A model for insect locomotion in the horizontal plane: Feedforward activation of fast muscles, stability, and robustness

We develop a neuromechanical model for running insects that includes a simplified hexapedal leg geometry with agonist-antagonist muscle pairs actuating each leg joint. Restricting to dynamics in the horizontal plane and neglecting leg masses, we reduce the model to three degrees of freedom describing translational and yawing motions of the body. Muscles are driven by stylized action potentials characteristic of fast motoneurons, and modeled using an activation function and nonlinear length and shortening velocity dependence. Parameter values are based on measurements from depressor muscles and observations of kinematics and dynamics of the cockroach Blaberus discoidalis; in particular, motoneuronal inputs and muscle force levels are chosen to approximately achieve joint torques that are consistent with measured ground reaction forces. We show that the model has stable double-tripod gaits over the animal's speed range, that its dynamics at preferred speeds matches those observed, and that it maintains stable gaits, with low frequency yaw deviations, when subject to random perturbations in foot touchdown and lift-off timing and action potential input timing. We explain this in terms of the low-dimensional dynamics.     

Predictive and reactive tuning of the locomotor CPG

The neural control of locomotion involves a constant interplaybetween the actions of a central pattern generator (CPG) andsensory input elicited by bodily movement. With respect to theCPG, recent analysis of fictive locomotion has shown that durationsof flexion and extension tend to covary along specific linesin plots of phase duration versus cycle duration. The slopesof these lines evidently depend on internal states that varyamong preparations, but, within a preparation, remain rathersteady from one sequence to the next. These relationships canbe reproduced in a simple oscillator model having two pairsof preset parameters, suggesting that steady internal drivesto flexor and extensor half-centers determine how phase durationscovary. Regarding the role of sensory inputs, previous experimentshave revealed state-dependent rules that govern phase-switchingindependently of the CPG rhythm. In addition, sensory inputis known to modulate motoneuronal activation through stretchreflexes. To explore how sensory input combines with the locomotorCPG, we used a neuromechanical model with muscle actuators,proprioceptive feedback, sensory phase-switching rules, anda CPG. Interestingly, sequences of stable locomotion were alwaysassociated with phase durations that conformed to an extensor-dominatedphase-duration characteristic (where extension durations varymore than flexion durations). This is the characteristic seenin normal animals, but not necessarily in fictive locomotion,where movement and associated sensory input are absent. Thissuggests that to produce the biomechanical events required forstability, an extensor-dominated phase-duration characteristicis required. In the model, when the preset CPG phase durationswere well matched to coincide the biomechanical requirements,CPG-mediated phase switching produced stable cycles. When CPGphase durations were too short, phases switched prematurelyand the model soon fell. When CPG phase durations were too long,sensory rules fired and overrode the CPG, maintaining stability.We posit that under normal circumstances, descending input fromhigher centers continually adjusts the operating point of theCPG on the preset phase-duration characteristic according toanticipated biomechanical requirements. When the predictionsare good, CPG-generated phase durations closely match thoserequired by the kinetics and kinematics, and little or no sensoryadjustment occurs. We propose the term "neuromechanical tuning"to describe this process of matching the CPG to the biomechanicalrequirements.     

CPG Control for Biped Hopping Robot in Unpredictable Environment

A CPG control mechanism is proposed for hopping motion control of biped robot in unpredictable environment.Based on analysis of robot motion and biological observation of animal's control mechanism,the motion control task is divided into two simple parts:motion sequence control and output force control.Inspired by a two-level CPG model,a two-level CPG control mechanism is constructed to coordinate the drivers of robot joint,while various feedback information are introduced into the control mechanism.Interneurons within the control mechanism are modeled to generate motion rhythm and pattern promptly for motion sequence control; motoneurons are modeled to control output forces of joint drivers in real time according to feedbacks.The control system can perceive changes caused by unknown perturbations and environment changes according to feedback information,and adapt to unpredictable environment by adjusting outputs of neurons.The control mechanism is applied to a biped hopping robot in unpredictable environment on simulation platform,and stable adaptive motions are obtained.     

