Phasic and tonic stretch reflexes in muscles with few muscle spindles: human jaw-opener muscles

 We investigated phasic and tonic stretch reflexes in human jaw-opener muscles, which have few, if any, muscle spindles. Jaw-unloading reflexes were recorded for both opener and closer muscles. Surface electromyographic (EMG) activity was obtained from left and right digastric and superficial masseter muscles, and jaw orientation and torques were recorded. Unloading of jaw-opener muscles elicited a short-latency decrease in EMG activity (averaging 20 ms) followed by a short-duration silent period in these muscles and sometimes a short burst of activity in their antagonists. Similar behavior in response to unloading was observed for spindle-rich jaw-closer muscles, although the latency of the silent period was statistically shorter than that observed for jaw-opener muscles (averaging 13 ms). Control studies suggest that the jaw-opener reflex was not due to inputs from either cutaneous or periodontal mechanoreceptors. In the unloading response of the jaw openers, the tonic level of EMG activity observed after transition to the new jaw orientation was monotonically related to the residual torque and orientation. This is consistent with the idea that the tonic stretch reflex might mediate the change in muscle activation. In addition, the values of the static net joint torque and jaw orientation after the dynamic phase of unloading were related by a monotonic function resembling the invariant characteristic recorded in human limb joints. The torque-angle characteristics associated with different initial jaw orientations were similar in shape but spatially shifted, consistent with the idea that voluntary changes in jaw orientation might be associated with a change in a single parameter, which might be identified as the threshold of the tonic stretch reflex. It is suggested that functionally significant phasic and tonic stretch reflexes might not be mediated exclusively by muscle spindle afferents. Thus, the hypothesis that central modifications in the threshold of the tonic stretch reflex underlie the control of movement may be applied to the jaw system. Received: 11 October 1996 / Accepted: 17 March 1997     

Automatic detection and measurement of EMG silent periods in masticatory muscles during chewing in man.

Silent periods are transient stops of muscle activity that are induced by mechanical or electrical stimulus. The current report describes a new algorithm which enables automatic detection and measurement of silent periods that occur in the EMG of human masticatory muscles during chewing efforts together with jaw positions in space at the occurrence of the silent periods. The EMG signal for detection of the silent period is modeled based on the observation of the EMG records induced from the jaw-closing muscles during chewing efforts in seven volunteer subjects. The algorithm for automatic detection and measurement of the EMG silent period has been applied successfully to real EMG data and the performance of the software was confirmed to be sufficiently reliable. The results of the analysis are stored in a biosignal database for possible clinical use.     

Assessment of individual finger muscle activity in the extensor digitorum communis by surface EMG

The extensor digitorum communis (ED) is a slender muscle group in the dorsal forearm from which tendons arise to the index (D2), medius (D3), ring (D4), and little (D5) fingers. Limited independence has been attributed to the parts that actuate the individual fingers. However, in a detailed anatomical analysis, it was found that the ED parts to the different fingers have constant and widely spaced anatomical locations that promote independent function. These observations and the superficial muscle belly locations prompted the hypothesis that these ED parts would be individually assessable by small anatomically placed surface EMG electrodes. In the present study, this hypothesis was evaluated by measuring electromyography (EMG) from the ED parts and surrounding muscles during individual finger tapping tasks with the forearm resting on a flat surface. It was found that individual ED activity can be well measured in ED2, ED3, ED4, and extensor digiti minimi (EDM). ED3 did not give nor did its electrodes receive significant crosstalk from other ED parts. ED4 electrodes recorded an EMG level of 30 +/- 19% (mean +/- SD) ED2 EMG in D2 tapping and ED2 electrodes a level of 53 +/- 22% ED4 EMG in D4 tapping, by hypothesis mostly crosstalk. EDM electrodes may record EMG at the level of ED4 EMG in D4 tapping. In D2 tapping, the mutual ED2 and extensor indicis redundancy reflected in large intersubject EMG differences with sometimes one or the other almost silent. The results may expand the possibilities of EMG analysis and finger muscle electrostimulation in ergonomic and clinical applications.     

