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PubMed1997

Modulation of the mandibular stretch reflex sensitivity during various phases of rhythmic open-close movements in humans.

The muscle spindles of the jaw elevator muscles provide positive feedback to the alpha motoneurons. It is generally assumed that the feedback is modulated during chewing so that counterproductive forces of the jaw elevator muscles can be avoided during jaw opening. Our aim was to investigate the modulation of the muscle spindle input to the alpha motoneurons during various phases of open-close movements in man. To that end, subjects made rhythmic open-close movements at their natural chewing frequency. A force impulse (5 N, 10 ms), eliciting a jaw-jerk reflex, was unexpectedly applied. The impulse was applied to the mandible at 8 different phases during an open-close cycle, but only 1 impulse per cycle. Jaw movement and surface EMG of the masseter and temporal muscles on both sides were recorded during 3 cycles without an impulse and 3 succeeding cycles with an impulse. To examine whether the modulation of the mandibular stretch reflex sensitivity depends on the food resistance, we applied an additional external force on the mandible, counteracting closing of the jaw each cycle. Two experimental sessions were performed in random order, i.e., without force and with an additional force of 20 N. We observed pronounced reflexes at the onset of jaw closing, during the closing phase, and at occlusion. No or only weak jaw-jerk reflexes were present during jaw opening. The reflex amplitudes at occlusion were larger when an external force was present. This increase in reflex amplitude may be the result of an adjusted gamma motoneuron activity, from pre-motor inhibition, or from both. The reflex amplitudes elicited during jaw closing were not correlated with the phase of the movement.

Adult↗
PubMed1995

Inability to activate muscles maximally during cocontraction and the effect on joint stiffness.

In order to determine the maximum joint stiffness that could be produced by cocontraction of wrist flexor and extensor muscles, experiments were conducted in which healthy human subjects stabilized a wrist manipulandum that was made mechanically unstable by using positive position feedback to create a load with the characteristics of a negative spring. To determine a subject's limit of stability, the negative stiffness of the manipulandum was increased by increments until the subject could no longer reliably stabilize the manipulandum in a 1 degree target window. Static wrist stiffness was measured by applying a 3 degree rampand-hold displacement of the manipulandum, which stretched the wrist flexor muscles. As the load stiffness was made more and more negative, subjects responded by increasing the level of cocontraction of flexor and extensor muscles to increase the stiffness of the wrist. The stiffness measured at a subject's limit of stability was taken as the maximum stiffness that the subject could achieve by cocontraction of wrist flexor and extensor muscles. In almost all cases, this value was as large or larger than that measured when the subject was asked to cocontract maximally to stiffen the wrist in the absence of any load. Static wrist stiffness was also measured when subjects reciprocally activated flexor or extensor muscles to hold the manipulandum in the target window against a load generated by a stretched spring. We found a strong linear correlation between wrist stiffness and flexor torque over the range of torques used in this study (20-80% maximal voluntary contraction). The maximum stiffness achieved by cocontraction of wrist flexor and extensor muscles was less than 50% of the maximum value predicted from the joint stiffness measured during matched reciprocal activation of flexor and extensor muscles. EMG recorded from either wrist flexor or extensor muscles during maximal cocontraction confirmed that this reduced stiffness was due to lower levels of activation during cocontraction of flexor and extensor muscles than during reciprocal contraction.

Adult↗
PubMed1995

Control variables and proprioceptive feedback in fast single-joint movement.