A single muscle's multifunctional control potential of body dynamics for postural control and running

A neuromechanical approach to control requires understanding how mechanics alters the potential of neural feedback to control body dynamics. Here, we rewrite activation of individual motor units of a behaving animal to mimic the effects of neural feedback without concomitant changes in other muscles. We target a putative control muscle in the cockroach, Blaberus discoidalis (L.), and simultaneously capture limb and body dynamics through high-speed videography and a micro-accelerometer backpack. We test four neuromechanical control hypotheses. We supported the hypothesis that mechanics linearly translates neural feedback into accelerations and rotations during static postural control. However, during running, the same neural feedback produced a nonlinear acceleration control potential restricted to the vertical plane. Using this, we reject the hypothesis from previous work that this muscle acts primarily to absorb energy from the body. The conversion of the control potential is paralleled by nonlinear changes in limb kinematics, supporting the hypothesis that significant mechanical feedback filters the graded neural feedback for running control. Finally, we insert the same neural feedback signal but at different phases in the dynamics. In this context, mechanical feedback enables turning by changing the timing and direction of the accelerations produced by the graded neural feedback.     

Muscle type-specific myosin isoforms in crustacean muscles

Differential expression of multiple myosin heavy chain (MyHC) genes largely determines the diversity of critical physiological, histochemical, and enzymatic properties characteristic of skeletal muscle. Hypotheses to explain myofiber diversity range from intrinsic control of expression based on myoblast lineage to extrinsic control by innervation, hormones, and usage. The unique innervation and specialized function of crayfish (Procambarus clarkii) appendicular and abdominal musculature provide a model to test these hypotheses. The leg opener and superficial abdominal extensor muscles are innervated by tonic excitatory motoneurons. High resolution SDS-PAGE revealed that these two muscles express the same MyHC profile. In contrast, the deep abdominal extensor muscles, innervated by phasic motoneurons, express MyHC profiles different from the tonic profiles. The claw closer muscles are dually innervated by tonic and phasic motoneurons and a mixed phenotype was observed, albeit biased toward the phasic profile seen in the closer muscle. These results indicate that multiple MyHC isoforms are present in the crayfish and that differential expression is associated with diversity of muscle type and function.     

Affinity for MgADP and force of unbinding from actin of myosin purified from tonic and phasic smooth muscle

Smooth muscle is unique in its ability to maintain force at low MgATP consumption. This property, called the latch state, is more prominent in tonic than phasic smooth muscle. Studies performed at the muscle strip level have suggested that myosin from tonic muscle has a greater affinity for MgADP and therefore remains attached to actin longer than myosin from phasic muscle, allowing for cross-bridge dephosphorylation and latch-bridge formation. An alternative hypothesis is that after dephosphorylation, myosin reattaches to actin and maintains force. We investigated these fundamental properties of smooth muscle at the molecular level. We used an in vitro motility assay to measure actin filament velocity (nu(max)) when propelled by myosin purified from phasic or tonic muscle at increasing [MgADP]. Myosin was 25% thiophosphorylated and 75% unphosphorylated to approximate in vivo conditions. The slope of nu(max) versus [MgADP] was significantly greater for tonic (-0.51 +/- 0.04) than phasic muscle myosin (-0.15 +/- 0.04), demonstrating the greater MgADP affinity of myosin from tonic muscle. We then used a laser trap assay to measure the unbinding force from actin of populations of unphosphorylated tonic and phasic muscle myosin. Both myosin types attached to actin, and their unbinding force (0.092 +/- 0.022 pN for phasic muscle and 0.084 +/- 0.017 pN for tonic muscle) was not statistically different. We conclude that the greater affinity for MgADP of tonic muscle myosin and the reattachment of dephosphorylated myosin to actin may both contribute to the latch state.     

Cuticular receptor activation of postural motoneurons in the abdomen of the hermit crab,<Emphasis Type="Italic"> Pagurus pollicarus</Emphasis>

Displacement of the abdominal cuticle of the hermit crab, Pagurus pollicarus, activates motoneurons of the ventral superficial muscles that mediate posture and slow movements. Five excitatory motoneurons innervating the right ventral superficial muscle of the fourth abdominal segment were activated in a phasic stereotyped fashion in the isolated nervous system. Intracellular records from these motoneurons showed an initial monosynaptic burst, a period of inhibition in which inhibitory post-synaptic potentials were present and then a later period of increased spike frequency generated by excitatory post-synaptic potentials. The reflex response was maintained after severing all ganglionic roots from peripheral structures, isolating the nerve cord from peripheral feedback pathways. The two excitatory components of the response showed a dependence on strain that was much smaller than that found in sensory afferents. There was no relationship between the site of touch to the cuticle and the intensity or pattern of activation of the motoneurons. The reflex burst produced a transient activation of both longitudinal and transverse/circular layers of the muscle with forces that varied between 10% and 25% of the maximum muscle force. These results are consistent with a feedforward regulation of muscle stiffness.     