Relative activation of two human elbow flexors under isometric conditions: a cautionary note concerning flexor equivalence

Summary We examined the electromyographic (EMG) activity of two human elbow-flexor muscles, biceps brachii and brachioradialis, during isometric contractions. The task required subjects to match the EMG level of one of the muscles (the control muscle) to one of four target levels (5, 10, 15, or 20% of maximum) at various elbow angles. A new technique was developed for the target-matching task. The activity of the other muscle (the test muscle) was simultaneously recorded during the task. For the notion of flexor equivalence to be supported, the EMG levels for the two muscles should have covaried. This was not the case. The results revealed three features: (1) while the control-muscle EMG remained constant across joint angles, the test-muscle EMG varied with joint angle, and the trend of this variation differed among subjects; (2) in nine out of ten subjects the trend of test-muscle EMG variation with joint angle was reversed when the other muscle served as the test muscle; and (3) the testmuscle EMG associated with the four target levels was subject-, muscle-, and angle-dependent. These results caution against the generalization of the flexor equivalent concept to isometric conditions. In particular, the activity of one muscle is not a reliable indicator of the activity of other muscles subserving the same joint action.     

Electromyographic responses to constant position errors imposed during voluntary elbow joint movement in human

The role of reflexes in the control of stiffness during human elbow joint movement was investigated for a wide range of movement speeds (1.5–6 rad/s). The electromyographic (EMG) responses of the elbow joint muscles to step position errors (step amplitude 0.15 rad; rise time 100 ms) imposed at the onset of targeted flexion movements (1.0 rad amplitude) were recorded. For all speeds of movement, the step position disturbance produced large modulations of the usual triphasic EMG activity, both excitatory and inhibitory, with an onset latency of 25 ms. In the muscles stretched by the perturbation, the early EMG response (25–60 ms latency) magnitude was greater than 50% of the activity during the unperturbed movements (background activity). In all muscles the EMG responses integrated over the entire movement were greater than 25% of the background activity. The responses were relatively greater for slower movements. Perturbations assisting the movement caused a short-latency (25–60 ms) reflex response (in the antagonist muscle) that increased with movement speed and was constant as a percentage of the background EMG activity. In contrast, perturbations resisting the movement caused a reflex response (in the agonist muscle) that was of the same absolute magnitude at all movement speeds, and thus decreased with movement speed as a percentage of the background EMG activity. There was a directional asymmetry in the reflex response, which produced an asymmetry in the mechanical response during slow movements. When the step perturbation occurred in a direction assisting the flexion movement, the antagonist muscle activity increased, but the main component of this response was delayed until the normal time of onset of the antagonist burst. When the step perturbation resisted the movement the agonist muscles responded briskly at short latency (25 ms). A reflex reversal occurred in two of six subjects. A fixed reflex response occurred in the antagonist muscle, regardless of the perturbation direction. For the extension direction perturbations (resisting movement), this response represented a reflex reversal (50 ms onset latency) and it caused the torque resisting the imposed step (stiffness) to drop markedly (below zero for one subject). Reflex responses were larger when the subject was prevented from reaching the target. That is, when the perturbation remained on until after the normal time of reaching the target, the EMG activity increased, with a parallel increase in stiffness. Similarly, when the perturbations prevented the subject from reaching the target during a 1-rad voluntary cyclic movement, the EMG and stiffness increased markedly. Coactivation of the antagonist muscle with the agonist muscle was not prominent (<30% of antagonist activity) during unperturbed movements. The perturbations were resisted with reciprocal activity, and thus reflex action did not increase the coactivation. However, as a result of the low-pass properties of muscle, substantial cocontraction of the agonist and antagonists muscle forces may have occurred during rapid movements, thus leading to increased stiffness. As the relative changes in normal EMG activity produced by the perturbation were often comparable with the changes in mean muscle torque (stiffness) reported in the first paper of this series, we conclude that the action of reflexes produced a significant portion of the resistance to perturbations. This reflexive portion was greater for slower movements, it was greater when the subject neared the target, and it was variable according to the perturbation direction and the particular subject. Given that the perturbations were of similar frequency content to the movement itself (though of smaller amplitude) and that the reflexes contributed substantially to the resistance to these perturbations, we suggest that in normal unperturbed movements the observed EMG is likewise substantially determined by the reflex activity.     