Sensorimotor mechanisms were studied on the basis of kinematic and electromyographic data as well as the static torque developed by the muscles as a function of joint angle. The latter relationship is known as the torque/angle characteristic. Fast single-joint movement may result from a shift in this characteristic and a change in its slope. Such movements were studied at the wrist in 9 normal and 1 deafferented subject. After training to flex the wrist to a target, subjects repeated the same movements but in random test trials movements were opposed by the load generated by linear position feedback to a torque motor. At the end of the loaded trials, the load was suddenly removed. In the second experiment, subjects made wrist movements to the target that were opposed by the load and, on random test trials, the movements were not loaded. In these test trials, the wrist arrived in a static position outside the target zone. In both experiments, subjects were instructed not to correct errors. The final torque/angle characteristics specified in the movements were reconstructed on the basis of the static wrist positions and torques before and after unloading. Normal subjects made movements by shifting the position of the torque/angle characteristic and by increasing its slope. If subjects indeed maintained the same pattern of control variables (descending commands), the same final position of the characteristic would be reproduced from trial to trial regardless of load perturbations. This assumption of equifinality was tested by comparing the final position of the wrist in nonloaded movements with that after removal of the load in loaded movements. Equifinality was observed in normal subjects. Movements in the deafferented subject were also associated with a shift of the torque/angle characteristic and a change in its slope. However, she was unable to consistently reproduce its final position. In spite of muscle coactivation, her maximal stiffness was lower than in normal subjects. In the absence of vision, the subject made movements with the load by increasing the slope of the characteristic instead of by shifting its position far enough. Load perturbation affected her final wrist position (inequifinality), which may reflect the presence of a significant hysteresis of the characteristic as a result of the absence of stretch reflexes. The deficits following deafferentation presumably result from the destruction of biomechanical and sensorimotor mechanisms including the ability of control variables to specify the positional frame of reference for afferent and descending systems.

Adult↗
PubMed1998

A comparison of jaw-opener and jaw-closer muscle activity in humans to overcome an external force counteracting jaw movement.

In contrast to the jaw-closer muscles, no or very few spindles are present in the jaw-opening digastric muscle. Therefore sensory feedback to the digastric muscle may be different from feedback to the jaw-closer muscles, resulting in a different reaction when jaw movement is perturbed. This possible difference was investigated by comparing the reaction of the digastric muscle when jaw opening is perturbed, with the reaction of the masseter muscle when jaw closing is perturbed. Subjects made rhythmic, 1-Hz open-close movements of the jaw under control of a metronome. During jaw opening (digastric muscle) or, in the other experiments, during jaw closing (masseter muscle), an external force counteracting jaw movement could appear. Series of movements without the force were unexpectedly alternated by series with the force. In both muscles sensory induced activity started approximately 25 ms after the onset of the force and consisted of two phases. In the masseter muscle the maximum of the first increase was reached significantly sooner (37 +/- 2 ms SEM) than in the digastric muscle (54 +/- 3 ms). The second increase appeared much sooner in the masseter muscle (73 +/- 4 ms) than in the digastric muscle (159 +/- 10 ms). When the force was expected, in both muscles an increase in preprogrammed muscle activity was observed. Also an increase in reflex activity, generated before 120 ms after the onset of the force, was observed, compared with when the force appeared unexpectedly. The relative increase in reflex activity was approximately 2 times larger than the relative increase in preprogrammed activity. Therefore, the increase in reflex activity when the force was expected may have been caused not only by an increase in recruitment, but also by an increase in the gain of the reflex loops. Reflex activity relative to preprogrammed activity was on average 4 times larger in the masseter muscle than in the digastric muscle. This indicates that the masseter muscle can react more adequately to disturbances of jaw movement than the digastric muscle.

Adult↗
PubMed2003

Importance of body sway velocity information in controlling ankle extensor activities during quiet stance.

In literature, it has been suggested that the CNS anticipates spontaneous change in body position during quiet stance and continuously modulates ankle extensor muscle activity to compensate for the change. The purpose of this study was to investigate whether velocity feedback contributes by modulating ankle extensor activities in an anticipatory fashion, facilitating effective control of quiet stance. Both theoretical analysis and experiments were carried out to investigate to what extent velocity feedback contributes to controlling quiet stance. The experiments were carried out with 16 healthy subjects who were asked to stand quietly with their eyes open or closed. During the experiments, the center of pressure (COP) displacement (COPdis), the center of mass (COM) displacement (COMdis), and COM velocity (COMvel) in the anteroposterior direction were measured. Rectified electromyograms (EMGs) were used to measure muscle activity in the right soleus muscle, the medial gastrocnemius muscle, and the lateral gastrocnemius muscle. The simulations were performed using an inverted pendulum model that described the anteroposterior kinematics and dynamics of quiet stance. In the simulations, an assumption was made that the COMdis of the body would be regulated using a proportional-derivative (PD) controller. Two different PD controllers were evaluated in these simulations: 1) a controller with the high-derivative/velocity gain (HDG) and 2) a controller with the low-derivative/velocity gain (LDG). Cross-correlation analysis was applied to investigate the relationships between time series obtained in experiments 1) COMdis and EMGs and 2) COMvel and EMGs. Identical cross-correlation analysis was applied to investigate the relationships between time series obtained in simulations 3) COMdis and ankle torque and 4) COMvel and ankle torque. The results of these analyses showed that the COMdis was positively correlated with all three EMGs and that the EMGs temporally preceded the COMdis. These findings agree with the previously published studies in which it was shown that the lateral gastrocnemius muscle is actively modulated in anticipation of the body's COM position change. The COMvel and all three EMGs were also correlated and the cross-correlation function (CCF) had two peaks: one that was positive and another that was negative. The positive peaks were statistically significant, unlike the negative ones; they were larger than the negative peaks; and their time shifts were much shorter compared with the time shifts of the negative peaks. When these results were compared with the CCF results obtained for simulated time series, it was discovered that the cross-correlation results for the HDG controller closely matched cross-correlation results for the experimental time series. On the other hand, the simulation result obtained for LDG controller did not match the experimental results. These findings suggest that the actual postural control system during quiet stance adopts a control strategy that relies notably on velocity information and that such a controller can modulate muscle activity in anticipatory manner without using a feed-forward mechanism.