Immediate diaphragmatic electromyogram responses to imperceptible mechanical loads in conscious humans.

We used an esophageal electrode to measure the amplitude and neural inspiratory and expiratory (N TE) timing responses of crural diaphragmatic electrical activity in response to flow-resistive (R) and elastic (E) loads at or below the threshold for conscious detection, applied pseudorandomly to the oral airway of eight normal subjects. We observed a rapid first-breath neural reflex that modified respiratory timing such that N TE lengthened significantly in response to R loads in six of eight subjects and shortened in response to E loading in six of seven subjects. The prolongation of N TE with R loading resulted primarily from lengthening the portion of N TE during which phasic activity in the diaphragm is absent (TE NDIA), whereas E loading shortened N TE mainly by reducing TE NDIA. Most subjects responded to both types of loading by decreasing mean tonic diaphragmatic activity, the average level of muscle activity that exists when no phasic changes are occurring, as well as its variability. The observed timing responses are consistent in direction with optimally adaptive pattern regulation, whereas the modulation of tonic activity may be useful in neural regulation of end-expiratory lung volume.     

Computer simulation study on central pattern generator: from biology to engineering

Central pattern generator (CPG) is a neuronal circuit in the nervous system that can generate oscillatory patterns for the rhythmic movements. Its simplified format, neural oscillator, is wildly adopted in engineering application. This paper explores the CPG from an integral view that combines biology and engineering together. Biological CPG and simplified CPG are both studied. Computer simulation reveals the mechanism of CPG. Some properties, such as effect of tonic input and sensory feedback, stable oscillation, robustness, entrainment etc., are further studied. The promising results provide foundation for the potential engineering application in future.     

Entrainment, Instability, Quasi-periodicity, and Chaos in a Compound Neural Oscillator

We studied the dynamical behavior of a class of compound central pattern generator (CPG) models consisting of a simple neural network oscillator driven by both constant and periodic inputs of varying amplitudes, frequencies, and phases. We focused on a specific oscillator composed of two mutually inhibiting types of neuron (inspiratory and expiratory neurons) that may be considered as a minimal model of the mammalian respiratory rhythm generator. The simulation results demonstrated how a simple CPG model— with a minimum number of neurons and mild nonlinearities— may reproduce a host of complex dynamical behaviors under various periodic inputs. In particular, the network oscillated spontaneously only when both neurons received adequate and proportionate constant excitations. In the presence of a periodic source, the spontaneous rhythm was overriden by an entrained oscillation of varying forms depending on the nature of the source. Stable entrained oscillations were inducible by two types of inputs: (1) anti-phase periodic inputs with alternating agonist-antagonist drives to both neurons and (2) a single periodic drive to only one of the neurons. In-phase inputs, which exert periodic drives of similar magnitude and phase relationships to both neurons, resulted in varying disruptions of the entrained oscillations including magnitude attenuation, harmonic and phase distortions, and quasi-periodic interference. In the absence of significant phasic feedback, chaotic motion developed only when the CPG was driven by multiple periodic inputs. Apneic episodes with repetitive alternation of active (intrinsic oscillation) and inactive (cessation of oscillation) states developed when the network was driven by a moderate periodic input of low frequency. %and amplitudes of intermediate strength, Similar results were demonstrated in other, more complex oscillator models (that is, half-center oscillator and three-phase respiratory network model). These theoretical results may have important implications in elucidating the mechanisms of rhythmogenesis in the mature and developing respiratory CPG as well as other compound CPGs in mammalian and invertebrate nervous systems.     