Relation between electromyogram and torque of isometric reflex contractions in man

Human subjects maintained isometric plantar or dorsal flexions of the ankle in a matching task. H-reflexes of different sizes were superimposed on the steady activity. The peak-to-peak amplitude of the reflexes was measured on the electromyogram (EMG) of the soleus muscle. The size of the corresponding muscle contractions was determined on the isometric torque signal in relation to the maintained flexion force. The EMG-torque relation which was defined as the reflex muscle contraction as a function of the EMG reflex signal approximated a square root function for a given steady contraction level. It was not modulated by steady dorsal flexions, but it decreased continuously with stronger plantar steady torques. This dependence was caused by the silent period following the reflex discharge. Since the reflex discharge and the silent period were near in time to the duration of the contraction, the silent period had a direct effect on the reflex contraction amplitude.     

Three silent periods in the orbiculari oculi muscles of man: normal findings and some clinical vignettes.

PURPOSE: To investigate how many true silent periods could be found in the orbiculari oculi muscles of man. MATERIAL AND METHODS: 10 subjects, clinically healthy (5 male, 5 female), with a mean age of 34 years-old (range: 23 to 48) were evaluated by mean of the blink reflex at resting and during contraction of the orbiculari oculi reflex according to protocols validated internationally. RESULTS: Three responses called R1, R2 and R3 were obtained in the orbicular oculi muscle at resting state which had latencies and amplitudes within normal limits. What was new was to obtain three silent periods when the subjects were evaluated during muscle contraction. The duration of the first silent period was statistically longer than the second one (p < 0.004) and shorter than the third silent period (p < 0.0001). In addition, this test was found useful in detecting more specific findings in patients with hemifacial spasm and Meigge syndrome. CONCLUSION: This is by the first time that three silent periods in the orbicular oculi muscles are consistently demonstrated. The refractoriness of the alpha motoneurons and the action of gamma-collateral activity seem to be the main conditions leasing to display the first two periods of muscle suppression. The modification of gamma motoneurons firing as well as a pause of muscle spindles in facial muscles due to the action of nociceptive stimuli traveling unmyelinated C fibers of the supraorbital nerve might be the most important mechanisms involved in the production of the third silent period. These results enables further clinical application of this test.     

The contribution of transcortical pathways to long-latency stretch and tactile reflexes in human hand muscles

Long-latency electromyographic (EMG) responses can be evoked in the first dorsal interosseous muscle (FDI) by unexpected slips of an object (skin stretch) held between the index and thumb, or by forcible adduction of the metacarpophalangeal joint (muscle stretch). The former type of response is due to stimulation of tactile afferents in the skin of the digits, whereas the latter also activates muscle receptors. Previous studies have provided good evidence that long-latency reflex responses to stretch of distal muscles involve activity in a transcortical reflex pathway. The present experiments examined whether cutaneous reflexes also utilise a transcortical route. Transcranial magnetic or electrical stimuli were given over the motor cortex to evoke EMG activity during the period of the long-latency reflex response. When evoked by muscle stretch the responses to magnetic stimulation were facilitated more than those to electric stimulation. In contrast, facilitation was equal during the long-latency reflex elicited by cutaneous stimulation. Because of the different ways in which electrical and magnetic stimuli are believed to activate the motor cortex, we interpret these results to mean that the long-latency response to skin stretch is not mediated by a transcortical mechanism in the majority of subjects, whereas that following muscle stretch is. However, these are average data. In a few individual subjects, the opposite results were obtained. We suggest that there may be differences between subjects in the transcortical contribution to long-latency reflex responses. The implication is that, under normal circumstances, several pathways may contribute to these responses. If so, the relative roles of the pathways may change during different tasks, and in pathological states lesions in one system may well be accompanied by compensatory changes in other systems.     

