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Like most of the cranial muscles involved in speech, the trigeminally innervated anterior digastric muscles are controlled by descending corticobulbar projections from the primary motor cortex (M1) of each hemisphere. We hypothesized that changes in corticobulbar M1 excitability during speech production would show a hemispheric asymmetry favoring the left side, which is the dominant hemisphere for language processing in most strongly right handed subjects. Fifteen volunteers aged 24.5+/-5.3 (SD) yr participated. All subjects were strongly right handed as reported by questionnaire. A surface electromyograph (EMG) was recorded bilaterally from digastrics and jaw movement detected by an accelerometer attached to a lower incisor. Focal transcranial magnetic stimulation (TMS) was used to assess corticomotor excitability of the digastric representation in M1 of both hemispheres during four tasks: 1) static isometric contraction of digastrics; 2) speaking a single word; 3) visually guided, nonspeech jaw movement that matched the jaw kinematics recorded during task 2; and 4) reciting a sentence. Background EMG was well matched in all tasks and jaw kinematics were similar around the time of the TMS pulse for tasks 2-4. TMS resting thresholds and digastric muscle-evoked potential (MEP) size during isometric contraction did not differ for TMS over left versus right M1. MEPs elicited by TMS over left, but not right M1 increased in size during speech and nonspeech jaw movement compared with isometric contraction. We conclude that left corticobulbar M1 is preferentially engaged for descending control of digastric muscles during speech and the performance of a rapid jaw movement to match a target kinematic profile.

The authors sought to determine whether repeated practice of a skilled motor task reduced the tremor arising from pulsatile control that occurs during and after training. Participants flexed and extended their index finger at the metacarpophalangeal joint to track a screen cursor during skill training, in 6 training runs, each of 3-min duration. Nonskill training comprised voluntary flexion and extension movements. The authors measured performance by the average tracking error in a standard 10-s target pattern embedded in the training runs. Cross-correlation of the motor performance and the target pattern revealed that the improved ability to match the shape of the target pattern accounted for 63% of the improved motor performance and that the decreased time to respond to changes in the target line accounted for 10% of the improvement. Skill, but not nonskill training, reduced tremor after 3 min of training during the training movements and during movements 10 and 25 min afterwards. The authors observed no changes in resting tremor after either training protocol. Although training reduced the tremor, this reduction in itself did not significantly improve tracking performance. The authors conclude that visuomotor skill training produces a general reduction in finger tremor (pulsatile control) during voluntary movements that extends beyond the period of training.

Objective: To evaluate the time-course of changes in masseter motoneuron pool excitability following transcranial magnetic stimulation of motor cortex, and relate this to the duration of the masseter cortical silent period (CSP)., Methods: Surface EMG was recorded bilaterally from masseter and digastric muscles in 13 subjects. Focal TMS was applied at 1.3x active motor threshold (AMT) to motor cortex of one hemisphere to elicit a muscle evoked potential (MEP) and silent period bilaterally in masseter as subjects maintained an isometric bite at approximately 10% maximum. With jaw muscles relaxed, a servo-controlled stretcher evoked a stretch reflex in masseter which was conditioned by TMS (1.3x AMT) at 14 different conditioning-testing intervals. There were 20 trials at each interval, in random order. TMS evoked no MEP in resting masseter, but often produced a small MEP in digastric., Results: Mean (+/-SE) masseter CSP was 67+/-3ms. The masseter stretch reflex was facilitated when stretch preceded TMS by 8 and 10ms, which we attribute to spatial summation of corticobulbar and Ia-afferent excitatory inputs to masseter. Masseter stretch reflex amplitude was reduced when TMS was given up to 75ms before stretch, and for up to 2ms afterwards., Conclusions: We conclude that descending corticobulbar activity evoked by TMS acts bilaterally on brainstem interneurons that either inhibit masseter motoneurons or increase pre-synaptic inhibition of Ia-afferent terminals for up to 75ms after TMS. The reduction of masseter motoneuron pool excitability following TMS has a similar time-course to the CSP., Significance: In contrast to the situation for spinal and facial (CN VII) muscles, the masseter CSP appears to have no component that can be attributed exclusively to cortical mechanisms. Abnormalities in the masseter cortical silent period observed in neurological conditions may be due to pathophysiological changes at cortical and/or sub-cortical levels.

Objective: To determine whether a single hemisphere exerts distinct inhibitory influences over masseter muscles on each side, and to compare features of the masseter cortical silent period (CSP) evoked by transcranial magnetic stimulation (TMS) with previous reports from limb and other cranial muscles., Methods: Focal TMS was applied over the motor cortex jaw area in 14 normal subjects. In one experiment, TMS intensity was constant (1.1 or 1.3x active motor threshold, T) and masseter muscle activation varied from 10% to 100% of maximal. In another experiment, muscle activation was constant (20% maximal) and TMS intensity varied from 0.7 to 1.3T., Results: In all subjects, TMS evoked a silent period of similar duration in masseter muscles on both sides. Masseter CSP duration increased at higher TMS intensities, but was not affected by muscle activation level or the size of the excitatory response evoked by TMS. Weak TMS produced a bilateral CSP without short-latency excitation. The masseter CSP was short ( approximately 100ms at 1.3T), yet this was not due to maintenance of excitatory drive from the unstimulated hemisphere, as the masseter CSP was not prolonged with dual-hemisphere TMS., Conclusions: Intracortical inhibitory circuits activated by TMS have a relatively weak effect on corticotrigeminal neurons supplying masseter, and effects are equivalent for corticobulbar efferents directed to contralateral and ipsilateral masseter motoneuron pools., Significance: Trigeminally innervated masseter muscles exhibit weak, bilaterally symmetric inhibition following focal TMS. This method can be used to investigate abnormalities of intracortical inhibition in movement disorders or focal lesions affecting the masticatory muscles in humans.