An Attempt to Account for the Diversity of Crustacean Muscles

Crustacean muscles are known to contain muscle fibers of variableproperties and to be innervated by phasic and/or tonic motoneuronswhich may possess synapses of diverse physiological properties.Frequently, phasic motor axons innervate short-sarcomere phasicmuscle fibers and tonic motor axons innervate long-sarcomeretonic muscle fibers, but some muscles receiving a single (tonic)motor axon contain both phasic and tonic muscle fibers. Althoughit is not known whether neural trophic influences are involvedin muscle differentiation, some neural trophic effects havebeen found in crustaceans, and it is reasonable to assume thatsuch influences may be involved in establishing the definitiveproperties of the muscle. Several other postulates must be made:(1) Phasic and tonic motor axons differ in their trophic effectiveness:(2) muscle fibers innervated relatively early in developmentby a tonic motor axon acquire the properties of tonic musclefibers, while those innervated later become intermediate orphasic muscle fibers; (3) the developmental stage of a growingor regenerating axon terminal plays a role in determinationof synaptic properties. Studies on regenerating limb buds supportthe hypothesis, which can account for the genesis of all observedtypes of crustacean neuromuscular system. Further experimentalwork is necessary to test the hypothesis.     

An integrated model of active glenohumeral stability

We present the first model of the glenohumeral joint implementing active muscle driven humeral positioning and stabilization without a priori constraints on glenohumeral kinematics. Previously established methods were used to predetermine the path, activation timing and resultant force contribution of 27 individual muscle segments at any given joint position. Artificial boundary conditions were applied in a three-dimensional finite element model of the joint and progressively released until the humeral head was completely free to rotate and translate within the fixed glenoid according to the compressive component of the predetermined resultant force. The shear component was then added such that no boundary conditions other than muscular force were applied. The framework was exploited to simulate elevation as a composite of instantaneous positions and theoretically demonstrate that joint stability can be achieved exclusively through muscular activity. Predicted muscle moment arms, muscle activation timing, humeral head translations, joint contact forces and stability ratio were comparable with existing experimental and in vivo data. This framework could be valuable for subject specific modeling and may be used to address clinical hypotheses related to shoulder joint stability that cannot be pursued using simplified modeling approaches.     

The sensitivity of Renshaw cells to velocity of sinusoidal stretches of the triceps surae muscle.

1. The electrical activity of Renshaw cells monosynaptically excited by ventral root stimulation and disynaptically excited by electric stimulation of the group I afferents in the GS nerve has been recorded and their response to individual sinusoidal stretches of the deefferented GS muscle tested for different amplitudes and durations of the stimulus. 2. The experimental data indicate that the Rensahw cell responses are not only length dependent but also rate dependent. This finding indicates that the same Renshaw cells receive recurrent collaterals of both tonic and phasic motoneurons. 3. The observation that the discharge of Renshaw cells is particularly sensitive to the velocity of stretch suggests that the recurrent collaterals of large phasic motoneurons, which are recruited during high velocity stretches, exert a stronger excitatory action on Renshaw cells than do axon collaterals of the smaller tonic motoneurons, which are selectively stimulated during low velocity stretches.     

Quantitative differences among EMG activities of muscles innervated by subpopulations of hypoglossal and upper spinal motoneurons during non-REM sleep - REM sleep transitions: a window on neural processes in the sleeping brain

In the rat, a species widely used to study the neural mechanisms of sleep and motor control, lingual electromyographic activity (EMG) is minimal during non-rapid eye movement (non-REM) sleep and then phasic twitches gradually increase after the onset of REM sleep. To better characterize the central neural processes underlying this pattern, we quantified EMG of muscles innervated by distinct subpopulations of hypoglossal motoneurons and nuchal (N) EMG during transitions from non-REM sleep to REM sleep. In 8 chronically instrumented rats, we recorded cortical EEG, EMG at sites near the base of the tongue where genioglossal and intrinsic muscle fibers predominate (GG-I), EMG of the geniohyoid (GH) muscle, and N EMG. Sleep-wake states were identified and EMGs quantified relative to their mean levels in wakefulness in successive 10 s epochs. During non-REM sleep, the average EMG levels differed among the three muscles, with the order being N>GH>GG-I. During REM sleep, due to different magnitudes of phasic twitches, the order was reversed to GG-I>GH>N. GG-I and GH exhibited a gradual increase of twitching that peaked at 70-120 s after the onset of REM sleep and then declined if the REM sleep episode lasted longer. We propose that a common phasic excitatory generator impinges on motoneuron pools that innervate different muscles, but twitching magnitudes are different due to different levels of tonic motoneuronal hyperpolarization. We also propose that REM sleep episodes of average durations are terminated by intense activity of the central generator of phasic events, whereas long REM sleep episodes end as a result of a gradual waning of the tonic disfacilitatory and inhibitory processes.     