Differences in stretch reflex responses of elbow flexor muscles during shortening, lengthening and isometric contractions

Stretch reflexes were evoked in elbow flexor muscles undergoing three different muscle contractions, i.e. isotonic shortening (SHO) and lengthening (LEN), and isometric (ISO) contractions. The intermuscle relationships for the magnitude of the stretch reflex component in the eletromyographic (EMG) activities of two main elbow flexor muscles, i.e. the biceps brachii (BB) and the brachioradialis (BRD), were compared among the three types of contractions. The subjects were requested to move their forearms sinusoidally (0.1?Hz) against a constant pre-load between elbow joint angles of 10° (0°?=?full extension) and 80° during SHO and LEN, and to keep an angle of 45° during the ISO. The perturbations were applied at the elbow angle of 45° in pseudo-random order. The EMG signals were rectified and averaged over a period of 100?ms before and 400?ms after the onset of the perturbation 40–50 times. From the ensemble averaged EMG waveform, the background activity (BGA), short (20–50?ms) and long latency (M2, 50–80, M3, 80–100?ms) reflex and voluntary activity (100–150?ms) components were measured. The results showed that both BGA and reflex EMG activity of the two elbow flexor muscles were markedly decreased during the lengthening contraction compared to the SHO and ISO contractions. Furthermore, the changes of reflex EMG components in the BRD muscle were more pronounced than those in the BB muscle, i.e. the ratios of M2 and M3 magnitudes between BRD and BB (BRD:BB) were significantly reduced during the LEN contractions. These results would suggest that the gain of long latency stretch reflex EMG activities in synergistic muscles might be modulated independently according to the model of muscle contraction.     

Patterns of neck muscle activation in cats during reflex and voluntary head movements

Summary When the head rotates, vestibulocollic reflexes counteract the rotation by causing contraction of the neck muscles that pull against the imposed motion. With voluntary head rotations, these same muscles contract and assist the movement of the head. The purpose of this study was to determine if an infinite variety of muscle activation patterns are available to generate a particular head movement, or if the CNS selects a consistent and unique muscle pattern for the same head movement whether performed in a voluntary or reflex mode. The relationship of neck muscle activity to reflex and voluntary head movements was examined by recording intramuscular EMG activity from six neck muscles in three alert cats during sinusoidal head rotations about 24 vertical and horizontal axes. The cats were trained to voluntarily follow a water spout with their heads. Vestibulocollic reflex (VCR) responses were recorded in the same cats by rotating them in an equivalent set of planes with the head stabilized to the trunk so that only the vestibular labyrinths were stimulated. Gain and phase of the EMG responses were calculated, and data analyzed to determine the directions of rotation for which specific muscles produced their greatest EMG output. Each muscle exhibited preferential activation for a unique direction of rotation, and weak responses during rotations orthogonal to that preferred direction. The direction of maximal activation could differ for reflex and voluntary responses. Also, the best excitation of the muscle was not always in the direction that would produce a maximum mechanical advantage for the muscle based on its line of pull. The results of this study suggest that a unique pattern of activity is selected for VCR and tracking responses in any one animal. Patterns for the two behaviors differ, indicating that the CNS can generate movements in the same direction using different muscle patterns.     

Do corticomotoneuronal cells predict target muscle EMG activity?

Data from two rhesus macaques were used to investigate the pattern of cortical cell activation during reach-to-grasp movements in relation to the corresponding activation pattern of the cell's facilitated target muscles. The presence of postspike facilitation (PSpF) in spike-triggered averages (SpTAs) of electromyographic (EMG) activity was used to identify cortical neurons with excitatory synaptic linkages with motoneurons. EMG activity from 22 to 24 muscles of the forelimb was recorded together with the activity of M1 cortical neurons. The extent of covariation was characterized by 1) identifying the task segment containing the cell and target muscle activity peaks, 2) quantifying the timing and overlap between corticomotoneuronal (CM) cell and EMG peaks, and 3) applying Pearson correlation analysis to plots of CM cell firing rate versus EMG activity of the cell's facilitated muscles. At least one firing rate peak, for nearly all (95%) CM cells tested, matched a corresponding peak in the EMG activity of the cell's target muscles. Although some individual CM cells had very strong correlations with target muscles, overall, substantial disparities were common. We also investigated correlations for ensembles of CM cells sharing the same target muscle. The ensemble population activity of even a small number of CM cells influencing the same target muscle produced a relatively good match (r >/= 0.8) to target muscle EMG activity. Our results provide evidence in support of the notion that corticomotoneuronal output from primary motor cortex encodes movement in a framework of muscle-based parameters, specifically muscle-activation patterns as reflected in EMG activity.     

