EVOLUTIONARY MORPHOLOGY AND PHYLOGENY OF ARAGHNIDA

Abstract— This paper reports results from a cladistic analysis of the 11 Recent arachnid orders. The polarities of 64 newly discovered and traditional characters were determined through outgroup comparisons that included Eurypterida, Xiphosura, Trilobita and Crustacea. A branch-and-bound algorithm was used to discover a single tree (consistency index 0–59). The relationships suggested by this analysis differ substantially from previous interpretations of arachnid phylogeny, and a new taxonomic system is introduced to accommodate these results. This analysis suggests that Arachnida is monophyletic and composed of two principal lineages, Micrura and Dromopoda. Possible synapomorphies of Micrura include a pygidmm, tntosternum, six principal lateral eyes, poorly sclerotized postgenital appendages, coxal gland orifices near leg 1, an array of micxotubules associated with the spermatozoan nucleus, and absence of coxal endites on the walking legs. The micruran orders appear to have the following relationships: (Palpigradi (Araneae (Amblypygi (I helyphonida, Schizomida)))) (Ricinulei, Acari). Possible synapomorphies of Dromopoda include transverse carapaeal furrows, greatly reduced prosomal sternum, prosomal endosternite with two segmental components, stomotheca, bicondylar femoropatellar and patellotibial joints and extensor muscles. The dromopodan orders appear to have the following relationships: Opiliones (Scorpioncs (Pscudo-scorpiones, Solifugae)).     

Evolutionary and functional substitution of extrinsic musculature in Solifugae (Arachnida)

The locomotory system of Solifugae is distinct from that of other Arachnida in several ways. Only three pairs of legs are involved in locomotion, while the first pair function as sensory appendages. Morphologically, the proximal region of the locomotory system in Solifugae is characterized by fused coxae. Within the prosoma of Solifugae, an endosternite is missing: in other Arachnida, this endosternite serves as the proximal attachment site for a portion of the extrinsic musculature. How then do these skeletal modifications influence the muscular anatomy in the proximal region of the locomotory system? To answer this question, we studied the skeletomuscular anatomy of Galeodes granti at the interface between the prosoma and legs, reinvestigating the complex muscular anatomy of this body region for the first time in over 80 years and—for the first time—using detailed micro-computed tomography scans to analyze the skeletomuscular morphology. Specimens of three further species were checked for comparison. The analysis revealed differences in the number and composition of coxa-trochanter muscles in each of the four pairs of legs. These are compared in the light of serial homology. The comparison between the proximal locomotory system of Solifugae and that of other Arachnida unveils a series of analogies. Primarily, the coxa-trochanter joint is the most proximal joint to move the leg relative to the prosoma. Therefore, we argue that from a morpho-functional point of view, the coxa-trochanter muscles in Solifugae should be considered secondary extrinsic musculature. Thus, the legs gain a stable, articulated joint in the most proximal region of the leg to the prosoma, which might be advantageous for agile locomotion.     

Cupiennius salei: biomechanical properties of the tibia–metatarsus joint and its flexing muscles

Hunting spiders are well adapted to fast locomotion. Space saving hydraulic leg extension enables leg segments, which consist almost soley of flexor muscles. As a result, the muscle cross sectional area is high despite slender legs. Considering these morphological features in context with the spider’s segmented C-shaped legs, these specifics might influence the spider’s muscle properties. Moreover, these properties have to be known for modeling of spider locomotion. Cupiennius salei (n = 5) were fixed in a metal frame allowing exclusive flexion of the tibia–metatarsus joint of the second leg (counted from anterior). Its flexing muscles were stimulated supramaximally using needle electrodes. Accounting for the joint geometry, the force–length and the force–velocity relationships were determined. The spider muscles produce 0.07 N cm maximum isometric moment (corresponding to 25 N/cm² maximum stress) at 160° tibia–metatarsus joint angle. When overextended to the dorsal limit at approximately 200°, the maximum isometric moments decrease to 72%, and, when flexed to the ventral hinge stop at 85°, they drop to 11%. The force–velocity relation shows the typical hyperbolic shape. The mean maximum shortening velocity is 5.7 optimum muscle lengths per second and the mean curvature (a/F _iso) of the Hill-function is 0.34. The spider muscle’s properties which were determined are similar to those of other species acting as motors during locomotion (working range, curvature of Hill hyperbola, peak power at the preferred speeds), but they are relatively slow. In conjunction with the low mechanical advantage (muscle lever/load arm), the arrangement of three considerably actuated joints in series may nonetheless enable high locomotion velocities.     