Normal physiological tremor of the jaw has a frequency of 6 to 8 Hz. A patient is described with jaw tremor at frequencies of 12 Hz during jaw movement and 15 Hz when the jaw was relaxed. The 15 Hz tremor was driven by synchronous, bilateral bursts of activity in the temporalis and masseter muscles, which alternated with digastric bursts. Coherence analysis indicated the tremor was highly correlated with both opening and closing muscle activity, and that the opening and closing muscles were about 180 degrees out of phase. The existence of two tremors with different, nonphysiological peak frequencies and the influence of attention, relaxation, and movement in switching from one tremor frequency to the other, suggest that more than one generator may be operating., (2007 Movement Disorder Society)

Human precision grip requires precise scaling of the grip force to match the weight and frictional conditions of the object. The ability to produce an accurately scaled grip force prior to lifting an object is thought to be the result of an internal feedforward model. However, relatively little is known about the roles of various brain regions in the control of such precision grip-lift synergies. Here we investigate the role of the primary motor (M1) and sensory (S1) cortices during a grip-lift task using inhibitory transcranial magnetic theta-burst stimulation (TBS). Fifteen healthy individuals received 40 s of either (i) M1 TBS, (ii) S1 TBS or (iii) sham stimulation. Following a 5-min rest, subjects lifted a manipulandum five times using a precision grip or completed a simple reaction time task. Following S1 stimulation, the duration of the pre-load phase was significantly longer than following sham stimulation. Following M1 stimulation, the temporal relationship between changes in grip and load force was altered, with changes in grip force coming to lag behind changes in load force. This result contrasts with that seen in the sham condition where changes in grip force preceded changes in load force. No significant difference was observed in the simple reaction task following either M1 or S1 stimulation. These results further quantify the contribution of the M1 to anticipatory grip-force scaling. In addition, they provide the first evidence for the contribution of S1 to object manipulation, suggesting that sensory information is not necessary for optimal functioning of anticipatory control.

Background: Reorganization of the human motor cortex can be induced by specific patterns of peripheral afferent stimulation. The potential for afferent stimulation to facilitate the functional recovery associated with conventional rehabilitative techniques has not previously been investigated., Objective: The authors sought to determine whether combining appropriate afferent stimulation with task-specific training resulted in greater improvements than training alone in patients with impaired upper limb function in the subacute phase following stroke., Method: Twenty patients with hemiparesis due to stroke were allocated randomly to either a stimulation or control group. All received 9 sessions of task-specific physiotherapy training over 3 weeks. Prior to each training session, associative electrical stimulation of the motor point of 2 hand muscles was given in the stimulation group, whereas the control group received sham stimulation. Changes in dexterity were assessed using a grip-lift task, and standard measures of upper-limb function were made before and following the intervention. Corticospinal excitability was examined using transcranial magnetic stimulation., Results: Both groups showed comparable improvements in functional measures of upper-limb function. Of the 20 patients, only 14 could perform the grip-lift task, which is an objective measure of dexterity. Patients in the stimulation group exhibited significantly greater improvements in this task than the control group. There was no significant change in corticospinal excitability in either group., Conclusion: This pilot study provides preliminary data suggesting that targeted afferent stimulation may facilitate the response to conventional rehabilitation in patients with hemiparesis due to stroke, but these results need to be confirmed in a larger scale study.

Objective: To investigate the presence and features of short-interval intracortical inhibition (SICI) in the human trigeminal motor system., Methods: Surface electromyogram (EMG) was recorded from left and right digastric muscles in 7 subjects, along with additional experiments with intramuscular EMG in 2 subjects. Focal transcranial magnetic stimulation (TMS) was used to activate the motor cortex of one hemisphere and elicit motor evoked potentials (MEPs) in digastric muscles on each side, at rest and while subjects activated the muscles at 10% maximal EMG. Paired or single TMS pulses were delivered in blocks of trials, while conditioning TMS intensity and interstimulus interval (ISI) were varied., Results: At rest, paired TMS (3-ms ISI) with conditioning intensities 0.8-0.9x active motor threshold (TA) reduced the digastric MEP amplitude to a similar extent bilaterally. Conditioning at 0.5-0.7TA did not significantly reduce the MEP. MEP amplitude was reduced to a similar extent in both digastric muscles by ISIs between 1 and 4 ms (0.8TA). Voluntary bilateral activation of digastric muscles reduced the effectiveness of conditioning TMS compared to the resting state, with no differences between sides. The similarity of the responses in both digastric muscles was not due to EMG cross-talk (estimated to be approximately 10% in surface records and approximately 2% in intramuscular records), as the intramuscular records showed the same pattern as the surface records., Conclusions: The effects of paired-pulse TMS on digastric are similar to those reported for contralateral hand muscles, and are consistent with activation of SICI circuits in M1 by conditioning TMS. Our evidence further suggests that the corticomotor representations of left and right digastric muscles in M1 of a single hemisphere receive analogous inhibitory modulation from SICI circuits., Significance: SICI has been demonstrated in the face area of motor cortex controlling the trigeminal motor system in normal subjects. This method can be used to investigate abnormalities of SICI in movement disorders affecting the masticatory muscles in humans.

This article reviews recent experimental evidence explaining the mechanisms that support the mandible in its rest or postural position when the head is stationary and during locomotion. At rest, and during slow jaw movements, there is alternating activation of the jaw-opening and jaw-closing muscles which arises from a central pattern generator. However, this cannot account for the rest position of the mandible even when the head is stationary. Jaw movements and masticatory muscle activity were measured in subjects who stood, walked and ran on a treadmill. Even during walking, there are no bursts of masseter EMG time-locked to heel-landing. However, when subjects ran, the downward movement of the mandible in each step evokes a burst of EMG in the masseters. This is a stretch reflex in the jaw-closing muscles, which acts to limit the downward movement of the mandible relative to the maxilla during locomotion, and to restore the mandibular position towards its rest position. Thus, when the head is stationary, the low-level activity in the jaw-opening and jaw-closing muscles does not contribute to the rest position. Instead, the mandible is supported by passive viscoelastic forces in perioral soft tissues which limit vertical jaw movements even when the head moves gently up and down during walking. When the head moves more vigorously up and down, stretch reflexes in the jaw-closing muscles limit the movement of the mandible. That is, both passive forces and active reflex responses maintain jaw posture within narrow limits during brisk head movements.

Conditions known to cause hand pain, such as arthritis, are often accompanied by impaired dexterity. The aim of this study was to determine whether this association is coincident, or whether pain affects dexterity directly. In the first part of the study, several tests of dexterity based on pegboard skills were compared with a precision-grip-lift task: the correlations between the results of any of these tests were not significant at the 0.01 level. Nineteen subjects were then tested with a modified Purdue pegboard test and the precision grip-lift task, both without pain and during pain induced by injection of 5% hypertonic saline into the first dorsal interosseous muscle of the non-dominant hand. There was no significant difference in the performance of either task when the muscle was painful, indicating that acute experimental muscle pain does not affect dexterity.

