Effect of stance width on multidirectional postural responses

The effect of stance width on postural responses to 12 different directions of surface translations was examined. Postural responses were characterized by recording 11 lower limb and trunk muscles, body kinematics, and forces exerted under each foot of 7 healthy subjects while they were subjected to horizontal surface translations in 12 different, randomly presented directions. A quasi-static approach of force analysis was done, examining force integrals in three different epochs (background, passive, and active periods). The latency and amplitude of muscle responses were quantified for each direction, and muscle tuning curves were used to determine the spatial activation patterns for each muscle. The results demonstrate that the horizontal force constraint exerted at the ground was lessened in the wide, compared with narrow, stance for humans, a similar finding to that reported by Macpherson for cats. Despite more trunk displacement in narrow stance, there were no significant changes in body center of mass (CoM) displacement due to large changes in center of pressure (CoP), especially in response to lateral translations. Electromyographic (EMG) magnitude decreased for all directions in wide stance, particularly for the more proximal muscles, whereas latencies remained the same from narrow to wide stance. Equilibrium control in narrow stance was more of an active postural strategy that included regulating the loading/unloading of the limbs and the direction of horizontal force vectors. In wide stance, equilibrium control relied more on an increase in passive stiffness resulting from changes in limb geometry. The selective latency modulation of the proximal muscles with translation direction suggests that the trunk was being actively controlled in all directions. The similar EMG latencies for both narrow and wide stance, with modulation of only the muscle activation magnitude as stance width changed, suggest that the same postural synergy was only slightly modified for a change in stance width. Nevertheless, the magnitude of the trunk displacement, as well as of CoP displacement, was modified based on the degree of passive stiffness in the musculoskeletal system, which increased with stance width. The change from a more passive to an active horizontal force constraint, to larger EMG magnitudes especially in the trunk muscles and larger trunk and CoP excursions in narrow stance are consistent with a more effortful response for equilibrium control in narrow stance to perturbations in all directions.     

Strategies that simplify the control of quadrupedal stance. I. Forces at the ground

1. Postural reactions were studied in six cats subjected to small, linear translations of the supporting surface in each of 16 different directions in the horizontal plane. Directions were specified in a polar coordinate system, with posterior translations being 0 degrees and leftward translations, 90 degrees. The data consisted of the forces exerted by each paw of the cat against the ground, measured in three orthogonal directions, vertical (z-axis), longitudinal (y-axis), and lateral (x-axis). 2. The force traces were analyzed by measuring the area under the curve during the postural reaction and dividing by the time of integration to give an average change in force. These values were normalized and plotted against direction of translation in polar coordinates, to give force tuning curves. The longitudinal and lateral force components were combined to generate force vectors in the horizontal plane. 3. Every cat responded to the platform translations with the same, simple strategy in which each hindlimb actively produced a correction force vector in one of only two possible directions. Participation of the forelimbs in the horizontal plane correction was not obligatory. While the direction of each hindlimb force vector was invariant, the amplitude was modulated according to the direction of platform movement. The resultant force vector, that acts through the center of mass of the animal, was in a direction opposite to the platform movement and directly opposed the perturbation. By this strategy, the cat was able to correct for destabilizing movements of the supporting surface in any direction in the horizontal plane. 4. It is concluded that the generation of forces between the paws and the ground is a high-level parameter that is controlled by the nervous system in a task-dependent manner. By using the strategy of restricting these forces to a set of two direction-invariant vectors, the problem of maintaining stance in the face of horizontal plane disturbances is greatly simplified.     