Skeletomuscular anatomy of the harvestman Leiobunum aldrichi (Weed, 1893) (Arachnida: Opiliones: Palpatores) and its evolutionary significance

Skeletal muscles of the North American harvestman Leiobunum aldrichi are exhaustively surveyed and compared with other chelicerates to clarify the evolutionary morphology and phylogenetic relationships of arachnids. Representatives of 104 muscle groups are described and illustrated, and their possible functions are proposed. Comparisons of the feeding apparatus of L. aldrichi with that of other opilions, especially Sim (Cyphophthalmi) and Acromares (Laniatores), and two scorpion genera ( Centruroides, Pandinus ) indicate that the pharyngeal apparatus in L. aldrichi is derived and that its ability to accommodate large food particles is a secondary rather than primitive condition. Comparisons reveal several possible synapomorphies between Opiliones and Scorpiones suggesting that these orders may be sister groups. Apparently unique synapomorphies include an extrinsic cheliceral muscle that arises from the carapace and inserts on the second cheliceral article (deutomerite); an epistome divided into distal and proximal parts by a transverse sulcus; pharyngeal dilator muscles supported by a peripharyngeal skeleton formed by one dorsomedial and two ventrolateral epistomal processes, the latter also with muscular attachments to the endosternite; a specialized preoral chamber (stomodieca) derived from extensions (coxapophyses) of the coxae of the pedipalp and first two leg pairs; internal processes associated with the coxapophyses that serve, in part, as an attachment for muscles operating the coxa-trochanter joints, and lateral endosternal suspensor muscles that insert on the arthrodial membrane between the leg coxae. These are the first observations providing explicit support for an Opiliones-Scorpiones clade.     

Body musculature of Stylochaeta scirtetica Brunson, 1950 and Dasydytes (Setodytes) tongiorgii (Balsamo, 1982) (Gastrotricha: Dasydytidae): A functional approach

Species of the freshwater gastrotrich taxon Dasydytidae show a set of conspicuous structural and behavioural adaptations to a semi-planktonic life. Conspicuously, most dasydytids have several groups of strong, moveable spines that can actively be abducted to perform saltatory movements, change the overall direction of locomotion, or enable the animals to rest in a defensive position. So far, there are only vague ideas of how these spine movements are achieved in dasydytid species. In order to gain insight into the possible morpho-functional coupling of body musculature and motile spines, we have carried out a study targeting the muscular system in two species of Dasydytidae by means of phalloidin staining and confocal laser scanning microscopy.

For spine movements in both species studied, Stylochaeta scirtetica and Dasydytes (Setodytes) tongiorgii, we have identified an antagonistic system of segmented longitudinal and oblique somatic muscle pairs. In both species, contraction of the musculi obliqua abduct the paired groups of ventro-lateral spines; contraction of the segments of musculi laterales causes their adduction.

A comparison of the muscular system of the studied species to that of other gastrotrichs reveals several homologous muscle pairs, visceral as well as somatic, that might be features of the stem species of a clade comprising all Paucitubulatina exclusive of Xenotrichulidae. The pairs of oblique somatic muscles are most probably an autapomorphy of Dasydytidae.     

The mechanism of generation and transmission of forces in leg extension.

This paper deals with the mechanical and electromyographic evaluation of the mechanism generating and transmitting the resultant leg extension force by maximal isometric contraction in two directions, the knee and hip joint being kept at 90 degrees. The two directions were a) from the center of gravity of the body to the ankle joint and b) from a point near the knee to the ankle. Six male subjects in a supine position exerted a maximal leg extension force of 47-112 kg for a) and 51-73 kg for b). These values were close to the smaller values of two forces estimated at the knee and at the hip from maximal isometric forces at the corresponding joint of the same joint angle. It was thus suggested that the joint limiting the resultant leg extension force was the knee for a) and the hip for b). The single joint muscles exhibited almost maximal activities when they concerned the joint which limited the resultant leg extension force. The double joint muscles were often contracted only moderately during the maximal isometric leg extension, indicating a different role of double joint muscles even at the maximal force production at a particular joint.     