Adult↗
PubMed1997

Neural correlates of a spatial sensory-to-motor transformation in primary motor cortex.

Primary motor cortex (MC) has been strongly implicated in motor processing, but there have been relatively few attempts to determine whether MC may also play a role in either sensory or context-dependent processing. In the present study, neuronal activity in MC was characterized in relation to the planning and execution of visually instructed limb movements whose trajectories were dissociated from their spatial targets. This design permitted the dissociation of neuronal activity related to motor processing from activity related to sensory or context-dependent processing. Two macaque monkeys were trained to perform a visually instructed, delayed reaching paradigm with indirect visual feedback. Subjects used the right forelimb to capture targets presented on a video display by moving a two-dimensional joystick whose position was reflected by a cursor. The target to be capture on each trial was indicated by a visual instruction stimulus (IS), which was separated from a movement-triggering stimulus (TS) by a variable delay. The direction of forelimb movement was dissociated from the location of the target by varying the spatial mappings between joystick and cursor across two conditions, unrotated (0 degrees offset between cursor and limb direction) and rotated (90 degrees offset). Task-related activity was recorded from a total of 180 MC neurons. The focus of this study was on directionally tuned neuronal activity that included phasic, stimulus-related activity following the IS; tonic, set-related activity between IS and TS; and phasic, movement-related activity following the TS. Of the entire sample of MC neurons with directionally tuned activity, 119 cells were tested both under rotation conditions, permitting dissociation of directional responses that depended on target locations from those that depended on limb trajectory. Task-related neuronal activity was classified as target dependent if it covaried exclusively with target location across both conditions, and as limb dependent if it covaried exclusively with limb trajectory. Directional activity that did not fulfill criteria for either target or limb dependence, because of changes across rotation conditions, was classified as complex. Approximately one quarter of MC neurons showed weak, but consistent, stimulus-related activity that was directionally tuned (24%, 29 of 119). Nearly all of the directionally classifiable stimulus-related activity was target dependent (94%, 15 of 16 responses), with the exception of a single limb-dependent response (6%, 1 of 16). A majority of MC neurons showed set-related activity that was directionally tuned (61%, 72 of 119). Of the directionally classifiable set-related activity, there were comparable numbers of target-dependent (37%, 16 of 43) and limb-dependent responses (35%, 15 of 43), with the remainder being complex (27%, 12 of 43). Movement-related activity following the TS was considered to be early or late, depending on whether it preceded or followed the onset of movement. The large majority of MC neurons showed early movement-related activity that was directionally tuned (86%, 102 of 119): among those whose neurons early activity was directionally classifiable, there were only one third as many target-dependent responses (14%, 11 of 79) as limb-dependent responses (43%, 34 of 79), with the remainder being complex (43%, 34 of 79). There was also a large majority of MC neurons that showed late movement-related activity that was directionally tuned (84%, 100 of 119): among those whose late activity was directionally classifiable, there were only one ninth as many target-dependent responses (5%, 4 of 88) as there were limb-dependent responses (41%, 36 of 88), with the remainder being complex (55%, 48 of 88). The instructed delay task employed in this study required a sensory-to-motor transformation through which the instructed target location was associated with a limb movement of the appropriate direction. (ABSTRACT TRUNCATED)

Action Potentials↗
PubMed1999

Trick maneuvers in cervical dystonia: investigation of movement- and touch-related changes in polymyographic activity.