Short-latency trigemino-cervical ref

We describe a reflex evoked in neck muscles by stimulation of afferent fibres in the trigeminal nerve. The clearest responses were seen in averaged, unrectified, monopolar surface electromyographic (EMG) recordings from active sternocleidomastoid muscles after stimulation of the infraorbital nerve. They consisted of a bilateral positive/negative (p19, n31) wave with a mean onset latency of 12.9 ms which corresponded to a period of inhibition in the underlying motor unit activity. Responses also could be seen in splenius and trapezius, but not in arm muscles. Stimuli to other branches of the trigeminal nerve (supraorbital or mental) did not produce such clear effects. The threshold for the reflex was relatively low (2–4 times perceptual threshold) and its size scaled with the level of background EMG in an approximately linear fashion. Responses to infraorbital stimulation did not interact with other short-latency inhibitory responses in the sternocleidomastoid muscle evoked by loud acoustic clicks or stimulation of the median nerve at the wrist. We suggest that the infraorbital response is part of a head withdrawal reflex involving an oligosynaptic trigemino-cervical system similar to that described in the cat.     

Referent configuration of the body: a global factor in the control of multiple skeletal muscles

In addition to local biomechanical and reflex factors influencing muscle activation, global factors may be used by the nervous system to control all muscles in a coherent and task-specific way. It has been hypothesized that a virtual or referent (R) configuration of the body determined by muscle recruitment thresholds specified by neural control levels is such a factor. Due to the threshold nature of the R configuration, the activity of each muscle depends on the difference between the actual (Q) and the R configuration of the body. The nervous system modifies the R configuration to produce movement. One prediction of this hypothesis is that the Q and R configurations may match each other, most likely in movements with reversals in direction, resulting in a minimum in the electromyographic (EMG) activity level of muscles involved. The depth of the minima is constrained by the degree of coactivation of opposing muscle groups. Another prediction is that EMG minima in the activity of multiple muscles may occur not only when the movement is assisted but also when it is opposed by external forces (e.g., gravity). To verify these predictions, we analyzed EMG patterns of 16–21 functionally diverse muscles of the legs, trunk, and arms during jumping and stepping in place. One EMG minimum in the activity of all muscles regularly occurred near the apex of the jump. A minimum was also observed near the point of transition of the body from flexion to extension leading to a jump. During stepping in place, the activity of muscles of each side of the body was usually minimized near the beginning and near the end of the stance phase as well as during the maximum elevation of the foot. Since EMG minima occurred not only during gravity-assisted but also gravity-opposed movement reversals, it is concluded that neural factors (such as matching between the Q and R) rather than mechanical factors are responsible for minimizing the EMG activity in these movements.     

