucleus effects were generally weaker and showed a strong bias toward facilitation of extensor muscles and a greater tendency to facilitate synergies involving muscles at noncontiguous joints.     

Anatomical,architectural, and biochemical diversity of the murine forelimb muscles

We characterized the architecture, fiber type, titin isoform distribution, and collagen content of 27 portions of 22 muscles in the murine forelimb. The mouse forelimb was different from the human arm in that it had the extensor digitorum lateralis muscle and no brachioradialis muscle. Architecturally, the mouse forelimb differed from humans with regard to load bearing, having a much larger contribution from extensors than flexors. In mice, the extensor : flexor PCSA ratio is 2.7, whereas in humans it is only 1.4. When the architectural difference index was calculated, similarities became especially apparent between flexors and extensors of the distal forelimb, as well as pronators. Discriminant analysis revealed that biochemical measures of collagen, titin, and myosin heavy chain were all strong between‐species discriminators. In terms of composition, when compared with similar muscles in humans, mice had, on average, faster muscles with higher collagen content and larger titin isoforms. This report establishes the anatomical and biochemical properties of mouse forelimb muscles. Given the prevalence of this species in biological studies, these data will be invaluable for studying the biological basis of mouse muscle structure and function.     

Relationship between humeral geometry and shoulder muscle power among suspensory, knuckle-walking, and digitigrade/palmigrade quadrupedal primates

Shoulder morphology is functionally related to different patterns of locomotion in primates. To investigate this we performed a quantitative analysis of the relationship between cortical bone thickness (Cbt) of the muscle/tendon attachment site on the humerus and physiological cross-sectional area (PCSA) of the shoulder muscle in primates with different locomotory habits. The deltoid, subscapularis, supraspinatus, and infraspinatus were investigated. A chimpanzee, a gibbon, a baboon, two species of macaque, a lutong, a capuchin, and a squirrel monkey were included in the study. The total length of the humerus was measured and the values were converted into three-dimensional reconstructed data on a computer by computed tomography. The Cbt values were obtained from the volumes divided by the areas of the muscle/tendon attachment sites of the humerus by computer analysis. Muscle mass, muscle fascicle length, and muscle pennation angle were measured and PCSA was calculated using these parameters. A relatively high Cbt and small PCSA were characteristic of the gibbon. The gibbon's high Cbt suggests that passive tension in the muscle/tendon attachment site of suspensory primates (brachiators) may be greater than that of quadrupedal primates, whereas the relatively small PCSA indicates an association with a large amount of internal muscle fascia to endure the passive stress of brachiation. Although chimpanzees undertake some suspensory locomotion, the results for this species resemble those of the digitigrade/palmigrade quadrupedal primates rather than those of the suspensory primate. However, the deltoid and subscapularis in chimpanzee differ from those of the other primates and appear to be affected by the peculiar locomotion of knuckle-walking, i.e. the moment arm of forelimb in chimpanzees is relatively longer than that of digitigrade/palmigrade quadrupedal primates. Hence, a large PCSA in the deltoid and subscapularis may contribute to sustaining the body weight during locomotion. On the other hand, a thin cortical bone relative to a large PCSA was a feature of the cercopithecids, indicating that digitigrade/palmigrade quadrupedal locomotion produces less tension at the muscle/tendon attachment sites compared with that produced by brachiators.     

Dimensions of forelimb muscles in orangutans and chimpanzees

Eight forelimbs of three orangutans and four chimpanzees were dissected and the muscle mass, fascicle length and physiological cross-sectional area (PCSA) of all forelimb muscles were systematically recorded to explore possible interspecies variation in muscle dimensions. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire forelimb muscles for normalization. The results indicate that the mass and PCSA ratios of the monoarticular elbow flexors ( M. brachialis and M. brachioradialis ) are significantly larger in orangutans. In contrast, the mass ratios of the biarticular muscles in the upper arm (the short head of M. biceps brachii and the long head of M. triceps brachii ) are significantly larger in chimpanzees. For the rotator cuff muscles, the force-generating capacity of M. subscapularis is significantly larger in orangutans, whereas the opposite rotator cuff muscle, M. infraspinatus , is larger in chimpanzees. These differences in forelimb muscle dimensions of the two species may reflect functional specialization for their different positional and locomotor behaviors.     