Objective: To determine the pattern of organization of common inputs to the motoneuron pools of individual muscles in the masticatory system., Methods: Six subjects bit on a rubber-coated wooden splint placed between the upper and lower incisor teeth. We recorded the surface electromyogram (EMG) of co-contracting masseter, temporalis and digastric muscles bilaterally during isometric jaw closing at 5%, 10%, 20% and 40% of maximal voluntary masseter EMG., Results: The cross-correlograms of the EMGs of homologous muscle pairs indicate that there are common synaptic inputs to the motoneuron pools of the left and right masseter, and left and right digastric muscles, but not to left and right temporalis. The amplitude of the central peak in masseter and digastric correlograms increased with bite force. When the activity of ipsilateral muscle pairs was cross-correlated, central peaks were prominent for masseter-digastric and masseter-temporalis muscle pairs, and the peak amplitudes increased significantly with bite force. In contrast, no significant central peak was observed for temporalis-digastric muscle pairs at any level of voluntary biting., Conclusions: We conclude that there is synchronous modulation of input bilaterally to the masseter muscles and to the digastric muscles but not to the temporalis muscles. There is synchronous modulation of input to ipsilateral masseter-digastric and masseter-temporalis muscle pairs but not to temporalis and digastric muscles., Significance: The extent of common input to motoneuron pools of muscles acting around a common joint varies for different muscle pairs, and is not simply a function of whether the muscles of the pair are synergists or antagonists.

Objective: Analysis of a precision grip-lift task provides measures to assess functional disability of the hand, but the correlation between these measures and accepted tests of motor function in stroke patients has not been established., Methods: Seventeen subacute stroke patients were studied to compare parameters of a precision grip-lift task between the affected and unaffected side, and to correlate them with function. Functional impairment was assessed with the Action Research Arm Test and the Fugl-Meyer assessment, as well as grip strength and maximal finger-tapping speed. The grip force (GF) and load force (LF) were recorded as patients lifted a custom-built manipulandum. All measures were recorded on two separate occasions, at least 1 week apart., Results: There was good reproducibility between testing sessions for the grip-lift and functional measures. The affected hand gripped the manipulandum for longer prior to lift-off than the unaffected hand, and the normal close temporal coupling between the rate of change of GF and LF during the lift was disrupted. These two measures correlated more highly with the ARAT than the FMA and, when combined with measures of grip strength and tapping speed, explained 71% of the variance of the ARAT., Conclusions: The grip-lift task is a sensitive measure of impaired dexterity following stroke and provides measures which correlate well with a commonly applied functional assessment scale., Significance: This task may be used clinically to detect changes in the hemiplegic upper limb during rehabilitation and recovery.

Transcranial magnetic stimulation was used to map hand muscle representations in the motor cortex of a patient in whom infarction of the sensory thalamus deprived the sensorimotor cortex of sensory input. The threshold for activation of the motor cortex on the affected side was higher and the cortical representational maps of individual muscles were less well defined than those on the normal side. It is concluded that electrophysiological changes in cortical organisation can be demonstrated following withdrawal of, or imbalance in sensory afferent activity to the cerebral cortex in humans.

Alternate grip strategies are often used for object manipulation in individuals with sensorimotor deficits. To determine the effect of grip type on force control, ten healthy adult subjects were asked to grip and lift a small manipulandum using a traditional precision grip (lateral pinch), a pinch grip with the fingers oriented downwards (downward pinch) and a "key grip" between the thumb and the side of the index finger. The sequence of grip type and hand used was varied randomly after every ten lifts. Each of the three grips resulted in different levels of force, with the key grip strategy resulting in the greatest grip force and the downward pinch grip using the least amount of grip force to lift the device. Cross-correlation analysis revealed that the ability to scale accurately the rate of grip force and load force changes was lowest in the downward pinch grip. This was also associated with a more variable time-shift between the two forces, indicating that the precise anticipatory control when lifting an object is diminished in this grip strategy. There was a difference between hands across all grips, with the left non-dominant hand using greater grip force during the lift but not the hold phase. Further, in contrast with the right hand, the left hand did not reduce grip force during the lift or the hold phase over the ten lifts, suggesting that the non-dominant hand did not quickly learn to optimise grip force. These findings suggest that the alternate grip strategies used by patients with limited fine motor control, such as following stroke, may partly explain the disruption of force control during object manipulation.

1. The normal human motor cortex can be made to reorganize by repeated stimulation of proprioceptive inputs, with or without concurrent stimulation of the motor cortex by transcranial magnetic nerve stimulation. Appropriate stimulation induces a focal increase in the excitability of corticospinal projections to specific muscles and, possibly, an increase in the area of the cortex projecting to those muscles. 2. We have shown that repeated stimulation on several successive days causes this 'plastic' reorganization to persist for at least several days. We have also used this approach to determine whether increases in the excitability of the motor cortex can be induced in stroke patients (in whom cortical excitability is usually depressed) and whether this is accompanied by functional changes. 3. The results of these studies were mixed but, in patients in whom plastic changes were induced, there were improvements and sometimes marked improvements in both motor function and some electrophysiological parameters. The reasons for the inconsistent results are not clear, but do not appear to relate to the site, size or nature of the lesion.

Stretch reflexes play a vital role in fine-tuning movements and in automatically maintaining posture. This article briefly reviews the operation of the stretch reflex in the human masticatory system. The conventional approach of stretching muscles in an open-loop manner has yielded much valuable information on the operation of this reflex. In particular, it has revealed that stretching the jaw-closing muscles evokes a reflex response with two major components. The short-latency reflex is favoured when stretches are brisk, but slower stretches evoke an additional long-latency component. In the hand muscles, the long-latency response is transcortical: in the masticatory muscles, it is not. In addition to its role in servo-control of muscle length during chewing, the stretch reflex in the jaw-closing muscles maintains the vertical position of the mandible during vigorous head movements such as those that occur during running, jumping, hopping and other vigorous whole-body movements in which the head moves briskly up and down. This is an interesting model system in which to investigate stretch reflexes with natural stimuli under unrestrained, physiological conditions.

This study assessed the reliability of alternate methods of analysis of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). We recorded two sets of MEPs (Time 1 and Time 2) at the optimal scalp sites for both the right first dorsal interosseous (FDI) and flexor carpi ulnaris (FCU) at two different stimulation intensities in 10 healthy subjects. MEP magnitude was determined in each of the following three ways: the mean peak-to-peak amplitude and area of the 20 individual responses; the amplitude and area of the ensemble averaged waveform; and the amplitude and area of the maximal response. There was no significant difference in amplitude or area for either muscle using any of the three methods between Time 1 and 2. However, the ensemble average (area and amplitude) was significantly smaller that the mean MEP, and the maximal MEP amplitude was significantly larger. Intraclass correlation analysis demonstrated that reliability of MEP measures over time was poor regardless of method. Reliability was similar between methods for FDI, but FCU had lower reliability values for the mean and ensemble average methods than the maximal method.