Nonlinear properties of stretch reflex studied in the decerebrate cat

1. Pairs of brief stretches or a series of stretches at random intervals (Poisson process) were applied to a slow (soleus) and a fast (plantaris) muscle in decerebrate cats to analyze the nonlinear effects of one stretch on the reflex responses to subsequent stretches. Neural activity, electromyogram (EMG), and force were recorded. The reflex responses due to stretch were compared with reflexes as a result of electrical stimulation of nerves. Nonlinearities of muscle were also examined in the absence of reflexes. Short-latency neural activity produced by the stimuli at all intervals studied was quite constant, so changes in sensory activity cannot account for the nonlinearities. Three phases of nonlinear interactions were observed, and mechanisms for these nonlinearities are suggested. 2. For short intervals (less than 100 ms) following a stretch the force and EMG produced by a second stretch is depressed. This early depression could be due to the after hyperpolarization of the motoneuron cel body or to synaptic mechanisms, since the depression of EMG is seen with electrical stimulation of Ia sensory, but not alpha-motor axons. In addition, a second stretch can disrupt the reflex contraction produced by the first stretch if it occurs at a time when new actomyosin bonds are not readily formed. Because of this force suppression, the total reflex force produced in response to two stretches may be less than the response to a single stretch. 3. For intervals between 100 and 300 ms the force and EMG produced by a second stretch is enhanced. This potentiation is also seen with electrical stimulation of large sensory but not motor axons and could result from a synchronization of motoneuronal excitability cycles. It is more prominent in the homogeneous (soleus) muscle than the mixed (plantaris) muscle, probably because the motoneuron cell bodies will reach a period of high excitability at more nearly the same time in the homogeneous muscle. 4. For longer intervals the force produced by a second stretch is reduced even when the EMG is close to control values. This late depression is also observed with electrical stimulation of cut motor axons and therefore arises from the contractile properties of muscles. 5. With a random series of stretches, the same time course of nonlinear interactions is observed. However, as the mean rate of the random stretches is increased, the average response of the reflex decreases. Thus, the stretch reflex will be most effective in correcting for occasional perturbations to a movement, rather than for continuously varying disturbances.     

Reflex inhibition of normal cramp following electrical stimulation of the muscle tendon

Muscle cramp was induced in one head of the gastrocnemius muscle (GA) in eight of thirteen subjects using maximum voluntary contraction when the muscle was in the shortened position. Cramp in GA was painful, involuntary, and localized. Induction of cramp was indicated by the presence of electromyographic (EMG) activity in one head of GA while the other head remained silent. In all cramping subjects, reflex inhibition of cramp electrical activity was observed following Achilles tendon electrical stimulation and they all reported subjective relief of cramp. Thus muscle cramp can be inhibited by stimulation of tendon afferents in the cramped muscle. When the inhibition of cramp-generated EMG and voluntary EMG was compared at similar mean EMG levels, the area and timing of the two phases of inhibition (I(1), I(2)) did not differ significantly. This strongly suggests that the same reflex pathway was the source of the inhibition in both cases. Thus the cramp-generated EMG is also likely to be driven by spinal synaptic input to the motorneurons. We have found that the muscle conditions that appear necessary to facilitate cramp, a near to maximal contraction of the shortened muscle, are also the conditions that render the inhibition generated by tendon afferents ineffective. When the strength of tendon inhibition in cramping subjects was compared with that in subjects that failed to cramp, it was found to be significantly weaker under the same experimental conditions. It is likely that reduced inhibitory feedback from tendon afferents has an important role in generating cramp.     

Neural control: novel evaluation of stretch reflex sensitivity

We evaluated the stretch reflex activities of the elbow flexor and extensor muscles considering the relationship between the reflex electromyographic (EMG) responses and their corresponding standardized muscle stretch velocities. Specifically, muscular stretch velocity was estimated by using ultrasonograms. Stretch reflex EMG responses were elicited in the biceps brachii, brachioradialis and triceps brachii with a ramp-and-hold rotation at the elbow joint, which consisted of various angular velocities for the extension- or flexion-direction. The whole muscle stretch velocity induced by each ramp-and-hold rotation was calculated on the basis of fibre length changes associated with the elbow joint angle. A linear regression equation was fitted to the relation between the whole muscle stretch velocity and the reflex EMG responses, and the variables from the equation were used to quantify sensitivity of each reflex EMG component. The reflex EMG responses were increased as the ramp-and-hold rotational velocity increased. There were no significant differences in the recorded magnitudes of reflex EMG responses with equivalent joint rotational velocity between the brachioradialis and the triceps brachii medial head. These muscles showed the highest reflex responses in the flexor and extensor muscles, respectively. To the contrary, the reflex EMG response elicited by the standardized muscle stretches was significantly greater in the extensor muscles, indicating a higher reflex sensitivity. This was because of the lower muscle stretch velocity of the triceps brachii with an equivalent elbow joint rotation. The stretch reflex sensitivity in both the elbow flexor and extensor muscles might be regulated so as to make the reflex responses the same when the equivalent joint rotational velocity is applied to these muscles.     