A kinematic and electromyographic study of cutaneous reflexes evoked from the forelimb of unrestrained walking cats

A kinematic and electromyographic (EMG) analysis was undertaken of the responses evoked in the forelimb of the cat by either mechanical obstruction of the forelimb during the swing phase of locomotion or by electrical stimulation of low-threshold cutaneous afferents during both swing and stance. Mechanical obstruction of the forelimb with a stiff metal rod evoked a complex response that allowed the cat to smoothly negotiate the obstacle without undue disruption of the overall locomotor rhythm. The initial movements were a flexion of the shoulder, together with a locking of the elbow joint, and a dorsiflexion of the wrist, which caused the limb to withdraw from the obstacle. They were followed by an extension of the shoulder, a flexion of the elbow, and a ventroflexion of the wrist, which together brought the limb forward and above the obstacle. The associated and complex pattern of short- and long-latency EMG responses was shown to be related to different aspects of the movement. At the shoulder there was a strong activation of flexor muscles; these responses were of long duration (greater than or equal to 100 ms) and generally lasted throughout the period of shoulder flexion. At the elbow, both flexor and extensor muscles were activated at short latency (9-13 ms). In flexors, this was followed by a cessation and subsequently an augmentation and prolongation of their activity. Dorsiflexors of both the wrist and digits were activated at short latency (10-12 ms) and remained active throughout the period of dorsiflexion of these joints. An injection of a local anesthetic into the area of skin contacted by the metal rod reduced or abolished all of the reflex responses, which suggests that the integrity of cutaneous reflex pathways is essential for the elaboration of these responses. Electrical stimulation of a cutaneous nerve innervating the distal forelimb (the superficial radial nerve) resulted in qualitatively similar, although weaker, responses to those obtained with the mechanical stimulation. Terminal experiments confirmed that these responses were mediated by low-threshold cutaneous afferents. Electrical stimulation also evoked short-latency excitatory responses (10-12 ms) in extensor muscles of the elbow. Generally, the largest reflex effects were obtained during the period of swing for flexor, extensor, and bifunctional muscles. During stance the stimulus was normally ineffective in exciting flexor muscles and in extensors evoked a short-latency inhibition, which was frequently followed by an increase in activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Relationship between stretch reflex thresholds and voluntary arm muscle activation in patients with spasticity

Previous studies have shown that deficits in agonist–antagonist muscle activation in the single-joint elbow system in patients with spastic hemiparesis are directly related to limitations in the range of regulation of the thresholds of muscle activation. We extended these findings to the double-joint, shoulder-elbow system in these patients. Ten non-disabled individuals and 11 stroke survivors with spasticity in upper limb muscles participated. Stroke survivors had sustained a single unilateral stroke 6–36 months previously, had full pain-free passive range of motion of the affected shoulder and elbow and had some voluntary control of the arm. EMG activity from four elbow and two shoulder muscles was recorded during quasi-static (<5°/s) stretching of elbow flexors/extensors and during slow voluntary elbow flexion/extension movement through full range. Stretches and active movements were initiated from full elbow flexion or extension with the shoulder in three different initial positions (60°, 90°, 145° horizontal abduction). SRTs were defined as the elbow angle at which EMG signals began to exceed 2SD of background noise. SRT angles obtained by passive muscle stretch were compared with the angles at which the respective muscles became activated during voluntary elbow movements. SRTs in elbow flexors were correlated with clinical spasticity scores. SRTs of elbow flexors and extensors were within the biomechanical range of the joint and varied with changes in the shoulder angle in all subjects with hemiparesis but could not be reached in this range in all healthy subjects when muscles were initially relaxed. In patients, limitations in the regulation of SRTs resulted in a subdivision of all-possible shoulder-elbow arm configurations into two areas, one in which spasticity was present (“spatial spasticity zone”) and another in which it was absent. Spatial spasticity zones were different for different muscles in different patients but, taken together, for all elbow muscles, the zones occupied a large part of elbow-shoulder joint space in each patient. The shape of the boundary between the spasticity and no-spasticity zones depended on the state of reflex inter-joint interaction. SRTs in single- and double-joint flexor muscles correlated with the positions at which muscles were activated during voluntary movements, for all shoulder angles, and this effect was greater in elbow flexor muscles (brachioradialis, biceps brachii). Flexor SRTs correlated with clinical spasticity in elbow flexors only when elbow muscles were at mid-length (90°). These findings support the notion that motor impairments after CNS damage are related to deficits in the specification and regulation of SRTs, resulting in the occurrence of spasticity zones in the space of elbow-shoulder configurations. It is suggested that the presence of spatial spasticity zones might be a major cause of motor impairments in general and deficits in inter-joint coordination in particular in patients with spasticity.     

