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.     

The preferred sensory direction of muscle spindle primary endings influences the velocity coding of two-dimensional limb movements in humans

The present study compares how accurately two different but close velocities of movement are discriminated by populations of muscle spindle primary afferents whether or not one takes into account the direction of the movement and the preferred sensory directions of the units (i.e., the direction of movement to which the afferents are the most sensitive). The activities of 26 muscle spindle primary endings originating from the tibialis anterior, the extensor digitorum longus, the extensor hallucis longus, and the peroneus lateralis muscles were recorded in the lateral peroneal nerve. Their responses to movements imposed at two velocities (12.5 and 18 mm/s) were analyzed. These movements were straight-line movements imposed in eight directions and circular movements in both clockwise and anticlockwise directions. The encoding of the movement velocity was analyzed in two ways. First, the mean frequencies of discharge of the muscle spindle afferents were compared for the two velocities. Second, the data were analyzed using a "neuronal population vector model." This model is based on the idea that such neuronal coding can be analyzed in terms of a series of population vectors (i.e., mean contribution of all the muscle spindle afferents within one directionally tuned muscle) and by finally calculating a sum vector. The results showed no clear and consistent difference in the response frequency of the muscle spindle afferents for the two velocities of movement compared. Rather, the most consistently significant differences between the two velocities were in the lengths of the sum vectors. It is concluded that the encoding of two-dimensional movement velocity relies on populations of muscle spindle afferents coming from the whole set of muscles surrounding a particular joint, each muscle making an instantaneous, oriented, and weighted contribution to the sensory coding of the kinematics parameters.     

Proprioceptive feedback in humans expresses motor invariants during writing

Proprioceptive feedback from populations of muscle spindle afferents feeds the brain with information relating to the instantaneous velocity and direction of ongoing movements. In this paper, we investigate whether the invariant relationship between the velocity and curvature of a trajectory, i.e. the two-thirds power law, is reflected in this muscle spindle feedback. Sixty unitary muscle spindle afferents from six ankle muscle groups were recorded using intraneural microelectrodes during imposed writing-like movements. The movements had kinematic parameters obeying the two-thirds power law and were imposed so that the tip of the foot followed trajectories forming four different letters and six numbers. The responses of the muscle spindle afferent populations were analysed using the population vector model. The results demonstrate that the neuronal trajectories attained from populations of muscle spindles clearly depict the path and kinematic parameters and express the movement invariants, i.e. the trajectory segmentation into units of action and the two-thirds power law. The central vs peripheral origin of such constraints involved in the motor system is discussed.     

Movement speed effects on limb position drift

Previous research has shown that even when limb position drifts considerably during continuous blind performance, the topological and metrical properties of generated hand paths remain remarkably invariant. We tested two possible accounts of this intriguing effect. According to one hypothesis, position drift is due to degradation of limb-position information. This hypothesis predicted that drift of static hand positions at movement reversals should not depend on movement speed. According to the other hypothesis, position drift is due to degradation of movement information. This hypothesis predicted that drift of static hand positions at movement reversals should vary with movement speed. We tested these two hypotheses by varying the required movement speed when normal human adults performed back-and-forth manual positioning movements in the absence of visual feedback. Movement distance and direction were well preserved even though hand positions between movements drifted considerably. In accord with the movement error hypothesis, but not in accord with the position hypothesis, the rate at which hand positions drifted depended on movement speed. The data are consistent with the idea that hand position, which defines the origin of the trajectory control coordinate system, and movement trajectory are controlled by distinct neural mechanisms.     

Human involuntary postural aftercontractions are strongly modulated by limb position