Automatic postural responses in the cat: responses of hindlimb muscles to horizontal perturbations of stance in multiple directions

Summary The effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in cats. We recorded EMG activity in eight proximal and distal muscles of the hindlimb along with vertical forces imposed by the limbs in awake behaving cats while they stood on an hydraulic platform. Postural responses to motion of the platform in 16 different horizontal directions were recorded. Vertical force changes were consistent with passive shifts of the center of mass and active correction of stance by the animals. When the perturbation was in the sagittal plane, vertical force changes began about 65 ms following initial platform movement. When the perturbation contained a component in the lateral direction, latency for vertical force changes was about 25 ms and an inflection in the vertical force trace was observed at 65 ms. No EMG responses were observed with latencies that were short enough to account for the early force component and it was concluded that this force change was due to passive shifts of the center of mass. The amplitude of the EMG responses of each muscle recorded varied systematically as perturbation direction changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. No angular range of activation was oriented strictly in the A-P or lateral directions. Most muscles displayed angular ranges of activation that encompassed a range of less than 180°. Onset latencies of EMG responses also varied systematically with perturbation direction. The amplitude and latency relationships between muscles, which made up the organization of postural responses, also varied systematically as perturbation direction was changed. This result suggests that direction of perturbation determines organizational makeup of postural responses, and for displacements in the horizontal plane, is considered a continuous variable by the nervous system.     

Neural control of superficial and deep neck muscles in humans

Human neck muscles have a complex multi-layered architecture. The role and neural control of these neck muscles were examined in nine seated subjects performing three series of isometric neck muscle contractions: 50-N contractions in eight fixed horizontal directions, 25-N contractions, and 50-N contractions, both with a continuously changing horizontal force direction. Activity in the left sternocleidomastoid, trapezius, levator scapulae, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles was measured with wire electrodes inserted at the C(4)/C(5) level under ultrasound guidance. We hypothesized that deep and superficial neck muscles would function as postural and focal muscles, respectively, and would thus be controlled by different neural signals. To test these hypotheses, electromyographic (EMG) tuning curves and correlations in the temporal and frequency domains were computed. Three main results emerged from these analyses: EMG tuning curves from all muscles exhibited well-defined preferred directions of activation for the 50-N isometric forces, larger contractions (25 vs. 50 N) yielded more focused EMG tuning curves, and agonist neck muscles from all layers received a common neural drive in the range of 10-15 Hz. The current results demonstrate that all neck muscles can exhibit phasic activity during isometric neck muscle contractions. Similar oscillations in the EMG of neck muscles from different layers further suggest that neck motoneurons were activated by common neurons. The reticular formation appears a likely generator of the common drive to the neck motoneurons due to its widespread projections to different groups of neck motoneurons.     

Functionally complex muscles of the cat hindlimb

The biceps femoris (BF) muscle is divided into three neuromuscular compartments defined by the innervation patterns of the main nerve branches (English and Weeks 1987). The goals of this study were i) to determine how different regions of the biceps femoris muscle are activated in the intact cat during a broad range of limb movements evoked by perturbations of stance posture, and ii) to determine the relationship between the anatomical compartments of biceps femoris and the functional units as defined in this task. Cats were trained to stand on a moveable platform with each paw on a triaxial force plate. The animal's stance was perturbed by linear translation of the platform in each of sixteen different directions in the horizontal plane. EMG activity was recorded from eight sites across the width of the left biceps femoris muscle. During quiet stance only the anterior compartment was tonically active, presumably contributing to hip extensor torque in the maintenance of stance. During platform translation, evoked EMG activity was recorded from each electrode pair for a wide range of directions of perturbation; as direction changed progressively, the amplitude of evoked activity from any electrode pair increased to a maximum and then decreased. When the EMG amplitude was plotted in polar coordinates as a function of translation direction, the region of response formed a petal shaped area in the horizontal plane, termed the EMG tuning curve. The compartments of the BF muscle were not activated homogeneously. The tuning curve of the anterior BF compartment was similar to that of other hip extensors, and coincided with the region of postero-lateral force production by the hindlimb against the support. The tuning curve of the middle BF compartment was shifted in a counterclockwise direction from that of the anterior compartment, but overlapped extensively with it; the middle BF tuning curve was similar to that of anterior gracilis. The tuning curve of the posterior biceps compartment was rotated further counterclockwise and overlapped very little with that of the middle BF compartment. The posterior BF was activated in a pattern similar to that of other knee flexors. The functional units of BF activation were not identical with the neuromuscular compartments defined by the main nerve branches. As direction of the perturbation changed, the region of BF that was activated moved progressively across the muscle. This progression of the active region was continuous across BFa and BFm, whereas there was a jump, or discontinuity at the border between BFm and BFp.(ABSTRACT TRUNCATED AT 400 WORDS)     