Electromyographic analysis of reach in individuals with cerebral palsy

Electromyographic analysis in both the time domain (root mean square EMG) and the frequency domain (mean power frequency EMG) of the biceps, triceps, wrist extensors and wrist flexors were analysed in six young cerebral palsied adults and six normal individuals. The subjects sat in a Rifton positioning chair. Each subject's right arm was positioned with the shoulder adducted, the elbow at 90 degrees and the hand resting on the arm rest. The subject then reached the right arm forward to grasp a dowel which was placed at shoulder level in front of the subject. There was no significant difference between the time it took the two groups to do the required movement. The RMS analysis indicated the muscle activation was variable among subjects, with evidence of concontraction of the antagonist muscles for the disabled group. The frequency analysis indicated that the disabled group had significantly lower mean power for the biceps and the wrist extensor muscles compared to the normal group. Neurological differences or fibre type abnormalities may account for these differences.     

Heteronymous reflex connections in human upper limb muscles in response to stretch of forearm muscles

Torque motor produced stretch of upper limb muscles results in two distinct reflex peaks in the electromyographic activity. Whereas the short-latency reflex (SLR) response is mediated largely by the spinal monosynaptic reflex pathway, the longer-latency reflex (LLR) is suggested to involve a transcortical loop. For the SLRs, patterns of heteronymous monosynaptic Ia connections have been well-studied for a large number of muscles in the cat and in humans. For LLRs, information is available for perturbations to proximal joints, although the protocols for most of these studies did not focus on heteronymous connections. The main objective of the present study was to elicit both SLRs and LLRs in wrist flexors and extensors and to examine heteronymous connections from these muscles to elbow flexors (biceps brachii; BiBr) and extensors (triceps brachii; TriBr) and to selected distal muscles, including abductor pollicis longus (APL), first dorsal interosseous (FDI), abductor digiti minimi (ADM), and Thenars. The stretch of wrist flexors produced SLR and LLR peaks in APL, FDI, ADM, Thenars, and BiBr while simultaneously inducing inhibition of wrist extensors and TriBr. When wrist extensors were stretched, SLR and LLR peaks were observed in TriBr, whereas the primary wrist flexors, APL and BiBr, were inhibited; response patterns of FDI, ADM, and Thenars were less consistent. The main conclusions from the observed data are that: 1) as in the cat, afferents from wrist flexors and extensors make heteronymous connections with proximal and distal upper limb muscles; and 2) the strength of heteronymous connections is greater for LLRs than SLRs in the distal muscles, whereas the opposite is true for the proximal muscles. In the majority of observations, SLR and LLR excitatory peaks were observed together. However, on occasion, LLRs were observed without the SLR response in hand muscles when wrist extensors were stretched.     

Histochemical and morphometric investigations of the musculature of forelimb mass of sheep with reference to its function. 1. Bending and extension of elbow joints]

Muscle tissue was removed from the extensors and flexors of the elbow joint of six male sheep (180 days old) and stained for NADH tetrazolium oxidoreductase and myofibrillar ATPase after preincubation at pH 4.3 in order to identify three fiber types: slow twitch oxidative (STO), fast twitch oxidative (FTO) and fast twitch glycolytic fibers (FTG). The medial head of the M. triceps brachii and the anconaeus muscle had the largest fibers (> 50 and 60 microns). The smallest muscle fibers (35-43 microns) were found in the dorsal part of the long head of the triceps muscle. The medial head of the triceps muscle and the anconaeus muscle possessed a very high percentage of STO-fibers (90 and 100%) and FTG-fibers were absent in these muscles. In the other extensors and flexors of the elbow joint the STO-percentage amounted to less than 30%. The dorsal part of the long head of the triceps muscle contained only 13% STO-fibers, but had the highest percentage of FTG-fibers (49%), which is representative of fast-muscles. The muscles of the elbow joint perform both static and dynamic functions. The medial head of the triceps brachii muscle and the anconaeus muscle possess the complement of enzymes which permits them to fulfil the work of extensors in the standing position. Therefore, they are typical of antigravity muscles. The histochemical structure of the other extensors and flexors reflect their function in motion. The lateral and long head of triceps muscle oppose the flexors and extend the elbow joint of the raised limb in the swing phase, during the landing phase they also function to support the other extensors of the elbow.     