Mandibular movements and masseter muscle activity were measured in humans during hopping, walking and running to determine whether reflexes contribute to the maintenance of jaw position during locomotion. In initial experiments, subjects hopped so that they landed either on their toes or on their heel. Landing on the toes provoked only small mandibular movements and no reflex responses in the masseter electromyogram (EMG). Landing on the heels with the jaw muscles relaxed caused the mandible to move vertically downwards relative to the maxilla, and evoked a brisk reflex response in the masseter at monosynaptic latency. Neither this relative movement of the mandible nor the reflex was seen when the teeth were clenched: hence the reflex is not the result of vestibular activation during head movement. The same variables were measured in a second series of experiments while subjects stood, walked and ran at various speeds and at various inclinations on a treadmill. During walking, the vertical movements of the head and therefore the mandible were slow and small, and there was no tonic masseter EMG or gait-related activity in the jaw-closing muscles. When subjects ran, the vertical head and jaw movement depended on the running speed and the inclination of the treadmill. Landing on the heels induced larger movements than landing on the toes. About 10 ms after each foot-strike, the mandible moved downwards relative to the maxilla, thereby stretching the jaw-closing muscles and activating them at segmental reflex latency. This caused the mandible to move back upwards. The strength of the reflex response was related to the speed and amplitude of the vertical jaw movement following landing. It is concluded that, during walking, the small, slow movements of the mandible relative to the maxilla are subthreshold for stretch reflexes in the jaw muscles: i.e. the mandible is supported by visco-elasticity of the soft tissues in the masticatory system. However, the brisker downward movements of the mandible after heel-landing during hopping and running evoke segmental reflex responses which contribute to the active maintenance of the posture of the mandible. This is a unique demonstration of how a stretch reflex operates to maintain posture under entirely natural conditions.

Objective: To determine whether the plastic changes induced in human motor cortex by afferent stimulation depend on stimulus frequency., Methods: Transcranial magnetic stimulation was used to examine changes in corticospinal excitability in 20 subjects before and after combined peripheral (motor point) and central stimulation. Peripheral stimuli were given as either low frequency (3 Hz) or high frequency (30 Hz) trains., Results: Low frequency stimulation induced prolonged depression of corticospinal excitability, while high frequency stimulation induced prolonged facilitation. These effects persisted for approximately 40-50 min after stimulation ceased., Conclusions: Corticospinal plasticity induced by dual peripheral and central stimulation is bi-directionally-modifiable in the adult human, with the direction of change being frequency-dependent., Significance: Therapies using peripheral stimulation to alter human motor cortex excitability could be tailored to exploit the differential effects of stimulus frequency on the direction of the excitability change.

The corticobulbar inputs to single masseter motoneurons from the contra- and ipsilateral motor cortex were examined using focal transcranial magnetic stimulation (TMS) with a figure-of-eight stimulating coil. Fine-wire electrodes were inserted into the masseter muscle of six subjects, and the responses of 30 motor units were examined. All were tested with contralateral TMS, and 87 % showed a short-latency excitation in the peristimulus time histogram at 7.0 +/- 0.3 ms. The response was a single peak of 1.5 +/- 0.2 ms duration, consistent with monosynaptic excitation via a single D- or I1-wave volley elicited by the stimulus. Increased TMS intensity produced a higher response probability (n = 13, paired t test, P < 0.05) but did not affect response latency. Of the remaining motor units tested with contralateral TMS, 7 % did not respond at intensities tested, and 7 % had reduced firing probability without any preceding excitation. Sixteen of these motor units were also tested with ipsilateral TMS and four (25 %) showed short-latency excitation at 6.7 +/- 0.6 ms, with a duration of 1.5 +/- 0.3 ms. Latency and duration of excitatory peaks for these four motor units did not differ significantly with ipsilateral vs. contralateral TMS (paired t tests, P > 0.05). Of the motor units tested with ipsilateral TMS, 56 % responded with a reduced firing probability without a preceding excitation, and 19 % did not respond. These data suggest that masseter motoneurons receive monosynaptic input from the motor cortex that is asymmetrical from each hemisphere, with most low threshold motoneurons receiving short-latency excitatory input from the contralateral hemisphere only.

Resting tremor is seen in both the limbs and in the trigeminal motor system. These rhythmical perturbations are the result of alternating activation of antagonistic muscles, and these increase in amplitude during slow, voluntary movements. In the present study, we examined the effect of experimental muscle pain on finger and jaw tremor. The tremor in the mandible and in the middle finger was measured on separate occasions, at rest and during two constant-velocity movements. Pain was then induced by the infusion of hypertonic saline into a jaw-closing muscle (masseter) or into a finger extensor muscle (extensor digitorum longus, EDL). During masseter pain, the power at the peak tremor frequency of the mandible decreased significantly both when the jaw was at rest and during voluntary jaw movements at two velocities. In contrast, pain in EDL resulted in a significant increase in the power of finger tremor only during the two speeds of voluntary movement. No change in the peak tremor frequency was seen in either the finger or the jaw during pain. The most likely explanation for these data is that muscle pain tonically modulates the amplitude of the outputs from the central "pulsatile control" generators that drive the alternating activation of antagonistic muscles which produce tremor at rest and during movements. This modulation is in the opposite direction for systems controlling jaw and hand, suggesting a specific interaction of the nociceptive afferents with separate central oscillators.

Movements of the head and of the mandible relative to the head were measured in human subjects walking and running on a treadmill at various speeds and inclinations. A miniature magnet and piezo-electric accelerometer assembly was mounted on the mandibular incisors, and a Hall-effect sensor along with a second accelerometer mounted on a maxillary incisor along a common vertical axis. Signals from these sensors provided continuous records of vertical head and mandible acceleration, and relative jaw position. Landing on the heel or on the toe in different forms of locomotion was followed by rapid deceleration of the downward movement of the head and slightly less rapid deceleration of the downward movement of the mandible, i.e., the mandible moved downwards relative to the maxilla, then upwards again to near its normal posture within 200 ms. No tooth contact occurred in any forms of gait at any inclination. The movement of the mandible relative to the maxilla depended on the nature and velocity of the locomotion and their effects on head deceleration. The least deceleration and hence mandibular displacement occurred during toe-landing, for example, during "uphill" running. The maximum displacement of the mandible relative to the head was less than 1mm, even at the fastest running speed. The mechanisms that limit the vertical movements of the jaw within such a narrow range are not known, but are likely to include passive soft-tissue visco-elasticity and stretch reflexes in the jaw-closing muscles.