Directional invariance during loading-related modulations of muscle activity: evidence for motor equivalence

In the present study, we investigated the influence of external force manipulations on movements in different directions, while keeping the amplitude invariant. Subjects (n=10) performed a series of cyclical anteroposterior, mediolateral, and oblique line-drawing movements (star drawing task) with their dominant limb in the horizontal plane. To dissociate kinematics from the underlying patterns of muscle activation, spring loading was applied to the forearm of the moving limb. Whereas spring loading of the arm resulted in considerable changes in the overall amount of muscle activation in the elbow and shoulder muscles, invariance was largely maintained at the kinematic level. Subjects produced the required movement directions and amplitudes of the star drawing largely successfully, irrespective of the force bias induced by the spring. These observations demonstrate motor equivalence and strengthen the notion that the spatial representation of drawing movements is encoded in the higher brain regions in a rather abstract form that is dissociated from the concrete muscle activation patterns underlying a particular movement direction. To achieve this goal, the central nervous system shifted between two or more muscle grouping strategies to overcome modulations in the interaction among posture-dependent (joint stiffness), dynamic (inertial), and elastic (spring) torque components in the joints. Spring loading induced general changes in the overall amount of EMG activity, which was largely muscle but not direction specific, presumably to represent the posture-dependent biasing force of the spring. Loading was mainly shown to increase muscle coactivation in the elbow joint. This indicates that the subjects tended to increase stiffness in the elbow to compensate for changes in the spring bias forces in order to minimize trajectory errors. Changes in muscle grouping of the shoulder antagonists were mainly a consequence of movement direction but were also affected partly by loading, presumably reflecting the influence of dynamic force components. Taken together, the results confirmed the hypothesis that changes of movement direction and direction of force in the end-effector generated specific sets of muscle grouping to overcome the dynamic requirements in the joints while keeping the kinematics largely unchanged. This suggests that directional tuning in muscle activity and changes in muscle grouping reflects the formation of appropriate internal models in the CNS that give rise to motor equivalence. Electronic Publication     

Force path curvature and conserved features of muscle activation

This study examined the patterns of muscle activity that subserve the production of dynamic isometric forces in various directions. The isometric condition provided a test for basic features of neuromuscular control, since the task was analogous to reaching movement, but the behavior was not necessarily shaped by the anisotropy of inertial and viscoelastic resistance to movement. Electromyographic (EMG) activity was simultaneously recorded from nine elbow and/or shoulder muscles, and force pulses, steps, and ramps were monitored using a transducer fixed to the constrained wrists of human subjects. The force responses were produced by activating shoulder and elbow muscles; response direction was controlled by the relative intensity of activity in muscles with different mechanical actions. The primary objective was to characterize the EMG temporal pattern. Ideally, synchronous patterns of phasic muscle activation (and synchronous dynamic elbow and shoulder torques) would result in a straight force path; asynchronous muscle activation could result in substantial force path curvature. For both pulses and steps, asynchronous muscle activation was observed and was accompanied by substantial force path curvature. A second objective was to compare phasic and tonic EMG activity. The spatial tuning of EMG intensity was similar for the phasic and tonic activities of each muscle and also similar to the spatial tuning of tonic activity in a previous study where the arm was stationary but unconstrained.     