A three-dimensional model of the rat hindlimb: musculoskeletal geometry and muscle moment arms

As a first step towards developing a dynamic model of the rat hindlimb, we measured muscle attachment and joint center coordinates relative to bony landmarks using stereophotogrammetry. Using these measurements, we analyzed muscle moment arms as functions of joint angle for most hindlimb muscles, and tested the hypothesis that postural change alone is sufficient to alter the function of selected muscles of the leg. We described muscle attachment sites as second-order curves. The length of the fit parabola and residual errors in the orthogonal directions give an estimate of muscle attachment sizes, which are consistent with observations made during dissection. We modeled each joint as a moving point dependent on joint angle; relative endpoint errors less than 7% indicate this method as accurate. Most muscles have moment arms with a large range across the physiological domain of joint angles, but their moment arms peak and vary little within the locomotion domain. The small variation in moment arms during locomotion potentially simplifies the neural control requirements during this phase. The moment arms of a number of muscles cross zero as angle varies within the quadrupedal locomotion domain, indicating they are intrinsically stabilizing. However, in the bipedal locomotion domain, the moment arms of these muscles do not cross zero and thus are no longer intrinsically stabilizing. We found that muscle function is largely determined by the change in moment arm with joint angle, particularly the transition from quadrupedal to bipedal posture, which may alter an intrinsically stabilizing arrangement or change the control burden.     

Structure, function and evolution of somatic musculature in Dasydytidae (Paucitubulatina, Gastrotricha)

The musculature of two species of the gastrotrich taxon Dasydytidae, Dasydytes (Dasydytes) goniathrix and Haltidytes crassus, was investigated and described using phalloidin staining, confocal microscopy and computer-aided three-dimensional data analysis. Dasydytidae is a peculiar taxon of freshwater Gastrotricha, containing species that are characterized by different adaptations to a semiplanktonic lifestyle, a rather uncommon feature among primarily benthic Gastrotricha. Like other dasydytid species studied so far, D. goniathrix and H. crassus possess a system of movable cuticular spines with an associated system of somatic oblique and segmented lateral muscles. The presence of other somatic (dorsodermal muscles R1 and R2) and visceral muscles (musculi ventrales, m. ventrolaterales, m. dorsales, m. helicoidales) known from a wide range of gastrotrich species was confirmed. Regarded from a functional perspective, the earlier proposed antagonistic role of oblique muscles (as spine abductors) and segmented lateral muscles (as adductors) is questioned for the species studied herein. Alternatively, our structural and behavioral observations suggest that muscular spine abduction in D. goniathrix is brought about by synergistic contraction of the musculi obliqua and m. laterales, and a passive adduction due to muscle relaxation and elastic recoil of the trunk and cuticle. For H. crassus, we hypothesize active muscular spine abduction by contraction of the musculi obliqua plus the last segment of m. laterales accompanied by severe cuticle deformations close to the spine insertions. Adduction is achieved by cuticle reformation due to elasticity and increase in tissue pressure brought about by muscle action, possibly of enforced dorsodermal muscles. The newly obtained and published muscular data of further gastrotrich species were gathered in a species-character matrix. Based on this data set, a maximum parsimony analysis of representatives of the Dasydytidae has been conducted. According to this analysis, there are three well-supported monophyletic lineages within likewise monophyletic Dasydytidae. The first lineage comprises the taxa Anacanthoderma, Stylochaeta and Chitonodytes, the second comprises Dasydytes, Setopus and Ornamentula, and the third represents the taxon Haltidytes. Relationships between these clades could be resolved but are only weakly supported. The new phylogenetic hypothesis is used to reconstruct the ancestral character pattern and to infer possible evolutionary transformations within the Dasydytidae.     