A long-latency stretch reflex (LLSR) has been described in the human masseter muscle, but its pathway remains uncertain. To investigate this, the excitability of corticomotoneuronal (CM) cells projecting to masseter motoneurons during the LLSR was assessed with transcranial magnetic stimulation (TMS). A facilitated response to TMS would be evidence of a LLSR pathway that traverses the motor cortex. Surface electromyogram electrodes were placed over the left or right masseter, and subjects ( n=10) bit on bars with their incisor teeth at 10% of maximal electromyographic activity (EMG). Servo-controlled displacements were imposed on the lower jaw to evoke a short- and long-latency stretch reflex in masseter. TMS intensity was just suprathreshold for a response in contralateral masseter. Trials consisted of: (1) stretch alone, (2) TMS alone, and (3) TMS with a preceding conditioning stretch at varied conditioning-testing (C-T) intervals chosen to combine TMS with the short-latency stretch reflex (3 ms, 5 ms) and the LLSR (23-41 ms). Masseter EMG was rectified and averaged. With TMS alone, mean (+/- SE) MEP area above baseline was 56+/-9%. The area of masseter MEPs above baseline in the C-T trials was calculated from each EMG average following subtraction of the response to stretch alone. Conditioning muscle stretch had no significant effect on masseter MEPs evoked by TMS with any C-T interval (ANOVA; P=0.90). In addition, subjects were unable to modify the SLSR or LLSR by voluntary command. It is concluded that the long-latency stretch reflex in the masseter does not involve the motor cortex and is not influenced by "motor set".

This study sought to determine whether prolonged peripheral nerve stimulation was effective in inducing persistent "plastic" changes in the excitability of the human motor cortex. The amplitude of the electromyographic response evoked in resting intrinsic hand muscles by focal transcranial magnetic stimulation (TMS) was taken as an index of motor cortical excitability. Twelve subjects were stimulated with each of three protocols, one of which was given on each of three separate occasions. The protocols consisted of various schedules of electrical stimulation of the radial and ulnar nerves or the motor point of the first dorsal interosseous muscle (FDI), or stimulation of FDI motor point paired with low-frequency TMS. Amplitudes of TMS-elicited motor evoked potentials (MEPs) were measured before peripheral stimulation and for 2 h after stimulation. The data from one subject were unusable. In every other subject, all three protocols induced a prolonged, significant facilitation of MEPs in at least some of the three intrinsic hand muscles used. In some instances, MEPs were not enlarged and occasionally were significantly depressed. Different protocols based on peripheral afferent stimulation can induce plastic changes in the organisation of the motor cortex that persist for at least 2 h.

Magnetic stimulation of sensorimotor cortex interferes with the detection of electro-cutaneous stimulation. However, it is uncertain whether this interference is due to activation of the somatosensory or the motor cortex. Here, transcranial magnetic stimuli (TMS) were delivered separately over somatosensory and motor cortex contralateral to the right ulnar nerve in 12 subjects. In separate trials, TMS were given 100 ms before and 20 ms after 60 ms trains of electro-cutaneous ulnar nerve stimuli, and their effect on the subjective perception of peripheral stimuli was assessed. TMS of both motor and somatosensory cortex interfered with the perception of afferent stimuli when given before or after stimulation of the ulnar nerve. Perception was more strongly suppressed by motor cortex stimulation than by somatosensory cortex stimulation, when given before or after the peripheral stimulus. A similar proportion of errors was induced by sensory cortex stimulation between the two stimulus timing intervals. This study suggests that the inhibition of the afferent volley is unlikely to be the result of antidromic activation of thalamocortical connections or corticospinal gating. A phenomenon akin to sensory masking is the most plausible explanation for much of the suppression of sensory perception by stimulation of the motor or somatosensory cortex. The more powerful suppressive effect of motor cortex stimulation may be due to multiple mechanisms.

Spectral analysis of jaw acceleration confirmed that the human mandible 'trembles' at a peak frequency around 6 Hz when held in its rest position and at other stationary jaw openings. The 6 Hz tremor increased during very slow movements of the mandible, but other lower-frequency peaks became prominent during more rapid jaw movements. These lower-frequency peaks are likely to be the result of asymmetries in the underlying, voluntarily produced, 'saw-tooth' movements. In comparison, finger tremor at rest and during slow voluntary movements had a mean peak frequency of about 8 Hz: this frequency did not change during rhythmical finger flexion and extension movements, but the power of the tremor increased non-linearly with the speed of the movement. The resting jaw tremor was weakly coherent with the activity of the masseter and digastric muscles at the tremor frequency in about half the subjects, but was more strongly coherent during voluntary movements in all subjects. The masseter activity was at least 150 deg out of phase with the digastric activity at the tremor frequency (and at all frequencies from 2.5-15 Hz). The alternating pattern of activity in antagonistic muscles at rest and during slow voluntary movements supports the idea that the masticatory system is subject to pulsatile control in a manner analogous to that seen in the finger.

Stimulus-response curves for motor evoked potentials (MEPs) induced in a hand muscle by transcranial magnetic stimulation (TMS) were constructed for 42 subjects with the aim of identifying differences related to age and sex. There was no effect of age on the resting threshold to TMS, the maximal amplitude of the MEP that could be evoked (MEP(max)) or the maximal slope of the stimulus-response curve. However, higher stimulus intensities were required to achieve both MEP(max) and the maximal slope in the older subjects. The trial-to-trial variability of MEPs was greater in the older subjects, particularly at intensities near threshold. There was a significant interaction between age, threshold and trial-to-trial variability of MEP amplitude. Overall, MEP variability fell markedly as stimulus intensity increased above threshold but less rapidly in older than in younger subjects. Females tended to have larger MEP variability than males, but age and threshold were much stronger modulators than sex. These differences in input-output characteristics are likely to be due either to a decreased number of spinal motoneurones being activated synchronously in older subjects, or to the activation of the same number of motoneurones in a less synchronous manner, leading to phase cancellation in the surface electromyogram.