Postural responses in the cat to unexpected rotations of the supporting surface: evidence for a centrally generated synergic organization

Summary Postural reactions to disruptions of stance are rapid and automatic in both quadrupeds and bipeds. Current evidence suggests that these postural responses are generated by the central nervous system as patterns involving muscle synergies. This study attempted to test this hypothesis of a centrally generated postural mechanism by determining whether the same postural response could be evoked in the freely-standing cat under two different biomechanical conditions. The present work is an extension of previous experiments in which the stance of cats was perturbed by a horizontal translation of the supporting surface in the anterior and posterior directions (Rushmer et al. 1983). We now tested whether simple rotation of the metacarpo- and metatarsophalangeal (M-P) joints that mimics the digit rotation occurring during platform translation, was sufficient to evoke the translation postural response. The rotational perturbations were biomechanically different from translations in that the rotation did not cause displacement of the centre of mass of the animal, nor did it result in any significant movement about any but the M-P joints. Even so, rotational perturbations did evoke the appropriate translational muscle synergies in all four animals. Both plantar flexion rotation and headward translation activated the posterior hindlimb synergy (which included gluteus medius, semitendinosus and lateral gastrocnemius). Similarly, dorsiflexion rotation and tailward translation both activated the same anterior hindlimb synergy (iliopsoas, vastus lateralis and tibialis anterior) together with the forelimb synergy. The postural responses elicited by rotational perturbations were biomechanically inappropriate, and caused the animal to displace its own centre of mass away from the stable, control position. The most striking finding was that the group of muscles in which the medium latency postural response was evoked was different than the group from which short latency reflex responses were elicited. These data support the hypothesis that postural reactions are not merely reflex responses to local sensory inputs associated with the perturbation but, instead, represent a centrally generated response, with the muscle synergy being the controlled unit.Supported by NIH grants NS19484 and RR05593 as well as Good Samaritan Hospital     

Human automatic postural responses: responses to horizontal perturbations of stance in multiple directions

Summary The effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in human subjects. We recorded EMG activity from eight proximal and distal muscles acting on joints of the legs and hip known to be involved in postural corrections, while subjects stood on an hydraulic platform. Postural responses to horizontal motion of the platform in 16 different directions were recorded. The amplitude of the EMG responses of each muscle studied varied continuously as perturbation direction was changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. There were several differences in the response characteristics of the proximo-axial muscles as opposed to the distal ones. Angular ranges of activity of the distal muscles were unipolar and encompassed a range of less than 180°. These muscles responded with relatively constant onset latencies when they were active. Proximo-axial muscles, acting on the upper leg and hip showed larger angular ranges of activation with bimodal amplitude distributions and/ or onset latency shifts as perturbation direction changed. While there were indications of constant temporal relationships between muscles involved in responses to perturbations around the sagittal plane, the onset latency relationships for other directions and the response amplitude relationships for all directions varied continuously as perturbation direction was changed. Responses were discrete in that for any particular perturbation direction there appeared to be a single unique response. Thus, while the present results do not refute the hypothesis that automatic postural responses may be composed of mixtures of a few elemental synergies, they suggest that composition of postural responses is a complex process that includes perturbation direction as a continuous variable.     

Muscle synergy organization is robust across a variety of postural perturbations

We recently showed that four muscle synergies can reproduce multiple muscle activation patterns in cats during postural responses to support surface translations. We now test the robustness of functional muscle synergies, which specify muscle groupings and the active force vectors produced during postural responses under several biomechanically distinct conditions. We aimed to determine whether such synergies represent a generalized control strategy for postural control or if they are merely specific to each postural task. Postural responses to multidirectional translations at different fore-hind paw distances and to multidirectional rotations at the preferred stance distance were analyzed. Five synergies were required to adequately reconstruct responses to translation at the preferred stance distance—four were similar to our previous analysis of translation, whereas the fifth accounted for the newly added background activity during quiet stance. These five control synergies could account for >80% total variability or r² > 0.6 of the electromyographic and force tuning curves for all other experimental conditions. Forces were successfully reconstructed but only when they were referenced to a coordinate system that rotated with the limb axis as stance distance changed. Finally, most of the functional muscle synergies were similar across all of the six cats in terms of muscle synergy number, synergy activation patterns, and synergy force vectors. The robustness of synergy organization across perturbation types, postures, and animals suggests that muscle synergies controlling task-variables are a general construct used by the CNS for balance control.     