Residual force enhancement during voluntary contractions of knee extensors and flexors at short and long muscle lengths

Residual force enhancement (RFE) is a term describing the observation that muscle tension during a contraction that includes a stretch and hold remains above that during an isometric contraction at the hold length. RFE has been observed during in vitro and in vivo experiments, but results involving voluntary contractions are mixed, particularly with respect to large muscles. The purpose of this study was to determine if RFE can be observed in large muscles such as knee extensors and flexors at joint configurations corresponding to the ascending and descending limbs of the muscle force-length curve. Two groups of twenty participants (ten males and ten females per group) performed maximum voluntary contractions on a Biodex machine in purely isometric conditions and in isometric conditions immediately following eccentric stretch. Knee extension trials were performed at 40° (short muscles) and 100° (long muscles) flexion from full extension (0°), and knee flexion trials were performed at 70° (short muscles) and 10° (long muscles) flexion. Stretch-isometric trials terminated at these angles following 30° of eccentric motion at 30°/s. Statistically-significant RFE was observed for both tasks at long-muscle joint configurations, but was not observed for either task at short-muscle joint configurations. Passive torque enhancement was also observed following muscle relaxation at long-muscle joint configurations for both tasks, but for only knee flexion at short-muscle joint configurations. These results reinforce for voluntary contractions of large muscles the RFE behavior observed in smaller muscles, and provide further evidence that RFE occurs primarily on the descending limb of the muscle force-length curve.     

System identification of muscle–joint interactions of the cat hind limb during locomotion

Neurophysiological experiments in walking cats have shown that a number of neural control mechanisms are involved in regulating the movements of the hind legs during locomotion. It is experimentally hard to isolate individual mechanisms without disrupting the natural walking pattern and we therefore introduce a different approach where we use a model to identify what control is necessary to maintain stability in the musculo-skeletal system. We developed a computer simulation model of the cat hind legs in which the movements of each leg are produced by eight limb muscles whose activations follow a centrally generated pattern with no proprioceptive feedback. All linear transfer functions, from each muscle activation to each joint angle, were identified using the response of the joint angle to an impulse in the muscle activation at 65 postures of the leg covering the entire step cycle. We analyzed the sensitivity and stability of each muscle action on the joint angles by studying the gain and pole plots of these transfer functions. We found that the actions of most of the hindlimb muscles display inherent stability during stepping, even without the involvement of any proprioceptive feedback mechanisms, and that those musculo-skeletal systems are acting in a critically damped manner, enabling them to react quickly without unnecessary oscillations. We also found that during the late swing, the activity of the posterior biceps/semitendinosus (PB/ST) muscles causes the joints to be unstable. In addition, vastus lateralis (VL), tibialis anterior (TA) and sartorius (SAT) muscle-joint systems were found to be unstable during the late stance phase, and we conclude that those muscles require neuronal feedback to maintain stable stepping, especially during late swing and late stance phases. Moreover, we could see a clear distinction in the pole distribution (along the step cycle) for the systems related to the ankle joint from that of the other two joints, hip or knee. A similar pattern, i.e., a pattern in which the poles were scattered over the s-plane with no clear clustering according to the phase of the leg position, could be seen in the systems related to soleus (SOL) and TA muscles which would indicate that these muscles depend on neural control mechanisms, which may involve supraspinal structures, over the whole step cycle.     

Motor unit firing patterns of lower leg muscles during isometric plantar flexion with flexed knee joint position

The ankle flexor and extensor muscles are essential for pedal movements associated with car driving. Neuromuscular activation of lower leg muscles is influenced by the posture during a given task, such as the flexed knee joint angle during car driving. This study aimed to investigate the influence of flexion of the knee joint on recruitment threshold-dependent motor unit activity in lower leg muscles during isometric contraction. Twenty healthy participants performed plantar flexor and dorsiflexor isometric ramp contractions at 30 % of the maximal voluntary contraction (MVC) with extended (0°) and flexed (130°) knee joint angles. High-density surface electromyograms were recorded from medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) muscles and decomposed to extract individual motor units. The torque-dependent change (Δpps /Δ%MVC) of the motor unit activity of MG (recruited at 15 %MVC) and SOL (recruited at 5 %MVC) muscles was higher with a flexed compared with an extended knee joint (p < 0.05). The torque-dependent change of TA MU did not different between the knee joint angles. The motor units within certain limited recruitment thresholds recruited to exert plantar flexion torque can be excited to compensate for the loss of MG muscle torque output with a flexed knee joint.     

