Postural adjustments in arm and leg muscles associated with isodirectional and antidirectional coupling of upper limb movements in the horizontal plane

The hypothesis that anticipatory postural adjustments (APAs) may concur in generating the directional preference experienced during limb coupled movements was tested by measuring the electromyographic and mechanic postural actions elicited when moving: (1) one single arm/hand and, (2) both limbs, iso- or antidirectionally coupled. During fast adduction of the right arm in the horizontal plane (prime mover, pectoralis Major, rPM) APAs were recorded in the contralateral lPM as well as in the right ischiocruralis (rIC) muscle. This last action was associated to a transient increase of Tz (torque around body vertical axis) in the direction opposite to arm rotation. Both the APAs in rIC and the Tz changes nearly doubled in size when arms were coupled isodirectionally (adduction of one arm and abduction on the other) while they vanished when both arms were simultaneously adducted (antidirectional coupling). Conformably, during rhythmic arm oscillations APAs and Tz were cyclically modulated when movements were isodirectional, the modulation amplitude being strongly enhanced by increasing the movement frequency. When oscillations were antidirectional neither APAs nor Tz changes were observed, even if frequency was incremented. The postural actions linked to unidirectional or cyclic movements of the hand were affected by either coupling or frequency in the same way as arm movements, albeit much smaller in size. In conclusion, during antidirectional movements APAs in prime movers are synergic with voluntary activation and no postural engagement is requested to leg muscles. Conversely, during isodirectional movements, APAs in prime movers conflict with the voluntary commands and a strong, frequency-dependent, postural effort is required to leg muscles. How these factors may co-operate in determining the preference for antidirectional coupling is discussed.     

The role of anticipatory postural adjustments (APAs) in interlimb coordination of coupled arm movements in the parasagittal plane: I. APAs associated with fast discrete flexion and extension movements of one arm or of both arms ISO- and ANTI-directionally coupled

Coupling stability during cyclic arm movements in the horizontal (transverse) plane is lower in ISO- than in ANTI-directional coupling. We proposed that such impairment arises from the interference exerted in ISO by the anticipatory postural adjustments (APAs) linked to the primary movements. To evaluate if a link between coupling stability and postural adjustments also exist for arm movements with different postural requirements, we focused on arm(s) flexion–extension in the parasagittal plane and started by analysing the APAs distribution in arm, trunk and leg muscles. Fast flexion and extension of the right arm elicited APAs in the left anterior and posterior deltoid that replicated the excitation–inhibition of the homologous prime movers; this pattern would favour ISO and contrast ANTI-coupled movements. Instead, in the left latissimus dorsi, APAs were opposite to the voluntary actions in the right latissimus dorsi, thus favouring ANTI coupling. Symmetrical APAs were also elicited in right and left erector spinae (rES, lES) and asymmetrical APAs in Ischiocruralis (rIC, lIC), while an antero-posterior force (Fy) and a moment about the vertical axis (Tz) were discharged to the ground. When fast discrete movements were ISO-coupled, APAs were symmetrical in trunk (rES, lES) and leg (rIC, lIC) muscles and a large Fy but no Tz was generated. In ANTI coupling, APAs in rES and lES remained symmetrical, whereas they became antisymmetrical in rIC and lIC. A large Tz and a small Fy were recorded. In conclusion, during parasagittal movements, APAs in are elicited in both ISO and ANTI coupling, at variance with horizontal movements where they are only present in ISO. This would suggest that the difference in coupling stability between the two modes is smaller (or even reversed) in parasagittal with respect to horizontal arm movements.     

Coordination between postural and movement controls: effect of changes in body mass distribution on postural and focal component characteristics