Earlier investigations have shown that stimulation of peripheral afferent nerves induces prolonged changes in the excitability of the human motor cortex. The present study compared the effect of experimental pain and non-painful conditioning stimulation on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in the relaxed first dorsal interosseous (FDI) and flexor carpi ulnaris (FCU) muscles. The MEPs were measured in 10 healthy subjects, and stimulus-response curves were generated before and after each of four stimulation paradigms conducted in random order on separate occasions: (a) control; (b) "dual stimulation" consisting of electrical stimulation of the FDI motor point paired with TMS; (c) painful infusion of hypertonic saline in the FDI muscle; and (d) pain combined with dual stimulation. There were no significant changes in FDI MEPs following the control paradigm, and dual stimulation induced an increase in the FDI MEPs only inconsistently. In contrast, the painful stimulation and the combined pain and dual stimulation paradigms were followed by significant suppression of the FDI MEPs at higher stimulus intensities. No changes were observed in the FCU MEPs following the four paradigms. In two additional subjects, the responses evoked in FDI by direct stimulation of the descending corticospinal tracts were significantly depressed following painful stimulation of the FDI, although the ulnar-evoked M-waves remained constant. It is concluded that muscle pain is followed by a period with profound depression of MEPs amplitudes in the resting muscle, but that these changes are at least in part due to a lasting depression of the excitability of the motoneurones in the spinal cord. Hence, painful stimulation differs from non-painful, repetitive stimulation, which facilitates the corticomotor pathway.

Cortical representation maps derived by transcranial magnetic stimulation (TMS) are often used, inter alia, in studying the plasticity of the brain. Parameters such as map area, map volume, optimal stimulation site and centre of gravity are commonly used to quantify changes in the topography of the motor cortex. However, reports on the stability of these parameters over time has not been conclusive. In the present study, the areas of the scalp from which responses were evoked from corticospinal cells projecting to three intrinsic hand muscles were systematically mapped with TMS at intervals of 24 hours, one week and two weeks from eight normal subjects. The area, "volume" and centre of gravity of these maps did not change significantly over this period. It is concluded that mapping with TMS is suitable for studies which aim to study the effect of various interventions on the cortical representation of individual muscles in human subjects.

We aimed to determine whether postexercise depression of motor-evoked potentials (MEPs) could be demonstrated without voluntary muscle activation in humans. Voluntary fatigue was induced with a 2-min maximal voluntary contraction (MVC) of the first dorsal interosseous (FDI) muscle. On another occasion, "electrical fatigue" was induced with trains of shocks delivered for 2 min over the FDI motor point. Five of the twelve subjects also underwent "sequential fatigue" consisting of a 2-min MVC of FDI followed by 20 min of rest and then 2 min of motor point stimulation. Voluntary fatigue induced MEP depression that persisted for at least 20 min. Electrical fatigue induced a transient MEP facilitation that subsided 20 min after the stimulation and became depressed within 30 min. Thus MEP depression can be induced by both voluntary and electrical fatigue. With electrical fatigue, the initial depression is "masked" by transient MEP facilitation, reflecting cortical plasticity induced by the prolonged electrical stimulation. MEP depression probably reflects tonic afferent input from the exercising muscle that alters cortical excitability without altering spinal excitability.

A simple and inexpensive method for recording vertical movements of the human mandible relative to the maxilla is presented. Measurements are made from accelerometers and a Hall-effect device temporarily glued to the upper and lower anterior teeth. The accelerometer signals are integrated once to give velocity and a second time to give position. Movements of the mandible relative to the maxilla are obtained by integrating the difference between the two accelerometer signals. The (relative) velocity and position records derived in this way are linear, but subject to drift when the jaw is stationary. Steady mandibular position is obtained from the Hall-effect system, but this signal must be corrected for its inherent non-linearity. This device can record rapid movements of the mandible even when the head is unrestrained, and interferes minimally with normal jaw movements.

Motor learning must involve changes in the organisation of the brain, and it seems axiomatic that afferent signals generated during repeated motor practice contribute to this. In this study, motor-point stimulation of the first dorsal interosseous (FDI) muscle was paired with transcranial magnetic stimulation of the human motor cortex on three successive days to determine whether repeated stimulation sessions result in enduring reorganisation of the motor cortex. This repeated "dual" stimulation induced significant changes in the excitability of the motor cortex together with expansion of the area of scalp from which these responses were elicited. The expansion in muscle representation was accompanied by large movements in the centre of gravity (CoG), suggesting a true reorganisation of the underlying cortical representational zone. The changes persisted for at least 2 days following the last stimulation session. It is concluded that repeated dual stimulation is capable of inducing long-lasting reorganisation within the motor cortex. These changes may be similar in nature to those seen in the motor cortex during motor learning. Moreover, these observations suggest that it may be possible to induce the motor cortex of patients who have suffered strokes to reorganise in a way that improves the voluntary control of the weakened muscles.

Following a fatiguing muscle contraction, motor evoked potentials (MEPs) evoked by transcranial magnetic brain stimulation remain depressed for many minutes, reflecting a reduction in the excitability of the corticospinal projection. No functional significance has been linked to this observation. We postulated that dexterity would be affected when MEPs are depressed. MEPs were recorded from the first dorsal interosseous muscle of 11 healthy subjects in resting muscles before and after a fatiguing maximum voluntary contraction. This induced significant MEP depression in all subjects. No change in dexterity was seen when MEPs were depressed or at any time point throughout the experiment. We conclude that fatigue-induced MEP depression is not associated with a decline in dexterity.

Like most other muscles, the human masticatory muscles are controlled by descending signals from the cortex and other supraspinal structures, as well as afferent signals arising in receptors in muscles, skin and other tissues. However, the special functional roles of the masticatory system, and in particular the fact that the muscles on both sides are usually used together, has led to some special adaptations of function.

Objective: Manipulation of afferent input can induce reorganization within the sensorimotor cortex which may have important functional consequences. Here we investigate whether prolonged peripheral nerve stimulation can induce reorganization within the human motor cortex., Methods: Using transcranial magnetic stimulation, we mapped the scalp representation of the corticospinal projection to hand muscles in 8 normal subjects before and after 2h of simultaneous repetitive electrical stimulation of the ulnar and radial nerves at the wrist. Control mapping experiments were conducted in 6 subjects., Results: Following nerve stimulation, larger motor-evoked potentials were evoked from more scalp sites. The induced changes were most apparent in first dorsal interosseous, but were also seen in other hand muscles. The increases in area of the representational maps were accompanied by changes in the location of the optimal site for evoking responses in first dorsal interosseous, and changes in the centres of gravity of the maps., Conclusions: Prolonged afferent stimulation induces an increase in excitability of the corticospinal projection. This is accompanied by a significant shift in the centre of gravity of the stimulated muscles which we propose is evidence of a non-uniform expansion in their cortical representation.