Stance dependence of automatic postural adjustments in humans

Summary This study investigated the effect of initial stance configuration on automatic postural responses in humans. Subjects were tested in both bipedal and quadrupedal stance postures. The postural responses to horizontal translations of the supporting surface were measured in terms of the forces at the ground, movement of the body segments, and electromyographic (EMG) activity. Postural responses to the same perturbations changed with initial stance posture; these responses were biomechanically appropriate for restoring centre of mass. A change in stance configuration prior to platform movement led to a change in both the spatial and temporal organization of evoked muscle activation. Specifically, for the same direction of platform movement, during bipedal stance muscles on one side of the lower limb were activated in a distal to proximal sequence; during quadrupedal stance, muscles on the opposite side of the lower limb were activated and in a proximal to distal sequence. The most significant finding was an asymmetry in the use of the upper limbs and the lower limbs during postural corrections in quadrupedal stance. Whereas antagonists of the upper limb were either co-activated or co-inhibited, depending on the direction of translation, lower limb antagonists were reciprocally activated and inhibited. Human subjects in a quadrupedal stance posture used the lower limbs as levers, protracting or retracting the hips in order to propel the trunk back to its original position with respect to the hands and feet. Postural responses of the subjects during quadrupedal stance were remarkably similar to those of cats subjected to similar perturbations of the supporting surface. Furthermore, the same predominance of lower limb correction is characteristic of both species, suggesting that the standing cat is a good model for studying postural control in humans.     

A limited set of muscle synergies for force control during a postural task

Recently developed computational techniques have been used to reduce muscle activation patterns of high complexity to a simple synergy organization and to bring new insights to the long-standing degrees of freedom problem in motor control. We used a nonnegative factorization approach to identify muscle synergies during postural responses in the cat and to examine the functional significance of such synergies for natural behaviors. We hypothesized that the simplification of neural control afforded by muscle synergies must be matched by a similar reduction in degrees of freedom at the biomechanical level. Electromyographic data were recorded from 8-15 hindlimb muscles of cats exposed to 16 directions of support surface translation. Results showed that as few as four synergies could account for >95% of the automatic postural response across all muscles and all directions. Each synergy was activated for a specific set of perturbation directions, and moreover, each was correlated with a unique vector of endpoint force under the limb. We suggest that, within the context of active balance control, postural synergies reflect a neural command signal that specifies endpoint force of a limb.     

Proprioceptive population coding of two-dimensional limb movements in humans: I. Muscle spindle feedback during spatially oriented movements

The proprioceptive coding of multidirectional ankle joint movements was investigated, focusing in particular on the question as to how accurately the direction of a movement is encoded when all the proprioceptive information from all the muscles involved in the actual movement is taken into account. During ankle movements imposed on human subjects, the activity of 30 muscle spindle afferents originating in the extensor digitorum longus, tibialis anterior, extensor hallucis longus and peroneus lateralis muscles was recorded from the lateral peroneal nerve using the microneurographic technique. In the first part of the study, it was proposed to investigate whether muscle spindle afferents have a preferred direction, as previously found to occur in the case of cortical cells, and to analyze the neural coding of the movement trajectories using a "population vector model." This model is based on the idea that neuronal coding can be analyzed in terms of a series of vectors, each based on specific movement parameters. In the present case, each vector gives the mean contribution of a population of muscle spindle afferents within one directionally tuned muscle. A given population vector points in the "preferred sensory direction" of the muscle to which it corresponds, and its length is the mean frequency of all the afferents within that muscle. Our working hypothesis was that the sum of these weighted vectors points in the same direction as the ongoing movement. The results show that each muscle spindle afferent, and likewise each muscle, has a specific preferred sensory direction, as well as a preferred sensory sector within which it is capable of sending sensory information to the central nervous system. Interestingly, the results also demonstrate that the preferred directions are the same as the directions of vibration-induced illusions. In addition, the results show that the neuronal population vector model describes the multipopulation proprioceptive coding of spatially oriented 2D limb movements, even at the peripheral sensory level, based on the sum vectors calculated from all the muscles involved in the movement. In an accompanying paper, the coding of more complex 2D movements such as those involved in drawing rectilinear and curvilinear geometrical shapes was investigated.     

