Trunk muscle reactions to sudden unexpected and expected perturbations in the absence of upright postural demand

The aim was to increase the understanding of the multifunctional role of the trunk muscles in spine control, particularly transversus abdominis (TrA). In 11 healthy males, intramuscular fine-wire electromyography (EMG) was obtained bilaterally from TrA, obliquus externus (OE), rectus abdominis (RA) and erector spinae (ES). The subjects lay on their right side on a horizontal swivel-table with immobilized pelvis and lower limbs and the trunk strapped to a movable platform. Unexpected or expected release of loads attached to the table by steel cables produced a perturbation inducing either trunk flexion or extension. The timing and the amplitude of activation of TrA were independent of direction of induced trunk movement. Furthermore, timing of TrA activation was simultaneous to or later than that of the more superficial abdominal muscles. Expectation of the perturbation caused a general shortening of onset latencies. The results indicate a direction independent function of TrA in lumbar spine control. Balancing the trunk vertically appears to add specific demands, since the recruitment of TrA in relation to the other abdominal muscles differed from earlier experiments in standing.     

Neck,trunk and limb muscle responses during postural perturbations in humans

Summary This study examined the EMG onsets of leg, trunk, and neck muscles in 10 standing human subjects in response to support surface anterior and posterior translations, and to plantar and dorsiflexion rotations. The objective of the study was to test the hypothesis that the responses radiating upward from distal leg muscles represent part of a large ascending synergy encompassing axial muscles along the entire length of the body. If these responses are not ascending, then the muscles of the neck, and possibly the trunk, can be independently activated by vestibular, proprioceptive or visual inputs. We analysed the timing of postural muscle responses within and between body segments in order to determine whether they maintained a consistent temporal relationship under translational and rotational platform movement paradigms. Our results did not strongly support an ascending pattern of activation in all directions of platform perturbation. Temporal differences between activation patterns to platform perturbations in the forward or backward directions were revealed. In response to posterior platform translations we observed an ascending pattern of muscle responses along the extensor surface of the body. In addition, responses elicited in the neck flexor and abdominal muscles occurred as early as those of the stretched ankle muscles. This pattern of upward radiation from stretched ankle muscles was not as clear for anterior platform displacements, where early neck flexor muscle responses were observed during the ascending sequence on the flexor surface of the body. Platform rotations caused fewer responses in the neck and upper trunk muscles than translations, and all muscle responses occurred simultaneously rather than sequentially. Probable differences in the stimulation of vestibular and neck proprioceptive inputs and the mechanical demands of the rotation and translation paradigms are discussed.     

Perturbed upper limb movements cause short-latency postural responses in trunk muscles

Addition of a load to a moving upper limb produces a perturbation of the trunk due to transmission of mechanical forces. This experiment investigated the postural response of the trunk muscles in relation to unexpected limb loading. Subjects performed rapid, bilateral shoulder flexion in response to a stimulus. In one third of trials, an unexpected load was added bilaterally to the upper limbs in the first third of the movement. Trunk muscle electromyography, intra-abdominal pressure and upper limb and trunk motion were measured. A short-latency response of the erector spinae and transversus abdominis muscles occurred approximately 50 ms after the onset of the limb perturbation that resulted from addition of the load early in the movement and was coincident with the onset of the observed perturbation at the trunk. The results provide evidence of initiation of a complex postural response of the trunk muscles that is consistent with mediation by afferent input from a site distant to the lumbar spine, which may include afferents of the upper limb.     

Effects of knee and ankle muscle fatigue on postural control in the unipedal stance

The aim of this study was to compare the effects of acute muscle fatigue of the ankle and knee musculature on postural control by immediate measures after performing fatiguing tasks (POST condition). One group of subjects (n = 8) performed a fatiguing task by voluntary contractions of the triceps surae (group TRI) and the other (n = 9) performed a fatiguing task by voluntary contractions of the quadriceps femoris (group QUA). Each muscle group was exercised until the loss of maximal voluntary contraction torque reached 50% (isokinetic dynamometer). Posture was assessed by measuring the centre of foot pressure (COP) with a force platform during a test of unipedal quiet standing posture with eyes closed. Initially (in PRE condition), the mean COP velocity was not significantly different between group TRI and group QUA. In POST condition, the mean COP velocity increased more in group QUA than in group TRI. The postural control was more impaired by knee muscle fatigue than by ankle muscle fatigue.     