Active zones and receptor surfaces of freeze-fractured crayfish phasic and tonic motor synapses

Deep and superficial flexor muscles in the crayfish abdomen are innervated respectively by small populations of physiologically distinct phasic and tonic motoneurons. Phasic motoneurons typically produce large EPSP's, releasing 100 to 1000 times more transmitter per synapse than their tonic counterparts, and exhibiting more rapid synaptic depression with maintained stimulation. Freeze-fracturing the abdominal flexor muscles yielded images of phasic and tonic synapse-bearing terminals. The two types of synapse are qualitatively similar in ultrastructure, displaying on the presynaptic membrane's P-face synaptic contacts recognized by relatively particle-free oval plaques which are often framed by the muscle fiber's E-face leaflet with its associated receptor particles. Situated within these presynaptic plaques are discrete clusters of large intramembrane particles, forming active zone (AZ) sites specialized for transmitter release. AZs of phasic and tonic synapses are similar: 80% had a range of 15–40 large particles distributed in either paired spherical clusters or in linear form, with a few depressions denoting sites of synaptic vesicle fusion or retrieval around their perimeters. The packing density of particles is similar for phasic and tonic AZs. The E-face of the muscle membrane displays oval-shaped receptor-containing sites made up of tightly packed intramembranous particles. Phasic and tonic receptor particles are packed at similar densities and the measured values resemble those of several other crustacean and insect neuromuscular junctions. Overall, the similarity between phasic and tonic synapses in the packing density of particles at their presynaptic AZs and postsynaptic receptor surfaces suggests similar regulatory mechanisms for channel insertion and spacing. Furthermore, the findings suggest that morphological differences in active zones or receptor surfaces cannot account for large differences in transmitter release per synapse.     

Tonic contractions allow metabolic recuperation of the adductor muscle during escape responses of giant scallop Placopecten magellanicus

To escape from starfish predators, giant scallops, Placopecten magellanicus, swim using series of strong phasic contractions interrupted by tonic contractions. To investigate whether these tonic contractions allow metabolic recuperation of the adductor muscle, we sampled scallops at rest (Control), after an initial series of phasic contractions (Phasic) and after 1 min of tonic contraction following their initial phasic contractions (Phasic + Tonic) and compared muscle levels of phosphoarginine, adenylate nucleotides (ATP, ADP and AMP) and adenylate energy charge (AEC). Scallops in the two active groups did not differ in the numbers of phasic contractions or the mean phasic force production. Phosphoarginine concentrations in the adductor muscle decreased with phasic activity and remained low after 1 min of tonic contraction. ATP and ADP and total adenylate levels did not differ between the three groups, but AMP levels were higher in the scallops sampled after phasic contractions than in control scallops. The AEC was reduced by phasic contractions but returned to control levels after 1 min of tonic contraction. A significant negative correlation between AEC and the number of claps in the Phasic group disappeared in the Phasic + Tonic group. Thus, tonic contractions following phasic contractions allow partial metabolic recovery of the adductor muscle by returning AEC to control levels. However, phosphoarginine levels did not recover during tonic contractions, and a negative correlation between the number of claps and phosphoarginine levels remained in the Phasic + Tonic group. By interspersing tonic contractions between series of phasic contractions, scallops improved muscle energetic status, which should help maintain phasic force production during the remainder of the escape response.     

Modulation of laryngeal and respiratory pump muscle activities with upper airway pressure and flow.

The hypothesis that upper airway (UA) pressure and flow modulate respiratory muscle activity in a respiratory phase-specific fashion was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. We generated negative pressure and inspiratory flow in phase with tracheal inspiration or positive pressure and expiratory flow in phase with tracheal expiration in the isolated UA. Stimulation of UA negative pressure receptors with body temperature air resulted in a 10--15% enhancement of phasic moving-time-averaged posterior cricoarytenoid electromyographic (EMG) activity above tonic levels obtained without pressure and flow in the UA (baseline). Stimulation of UA positive pressure receptors increased phasic moving-time-averaged thyroarytenoid EMG activity above tonic levels by 45% from baseline. The same enhancement of posterior cricoarytenoid or thyroarytenoid EMG activity was observed with the addition of flow receptor stimulation with room temperature air. Tidal volume and diaphragmatic and abdominal muscle activity were unaffected by UA flow and/or pressure, whereas respiratory timing was minimally affected. We conclude that laryngeal afferents, mainly from pressure receptors, are important in modulating the respiratory activity of laryngeal muscles.     