Whole-body reaching movements are accomplished through a combination of anticipatory postural adjustments (APAs) and focal movements. Two different modes of central organization is usually proposed for this coordination: first, a single-process control, where the APAs and the focal movements would share a common command; second, where the APAs and the focal movements would be independently controlled through parallel commands (dual-process control). Here, we investigated which one of these modes of control could better explain the coordination between the trunk and the upper limb for standing subjects reaching for a target located beyond arm’s length. This was done evaluating the effect of changing the APAs settings on the arm movement. The APAs modification was achieved by shifting the subject’s centre of mass prior to the focal movement onset; this was done by adding an asymmetric load on either side of the head (a control condition with the load fixed centrally at the top of the head was also performed). As it changed the body mass distribution, the muscular torques and the orientation of the head inertia tensor, it is assumed that the addition of the asymmetric load led to a change in the APAs. Analyses indeed showed that both the initial head and trunk displacement towards the supporting side (during the unloading of the moving leg) were smaller when the load was fixed on the side of the supporting leg than when it was fixed on the side of the moving leg. However, changing the initial conditions, and therefore the APAs settings, had no significant effect on the path and kinematics of the focal hand movement. Therefore, subjects cancelled out the effect of the trunk motion on the hand-in-space motion through compensatory arm movements. These results support the dual-process control hypothesis for the postural and the focal components.

Anticipatory postural adjustments in children with typical motor development

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of vertical posture. However, little is known about how children utilize APAs during self-induced postural perturbations. A group of children, aged 7–16 years, with typical motor development, performed various arm movements while standing on a force platform. APAs were measured by recording the electromyographic activity of six trunk and leg muscles on both sides of the body and displacement of center of pressure (COP). Anticipatory bursts of activity in the dorsal muscle groups of the trunk and legs and suppression in the ventral muscle groups as well as posterior COP displacement were found during the performance of bilateral shoulder flexion. Conversely, during bilateral shoulder extension, the COP displacement was anterior, and APAs were reversed showing bursts of activity in the ventral muscle groups and suppression in the dorsal muscles. During right and left reciprocal arm movements, COP displacement was minimal and APAs were generated in the dorsal muscle groups on the side of the forward moving arm and in the ventral muscle groups on the side of the arm moving into extension. However this pattern reversed for lower leg muscles, where APAs were generated in the ventral muscles on the side of forward moving arm and in the dorsal muscle on the side of the arm moving into extension. The results of this study indicate that children with typical motor development are able to generate APAs, produce task-specific sequencing of muscle activity and differentiate between perturbations in the sagittal and transverse planes. The results of this study indicate that by at least age 7, children who are typically developing demonstrate the ability to generate patterns of anticipatory muscle activation and suppression, along with center of pressure changes, similar to those reported in healthy adults.     

Hemispheric specialization in the co-ordination of arm and trunk movements during pointing in patients with unilateral brain damage

During pointing movements involving trunk displacement, healthy subjects perform stereotypically, selecting a strategy in which the movement is initiated with either the hand or trunk, and where the trunk continues after the end of the hand movement. In a previous study, such temporal co-ordination was not found in patients with left-hemispheric brain lesions reaching with either their dominant paretic or with their non-dominant non-paretic arm. This co-ordination deficit may be associated in part with the presence of a lesion in the dominant left hemisphere. If so, then no deficit should be observed in patients with stroke-related damage in their non-dominant right hemisphere moving with their ipsilesional arm. To verify this, 21 right-hand dominant adults (7 who had had a stroke in the right hemisphere, 7 who had had a stroke in the left hemisphere and 7 healthy subjects) pointed to two targets located on a table in front of them in the ipsilateral and contralateral workspace. Pointing was done under three movement conditions: while not moving the trunk, while bending the trunk forward and while bending the trunk backwards. The experiment was repeated with the non-paretic arm of patients with stroke and for the right and left arms of healthy subjects. Kinematic data were recorded (Optotrak). Results showed that, compared to healthy subjects, arm-trunk timing was disrupted in patients with stroke for some conditions. As in patients with lesions in the dominant hemisphere, arm-trunk timing in those with lesions in the non-dominant hemisphere was equally more variable than movements in healthy subjects. However, patients with dominant hemisphere lesions used significantly less trunk displacement than those with non-dominant hemisphere lesions to accomplish the task. The deficit in trunk displacement was not due to problems of trunk control or sitting balance since, in control experiments, all subjects were able to move the trunk the required distance, with and without the added weight of the limb. Results support the hypothesis that the temporal co-ordination of trunk and arm recruitment during pointing movements is mediated bilaterally by each hemisphere. However, the difference in the range of trunk displacement between patients with left and right brain lesions suggests that the left (dominant) hemisphere plays a greater role than the right in the control of movements involving complex co-ordination between the arm and trunk. Electronic Publication     