Experimental muscle pain changes feedforward postural responses of the trunk muscles

Many studies have identified changes in trunk muscle recruitment in clinical low back pain (LBP). However, due to the heterogeneity of the LBP population these changes have been variable and it has been impossible to identify a cause-effect relationship. Several studies have identified a consistent change in the feedforward postural response of transversus abdominis (TrA), the deepest abdominal muscle, in association with arm movements in chronic LBP. This study aimed to determine whether the feedforward recruitment of the trunk muscles in a postural task could be altered by acute experimentally induced LBP. Electromyographic (EMG) recordings of the abdominal and paraspinal muscles were made during arm movements in a control trial, following the injection of isotonic (non-painful) and hypertonic (painful) saline into the longissimus muscle at L4, and during a 1-h follow-up. Movements included rapid arm flexion in response to a light and repetitive arm flexion-extension. Temporal and spatial EMG parameters were measured. The onset and amplitude of EMG of most muscles was changed in a variable manner during the period of experimentally induced pain. However, across movement trials and subjects the activation of TrA was consistently reduced in amplitude or delayed. Analyses in the time and frequency domain were used to confirm these findings. The results suggest that acute experimentally induced pain may affect feedforward postural activity of the trunk muscles. Although the response was variable, pain produced differential changes in the motor control of the trunk muscles, with consistent impairment of TrA activity.     

Scaling of plantarflexor muscle activity and postural time-to-contact in response to upper-body perturbations in young and older adults

In this study, we describe and compare the compensatory responses of healthy young and older adults to sequentially increasing upper-body perturbations. The scaling of plantarflexor muscular activity and minimum time-to-contact (TtC_MIN) was examined, and we determined whether TtC_MIN predictions of instability (stepping transitions) for the older subjects were similar to those we previously reported for younger subjects (Hasson et al. in J Biomech 41:2121–2129, 2008). We found that the older subjects stepped at a lower perturbation level than the younger subjects; however, this response was appropriate based on their greater center of mass (CoM) accelerations, which may have been caused by differences in pre-perturbation states between the age groups. Although the CoM acceleration increased linearly with perturbation magnitude, the amount of gastrocnemius and soleus muscular activity increased nonlinearly in both age groups. There were no differences in the maximum plantarflexor torque responses, suggesting that the maximum torque capabilities of the older subjects were not limiting factors. As previously demonstrated in the younger subjects, the older subjects showed a quadratic decrease in TtC_MIN with increasing perturbation magnitude. The vertices of the quadratics gave accurate predictions of stepping transitions in both age groups, even though the older subjects stepped at lower perturbation magnitudes. By probing the postural system’s behavior through sequentially increasing upper-body perturbations, we observed a complementary nonlinear scaling of muscle activity and TtC_MIN, which suggests that subjects could use TtC or a correlate as an informational variable to help determine whether a step is necessary.

Sternocleidomastoid muscle deoxygenation in response to incremental inspiratory threshold loading measured by near infrared spectroscopy

This study investigated the pattern of changes in muscle oxygenation, deoxygenation and blood volume in the sternocleidomastoid (SCM) in comparison with the parasternal (PS) and intercostal (IC) muscles during a bout of incremental inspiratory threshold loading (ITL) in healthy subjects using near-infrared spectroscopy. During progressive loading, the PS and IC showed a significant increase in oxygenated hemoglobin (5.9 ± 2.3 and 6.8 ± 2.4 μM, P<0.05) and the SCM showed an increase in deoxygenated hemoglobin (17.3 ± 3.8 μM, P<0.05). Total hemoglobin also steadily increased in the SCM whereas it decreased in the quiescent vastus lateralis muscle (20.7 ± 6.1μM vs. -6.6 ± 2.4 μM, P<0.05), which was used as the control muscle during the ITL. Our data suggests that the SCM is recruited progressively during progressive ITL and is accompanied by an increased blood volume and maintenance of O(2)Hb. Blood redistribution away from the nonactive limb muscles during the ITL may provide one source of maintaining inspiratory muscle blood volume and oxygenation during high respiratory motor output.     

Automatic postural responses in the cat: responses of distal hindlimb muscles to paired vertical perturbations of stance

Summary The active components of the quadrupedal diagonal stance response to rapid removal of the support from beneath a single limb were studied in cats to further define the mechanisms that trigger and generate the response. We recorded EMG activity from lateral gastrocnemius and tibialis anterior muscles in awake, behaving cats while they stood on an hydraulic posture platform. By dropping the support from beneath a single limb, we evoked the diagonal stance response, with its characteristic changes in vertical force and EMG patterns. As the animal responded to this drop, a second perturbation of posture was then presented at intervals of 10 to 100 ms following the first. This second perturbation, which consisted of dropping the support from beneath the two limbs that were loaded as a result of the initial limb drop, made the first response biomechanically inappropriate. The EMG responses observed in both muscles during paired perturbations were triggered by the somatosensory events related to the perturbations. Muscle responses that were appropriate for the first perturbation always occurred with amplitudes and latencies similar to control trials. This was true even when the second perturbation occurred 10–20 ms after the first, that is, when this perturbation either preceded or was coincident with the response to the initial limb drop. The EMG responses that were normally associated with the second perturbation were delayed and/or reduced in amplitude when the time interval between perturbations was short. As the inter-perturbation interval was lengthened beyond 60–100 ms, however, EMG responses to the second perturbation were unaffected by the occurrence of the first perturbation. When the hindlimb containing the recording electrodes was dropped as part of the second perturbation, a myotatic latency response was observed in tibialis anterior. The amplitude of this response to the second perturbation was greater than controls when this displacement was presented during the period between initiation of the first perturbation and execution of the response to it. When the second displacement was presented after execution of the first response began, the amplitude of the myotatic response was reduced below control levels. While the results do not preclude the possibility that these automatic postural responses are segmental or suprasegmental reflexes, they support the hypothesis that the active component of the response to drop of the support beneath a single limb is centrally programmed and that the appropriate response can be riggered very rapidly by the somatosensory information signalling the perturbation.Supported by NIH grants NS19484 and RR05593 as well as the Medical Research Foundation of Oregon, and the Neurological Sciences Center of Good Samaritan Hospital and Medical Center     