The disynaptic group I inhibition between wrist flexor and extensor muscles revisited in humans

The present studies are designed to further characterise the interneuronal pathway mediating the disynaptic reciprocal group I inhibition between flexors and extensors at the wrist and the elbow levels in humans. In the first series of experiments, we compared the electrical threshold of the reciprocal group I inhibition at the wrist and the elbow level after a prolonged vibration aimed at raising the electrical threshold of the antagonistic activated Ia afferents. Prolonged vibration to the ‘conditioning’ tendon, which raised significantly the electrical threshold of the inhibition at the elbow level, did not alter it at the wrist level. These results suggest that the dominant input to the relevant interneurones is Ia in origin at the elbow level but Ib in origin at the wrist level. In the second series of experiments, using the spatial facilitation method, we compared the effects on the post-stimulus time histograms of single voluntarily activated motor units of two volleys delivered both separately and together to group I afferents in the nerves supplying the homonymous and antagonistic muscles. At the wrist, but not at the elbow level, the peak of homonymous monosynaptic group I excitation was reduced on combined stimulation, although the antagonistic IPSP was just at the threshold. Because the suppression did not involve the initial bins of the peak, it is argued that the suppression is not due to presynaptic inhibition of Ia terminals, but probably reflects convergence between the homonymous and antagonistic volleys onto the interneurones mediating the disynaptic inhibition. Taken together with the previously reported effects of recurrent inhibition on reciprocal inhibition, these results suggest that inhibition between flexors and extensors is differently organised at the elbow (reciprocal Ia inhibition) and the wrist (non-reciprocal group I inhibition) levels. It is argued that the particular connectivity at the wrist level might correspond to some functional requirements at this ball joint.     

A Model of the Upper Extremity for Simulating Musculoskeletal Surgery and Analyzing Neuromuscular Control

Biomechanical models of the musculoskeletal system are frequently used to study neuromuscular control and simulate surgical procedures. To be broadly applicable, a model must be accessible to users, provide accurate representations of muscles and joints, and capture important interactions between joints. We have developed a model of the upper extremity that includes 15 degrees of freedom representing the shoulder, elbow, forearm, wrist, thumb, and index finger, and 50 muscle compartments crossing these joints. The kinematics of each joint and the force-generating parameters for each muscle were derived from experimental data. The model estimates the muscle–tendon lengths and moment arms for each of the muscles over a wide range of postures. Given a pattern of muscle activations, the model also estimates muscle forces and joint moments. The moment arms and maximum moment-generating capacity of each muscle group (e.g., elbow flexors) were compared to experimental data to assess the accuracy of the model. These comparisons showed that moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension. The computer model is available to researchers at .     

Descriptive and functional morphology of the neck and forelimb of the striped hyena (Hyaena hyaena, L. 1758)

The musculature of the neck and the forelimb of Hyaena hyaena is described and the biomechanical implications of some morphological aspects of muscles and skeleton are discussed. The extensors of the head and neck, the protractors and retractors of the forelimb and the extensors of the shoulder, elbow and carpal joints are relatively stronger developed than those in Canidae and Felidae. Moreover these muscles have larger moments about the joints. This is considered as an adaptation to lifting and carrying large and heavy prey or carrion.     

The scaling of postcranial muscles in cats (Felidae) I: forelimb,cervical, and thoracic muscles

The body masses of cats (Mammalia, Carnivora, Felidae) span a ~300‐fold range from the smallest to largest species. Despite this range, felid musculoskeletal anatomy remains remarkably conservative, including the maintenance of a crouched limb posture at unusually large sizes. The forelimbs in felids are important for body support and other aspects of locomotion, as well as climbing and prey capture, with the assistance of the vertebral (and hindlimb) muscles. Here, we examine the scaling of the anterior postcranial musculature across felids to assess scaling patterns between different species spanning the range of felid body sizes. The muscle architecture (lengths and masses of the muscle‐tendon unit components) for the forelimb, cervical and thoracic muscles was quantified to analyse how the muscles scale with body mass. Our results demonstrate that physiological cross‐sectional areas of the forelimb muscles scale positively with increasing body mass (i.e. becoming relatively larger). Many significantly allometric variables pertain to shoulder support, whereas the rest of the limb muscles become relatively weaker in larger felid species. However, when phylogenetic relationships were corrected for, most of these significant relationships disappeared, leaving no significantly allometric muscle metrics. The majority of cervical and thoracic muscle metrics are not significantly allometric, despite there being many allometric skeletal elements in these regions. When forelimb muscle data were considered in isolation or in combination with those of the vertebral muscles in principal components analyses and MANOVAs, there was no significant discrimination among species by either size or locomotory mode. Our results support the inference that larger felid species have relatively weaker anterior postcranial musculature compared with smaller species, due to an absence of significant positive allometry of forelimb or vertebral muscle architecture. This difference in strength is consistent with behavioural changes in larger felids, such as a reduction of maximal speed and other aspects of locomotor abilities.     