Proprioceptive population coding of two-dimensional limb movements in humans: II. Muscle-spindle feedback during "drawing-like" movements

It was proposed to study the proprioceptive sensory coding of movement trajectories during the performance of two-dimensional "drawing-like" movements imposed on the tip of the foot. For this purpose, the activity of the muscle-spindle afferents from the Extensor digitorum longus, Tibialis anterior, Extensor hallucis longus, and Peroneus lateralis muscles was recorded from the lateral peroneal nerve using the microneurographic technique. The drawing movements, describing geometrical shapes such as squares, triangles, ellipses, and circles, were imposed at a constant velocity in both the clockwise and counterclockwise directions. A total number of 44 muscle-spindle afferents were tested, 36 of which were identified as primary and eight as secondary afferents. Whatever the shape of the imposed foot movement, the primary endings from one muscle never discharged throughout the whole trajectory (on average, they discharged for only 49.2% of the length of the trajectory), whereas all the secondary endings discharged for most part of the drawing trajectories (average: 84.8%). The relationship between afferent discharge rate and direction could be described with a cosine-shaped tuning function. The peak of this function corresponded to the preferred sensory direction of the receptor-bearing muscles. The whole path of a given geometrical drawing movement was found to be coded in turn by each of the primary afferents originating from each of the muscles successively stretched. The contribution of each population of muscle afferents from each ankle muscle was represented by a "population vector", whose orientation was the preferred direction of the muscle under consideration and whose length was the mean instantaneous frequency of the afferent population. The "sum vector" corresponding to the sum of all these weighted "population vectors" was found to point in the instantaneous direction of the drawing trajectory, i.e., the tangent to the trajectory. These findings suggest that trajectory information is already encoded at the peripheral level on the basis of the integrated inputs provided by sets of receptors belonging to all the muscles acting on a given joint.     

Electromyographic responses to a mechanical perturbation applied during impending arm movements in different directions: one-joint and two-joint conditions

Directional tuning is a common finding for many physiological features of arm movements and related neuronal activity. We investigated whether the electromyographic response to a brief (30 ms) torque perturbation prior to voluntary movement depends on the direction of the impending movement. Pointing movements with the elbow joint alone and those involving both the shoulder and elbow joints were studied in separate experiments. Target direction was varied between flexion and extension for the one-joint experiments and among four spatial directions for the two-joint experiments. Movement trials in which a perturbation stretched the flexor muscles just prior to the pointing movement were randomly interspersed among unperturbed movement trials in each experiment. A small pre-load ensured some background activity of the flexor muscles. Results were remarkably similar for the one- and two-joint conditions. The short-latency reflex response of the stretched muscles (in a 30-60 ms window after perturbation onset) was not modulated with direction of target-reaching movement in a statistically significant manner, which confirms earlier findings for one-joint movements and extends these to the two-joint condition. Beyond the short-latency window, the perturbation provoked earlier onsets of target-reaching muscle activities for the agonist muscles, whether or not the muscle had been stretched by the perturbation. The onset of the braking activity of the antagonist muscles also occurred earlier in the presence of the brief perturbation prior to movement, irrespective of whether the muscle had been stretched or not. The magnitude of target-reaching muscle activity, in general, was greater for the perturbed trials, though not consistently for all muscles or all directions. These results suggest that, when movement is about to be initiated, in either single- or multi-joint conditions, the long-latency effects of the stretch strongly depend on the intended direction of movement. The dependence is such that the response serves to hasten and augment the intended movement, but not necessarily to oppose the perturbation.     