Whole-body vibration induced adaptation in knee extensors; consequences of initial strength, vibration frequency, and joint angle

It was hypothesized that both vibration frequency and muscle length modulate the strengthening of muscles that is assumed to result from whole-body vibration (WBV). Length of knee extensor muscles during vibration is affected by the knee joint angle; the lengths of the knee extensors increase with more flexed knee joint angles. In an intervention study 28 volunteers were randomly assigned to 1 of 4 groups. Each group received 4 weeks of WBV at 1 of 3 different frequencies (20, 27, or 34 Hz) or 1 of 2 different lengths of knee extensors. Voluntary, isometric knee extension moment-angle relationship was determined. Initially, stronger subjects reacted differently to WBV than weaker participants. In stronger subjects knee extension moment did not improve; in the weaker subjects considerable improvements were observed ranging from 10 to 50%. Neither vibration frequency nor muscle length during the intervention affected the improvements. In addition to strength, the knee joint angle at which the maximal joint moment was generated (optimal joint angle) was affected. When trained at short muscle lengths, optimal angle shifted to more extend joint position. WBV training at long muscle lengths tended to induce an opposite shift. The amount of this shift tended to be influenced by vibration frequency; the lower the vibration frequency the larger the shift. Shifts of optimal lengths occurred in both weaker and stronger subjects. This study shows that muscle length during training affects the angle of knee joint at which the maximal extension moment was generated. Moreover, in weaker subjects WBV resulted in higher maximal knee joint extension moments. Vibration frequency and muscle length during vibration did not affect this joint moment gain.     

Evidence of residual force enhancement for multi-joint leg extension

Force enhancement is a well accepted property of skeletal muscle and has been observed at all structural levels ranging from single myofibrils to voluntarily activated m. quadriceps femoris in vivo. However, force enhancement has not been studied for multi-joint movements like human leg extension; therefore knowledge about its relevance in daily living remains limited. The purpose of this study was to determine whether there is force enhancement during maximal voluntary multi-joint leg extension. Human leg extension was studied (n=22) on a motor driven leg press dynamometer where external reaction forces under the feet as well as activity of 8 lower extremity muscles were measured. In addition, torque in the ankle and knee joints was calculated using inverse dynamics. The steady-state isometric force, joint torques, and muscle activation after active stretch (20° stretch amplitude at 60°/s) were compared with the corresponding values obtained during isometric reference contractions. There was consistent force enhancement during and following stretch for both forces and joint torques. Potentiation during stretch reached values between 26% and 30%, while a significant force enhancement of 10.5–12.3% and 4.3–7.4% remained 0.5–1 and 2.5–3 s after stretch, respectively. During stretch, EMG signals of m. gastrocnemius medialis and lateralis were significantly increased, while following stretch all analyzed muscles showed the same activity as during the reference contractions. We conclude from these results that force enhancement exists in everyday movements and should be accounted for when analyzing or modelling human movement.     

Internal and external morphology of the deutonymph of Caloglyphus boharti (Arachnida: Acari)

A band of flexible cuticle encircles the deutonymph, separating the dorsal and ventral plates. The coxae are large, flat and fused with one another to form most of the ventor. Individual coxal margins are redefined as sternites, epimerites or simply apodemes according to which margins fuse with which others. A given area of cuticle may have patches of dark or light cuticle not corresponding to particular structures or cuticular contours; this is a source of confusion to taxonomists. Each leg has a dicondylic coxal-trochantal (adduction-abduction) and trochantal-femoral (promotion-remotion) joint with opposing muscles. The three more distal monocondylic joints (flexion-extension) have only flexor muscles; extension is by increased haemolymph pressure. The five apodemes of the sucker plate provide rigidity; the four suckers attach by a flexible cuticular ring to a solid flange or socket in the sucker plate. The sucker muscles attach to the center of each sucker. The flat, external face of the sucker plate apodemes may complement sucker action by adhesion. Coxal discs and sucker plate discs are identical, contain birefringent cuticular elements, and are considered modified setae. Functional mouthparts and a pharynx are lacking, but a cheliceral anlage is present. The esophagus, midgut and caecae, and malpighian tubules are lumenless and the cells small. The hindgut has a lumen, larger cells and opens externally via the anus. Whereas the digestive tract is regressed, the reproductive system is yet incompletely developed. In older deutonymphs anlagen of ducts, accessory glands and gonads are discernible. The nature of the haemocoel and peritoneum remains nuclear. The central nerve mass is conspicuously large for the size of the deutonymph. The supraesophageal ganglion gives rise to the cheliceral nerves; all other nerves arise from the subesophageal ganglion. Most major nerves were traced to the effector organs. The muscles are divided into leg, dorso-ventral (derived from coxal muscles), dorsal, sucker, and anogenital muscles. The trochantal adductor muscles originate on an endosternite, which is supported by muscles running to the dorsal hysterosoma. The dorso-ventral and propodosomal retractor muscles affect haemolymph pressure. The massive sucker retractor muscles are unique to this instar. Anogenital muscles are not well developed.     