Involuntary muscle activations called aftercontractions occur in skeletal muscles following sustained voluntary contractions. They are strongest following high-force voluntary contractions in proximal muscles. Their mechanism is unknown. Some authors have hypothesised that they are dependent on proprioceptive feedback; others believe that they are independent of such influences. These experiments tested this hypothesis by examining the effect of shoulder joint excursion magnitude and direction on aftercontraction amplitude. A 1-min maximal isometric voluntary abduction of the shoulder joint was used to evoke a postural involuntary aftercontraction in the deltoid muscle. During the 20-s aftercontraction which followed the arm was allowed to abduct in the coronal plane and active muscle shortening took place. The maximum amplitude of EMG activity during the aftercontraction of the deltoid muscle was equal to 20–50% of the EMG amplitude of the maximal voluntary contraction. The aftercontraction EMG amplitude grew as the angle of shoulder joint abduction increased. This growth ceased and the activity levelled off if arm movement was blocked. The results showed that the final EMG amplitude reached depended linearly on the final shoulder angle allowed—it did not grow purely as a function of time. Forcible adduction of the arm by the experimenter and consequent lengthening of the muscle caused the EMG of the aftercontraction to fall with decreasing shoulder joint angle. It is concluded that the neural centres controlling the involuntary aftercontraction are strongly modulated by proprioceptive feedback. Results are given as mean (SD) unless otherwise stated.     

Temporal discrimination of two passive movements in humans: a new psychophysical approach to assessing kinaesthesia

Percutaneous electrical stimulation of the motor point of the first dorsal interosseous muscle (FDI) was used to produce a non-painful contraction of the FDI muscle that caused index finger abduction movement but no radiating cutaneous paraesthesias or sharp sensations localized to joints. Pairs of stimuli separated by different time intervals were given and subjects were asked to report whether they perceived a single or a double index finger abduction movement. The threshold value was the shortest interval for which the subjects reported two separate index finger abduction movements. Temporal discrimination movement thresholds (TDMT) were measured for both right and left hand. To assess the possible role of muscle and cutaneous afferents in temporal discrimination, we investigated the effects of high-frequency (20 Hz) electrical stimulation of the right ulnar and radial nerves on TDMT. In humans, muscle afferents from FDI are supplied by the ulnar nerve whereas the cutaneous territory overlying the muscle and joint is supplied by the radial and median nerves. Threshold values were not significantly different for right (75.1 ms) and left (75.6 ms) hands. During ulnar and to a lesser extent during radial nerve stimulation, TDMT values were significantly increased (119.2 and 93.5 ms, respectively) compared with baseline conditions (78.0 ms) whereas no changes were observed during median nerve stimulation (80.5 ms). These results suggest that muscle, and in part cutaneous, afferents contribute to temporal discrimination of a dual movement. The technique may provide a useful way of measuring temporal discrimination of kinaesthetic inputs in humans.     

Kinaesthetic role of muscle afferents in man,studied by tendon vibration and microneurography

Summary The characteristics of vibration-induced illusory joint movements were studied in healthy human subjects. Unseen by the subject, constant frequency vibration trains applied to the distal tendon of the Triceps or Biceps induced an almost constant velocity illusory movement of the elbow whose direction corresponded to that of a joint rotation stretching the vibrated muscle. Vibration trains of the same duration and frequency applied alternatively to the Biceps and Triceps evoked alternating flexion-extension illusory movements.During successive application of vibration trains at frequencies from 10 to 120 Hz, the perceived velocity of the illusory movements increased progressively from 10 to 70–80 Hz, then decreased from 80 to 120 Hz. The maximal perceived velocity was three times higher during alternating vibration of the Biceps and Triceps than during single muscle stimulation.Unit activity from 15 muscle spindle primary endings and five secondary endings located in Tibialis anterior and Extensor digitorum longus muscles were recorded using microneurography in order to study their responses to tendon vibration and passive and active movements of the ankle.Primary endings were all activated by low amplitude tendon vibration (0.2–0.5 mm) previously used to induce illusory movements of the elbow. The discharge of some was phase-locked with the vibration cycle up to 120 Hz, while others responded one-to-one to the vibration cycle up to 30–50 Hz, then fired in a sub-harmonic manner at higher frequencies. Secondary endings were much less sensitive to low amplitude tendon vibration.Primary and secondary ending responses to ramp and sinusoïdal movements of the ankle joint were compared. During the movement, the primary ending discharge frequency was almost constant, while the secondary ending activity progressively increased. During ankle movements the primary ending discharge appeared mainly related to velocity, while some secondary activities seemed related to both movement velocity and joint angle position.Muscle spindle sensory ending responses to active and passive ankle movements stretching the receptor-bearing muscle (plantar flexion) were qualitatively and quantitatively similar. During passive reverse movements (dorsiflexion) most of the sensory endings stopped firing when their muscle shortened. Active muscle shortening (isotonic contraction) modulated differently the muscle spindle sensory ending discharge, which could stop completely, decrease or some times increase during active ankle dorsiflexion. During isometric contraction most of the muscle spindle sensory endings were activated.The characteristics of the vibration-induced illusory movements and the muscle spindle responses to tendon vibration and to active and passive joint movements strengthened the possibility of the contribution of primary endings to kinaesthesia, as suggested by several previous works. Moreover, the present results led us to attribute to proprioception in the muscle stretched during joint movement a predominant, but not exclusive, role in this kind of perception.     