Effects produced in human arm and forearm motoneurones after electrical stimulation of ulnar and median nerves at wrist level

Effects of electrical stimulation of ulnar and median nerves at wrist level were investigated in post-stimulus time histograms (PSTHs) of single motor units from both flexors and extensors in human arm and forearm. Stimulation of ulnar nerve produced late (mean extra time—after monosynaptic group Ia excitation—10.7 ± 0.1 ms) high-threshold (>1.2 × motor threshold, MT) excitation, which was not reproduced by purely cutaneous stimulation, in all the investigated motor nuclei except in Extensor Carpi Radialis. Stimulation of median nerve, and of the skin of fingers II and III (at palmar side level), produced short latency inhibition (mean extra time 3.8 ± 0.3 ms), which was most often truncated or followed by late excitation (mean extra time 11.8 ± 0.3 ms); both effects were of low threshold (0.8 × MT). Short latency inhibition was very strong, and late excitation was rare and weak in almost all the investigated motor units except in those supplying flexors in forearm, in which the main effect was the late facilitation (stronger than in other motoneurones). Since extra time was not more than 13 ms, it is suggested that the late effects may be mediated through spinal pathways, at least during their 3–5 first ms. Based on the electrophysiological results and on the anatomical characteristics of ulnar and median nerves, it is assumed that ulnar-induced late high-threshold peak in PSTHs might reflect group II excitation in spinal motoneurones, and median-induced modifications in motor unit discharge, mainly cutaneous control of motoneurone discharge. Since the central delay of median-induced inhibition is longer the more caudal the motoneurone, inhibitory propriospinal-like interneurones are supposed to mediate cutaneous inhibitory control from hand upon muscles in arm and forearm. Potential roles of proprioceptive and cutaneous control from hand to more proximal musculature, provided by ulnar and median nerve, respectively, during precise hand movements are discussed.     

Mirror,mirror on the wall: viewing a mirror reflection of unilateral hand movements facilitates ipsilateral M1 excitability

Primary motor cortex (M1) excitability is modulated by both ipsilateral limb movement and passive observation of movement of the contralateral limb. An interaction of these effects within M1 may account for recent research suggesting improved functional recovery of the impaired arm following stroke by viewing a mirror reflection of movements of the unimpaired arm superimposed over the (unseen) impaired arm. This hypothesis was tested in the present study using single-pulse transcranial magnetic stimulation (TMS) in eight neurologically healthy subjects. Excitability of M1 ipsilateral to a phasic, unilateral hand movement was measured while subjects performed paced (1 Hz), unilateral index finger-thumb opposition movements. Motor evoked potentials (MEPs) were obtained from the inactive first dorsal interosseous (FDI) in each of four viewing conditions: Active (viewing the active hand), Central (viewing a mark positioned between hands), Inactive (viewing the inactive hand) and Mirror (viewing a mirror-reflection of the active hand in a mirror oriented in the mid-sagittal plane) and with both hands at rest (Rest). MEPs were significantly enhanced during ipsilateral hand movement compared with the Rest condition (P<0.05). Largest MEPs were obtained in the Mirror condition, and this was significant compared with both the Inactive and Central viewing conditions (P<0.05). There was no difference between the dominant and non-dominant hand. Excitability of M1 ipsilateral to a unilateral hand movement is facilitated by viewing a mirror reflection of the moving hand. This finding provides neurophysiological evidence supporting the application of mirror therapy in stroke rehabilitation.     

Neural coupling between the upper and lower limbs in humans

The aim of this study was to establish the effects of active sinusoidal ipsilateral and contralateral upper limb flexion, extension, abduction, and adduction with elbows extended on the right soleus H-reflex with subjects seated and standing. Reflex effects were also established when both arms moved synchronously in a reciprocal pattern with elbows flexed in seated and standing subjects. Sinusoidal arm movements were timed to a metronome and performed at 0.2 Hz. Soleus H-reflexes were elicited only once (every 4s) in every movement cycle of the upper limbs. Position of arms, and activity of shoulder muscles were recorded through twin-axis goniometers and surface electromyography (EMG), respectively. We found that in seated subjects, regardless the direction of the active movement or the upper limb being moved, the soleus H-reflex was depressed. In standing subjects, a reflex depression was observed during extension, abduction, and adduction of the ipsilateral and contralateral upper limbs. Muscles were active during arm flexion and abduction in all directions of arm movement with subjects either seated or standing. It is suggested that arm movement might be incorporated in the rehabilitation training of people with a supraspinal or spinal cord lesion, since it can benefit motor recovery by decreasing spinal reflex excitability of the legs in these patients.     