Control of the wrist in three-joint arm movements to multiple directions in the horizontal plane

In a reaching movement, the wrist joint is subject to inertial effects from proximal joint motion. However, precise control of the wrist is important for reaching accuracy. Studies of three-joint arm movements report that the wrist joint moves little during point-to-point reaches, but muscle activities and kinetics have not yet been described across a range of movement directions. We hypothesized that to minimize wrist motion, muscle torques at the wrist must perfectly counteract inertial effects arising from proximal joint motion. Subjects were given no instructions regarding joint movement and were observed to keep the wrist nearly motionless during center-out reaches to directions throughout the horizontal plane. Consistent with this, wrist muscle torques exactly mirrored interaction torques, in contrast to muscle torques at proximal joints. These findings suggest that in this reaching task the nervous system chooses to minimize wrist motion by anticipating dynamic inertial effects. The wrist muscle torques were associated with a direction-dependent choice of muscles, also characterized by initial reciprocal activation rather than initial coactivation to stiffen the wrist joint. In a second experiment, the same pattern of muscle activities persisted even after many trials reaching with the wrist joint immobilized. These results, combined with similar features at the three joints, such as cosine-like tuning of muscle torques and of muscle onsets across direction, suggest that the nervous system uses similar rules for muscles at each joint, as part of one plan for the arm during a point-to-point reach.     

Activation of intrinsic and extrinsic finger muscles in relation to the fingertip force vector

Surface EMG was recorded from two intrinsic and two extrinsic muscles of the index finger during a two-dimensional isometric force task in the plane of flexion and extension. Subjects applied force isometrically at the fingertip in eight equally spaced directions, encompassing 360 degrees. Target forces spanned the range from 20% to 50% of maximum for each direction. The effect of varying the metacarpophalangeal (MCP) and interphalangeal (IP) joint angles was investigated. We found that when applying isometric force with the fingertip, the intrinsic muscles of the index finger behaved as a single unit whose region of activation overlapped that of the extrinsic flexor and extensor muscles. The activation region of the intrinsic muscles also spanned a range of force directions for which the extrinsic muscles were virtually inactive. The activation of all muscles, with the exception of the extrinsic extensor, was modified by changing the MCP and IP joint angles. Both IP flexion and MCP extension produced rotation of the resultant activity vector in the direction of MCP flexion. However, the relative rotation was much greater with IP flexion than MCP extension. The effect of IP flexion is linked to rotation of the force direction where joint torque switches from extension to flexion, while the effect of MCP extension is more likely related to changes in muscle length and MCP moment arm. Our results suggest that the primary role of intrinsic finger muscles is to precisely control the direction of fingertip force, while extrinsic muscles provide stability of the joints.     

The force constraint strategy for stance is independent of prior experience

The purpose of this study was to examine the effect of prior experience concerning direction of a postural perturbation on the balance response of cats to translations of their support surface. Previous work has shown that, when cats are translated in many directions in the horizontal plane, they respond by exerting active forces with each paw in only two directions, termed the force constraint strategy. This study examined whether the force constraint strategy could be modified based on predictability of the direction of translation and whether this strategy is used by the naive animal with no prior experience of platform translation. Four cats were trained to stand quietly on the force platform using positive reinforcement, and then were implanted with chronically indwelling electrodes for recording electromyographic (EMG) activity. The first experiment concerned the response of the naive cats to their first exposure to platform translation and consisted of translations presented randomly in four different directions in the horizontal plane. The second experiment consisted of two complete sets of 16 directions of translation (15 trials per direction), with the direction of translation randomized in one set and serially ordered in the other, to make the direction of translation unpredictable or predictable, respectively. Forces exerted by the cat, EMG activity, and platform position were recorded during the 1-s trials. The use of the force constraint strategy was independent of prior experience with direction of translation, as was the amplitude of the response. Moreover, this strategy was observed in the naive cat. These findings suggest that the force constraint is a robust and consistent response to translational perturbations of stance in the cat and is part of its natural behavioral repertoire. The accuracy in specification of the direction of a postural response must be based on the sensory information that is obtained within a very short time after the onset of platform acceleration (loop time 40–70 ms). On the other hand, the amplitude of the postural response tended to decrease with experience and practice, suggesting a long-term change in central set that may manifest as a reduction in sensorimotor gain.     