There is strong evidence that the predominant pathway of the long-latency stretch reflex for flexor pollicis longus crosses the motor cortex. This reflex response is diminished during active thumb movements. We tested the hypothesis that this could be due to a decrease in the excitability of the transcortical component during movement. During isometric, concentric and eccentric thumb movements, transcranial magnetic stimulation (TMS) of the motor cortex was given at a time when the reflex signal was traversing the motor cortex. TMS was also given earlier in separate runs when the signal was traversing the spinal cord under each of the three contractile conditions. The electromyogram was analysed for non-linear summation between stretch responses and the potential evoked by the cortical stimulus. The response to TMS alone was uniform across the three types of contraction, and the lack of cortical involvement in the short-latency reflex was confirmed. The TMS-evoked response summed in a non-linear manner with the long-latency reflex response, confirming that the excitability of the motor cortex was increased as the reflex signal passed through it. The long-latency response was markedly depressed during isotonic compared with isometric contractions. However, the non-linear summation was not greater during the isometric contractions. Thus, the depressed reflex responses during isotonic movements do not stem from reduced motor cortical responsiveness or afferent input to the transcortical pathway, and may instead reflect modulation of cutaneous reflexes during isotonic contractions.

In the present study, we examined the electromyogram (EMG) patterns of the soleus and medial gastrocnemius (MG) muscles during rhythmical, two-legged hopping to investigate the contributions of the monosynaptic short- and long-latency stretch reflexes during such a natural movement in human. During rhythmical hopping, soleus muscle is activated reflexly at near-monosynaptic latency by stretch resulting from passive ankle flexion upon landing. Soleus muscle also contracts voluntarily in order to launch the body into the next hop. This is part of the rhythmical bursts of activity producing the hops. Depending on the hopping interval, this phase of activation can follow the short-latency phase or precede landing at very short hopping intervals. In MG, there is an initial phase of activity that stiffens the muscle in preparation for landing, and continues through the contact phase. The monosynaptic reflex response to landing is usually superimposed on this activity. Depending on the hopping interval, both of these responses may be overlaid with activity that is time-locked to the take-off into the next hop, and serves to launch the body into the next hop. However, no evidence for a long-latency stretch reflex was found. In addition, the preferred hopping frequency for all subjects was about 2 Hz. This frequency is associated with a pattern of EMG activity the timing of which indicates that it balances the requirement for a comfortable landing from a hop with the optimal muscle activation required for launching the following hop.

Corticotrigeminal projections to human masseter motoneuron pools were investigated with focal transcranial magnetic stimulation (TMS). Responses in left and right masseter muscles were quantified from the surface electromyogram (EMG) during different biting tasks. During bilateral biting, TMS elicited motor evoked potentials (MEPs) in both masseter muscles. On average, the MEP area in the masseter contralateral to the stimulus was 39% larger than in the ipsilateral muscle, despite comparable pre-stimulus EMG in both muscles. MEPs elicited while subjects attempted unilateral activation of one masseter muscle were compared with those obtained in the same muscle during a bilateral bite at an equivalent EMG level. MEPs in the masseter contralateral to the stimulated hemisphere were significantly smaller during unilateral compared with bilateral biting. There was no significant difference in the size of ipsilateral MEPs during ipsilateral and bilateral biting. We conclude that the corticotrigeminal projections to masseter are bilateral, with a stronger contralateral projection. The command for unilateral biting is associated with a reduced excitability of corticotrigeminal neurons in the contralateral, but not the ipsilateral motor cortex. We suggest that this may be accomplished by reduced activity of a population of corticotrigeminal neurons which branch to innervate both masseter motoneuron pools.

The interaction between muscle pain and motor function of the jaw has been examined in recent years, but the nature of the modulation of the short-latency stretch reflex by pain is not fully understood. In this study, the reflex responses to stretch were measured in single low-threshold motor units that were kept discharging at a constant frequency, before, during and after the induction of experimental pain in one masseter muscle by controlled infusion of hypertonic saline. The probability of evoking a reflex response in individual motor units in the painful muscle at near-monosynaptic latency was reduced by a mean of about 20%. However, the overall reflex response in the surface electromyogram of both the ipsi- and contralateral masseter muscles was greater during pain. This was apparently a secondary response to the pain-induced increase in pre-stimulus activity in the motoneurone pools of both muscles, because increased motoneurone excitability may facilitate stretch reflexes. It is concluded that the most likely explanation for the reduced reflex response of low-threshold masseter motor units during experimental pain is a tonic reduction in the fusimotor drive to the masseter spindles.

The aim of this study was to determine whether prolonged, repetitive mixed nerve stimulation (duty cycle 1 s, 500 ms on-500 ms off, 10 Hz) of the ulnar nerve leads to a change in excitability of primary motor cortex in normal human subjects. Motor-evoked potentials (MEPs) generated in three intrinsic hand muscles [abductor digiti minimi (ADM), first dorsal interosseous (FDI) and abductor pollicis brevis (APB)] by focal transcranial magnetic stimulation were recorded during complete relaxation before and after a period of prolonged repetitive ulnar nerve stimulation at the wrist. Transcranial magnetic stimuli were applied at seven scalp sites separated by 1 cm: the optimal scalp site for eliciting MEPs in the target muscle (FDI), three sites medial to the optimal site and three sites lateral to the optimal stimulation site. The area of the MEPs evoked in the ulnar-(FDI, ADM) but not the median-innervated (APB) muscles was increased after prolonged ulnar nerve stimulation. Centre of gravity measures demonstrated that there was no significant difference in the distribution of cortical excitability after the peripheral stimulation. F-wave responses in the intrinsic hand muscles were not altered after prolonged ulnar nerve stimulation, suggesting that the changes in MEP areas were not the result of stimulus-induced increases in the excitability of spinal motoneurones. Control experiments employing transcranial electric stimulation provided no evidence for a spinal origin for the excitability changes. These results demonstrate that in normal human subjects the excitability of the cortical projection to hand muscles can be altered in a manner determined by the peripheral stimulus applied.