Systematic changes in the perceived posture of the wrist and elbow during formation of a phantom hand and arm

Our previous study showed that a fully flexed or extended hand became perceived as an extended or flexed ‘phantom’ hand as ischemic anesthesia progressed (Inui et al. in J Physiol 589:5775–5784, 2011). Here, we examined what happened if the hand was held in the midposition before and during the anesthesia. Twenty healthy participants reported the perceived postures of their right wrist and elbow during an ischemic block of the right upper arm using the left hand and arm. If the actual arm and hand were fully extended, then the perceived position of the elbow and wrist moved toward flexion. Conversely, if they were fully flexed, then the perceived position of the joints moved toward extension. However, when the hand was held in the midposition before and during the anesthesia, the position of the wrist was perceived to be in the same position. Hence, the fully flexed or extended position of a limb was essential for systematic changes in the perceived posture of the limb during the anesthesia. Because the start of these changes occurred as somatosensory inputs were declining, the changes depended on the fading inputs from strongly stretched muscle and skin during the anesthesia.     

The effect of voluntary arm abduction on balance recovery following multidirectional stance perturbations

The goal of this study was to investigate how voluntarily abducting one arm, 90° at onset of a rotational perturbation of the support surface, influences the recovery of upright stance. Young adults were tested under four stance conditions: abducting one arm to the horizontal only (AO); perturbation of stance using a support surface rotation only (PO); combined support surface rotation and abduction of the downhill arm, ipsilateral to tilt (IPS); and fourth abduction of the uphill, contralateral arm (CON). Simultaneous auditory and visual trigger cues were used for arm raising. Perturbations consisted of six directions of combined support surface roll and pitch rotation (7.5° and 60°/s). Outcome measures were whole body centre of mass (COM) movements and body segment angular displacements recorded with a motion analysis system, as well as leg, trunk, and arm EMG responses. Arm raises contralateral and ipsilateral to the direction of support surface roll were more rapid than in the AO condition and significantly reduced or increased, respectively, COM lateral displacements relative to the PO condition. The changes in COM displacements and velocities during combined CON arm raise and perturbation were greater than expected from the sum of displacements for AO and PO conditions alone, but less for the IPS condition. Arm raising increased trunk roll in a direction opposite arm raising was more than for the AO and PO conditions. Robust effects were also observed for hip abduction but not for leg flexion. Early balance correcting activity was enhanced on the side opposite arm raising and later stabilising activity reduced bilaterally in lower trunk muscles compared to summed activity for the AO and PO conditions. Similar effects were observed in gluteus medius muscles but effects were weak in ankle muscles. EMG onsets in muscles of the raised arm were earlier than in the AO conditions. We conclude that triggered arm abduction, contralateral to the direction of support surface rotation, had significant stabilization benefits for young adults and ipsilateral arm movements had destabilizing effects. The arm raises could be simultaneously executed with balance corrections. These results provide insights into the integration of balance corrections and voluntary commands into one automatic reaction that may be useful in training fall avoidance.     

Cervical muscle response to trunk flexion in whiplash-type lateral impacts

The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity, whiplash-type lateral impacts when the occupant is seated out of the recommended driving position (neutral posture). Twenty healthy volunteers were subjected to left lateral impacts of 4.1, 7.7, 10.5, and 13.7 m/s² acceleration, with their trunk flexed by 45° and laterally flexed to the right and left also by 45° at the time of impact. Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded. Under these conditions of trunk-flexed postures, in a left lateral impact, muscle responses were of generally low magnitude with the trunk flexed to either the left or right. Even at the highest acceleration of 13.7 m/s², all muscles generated less than 37% of their known maximal voluntary contraction electromyogram. Also, in these left lateral impacts, the right splenius capitis showed a greater EMG response than the left splenius capitis regardless of whether the subject was flexed to the right or left at the time of impact. The right splenius capitis (the one contralateral to the left lateral impact direction) was more active than its counterpart. Compared to what is known for EMG responses with an occupant in the neutral posture, the right sternocleidomastoid (usually the most active muscle in a left lateral collision) was significantly less-active with trunk flexion than with neutral posture conditions (P<0.01). In the absence of bodily impact, the flexed trunk posture does not produce a biomechanical response that would increase the likelihood of cervical muscle injury in low velocity lateral impacts, and may lessen the risk of injury for some muscles.     