Regeneration from crayfish phasic and tonic motor axons in vitro

An explant culture system is described that allows examination of axonal growth from the tonically and phasically active motoneurons of the abdominal nerve cord of the crayfish. In this preparation, growth occurs from the cut end of the axon while the remainder of the motoneuron is undisturbed. In vitro growth from the branches of the third roots, which contain the axons from the tonic and phasic motoneurons of abdominal ganglia one through four, was verified as axonal by retrograde labeling of axons and neuronal somata within the nerve cord. Growth from the axons of phasic and tonic cells was observed as early as 24 h after plating and continued for an additional 7–10 days. The morphology and growth rates of the motor terminals differed between the tonic and phasic axons. The phasic axons grew significantly faster and branched more often than did the tonic motor axons. These differences in growth may be related to differences in motoneuron size or, may result from differences in electrical activity. Tonic motoneurons show spontaneous impulse activity for up to 6 days in culture, whereas phasic motoneurons show no spontaneous impulse activity. In addition, the differences in growth may be related to the morphological differences in tonic and phasic motor terminals observed in situ. © 1993 John Wiley & Sons, Inc.     

Altered control of submaximal bite force during bruxism in humans

The control of bite force during varying submaximal loads was examined in patients suffering from bruxism compared to healthy humans not showing these symptoms. The subjects raised a bar (preload) with their incisor teeth and held it between their upper and lower incisors using the minimal bite force required to keep the bar in a horizontal position. Further loading was added during the preload phase. A sham load was also used. Depending on the session, the teeth were loaded by the experimenter or the subject and in one session the subject did not see the load (no visual feedback). The bite force was measured continuously using a calibrated force transducer. In all the subjects, the bite force increased with increasing load. Following the addition of the load, the level of the tonic bite force was reached rapidly with no marked overshoot. The patients with bruxism used significantly higher bite forces to hold the submaximal loads compared to the control subjects. In the control subjects, the holding forces for each submaximal load were identical in the men and the women and were independent of subject maximal bite force. Sham loading evoked no marked responses in biting force. Whether the subject or the experimenter added the load or whether the subject had visual feedback or not were not significant factors in determining the level of bite force. The results indicated that the patients with bruxism used excessively large biting forces for each given submaximal load. This study showed no evidence that the inappropriate control of bite force by patients with bruxism was due to an abnormality in the higher cortical circuits that regulates the function of trigeminal motoneurons in the brainstem. This was shown by a lack of abnormality in coordination of voluntary hand movement with biting force, a lack of abnormal anticipation response to a sham load and a lack of any effect of visual feedback. The results were in line with the hypothesis that afferent input from oral (periodontal or masticatory muscle) tissues does not provide an appropriate control of motor command in bruxism.     

Reduction of neuromuscular redundancy for postural force generation using an intrinsic stability criterion

Postural control requires the coordination of multiple muscles to achieve both endpoint force production and postural stability. Multiple muscle activation patterns can produce the required force for standing, but the mechanical stability associated with any given pattern may vary, and has implications for the degree of delayed neural feedback necessary for postural stability. We hypothesized that muscular redundancy is reduced when muscle activation patterns are chosen with respect to intrinsic musculoskeletal stability as well as endpoint force production. We used a three-dimensional musculoskeletal model of the cat hindlimb with 31 muscles to determine the possible contributions of intrinsic muscle properties to limb stability during isometric force generation. Using dynamic stability analysis we demonstrate that within the large set of activation patterns that satisfy the force requirement for posture, only a reduced subset produce a mechanically stable limb configuration. Greater stability in the frontal-plane suggests that neural control mechanisms are more highly active for sagittal-plane and for ankle joint control. Even when the limb was unstable, the time-constants of instability were sufficiently great to allow long-latency neural feedback mechanisms to intervene, which may be preferential for movements requiring maneuverability versus stability. Local joint stiffness of muscles was determined by the stabilizing or destabilizing effects of moment-arm versus joint angle relationships. By preferentially activating muscles with high local stiffness, muscle activation patterns with feedforward stabilizing properties could be selected. Such a strategy may increase intrinsic postural stability without co-contraction, and may be useful criteria in the force-sharing problem.