Antagonistic gestures or trick maneuvers are well-known clinical features to reduce or abolish dystonic posturing in cervical dystonia (CD). The maneuvers typically consist of a finger touch to the facial skin but their physiology remains unknown. To determine the temporal profile of geste maneuver performance, 25 patients with idiopathic CD were studied by means of polymyography of six cervical muscles prior to any botulinum toxin treatment. Two piezoelectric elements fixed to a fingertip of the hand involved in the trick maneuver and to the facial target region, respectively, were used to relate the essential points of the trick maneuver time course (start of geste-arm movement, facial contact, end of contact, end of movement) to changes in polymyographic activity. Thirteen patients (52%) showed marked reductions of electromyographic (EMG) activity (> or =50% in at least one muscle) during arm movement, definitely prior to contact between fingers and facial target area; in the remaining 12 patients (48%), geste-related EMG effects were confined to facial-finger contact. These results might indicate different physiological mechanisms in clinically indistinguishable antagonistic gestures.

Adult↗
PubMed2005

Task-dependent intracortical inhibition is impaired in focal hand dystonia.

We tested whether task-dependent modulation of inhibition within the motor cortex is impaired in patients with dystonia. Paired-pulse transcranial magnetic stimulation (TMS) at an interstimulus interval of 2 msec was used to measure the effect of two different tasks on short ISI intracortical inhibition (SICI) in dystonic and normal subjects. In two experiments, SICI of the fourth dorsal interosseus (4DIO) and abductor pollicis brevis (APB) muscles were measured before and at the end of the training task. In the first experiment, subjects performed a nonselective task consisting of abducting the thumb, where the APB acted as agonist and the 4DIO as synergist. In the second experiment, the function of the 4DIO was changed as the subjects were asked to consciously inhibit this muscle while abducting the thumb (selective task). Therefore, while the APB was activated in both tasks, the 4DIO was activated in the nonselective task but was in the inhibitory surround in the selective task. We found that performance of the selective but not the nonselective task resulted in increased SICI in the 4DIO of normal but not in dystonic subjects. We conclude that task-dependent SICI is disturbed in patients with dystonia.

Dystonic Disorders↗
PubMed1994

Neural organization of the ventilatory activity in the frog, Rana catesbeiana. I.

In order to elucidate the neural basis for lung ventilation in the frog, we have investigated the efferent neural activity to oropharyngeal muscles in the decerebrate, paralyzed, unanesthetized bullfrog, Rana catesbeiana. Efferent motor output was recorded from the mandibular branch of the trigeminal (Vmd), the laryngeal branch of the vagus (Xl), and the main and sternohyoid branches of the hypoglossal nerve (Hm and Hsh, respectively). Two types of rhythmic bursting outputs were observed: (1) a high-frequency, low-amplitude, reciprocal oscillation between Vmd, a buccal levator nerve, and Hsh, a buccal depressor nerve; and (2) a low-frequency, high-amplitude, synchronous bursting of Vmd, Hm, Hsh, and Xl. The first type is inferred to represent fictive oropharyngeal ventilation. The second type of burst was divided into four intervals: (a) augmenting activity of Hsh; (b) activation of Xl with continued activation of Hsh; (c) activation of Vmd and Hm (a buccal levator nerve), continued activation of Xl, and termination of Hsh activity; and (d) warning activity in Vmd and Hm associated with a prominent second wave in Xl. This coordinated activity is inferred to represent fictive pulmonary ventilation because the neurograms in these four intervals correspond closely to EMGs and neurograms recorded in the intact frog during the four phases of pulmonary ventilation, namely, buccal depression, pulmonary expiration, pulmonary inspiration, and glottal closure. Hypercapnia, vagotomy, and cutaneous pinching enhanced the high-amplitude, low-frequency rhythm, but not the low-amplitude, high-frequency oscillation. Lung inflation generally inhibited the former and not the latter, but in some cases lung inflation stimulated pulmonary ventilation. We conclude that oropharyngeal and pulmonary ventilation of the frog are produced by one or, possibly, two intrinsically active generators.