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed “associated” contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were ∼1.5–2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.     

Independent coactivation of shoulder and elbow muscles

 The aim of this study was to examine the possibility of independent muscle coactivation at the shoulder and elbow. Subjects performed rapid point-to-point movements in a horizontal plane from different initial limb configurations to a single target. EMG activity was measured from flexor and extensor muscles acting at the shoulder (pectoralis clavicular head and posterior deltoid) and elbow (biceps long head and triceps lateral head) and flexor and extensor muscles acting at both joints (biceps short head and triceps long head). Muscle coactivation was assessed by measuring tonic levels of electromyographic (EMG) activity after limb position stabilized following the end of the movements. It was observed that tonic EMG levels following movements to the same target varied as a function of the amplitude of shoulder and elbow motion. Moreover, for the movements tested here, the coactivation of shoulder and elbow muscles was found to be independent – tonic EMG activity of shoulder muscles increased in proportion to shoulder movement, but was unrelated to elbow motion, whereas elbow and double-joint muscle coactivation varied with the amplitude of elbow movement and were not correlated with shoulder motion. In addition, tonic EMG levels were higher for movements in which the shoulder and elbow rotated in the same direction than for those in which the joints rotated in opposite directions. In this respect, muscle coactivation may reflect a simple strategy to compensate for forces introduced by multijoint limb dynamics. Received: 7 July 1998 / Accepted: 28 July 1998     

Exteroceptive reflexes in jaw-closing muscle EMG during rhythmic jaw closing and clenching in man

Exteroceptive jaw reflexes might play a role in normal functions of the mouth such as mastication. Until now these reflexes have only been studied under isometric conditions. The aim of this study was to compare exteroceptive reflexes in jaw muscle EMG during the closing phase of rhythmic open-close movements and clenching, at the same jaw gape and with similar muscle EMG. Reflexes consisting of successive waves of decreased and increased muscle activity (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by light noxious electrical stimulation of the vermillion border of the lower lip, were recorded from the jaw closing muscles of 17 subjects. Differences between the two tasks occurred in two phases of the PSEC: (1) in an early phase, around the R wave, there was significantly less EMG during jaw closing (mean EMG ratio between jaw-closing and clenching 0.71), and (2) in a late phase, around the transition between the S to the T wave, there was significantly more EMG during jaw closing (mean EMG ratio: 1.40). The decrease in EMG activity around the R wave during jaw closing may be due to a change in reflex sensitivity at an interneuron level. The increase in EMG activity around the transition between the S and T waves during jaw closing might, at least in part, be due to a proprioceptive stretch reflex. This reflex is mediated by muscles spindles that are activated by the deceleration of the jaw evoked by the lip stimulus. The finding of inhibitory reflex mechanisms that predominate more during rhythmic jaw movements than during clenching in an early phase of the PSEC might be related to protecting oral tissues from trauma when the jaw is closing with potentially a large muscle force. In contrast, when food is held between the teeth, a possible inhibitory influence of light noxious stimuli is diminished.     

Simultaneous EMGs From Six Sites During Muscular Relaxation: A Comparison Between Forehead and Forearm Feedback

In light of the continuing popularity of frontalis EMG feedback for general relaxation purposes, a pertinent question is whether this procedure is superior to feedback from other muscles. This question was addressed by comparing a frontalis feedback group (n = 12) with a forearm feedback group (n = 10) in a one-session experiment. The experimental session included a 10-min rest period preceded by verbal relaxation instructions and demonstration of the respective feedback contingencies, and followed by a 30-min feedback period. EMG records were obtained from forehead, forearm, neck, masseter, sternomastoid, and lower leg. In the two target muscles, significant group differences developed during rest, but no additional significant effects were produced during feedback. Regarding the four untrained sites, EMG tended to decrease in the forehead group, but not significantly below the levels of the forearm group. Simultaneous EMG cross-correlations within 5-min intervals based on successive 1-sec averages showed significant covariation between muscles in the head and neck area; the source of this covariation was uncertain, however—electrical “crosstalk” and generalized muscular responding were two possibilities discussed.