Knee angle-specific EMG normalization: The use of polynomial based EMG-angle relationships

The normalization of EMG signals to those recorded during a maximal voluntary contraction provides a valid construct for comparisons of relative muscle activity. However, the length dependence of muscle activation and purported, substantial, muscle translocation and changes in muscle architecture during dynamic movements presents a need for joint angle-dependent normalization processes. The purposes of the present study were to: (1) quantify variations in muscle activity across a large ROM, (2) determine the accuracy with which fitted EMG-joint angle curves accurately characterized these variations, and (3) compare peak (EMG-P) and average (EMG-A) EMG amplitudes obtained during a countermovement leg extension when normalized to both absolute peak and joint angle-specific muscle activity. Fifteen subjects performed a large ROM (110°) isokinetic (30° s^?1) leg extension from which EMG-joint angle relationships were derived using polynomial fitting of different complexities. Ten subjects also performed loaded countermovement leg extensions from which EMG signals were normalized using peak muscle activity and EMG-angle curves. EMG amplitude varied significantly over the ROM and the use of EMG-angle curves for signal normalization resulted in significantly greater EMG-P and EMG-A than those normalized using the absolute peak EMG. Higher-order polynomial fitting better matched the filtered EMG amplitudes. Thus, there is a strong rationale for using EMG-angle polynomial fits to normalize EMG signals for large ROM movements.     

Ankle flexor muscles in the cat: Length-active tension and muscle unit properties as related to locomotion

The mechanical properties of the whole muscle and fast-twitch muscle units of the cat hindlimb pretibial flexors have been explored and related to normal locomotion. Tibialis anterior (TA) is parallel-fibered and functionally crosses a single joint, the ankle, whereas extensor digitorum longus (EDL) is pinnate and spans the ankle, knee, metatarsophalangeal and interphalangeal joints. The active tetanic tension of TA remains near its peak value over a range of muscle lengths associated with normal ankle movement. In contrast, the length-tension curve of EDL is sharply peaked. However, normal corollary action of the knee, ankle and metatarsophalangeal joints during stepping minimizes EDL's excursion and maintains it at or near a length optimal for peak tension development. EDL is capable of producing synchronous but sterotyped digit and ankle movements while TA provides for independent ankle flexion at all relevant joint angles. The mechanical properties of 84 TA and 98 EDL fast-twitch muscle units were studied by measuring twitch contraction time (≤45 msec), peak tetanic tension, response to repetitive stimulation, and contractile fatigue resistance during electrical stimulation of single alpha axons, functionally isolated from ventral root filaments. These mechanical properties were essentially similar for both muscles with the exception of mean peak tetanic tension which was 30% lower for TA units (14 gm-wt) than for EDL units (20 gm-wt). A high proportion of units in both muscles demonstrated fatigue resistance which is reflective of the repetitive, phasic demand upon these muscles during locomotion.     

Prediction of antagonistic muscle forces using inverse dynamic optimization during flexion/extension of the knee.