Transcranial magnetic stimulation (TCMS) was used to determine the organization of cortical motor projections to the anterior digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in anterior digastric with a figure-of-eight coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilaterally on intramuscular recordings: this was the result of direct stimulation of the ipsilateral trigeminal motor root. Motor evoked potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencies of around 10 ms by stimulation of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bilaterally in relaxed anterior digastric muscles in six of the seven subjects. In the other subject, the responses in the relaxed muscle were exclusively ipsilateral. However, when the anterior digastric muscles were contracted, the responses were bilateral in all subjects. TCMS and spike-triggered averaging revealed that the bilateral responses were not due to the branching of axons from individual digastric motoneurones to muscles on each side. Because the digastric motor nucleus may contain separate populations of ipsi- and contralateral projecting motoneurones, it was necessary to study single motor-unit responses to TCMS to demonstrate a bilateral corticobulbar projection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were recorded. All were activated by contralateral stimulation. Approximately 80% were also activated by ipsilateral TCMS, although one well-characterised motor unit was inhibited by ipsilateral TCMS. When bilateral activation was present, the ipsilateral responses were more secure than the contralateral responses, which may indicate an additional interneurone in the pathway to the contralateral motoneurone. The major conclusions from this study are that (1) the cortical representation of the anterior digastric muscle is antero-lateral to hand muscles; (2) the cortical projection to the anterior digastric muscles is bilateral; (3) the corticobulbar projection is stronger contralaterally than ipsilaterally but may involve at least one additional synapse; and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally.

The H-reflexes and the motor potentials (MEPs) evoked by electromagnetic brain stimulation in the human wrist flexor were recorded over many trials. The responses from each stimulus at two steady levels of muscle activation were sorted into three groups, based on their amplitudes. The electromyogram (EMG) in each of these groups was rectified and averaged. The level of pre-response muscle activity was found to correlate with the amplitude of both the averaged H-reflexes and the averaged MEPs. This suggests that much of the amplitude fluctuations of both H-reflexes and MEPs can be attributed to moment-to-moment changes in the level of activity of the motoneurone pool. Overall, however, the amplitude of MEPs increased more rapidly than the amplitude of H-reflexes as the pre-stimulus EMG activity increased. This is probably because, while the amplitude of H-reflexes depends primarily on the level of motoneurone pool excitability, the amplitude of an MEP depends not only on this, but also on the excitability of the motor cortex, and the former is to some extent also dependent on the latter.

H reflexes were evoked in human soleus by stimulating the tibial nerve at a constant intensity. Each trial was then assigned to one of three groups on the basis of the amplitude of its H reflex; all trials in each group were then full-wave rectified and reaveraged. There was a strong positive relationship between the amplitude of the H reflex and the level of electromyographic activity in the muscle at the time of onset of the H reflex, which reflects the activity of the motoneuronal pool when the afferent volley arrived. Thus, much of the variability of the H reflex is due to small changes in the level of activation of the motoneuronal pool during repeated trials. The steady torque preceding the H reflex was a poor predictor of the H-reflex amplitude, presumably because of the delay between the changes in the electrical activity of motoneurons and the mechanical outcome thereof.

Surface electromyography (EMG) has been a powerful technique for studying reflex and other stimulus-evoked responses in the human nervous system. However, important additional insights can be gained into the operation of neural circuits by studying the responses of single motor units to various stimuli. In this paper, some of the advantages of single motor unit recording will be canvassed, and some examples of the application to this method to the study of reflex responses to sensory stimuli and brain stimulation will be presented.

The corticotrigeminal projections to masseter and anterior digastric motoneuron pools that are activated by TMS are bilateral, but not symmetrical. This conclusion is supported by whole-muscle data showing larger MEPs in the contralateral muscle with unilateral focal TMS, as well as evidence that TMS stimulation of one hemisphere may produce excitation in a masseter or digastric single motor unit while stimulation of the opposite hemisphere produced inhibition of the same motor unit. The asymmetry is particularly marked for masseter, in which the low-threshold motor units were most commonly excited with contralateral TMS and inhibited with ipsilateral TMS. Spike-triggered averaging of digastric motor unit activity revealed cross-talk in surface EMG recordings from digastric muscles, and no evidence that muscle fibres in both digastric muscles were innervated by a common motor axon. Narrow excitatory peaks in the PSTH of motor unit discharge elicited by TMS in masseter (either hemisphere) and digastric motor units (ipsilateral hemisphere) suggest a direct corticomotoneuronal projection. The contralateral projection to digastric motoneurons may include additional oligosynaptic connections, as judged by the broader peaks in the PSTH with contralateral TMS. The organisation of bilateral corticotrigeminal inputs revealed with TMS suggests that: (a) the contralateral hemisphere provides relatively more of the excitatory input delivered via the fast corticotrigeminal pathway for both masseter and digastric motoneuron pools, and (b) corticotrigeminal projections from either hemisphere are capable of contributing to the voluntary command mediating activation of masseter, and (to a lesser extent) anterior digastric muscles on one side, that is independent of the homologous muscles on the other side.

The reflex responses to brisk, ramp stretch perturbations of the human flexor pollicis longus muscle (FPL) were recorded during isometric and slow concentric or eccentric contractions at similar levels of muscle excitation. The subjects flexed their thumb to push down against a thumb-rest, whose position was controlled by a servo-controlled motor. In different runs, the stretch perturbations were imposed when the thumb-rest was stationary (isometric) or was flexing or extending the interphalangeal joint of the thumb at a constant velocity, i.e. during concentric or eccentric contractions of FPL. The latency of the most prominent component of the electromyographic reflex in the isometrically contracting muscle was about 60 ms, measured from the command signal. The amplitude of this response was sharply reduced during the non-isometric contractions. While not dependent on the direction, this modulation of the reflex response increased with the speed of active movement of the interphalangeal joint (flexion or extension). The response was greatly reduced during concentric or eccentric movements as slow as 1.6 mm x s[-1] (approximately 5 degrees x s (-1) at the joint). When the force rather than the position of the thumb-rest was servo-controlled, the stretch response to perturbation again diminished with speed in a self-paced flexion task, compared with an isometric "hold" condition.

1. Changing the posture of the human fingers can functionally 'disengage' the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs. 2. Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the 'controlled' unit) discharging at the same target frequency in both postures. The strength of short-term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. 3. All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture. 4. Short-term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. 5. Although the discharge rate of the 'uncontrolled' motor units increased when the muscle was disengaged, the variability of discharge of these and the 'controlled' motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.