Animals↗
PubMed1993

The effect of fasciculus cuneatus lesions on finger positioning and long-latency reflexes in monkeys.

Previous studies have reported abnormalities in fine hand and finger movements following interruption of the fasciculus cuneatus (FC) in primates. We report here that many of these deficits could be caused by an inability to actively regulate the position of the finger. Three macaques were trained to maintain the index finger in one position against constant or changing loads. Periodically, torque pulses were used to elicit reflexes in finger muscles. Following unilateral FC lesions, the monkeys failed to adjust finger position during the trials, and the normal M2 long-latency response was absent in the finger muscles. Performance on the task was impaired only in monkeys with complete lesions that included the deep ventral portion of the FC. These results suggest that afferent fibers in the FC regulate finger position, and do so partly through reflexive mechanisms. When the FC is interrupted, the inability to control finger position disturbs fine motor activities.

Animals↗
PubMed1994

Control of fast elbow movement: a study of electromyographic patterns during movements against unexpectedly decreased inertial load.

Predictions of three models of single-joint motor control were compared with experimental observations of the changes in electromyographic (EMG) patterns during fast voluntary movements against an unexpectedly reduced inertial load. The subjects performed elbow flexions over 40 degrees "as fast as possible" in two series. During the first series, an approximately 40% decrease in inertia, simulated by a torque-motor, might occur unpredictably on half of the trials (unloaded trials). During the second series, all the trials were unloaded. The major findings are: (1) no differences in the antagonist burst latency in unexpectedly unloaded and unperturbed trials; (2) a decrease in the antagonist latency during expected unloadings; (3) a small, statistically non significant decrease in the first agonist burst EMG integral; and (4) a larger, statistically significant increase in the antagonist burst EMG integral in unexpectedly unloaded trials as compared to unperturbed trials. The data are in good correspondence with a version of the equilibrium-point hypothesis that assumes central programming of the beginning of the antagonist burst and incorporates the possibility of reflex-induced changes in EMG amplitudes.

Acceleration↗
PubMed1992

Feedforward postural stabilization in a distal bimanual unloading task.

The aim of the present study was to investigate postural adjustments and positional stability in a bimanual unloading task, involving essentially the index finger, in order to test whether proactive adjustments are also observed in distal body segments. A second goal of the study was to evaluate the concept of a central command that would be responsible for coupling movement and posture. The positional disturbance of the right load-bearing index finger of healthy human subjects was studied under two types of manipulations: passive, i.e., imposed, unloading and active unloading, by the subject's left index finger. It was found that, in such a distal task, positional stabilization of the load-bearing finger was much better (by a factor of 6) in the active situation than the passive situation. This improvement was greater than previously reported for a proximal task. An electromyogram (EMG) analysis of the mostly implicated dorsal interosseous muscles revealed a typical unloading reflex in the passive situation (reactive mode) and a suppression of EMG before unloading onset in the active situation (proactive mode). Averaged records showed an almost perfect synchronization between the EMG suppression in the load-bearing interosseous muscle and the onset of the EMG burst of the unloading index finger. A trial-by-trial analysis, however, revealed a considerable scatter in intervals of the two EMG events, with a tendency of the activity burst in the left finger to occur slightly before the suppression of EMG in the load-bearing muscle. No positive correlation was found between the precision of synchronization (intervals near zero time) and the accuracy of performance, i.e., positional stability of the unloaded finger. Although the trial-by-trial variability was large, it is suggested that at least some of this variability is caused by a nonsteady state of motoneuronal excitability. In view of the low-pass property of the muscle, the observed variability in synchronization may be sufficiently precise to maintain the hypothesis of a central temporal coupling of the events in the two hands through a common command. However, the lack of a correlation between the degree of synchronization and the performance in stability argues rather in favor of separate commands to the two hands that select the parameters in the spatial domain. Finally, an intermanual EMG or torque analysis is proposed that might be useful in assessing the accuracy in goal achievement, i.e., the maintenance of a stable finger position in spite of the "internal" perturbation.

Electromyography↗
PubMed1995

Directional specificity of postural muscles in feed-forward postural reactions during fast voluntary arm movements.