This paper examined the feasibility of using different optimization criteria in inverse dynamic optimization to predict antagonistic muscle forces and joint reaction forces during isokinetic flexion/extension and isometric extension exercises of the knee. Both quadriceps and hamstrings muscle groups were included in this study. The knee joint motion included flexion/extension, varus/valgus, and internal/external rotations. Four linear, nonlinear, and physiological optimization criteria were utilized in the optimization procedure. All optimization criteria adopted in this paper were shown to be able to predict antagonistic muscle contraction during flexion and extension of the knee. The predicted muscle forces were compared in temporal patterns with EMG activities (averaged data measured from five subjects). Joint reaction forces were predicted to be similar using all optimization criteria. In comparison with previous studies, these results suggested that the kinematic information involved in the inverse dynamic optimization plays an important role in prediction of the recruitment of antagonistic muscles rather than the selection of a particular optimization criterion. Therefore, it might be concluded that a properly formulated inverse dynamic optimization procedure should describe the knee joint rotation in three orthogonal planes.     

The influence of vibration on spinal alpha-motoneurons excitability in static conditions and during evoked stepping in human

In healthy human the excitability of spinal alpha-motoneurons under application of vibrostimulation (20-60 Hz) to different leg muscles was investigated both in stationary condition and during stepping movements caused by vibration in the condition of suspended leg. In 15 subjects the amplitude of H-reflex were compared under vibration of rectus femoris (RF) and biceps femoris (BF) muscles of left leg as well during vibration of rectus femoris of contralateral, motionless leg in three spatial positions: upright, supine and on right side of body with suspended left leg. In dynamic conditions the amount of H-reflex was compared during evoked and voluntary stepping at 8 intervals of step cycle. In all body positions the vibration of each ipsilateral leg muscles caused significant suppression of H-reflex, this suppression was more prominent in the air-stepping conditions. The vibration of contralateral leg RF muscle had a weak influence on the amplitude of H-reflex. In 7 subjects the muscle vibration of ipsilateral and contralateral legs generated stepping movements. During evoked "air-stepping" H-reflex had different amplitudes in different phases of step cycle. At the same time the differences between responses under voluntary and non-voluntary stepping were revealed only in stance phase. Thus, different degree of H-reflex suppression by vibration under different body position in space depends on, it seems to be, from summary afferent inflows to spinal cord interneurons, which participate in regulation of posture and locomotion. Seemingly, the increasing of spinal cord neurons excitability occurs under involuntary air-stepping in swing phase, which is necessary for activation of locomotor automatism under unloading leg conditions.     

Dimensions and moment arms of the hind- and forelimb muscles of common chimpanzees (Pan troglodytes).

This paper supplies quantitative data on the hind- and forelimb musculature of common chimpanzees (Pan troglodytes) and calculates maximum joint moments of force as a contribution to a better understanding of the differences between chimpanzee and human locomotion. We dissected three chimpanzees, and recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA). We also obtained flexion/extension moment arms of the major muscles about the limb joints. We find that in the hindlimb, chimpanzees possess longer fascicles in most muscles but smaller PCSAs than are predicted for humans of equal body mass, suggesting that the adaptive emphasis in chimpanzees is on joint mobility at the expense of tension production. In common chimpanzee bipedalism, both hips and knees are significantly more flexed than in humans, necessitating muscles capable of exerting larger moments at the joints for the same ground force. However, we find that when subject to the same stresses, chimpanzee hindlimb muscles provide far smaller moments at the joints than humans, particularly the quadriceps and plantar flexors. In contrast, all forelimb muscle masses, fascicle lengths, and PCSAs are smaller in humans than in chimpanzees, reflecting the use of the forelimbs in chimpanzee, but not human, locomotion. When subject to the same stresses, chimpanzee forelimb muscles provide larger moments at the joints than humans, presumably because of the demands on the forelimbs during locomotion. These differences in muscle architecture and function help to explain why chimpanzees are restricted in their ability to walk, and particularly to run bipedally.     

The anatomical arrangement of muscle and tendon enhances limb versatility and locomotor performance

The arrangement of muscles and tendons has been studied in detail by anatomists, surgeons and biomechanists for over a century, and the energetics and mechanics of muscle contraction for almost as long. Investigation of how muscles function during locomotion and the relative length change in muscle fibres and the associated elastic tendon has, however, been more challenging. In recent years, novel in vivo measurement methods such as ultrasound and sonomicrometry have contributed to our understanding of the dynamics of the muscle tendon unit during locomotion. Here, we examine both published and new data to explore how muscles are arranged to deliver the wide repertoire of locomotor function and the trade-offs between performance and economy that result.