Multijoint muscle regulation mechanisms examined by measured human arm stiffness and EMG signals

Stiffness properties of the musculo-skeletal system can be controlled by regulating muscle activation and neural feedback gain. To understand the regulation of multijoint stiffness, we examined the relationship between human arm joint stiffness and muscle activation during static force control in the horizontal plane by means of surface electromyographic (EMG) studies. Subjects were asked to produce a specified force in a specified direction without cocontraction or they were asked to keep different cocontractions while producing or not producing an external force. The stiffness components of shoulder, elbow, and their cross-term and the EMG of six related muscles were measured during the tasks. Assuming that the EMG reflects the corresponding muscle stiffness, the joint stiffness was predicted from the EMG by using a two-link six-muscle arm model and a constrained least-square-error regression method. Using the parameters estimated in this regression, single-joint stiffness (diagonal terms of the joint-stiffness matrix) was decomposed successfully into biarticular and monoarticular muscle components. Although biarticular muscles act on both shoulder and elbow, they were found to covary strongly with elbow monoarticular muscles. The preferred force directions of biarticular muscles were biased to the directions of elbow monoarticular muscles. Namely, the elbow joint is regulated by the simultaneous activation of monoarticular and biarticular muscles, whereas the shoulder joint is regulated dominantly by monoarticular muscles. These results suggest that biarticular muscles are innervated mainly to control the elbow joint during static force-regulation tasks. In addition, muscle regulation mechanisms for static force control tasks were found to be quite different from those during movements previously reported. The elbow single-joint stiffness was always higher than cross-joint stiffness (off-diagonal terms of the matrix) in static tasks while elbow single-joint stiffness is reported to be sometimes as small as cross-joint stiffness during movement. That is, during movements, the elbow monoarticular muscles were occasionally not activated when biarticular muscles were activated. In static tasks, however, monoarticular muscle components in single-joint stiffness were increased considerably whenever biarticular muscle components in single- and cross-joint stiffness increased. These observations suggest that biarticular muscles are not simply coupled with the innervation of elbow monoarticular muscles but also are regulated independently according to the required task. During static force-regulation tasks, covariation between biarticular and elbow monoarticular muscles may be required to increase stability and/or controllability or to distribute effort among the appropriate muscles.     

Dynamic transitions in stance support accompanying leg flexion movements in man

Summary The control processes underlying dynamic transitions in stance support during single leg flexion movements were investigated in human subjects as a function of the intended speed of movement, by examining the vertical and lateral horizontal components of the ground reaction forces, the frontal plane trajectory of the body center of mass (CM) recorded via motion analysis, and the electromyographic (EMG) recordings of selected lower limb muscles. For the slowest movements, the measured vertical force beneath the flexing and single stance limbs closely matched the vertical force-time history predicted by a quasi-static mechanical model, whereas, the more rapid natural and fast speeds showed progressively larger discrepancies between measured and predicted forces. The initial resultant horizontal force component was exerted in the flexing to stance limb direction but was proportionately greater (41) beneath the flexing versus the stance limb during fast and natural speeds, and became equivalent for slow movements. Speed related EMG differences included an early phasic recruitment of the lateral hip muscle of the flexing limb which always preceded the ground reaction force changes for fast and natural but not slow movements, and a considerably earlier onset of the stance leg knee extensor relative to the flexing limb knee flexor for slow versus fast and natural speeds. Overall, the findings suggested two different speed related strategies for linking the postural and intentional movement components, where the choice of the strategy selected appeared to reflect the mechanical requirements needed to overcome the inertial force of the body mass during transitions from bipedal to single limb stance support.