Proprioceptive target matching asymmetries in left-handed individuals

In right-handers, the ability to reproduce proprioceptive targets has been shown to be asymmetric, favoring the non-preferred left arm. The present study sought to determine whether a similar arm/hemisphere asymmetry exists for left-handers. Ten strong left-handed adults used the left or right arm to perform proprioceptive target matching tasks that varied in processing demands (i.e., need for memory, interhemispheric transfer) and target amplitude (20, 40°). Similar to right-handers, left-handed individuals had smaller total errors when matching with the non-preferred arm. This asymmetry was greatest in conditions with increased processing demands and larger amplitude targets. These results provide the first evidence to date of right arm/left hemisphere dominance for proprioceptive target matching in left-handers that is the “mirror image” of right-handers.     

Responses in muscle sympathetic nerve activity to sustained hand-grips of different tensions in humans

Summary To clarify whether sympathetic nerve activity increases in relation to the tension of a sustained muscle contraction, muscle sympathetic nerve activity (MSA) was recorded directly from the peroneal nerve fascicle at the popliteal fossa by means of tungsten microelectrodes in five healthy male subjects. A sustained muscle contraction was performed by handgrip for two minutes in a supine position at tensions of 10, 30 and 45% of maximal grip strength (MGS). MSA, electrocardiogram (ECG) using bipolar electrodes from the chest and surface electromyogram (EMG) from the extensor pollicis longus were recorded simultaneously before and during the sustained handgrip. Arterial blood pressure was measured at the resting upper arm by auscultation. During handgrip with tensions of 10, 30 and 45% MGS, average MSA burst rate (bursts · min⁻¹) increased to 122, 152 and 230% of the resting value, respectively. During the same experimental procedures with tensions of 10, 30 and 45% MGS, average heart rate increased to 105, 110 and 111% of the resting value. These results confirm that sympathetic outflow to a resting muscle is increased with elevation of tension in an active muscle. This process would promote perfusion pressure in the active muscle.     

Control of single-joint movements in deafferented patients: evidence for amplitude coding rather than position control

Two deafferented patients and several control subjects participated in a series of experiments to investigate how accurate single-joint movements are programed, spatially calibrated, and updated in the absence of proprioceptive information. The deafferented patients suffered from a permanent and severe loss of large sensory myelinated fibers below the neck. Subjects performed, with and without vision, sequences of forearm supinations and pronations with two temporal delays between each movement (0 s and 8 s). Overall, the lack of proprioception did not yield any significant decrease in movement accuracy when vision was available. Without vision, the absence of proprioceptive afferents yielded (1) significantly larger spatial errors, (2) amplitude errors similar to those of control subjects, and (3) a significant drift when an 8-s delay was introduced between two successive movements. Subjects also performed, without vision, a 20 supination followed by a 20 pronation that brought back the wrist to the starting position. On some trials, the supination was blocked unexpectedly by way of a magnetic brake. When the supination was blocked, subjects were already on the second target and no pronation was required when the brake was released. The defferented patients, unaware of the procedure, always produced a 20 pronation. These data confirm that deafferented patients were not coding a final position. It rather suggests that they coded an amplitude and translated the spatial distance between the two targets in a corresponding force pulse. Overall, the results highlight the powerful and key role of proprioceptive afferents for calibrating the spatial motor frame of reference.     