Healthy subjects performed bilateral fast shoulder movements in different directions while standing on a force platform. Anticipatory postural adjustments were seen as changes in the electrical activity of postural muscles as well as displacements of the center of pressure and center of gravity. Postural muscle pairs of agonist-antagonist commonly demonstrated triphasic patterns starting prior to the first electromyographic (EMG) burst in the prime-mover muscle. Proximal postural muscles demonstrated the largest anticipatory increase in the background activity during movements in one of the two opposite directions (forward or backwards). These changes progressively decreased when movements deviated from the preferred direction and frequently disappeared during movements in the opposite direction. The patterns in distal muscles varied across subjects and could demonstrate larger anticipatory changes during movements forward and backwards as compared to movements in intermediate directions. Bilateral addition of inertial loads to the wrists did not change the general anticipatory patterns, while making some of their features more pronounced. Anticipatory postural adjustments were followed by later changes in the activity of postural muscles, also reflected in the mechanical variables. Changes in leg joint angles revealed a "hip-ankle strategy" during shoulder flexions and an "ankle strategy" during shoulder extensions. The study demonstrates different behaviors of proximal and distal muscles during anticipatory postural adjustments in preparation for fast arm movements. We suggest that the proximal muscles produce a general pattern of postural adjustments, while distal muscles take care of fine adjustments that are more likely to vary across subjects.

Adult↗
PubMed1975

Responses to load disturbances in human shoulder muscles: the hypothesis that one component is a pulse test information signal.

Human motor control has been investigated by applying displacements acting to rotate the shoulder while the subject was endeavouring to maintain a constant position against a pre-existing force delivered by a system of finite stiffness. Four separate stages of the force response were distinguished. First, for the initial 100 msec, an increase in force which was attributed to the viscoelastic properties of activated muscle. Second, after approximately 100 msec, a "medium latency" increase in force accompanied by an increase in EMG activity. Third, a "long latency" increase in force consistent with voluntary action restored the arm to its original position. Fourth, some 500 msec thereafter and dependent upon the final steady force level, a tremor might develop. No changes of force were seen that were of sufficiently short latency to be attributed either to Ia monosynaptic action or immediately following (within 15 msec) polysynaptic action of muscle mechano-receptors. Even the "medium latency" response was too weak to make an appreciable contribution to restoring the arm to its original position; at the best it provided only 15% of the force required. This response is often considered as a "stretch reflex" responsible for maintaining posture in its own right; for example, by means of a servo-assisted transcortical loop (Marsden et al., 1972). Instead, it is now suggested that it might be a test signal designed to inform the central nervous system of the current loading on the muscle and thus permit the CNS to select an appropriate pre-programmed response from its repertoire of motor actions.

Elasticity↗
PubMed1993

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle.

The effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression of the electromyogram (EMG) signal has been reported midway between the seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62:91-98) although this electrode position has been recommended (Zipp 1982, Eur J Appl Physiol 50: 41-54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical, abducted and flexed positions and they performed a dynamic movement test. The myo-electric signal was recorded along the length of the right upper trapezius muscle by a 16-channel bipolar array electrode and was integrated with a 0.2-s time resolution. A region just lateral to the midpoint between C7 and the lateral edge of acromion was found with high and stable amplitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion probably due to sliding of the skin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral and the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an interelectrode distance of 2 cm a centre position 2 cm lateral to the midpoint between C7 and acromion provided good repeatability and high signal yield.

Adult↗
PubMed1982

The role of sensory conflict on stair descent performance in humans.

Electromyographic (EMG) activity produced in the triceps surae (TS) and subsequent landing were examined under various visual conditions during stair descent with the following results: The amount of precontact TS EMG was reduced during each visual perturbation. Perturbations corresponded to no knowledge or visualization of stairs (B), no stair visualization during descent (A) and vertical movement of the surround during descent (M). Erroneous visual information was primarily responsible for altered EMG activity. The only known difference between the M data sets was that the surround moved up (U) or down (D) as the subject descended. However, TS EMG characteristics were different under these two conditions. Specific visual information appeared necessary for vision to override the other sensory systems. There was no difference in EMG when the room moved up (U) compared to the room not moving (NM). However, EMG activity was significantly different when the room moved down (D) compared to the room not moving (NM). The relationship between TS EMG activity and subsequent landing appeared related to landing strategy. Although the EMG was reduced during both the B and M test conditions compared to the control, the landing was "softer" for B and harder for M. The pre-contact EMG is apparently part of a preprogrammed movement pattern which can be modified by sensory information during task execution. Future studies should examine the neuronal mechanisms which provide the visual system access to the center controlling lower limb muscle activity during dynamic movement.