Functional specialisation of the thoracic limb of the hare (Lepus europeus)

We provide quantitative anatomical data on the muscle-tendon architecture of the hare thoracic limb (specifically muscle mass, fascicle length, pennation angle, tendon mass and length). In addition, moment arms of major thoracic limb muscles were measured. Maximum isometric force and power of muscles, the moment of force about a joint, and tendon stress and strain were estimated. Data are compared with those from other cursorial mammals. The thoracic limb of the hare consists predominantly of extrinsic musculature with long parallel fascicles, specialised for generating force over a large range. A large shoulder flexor/elbow extensor muscle mass is present, in particular Triceps brachii. The pennate nature of the long head of this muscle suggests it has an important role in stabilising the elbow joint during stance, whilst moment arm curves suggest that it may also play a role in initiating shoulder flexion. In addition, Supraspinatus and Infraspinatus are capable of generating high forces, potentially to stabilise the shoulder joint during the stance phase of locomotion. Supraspinatus may in addition play an important role in forelimb protraction. The Subscapularis muscle was capable of generating surprisingly high forces, suggesting that the hare must be able to withstand/produce high forces during activities that need medio-lateral stability, such as turning. Distally, tendons were relatively short, showing little potential for elastic energy storage when compared with both their pelvic limb counterparts and their equivalents in the horse thoracic limb. Thus, a 'stiffer' thoracic limb may be beneficial in terms of behaving like a strut, simply supporting and deflecting the body during high-speed running. This more distal/less proximal distribution of limb mass is also likely to be important in retaining the manipulative/adaptive/non-locomotor capabilities of the limb.     

Primate upper limb muscles exhibit activity patterns that differ from their anatomical action during a postural task

The present study examined muscular activity in the primate proximal forelimb during a posture task. By applying loads selectively to the shoulder, elbow, or both joints, we observed that monoarticular shoulder and elbow muscles varied their activity with loads at the unspanned joint. Shoulder monoarticulars changed activity with elbow torque and elbow monoarticulars changed activity with shoulder torque. Due to this additional modulation, the maximal activation of monoarticular muscles was deviated from their anatomical action toward either shoulder-extension/elbow-flexion or shoulder-flexion/elbow-extension. Biarticular muscles also expressed deviations in their preferred torque direction toward either shoulder-extension/elbow-flexion or shoulder-flexion/elbow-extension. The biased distribution of preferred torque directions in proximal forelimb muscles could be modeled by the minimization of a global measure of muscle activity. Moreover, arm-related neurons of primary motor cortex exhibit a similar bias in preferred torque directions consistent with the intimate relationship between the primary motor cortex and the motor periphery.     

Pattern of projections of group I afferents from elbow muscles to motoneurones supplying wrist muscles in man

Summary The pattern of projections of low threshold afferents from triceps and biceps brachii muscles onto motoneurones innervating muscles acting at the wrist was assessed by a reflex and a poststimulus time histogram (psth) technique. Activation of low-threshold afferents originating from elbow flexors or extensors resulted in an early, short-lasting inhibition of wrist flexor motoneurones (flexor carpi radialis, flexor carpi ulnaris). An inhibition was also found in the extensor carpi radialis (ECR) motoneurones after stimulation of low-threshold afferents from triceps. Evidence is presented that Ia fibres contribute to these effects. The inhibitory effects were found in all subjects, but they were constant in only 57% of the reflex experimental sessions and in 25% of the explored motor units. Stimulation of biceps low-threshold afferents was always ineffective on ECR motoneurones. No early facilitation was ever seen in motor nuclei innervating wrist muscles following stimulation of low threshold afferents from biceps and triceps. The pattern of transjoint projections of group I afferents from proximal to distal muscles and from distal to proximal ones (Cavallari and Katz 1989) is discussed in relation to that described in the cat forelimb.