Anticipatory postural adjustments stabilise the whole upper-limb prior to a gentle index finger tap

Little is known about anticipatory postural adjustments (APAs) developing when body segments of tiny mass are moved. Thus, APAs in the human upper-limb were investigated during a gentle and small index finger tap (35 mm stroke in 50 ms). This task was fulfilled by ten subjects either with prone or supine hand. EMG was recorded from Flexor Digitorum Superficialis (FDS), the prime mover, and from several upper-limb muscles under slight tonic contraction. Regardless of hand posture, EMG was inhibited in Flexor Carpi Radialis and facilitated in Extensor Carpi Radialis well before the FDS burst. With the prone hand, the prime mover activity was preceded by Biceps inhibition and Triceps facilitation; this effect reverted in sign with the supine hand. A postural reversal was also observed in Anterior Deltoid and Trapezius which were both inhibited with the prone hand. The effect in Trapezius was present only with the unsupported forearm. It is thus demonstrated that a gentle small finger tap produces well-defined anticipatory natural synergies behaving as the most “classical” APAs: (1) they are distributed to several upper-limb muscles creating a postural chain aiming to prevent the effects of the interaction torques generated by the voluntary movement; (2) they change in amplitude according to the level of postural stability and (3) they revert in sign when movement direction is reverted. These results are also corroborated by data obtained from a simple mechanical model simulating finger tapping in a fictive upper-limb. A possible role of APAs in controlling movements’ accuracy is also discussed.     

Determining natural arm configuration along a reaching trajectory

Owing to the flexibility and redundancy of neuromuscular and skeletal systems, humans can trace the same hand trajectory in space with various arm configurations. However, the joint trajectories of typical unrestrained movements tend to be consistent both within and across subjects. In this paper we propose a method to solve the 3-D inverse kinematics problem based on minimizing the magnitude of total work done by joint torques. We examined the fit of the joint-space trajectories against those observed from human performance in a variety of movement paths in 3-D workspace. The results showed that the joint-space trajectories produced by the method are in good agreement with the subjects’ arm movements (r ²>0.98), with the exception of shoulder adduction/abduction (where, in the worst case, r ² ∼0.8). Comparison of humeral rotation predicted by our algorithm with other models showed that the correlation coefficient (r ²) between actual data and our predictions is extremely high (mostly >0.98, 11 out of 15 cases, with a few exceptions, 4 of 15, in the range of 0.8–0.9) and the slope of linear regression is much closer to one (<0.05 distortion in 12 out of 15 cases, with only one case >0.15). However, the discrepancy in shoulder adduction/abduction indicated that when only the hand path is known, additional constraint(s) may be required to generate a complete match with human performance.     

Hemispheric differences in use-dependent corticomotor plasticity in young and old adults

The aim of this study was to examine corticomotor excitability and plasticity following repetitive thumb abduction training in left and right hands of young and old adults. Electromyographic recordings were obtained from the abductor pollicis brevis (APB) muscle of 12 young (aged 18–27 years) and 14 old (aged 63–75 years) adults. Motor training consisted of 300 ballistic abductions of the thumb to maximize peak abduction acceleration, with each hand tested in a separate session. Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) was used to assess changes in contralateral APB motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before and after training. For young and old adults, APB MEP amplitude increased for both hands after training, which is indicative of use-dependent plasticity. However, the increase in MEP amplitude was 21% (P = 0.04) greater in the left (non-dominant) hand compared with the right (dominant) hand. This occurred despite a 40% greater improvement in peak thumb abduction acceleration (motor learning) for the right hand in young subjects compared with the left hand in young subjects (P < 0.04) and the right hand in old subjects (P < 0.01). Furthermore, no difference in use-dependent plasticity was observed between young and old adults, and SICI remained unchanged following ballistic training for both hands in all subjects. These findings suggest that there is greater strengthening of corticomotor circuits for control of the left compared with the right hand during simple ballistic thumb training and that an age-related decline in motor learning was observed only in the dominant hand. In contrast to previous studies, these data also indicate that young and old adults can demonstrate similar use-dependent corticomotor plasticity during this simple thumb-training task.     