Tongue-placed tactile biofeedback suppresses the deleterious effects of muscle fatigue on joint position sense at the ankle

Whereas the acuity of the position sense at the ankle can be disturbed by muscle fatigue, it recently also has been shown to be improved, under normal ankle neuromuscular state, through the use of an artificial tongue-placed tactile biofeedback. The underlying principle of this biofeedback consisted of supplying individuals with supplementary information about the position of their matching ankle position relative to their reference ankle position through electrotactile stimulation of the tongue. Within this context, the purpose of the present experiment was to investigate whether this biofeedback could mitigate the deleterious effect of muscle fatigue on joint position sense at the ankle. To address this objective, sixteen young healthy university students were asked to perform an active ankle-matching task in two conditions of No-fatigue and Fatigue of the ankle muscles and two conditions of No-biofeedback and Biofeedback. Measures of the overall accuracy and the variability of the positioning were determined using the absolute error and the variable error, respectively. Results showed that the availability of the biofeedback allowed the subjects to suppress the deleterious effects of muscle fatigue on joint position sense at the ankle. In the context of sensory re-weighting process, these findings suggested that the central nervous system was able to integrate and increase the relative contribution of the artificial tongue-placed tactile biofeedback to compensate for a proprioceptive degradation at the ankle.     

Flexor bias of joint position in humans during spaceflight

The ability to estimate ankle and elbow joint position was tested before, during, and after a 17-day spaceflight. Subjects estimated targeted joint angles during isovelocity (IsoV) joint movements with agonist muscle groups either active or relaxed. These movements included elbow extension (EE) and elbow flexion (EF), and plantarflexion (PF) and dorsiflexion (DF) of the ankle. Subjects also estimated these joint positions while moving the dynamometer at their chosen (variable) velocity (VarV) during EE and PF. For IsoV tests, no differences were observed between active and passive movements for either the ankle or elbow. Compared with those of pre-flight test days, estimates of targeted elbow joint angles were ~5° to 15° more flexed in-flight, and returned toward the pre-flight values during recovery. The spaceflight effects for the ankle were inconsistent and less prevalent than those for the elbow. The VarV PF test condition for the 120° target angle at the ankle exhibited ~5° to 7° more DF target angle estimates in-flight compared with those pre- or post-flight. In contrast, during IsoV PF there was a tendency for ankle estimates to be ~2° to 3° more PF after 2–3 days exposure to spaceflight. These data indicate that during spaceflight the perception of elbow extension is greater than actuality, and are consistent with the interpretation that microgravity induced a flexor bias in the estimation of the actual elbow joint position. Moreover, these effects in joint proprioception during spaceflight were observed in individual isolated single-joint movements during tasks in which vestibular function in maintaining posture were minimal.     

Morphology of action potentials recorded from human nerves using microneurography

This study investigated the morphology of action potentials and the frequency of occurrence of the various waveforms encountered when using microneurography to record single-unit muscle afferent activity in humans. With 75% of the afferents recorded in this study (55 of 73 afferents), action potentials had a doublepeaked morphology. For action potentials with an initial, positive double peaked morphology, the relevant afferent conducts impulses past the microelectrode, with the second peak representing current fluctuations at the node of Ranvier proximal to the electrode. Accordingly, in the majority of recordings, the afferent is capable of conducting impulses to the spinal cord. The mean interpeak interval for these double-peaked units was 168 s (range 90–310 s). This represents marked prolongation of conduction time across the impaled internode. When the interpeak interval was relatively short (90–120 s), the double peaked morphology could be recognized only if the low pass filter was high (10 kHz). The probability of recording a double peaked unit was the same whether the recording was acquired early or late in a 3-h experiment. Conduction block developed in 6 of 73 single units during the recordings. These findings indicate that the majority of isolated single afferents and, indeed, the majority of afferents within the relevant fascicle are capable of transmitting impulses across the recording site, even though conduction across the impaled internode is slow. Conduction block due to direct injury or pressure is relatively uncommon.     

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level.

At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex.

Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation.

Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions.

As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited.

In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models.

Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters.

In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.     