Adolescent↗
PubMed2001

Triggering of balance corrections and compensatory strategies in a patient with total leg proprioceptive loss.

Triggering of balance corrections may depend on both leg and trunk proprioceptive inputs. To study this issue and to determine how a total proprioceptive loss in the legs (ToLPL) would affect postural reactions in different directions, we investigated the postural control of a patient with a long-standing dorsal root ganglionopathy. This patient had absent stretch reflexes at the ankle and knee joints, delayed reflexes at the hips, but normal muscle strength. Postural control was probed with support-surface movements driven by two different experimental protocols. The first protocol concentrated on leg muscle responses by varying ankle inputs during pitch plane perturbations. The second protocol focussed on the directional sensitivity of upper body responses using combined roll and pitch tilt perturbations. For both protocols, identical techniques were used to record ankle torques, angular velocities of the upper legs and trunk, and surface EMG from leg, hip and trunk muscles. For the first protocol, pitch plane stance perturbations with three different ankle inputs were imposed by a movable support surface. A simultaneous 4-cm rearward translation and 4-deg toe-up rotation produced an 80-deg/s "enhanced ankle input", a simple toe-up rotation gave a 40-deg/s "normal" ankle input and a simultaneous 4-cm rearward translation and 4-deg "toe-down" rotation yielding a 0-deg/s "nulled ankle input". Responses in the ToLPL patient were compared to those of healthy controls and those of patients with lower-leg proprioceptive loss (LLPL). Following normal and enhanced ankle input perturbations, stretch reflexes were absent in ankle and knee joint muscles of the ToLPL patient. Balance correcting responses in the lower legs were diminished and delayed by some 45 ms. In quadriceps, balance-correcting responses were larger than normal, peaked earlier and were not delayed. During the nulled ankle input condition, the ankle muscle responses in the ToLPL patient were again diminished and delayed by 40 ms with respect to both normal subjects and LLPL patients. However, the ToLPL patient again generated an earlier, larger, balance correcting response in quadriceps. For the second protocol, combinations of roll and pitch perturbations were also delivered by a moving support surface. The amplitude was 7.5 deg at 50 deg/s. Eight different directions were applied randomly (pure "toes down", pure "toes up" and directions at 45-deg intervals of roll). As with the first protocol pre-stimulus background muscle activity was excessive in all trunk and most leg muscles. Responses to roll tilt produced several striking changes from normal in the ToLPL patient. First reflexes in gluteus medius were delayed. Second, the trunk roll which commences around 30 ms in normals was in the opposite direction. This roll was accompanied by oppositely directed stretch reflexes in paraspinal muscles. Third, directional sensitivity of balance corrections was far more roll oriented in leg and trunk muscles. Fourth, some tilt directions caused a deactivation response of background activity. This "deactivation strategy" strongly contrasted with the strategy of controls who had low pre-stimulus background activity and activated responses around 100 ms to correct postural instability. These findings provide new insights into the generation of pitch and roll plane directed balance corrections based on the interaction of proprioceptive trigger signals from the ankles, knees and hips. Without proprioceptive input from the ankle and knee, ankle muscle responses are delayed but not absent. Upper leg and trunk responses are not delayed. This suggests that most, if not all, lower leg balance correcting responses are triggered by hip and, possibly, trunk proprioceptive inputs. When leg proprioceptive input is absent, balance correcting responses lose pitch plane sensitivity. The solution used by the patient to overcome these deficits was to markedly raise background muscle activity levels, presumably to provide a stiffer body structure. The lack of trunk flexibility and lateral instability this produced for roll tilts was offset by the ability to compensate by using a hitherto not described "deactivation response" strategy. The patient had a clinical picture usually described as "deafferented"; yet our roll tilt perturbations revealed delayed reflex responses in hip muscles. With vestibulospinal and neck-proprioceptive inputs, these responses may have helped with the development of compensation processes for the total leg proprioceptive deficit.

Adaptation, Physiological↗