Feedforward activity of the cervical flexor muscles during voluntary arm movements is delayed in chronic neck pain

The objective of this study was to compare onset of deep and superficial cervical flexor muscle activity during rapid, unilateral arm movements between ten patients with chronic neck pain and 12 control subjects. Deep cervical flexor (DCF) electromyographic activity (EMG) was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid (SCM) and anterior scalene (AS) muscles. While standing, subjects flexed and extended the right arm in response to a visual stimulus. For the control group, activation of DCF, SCM and AS muscles occurred less than 50 ms after the onset of deltoid activity, which is consistent with feedforward control of the neck during arm flexion and extension. When subjects with a history of neck pain flexed the arm, the onsets of DCF and contralateral SCM and AS muscles were significantly delayed (p<0.05). It is concluded that the delay in neck muscle activity associated with movement of the arm in patients with neck pain indicates a significant deficit in the automatic feedforward control of the cervical spine. As the deep cervical muscles are fundamentally important for support of the cervical lordosis and the cervical joints, change in the feedforward response may leave the cervical spine vulnerable to reactive forces from arm movement.     

Workspace location influences joint coordination during reaching in post-stroke hemiparesis

The purpose of this study was to determine the influence of workspace location on joint coordination in persons with post-stroke hemiparesis when trunk motion was required to complete reaches beyond the arm’s functional reach length. Seven subjects with mild right hemiparesis following a stroke and seven age and gender matched control subjects participated. Joint motions and characteristics of hand and trunk movement were measured over multiple repetitions. The variance (across trials) of joint combinations was partitioned into two components at every point in the hand’s trajectory using the uncontrolled manifold approach; the first component is a measure of the extent to which equivalent joint combinations are used to control a given hand path, and reflects performance flexibility. The second component of joint variance reflects the use of non-equivalent joint combinations, which lead to hand path error. Compared to the control subjects, persons with hemiparesis demonstrated a significantly greater amount of non-equivalent joint variability related to control of the hand’s path and of the hand’s position relative to the trunk when reaching toward the hemiparetic side (ipsilaterally), but not when reaching to the less involved side. The relative timing of the hand and trunk was also altered when reaching ipsilaterally. The current findings support the idea that the previously proposed “arm compensatory synergy” may be deficient in subjects with hemiparesis. This deficiency may be due to one or a combination of factors: changes in central commands that are thought to set the gain of the arm compensatory synergy; a limited ability to combine shoulder abduction and elbow extension that limits the expression of an appropriately set arm compensatory synergy; or a reduction of the necessary degrees-of-freedom needed to adequately compensate for poor trunk control when reaching ipsilaterally.     

Target size modifies anticipatory postural adjustments and subsequent elementary arm pointing

We herein studied whether arm-pointing movements from an upright posture (i.e. Belenkii’s paradigm) toward various targets demanding a low degree of precision could influence associated anticipatory postural adjustments (APAs) and subsequent arm movements. Six subjects were asked to use their right arm to point (without finger touch) to targets of 2, 4 and 8 cm in diameter (respectively, D2, D4 and D8). APAs were measured by recording the electromyographic activity of the right anterior deltoid and biceps femoris, as well as the kinematics of the right arm. Longer APA durations and lower values for the ratio between acceleration duration and total duration of the focal movement were observed for D4 compared to D2 and D8, whereas precision was constant across all three targets. Thus, the medium target seemed to engender an optimum motor strategy for accomplishing the accuracy and velocity requirements of the task. These results emphasize that subjects build perceptual representations of their environment as well as representations of the actions to be produced. We suggest that, even in this simple movement traditionally studied from a biomechanical perspective, APAs function not only to compensate for perturbations to equilibrium, but also reflect a cognitive representation of the precision requirements of the task.     