The contribution of gravitational torques to limb position sense

Summary An experiment is reported which examined whether gravitational torque acting about a joint is used by the CNS in elbow joint angle matching. Subjects were required to match the joint angles of their two limbs while the external torques acting about each elbow were systematically varied. It was found that when the matching limb was differentially loaded, the error in the produced reference angle corresponded to the directional prediction of a proposed gravitational torque hypothesis. The data suggest that torque sensation is an accessory source of information in limb positioning.This research was partially supported by grants from NATO Scientific Affiars Division, RG82/0227 and US Public Health Service, NS17421 and AG05154 awarded to G. E. Stelmach     

Effects of body orientation, load and vibration on sensing position and movement at the human elbow joint

Experiments were carried out to study the ability of human subjects to match the position of their forearms relative to the horizontal. The normal, arms-in-front position with the hands aligned and little forward flexion at the shoulder was called the reference position. When the arms were rotated to the side, one arm was raised, or both arms were raised, matching ability deteriorated compared with the reference position, when expressed as an increase in the standard deviation of matching errors. It was concluded that particular significance was assigned by the brain to the arms-in-front position, with the hands in their normal working space. Increases in errors were also observed when the reference arm was made weightless or its weight was increased by means of an adjustable load. This suggested that lifting the arm against gravity provided additional positional information. In a second experiment, dependence of the illusion of muscle lengthening evoked by vibration was tested after two different forms of muscle conditioning, a co-contraction of elbow muscles with the arm held flexed or with it held extended. The speed of the illusory extension of flexor muscles during their vibration increased three-fold after flexion conditioning compared with extension conditioning. Since after flexion conditioning, muscle spindles in flexor muscles are expected to be more sensitive to vibration than after extension conditioning, this observation provides additional support for the view that muscle spindles make an important contribution to kinaesthesia at the elbow joint.     

The combined effect of muscle contraction history and motor commands on human position sense

Along with afferent information, centrally generated motor command signals may play a role in joint position sense. Isometric muscle contractions can produce a perception of joint displacement in the same direction as the joint would move if unrestrained. Contradictory findings of perceived joint displacement in the opposite direction have been reported. As this only occurs if muscle spindle discharge in the contracting muscle is initially low, it may reflect increased muscle spindle firing from fusimotor activation, rather than central motor command signals. Methodological differences including the muscle contraction task and use of muscle conditioning could underlie the opposing findings. Hence, we tested perceived joint position during two contraction tasks (‘hold force’ and ‘hold position’) at the same joint (wrist) and controlled muscle spindle discharge with thixotropic muscle conditioning. We expected that prior conditioning of the contracting muscle would eliminate any effect of increased fusimotor activation, but not of central motor commands. Muscle conditioning altered perceived wrist position as expected. Further, during muscle contractions, subjects reported wrist positions displaced ~12° in the direction of contraction, despite no change in wrist position. This was similar for ‘hold force’ and ‘hold position’ tasks and occurred despite prior conditioning of the agonist muscle. However, conditioning of the antagonist muscle did reduce the effect of voluntary contraction on position sense. The errors in position sense cannot be explained by fusimotor activation. We propose that central signals combine with afferent signals to determine limb position and that multiple sources of information are weighted according to their reliability.     

Illusions of forearm displacement during vibration of elbow muscles in humans

Position matching ability at the forearm in young adults was measured after arm muscles had been placed in a defined mechanical state, called conditioning. With flexion conditioning, elbow flexors were contracted isometrically with the arm held flexed; with extension conditioning, extensors were contracted with the arm held extended. When both arms were flexion conditioned, vibration of the reference biceps produced significant position matching errors as shown by placement of the indicator arm. When the reference arm was flexion conditioned and the indicator arm extension conditioned, vibration no longer produced significant errors. Vibrating elbow flexors of the indicator arm produced significant illusions in the opposite direction from illusions produced by vibrating flexors of the reference arm. These observations show that in an arm matching task the way in which muscles of both arms are conditioned can have an influence on matching performance, including the ability to indicate a perceived illusion.     

Proprioceptive sensation in rotation of the trunk

Summary Proprioceptive sensation in rotation of the trunk about a vertical axis was investigated in normal human subjects. Subjects pointed at the big toe with the nose to test the accuracy of positioning of the trunk. Active rotation of the head and shoulders on the stationary hips and legs to align the nose and toe, was not significantly more accurate than moving the hips, legs and toe under the fixed head and shoulders. Passive displacements were imposed on the head and shoulders, or on the hips and legs. Thresholds for the detection of these displacements were unchanged by the exclusion of vestibular stimulation. Thresholds were highest (still less than 1°) at the slowest angular velocity (0.1 °/s) and became lower as the angular velocity was increased.