Electromyographyc evaluation of movements of lower limb in double pulley system equipment: comparison between gastrocnemius (caput laterale) and gluteus maximus

It was evaluated movements of lower limb in the double pulley system equipment on ten male volunteers during contraction of gastrocnemius (caput laterale) and gluteus maximus muscles in the following movements: 1) hip extension with extended knee and erect trunk, 2) hip extension with flexed knee and erect trunk, 3) hip extension with flexed knee and erect trunk, 3) hip extension with extended knee and inclined trunk, 5) hip abduction along the midline, 7) hip abduction with extension beyond the midline, 8) adduction with hip flexion beyond the midline, 8) adduction with hip flexion beyond the midline, and 9) adduction with hip extension beyond the midline. Myoelectric signals were taken up by Lec Tec surface electrodes connected to a 6-channel Lynx electromyographic signal amplifier coupled with a computer equipped with a model CAD 10/26 analogue digital conversion board and with a specific software for signal recording and analysis. We observed weak gastrocnemius muscle activity for all movements studied. In the case of gluteus maximus, the most important potentials were observed for movement 2, while for the remaining movements the actions were of reasonable intensity. Compared to gluteus, gastrocnemius was less required for all movements.     

Representation of the midline trunk, bilateral arms, and shoulders in the monkey postcentral somatosensory cortex

 Single neuronal activities were recorded in the arm/trunk region of the postcentral gyrus in awake Japanese monkeys. A total of 1608 units were isolated from four hemispheres of two animals, and receptive fields (RFs) and submodalities were identified in 1162 units. Deep or skin submodality neurons were dominant in area 3a or area 3b, respectively. The deep/skin ratio increased as the recording site moved from area 3b to the more caudal areas. In areas 3a and 3b, neuronal RFs were almost exclusively on either the arm or trunk. In areas 2 and 5, neurons with RFs on the trunk decreased and those with RFs on the hand or covering more than one body part, etc. increased. We found a total of 107 neurons with bilateral RFs and 56 with ipsilateral RFs, while the rest (n=999) were with contralateral RFs. Bilateral or ipsilateral neurons of skin submodality (n=37) were found in areas 1, 2, and 5. Twenty six (70%) had RFs on the trunk and/or occiput, five on the forelimb, and the rest (n=6) on both the trunk and forelimb (the combined type). Among 33 skin bilateral neurons, 90% (n=30) had RFs across the midline. Bilateral or ipsilateral neurons responding to joint manipulation (n=104) were found in areas 2 and 5. Most of them were activated by manipulation of the shoulder and/or elbow (the proximal type, n=72, 69%). There were 25 neurons of the combined type (both the proximal and distal joints were effective, 24%). Bilateral or ipsilateral neurons of deep-others submodality (n=20) were found in areas 1, 2, and 5. The forelimb type (n=12, 60%) was dominant in this category. The combined-type neurons in both the skin- and joint-manipulation categories were found only or mostly in area 5. These results indicate the presence of hierarchical processing for bilateral as well as contralateral information within the arm/trunk region of the postcentral gyrus. Received: 12 January 1998 / Accepted: 19 May 1998     

Modulation of anticipatory postural adjustments associated with unloading perturbation: effect of characteristics of a motor action

The purpose of the study was to determine whether characteristics of a motor action affect anticipatory postural adjustments (APAs). Standing subjects held a load between their hands with arms extended in front of their body. Next, subjects performed bilateral shoulder abduction movements (motor action) of three amplitudes at three instructed speeds. This motor action led to the release of the same load, inducing unloading perturbation in the sagittal plane. Electromyographic activities were recorded for the leg and trunk muscles. A change in the background muscle activity in these muscles was observed prior to the unloading perturbation and was quantified as APAs. APAs were dependant on instructed speed of the motor action; larger APA activities were observed in the leg and trunk muscles with a faster speed instruction. Meanwhile, the modulation of APAs was not observed by altering the movement amplitude. Moreover, experiments showed that motor action itself without a load release did not generate APA activity. Therefore, we concluded that the central nervous system selects information within a motor action (i.e., speed instruction) to approximate the magnitude of the forthcoming perturbation and modulate APAs, even when the unloading perturbation was unchanged.