Neck muscle responses to abrupt vertical acceleration in the seated human

The control of neck muscle during marked changes in "g" loading must be to protect head-trunk orientation. However, little is known about the organization of reflexes. We therefore investigated the shortest latency in neck muscle evoked by abrupt ascending and descending vertical acceleration with a stroke of 20 cm and peak acceleration 0.4×g in six healthy subjects. The subjects were seated upright and restrained on a chair driven by a hydraulic servo system. Ascent induced small responses in sternocleidomastoid muscles (SCMs) with a latency of 24.0 ms (SE 1.7) from onset of head acceleration, followed by larger responses of 100 ms in duration. In comparison the responses in SCMs evoked by descent were significantly delayed, with greater interindividual variations at 44.1 ms (SE 3.1) and smaller in amplitude than responses in ascent. Latencies were consistent and showed no habituation. In order to eliminate stretch components, we measured the neck responses to vertical acceleration with the head fixed by cervical collar. The latency of the fast response of SCM evoked by ascent was not significantly different from the latency of SCMs without the collar. These results may indicate that the fast responses of SCMs to sudden ascent may be composed of a vestibular-collic reflex for making the neck and head rigid for defense to sudden gravitational change. Electronic Publication     

Influence of muscle activation dynamics on reaction time in the elderly

The aim of this investigation was to determine whether age-related changes in the dynamics of muscle activation were, in part, responsible for longer reaction times (RT) in the elderly. A group of 12 young (mean age, 20.6 years) and 12 elderly (mean age, 64.3 years) women performed a series of ballistic forearm supination movements in response to an auditory stimulus while using a simple reaction time test. Surface electromyographic waveforms from biceps brachii (agonist) and pronator teres (antagonist) muscles were recorded, together with the angle-time curves representing the motion of the forearm, on to an IBM compatible microcomputer. The results showed that an age-related increase (P<0.05) in motor reaction time (MRT) contributed to longer RT in the elderly. In addition, the longer (P<0.05) MRTs in the elderly were associated with a significantly slower rate (P<0.05) of biceps brachii muscle activation and a significantly increased proportion (P<0.05) of the initial biceps brachii muscle burst required to initiate the movement. This data suggested that an important part of the slowing of motor behaviour, commonly observed with increasing age, may be due to either decreases in the ability of aged skeletal muscle to rapidly generate tension or to a reduction in motor drive.     

Walking modality affects respiratory muscle action and contribution to respiratory effort

We hypothesized that walking at increased speed or increasing gradient might have different effects on chest wall kinematics and respiratory muscle power components, and contribute differently to respiratory effort sensation. We measured the volumes of chest wall compartments by optoelectronic plethysmography, esophageal, gastric and transdiaphragmatic (P _di) pressures, and the sensation of the respiratory effort by a Borg scale in five normal subjects walking both at ascending gradient with constant speed (AG) and at ascending speed with constant gradient (AS). Chest wall kinematics, evaluated by displacement of chest wall compartments, did not show any significant difference between AS and AG. Muscle power, calculated as the product of mean flow and mean pressure, increased similarly, but its partitioning into pressure and velocity of shortening differed in the two modes. A greater increase in the pressure developed by the abdominal muscles (P _abm) (4.06-fold), and in the velocity of shortening of both rib cage inspiratory muscles (v _rcm,i) (2.01-fold) and the diaphragm (v _di) (1.90-fold) was associated with a lower increase in the pressure developed by the rib cage inspiratory muscles (P _rcm,i) (1.24-fold) and P _di (0.99-fold) with AG. Instead, with AS, a lower increase in P _abm (2.12-fold), v _rcm,i (1.66-fold) and v _di (1.54-fold) was associated with a greater increase in P _rcm,i (1.56-fold) and P _di (1.97-fold). A combination of P _abm and v _di during AG (Wald ²=23.19, P<0.0000), with the addition of P _rcm,i during AS (Wald ²=29.46, P<0.0000), was the best predictor of Borg score. In conclusion, the general strategy adopted by respiratory centers during different walking modes does not differ in terms of ventilation, chest wall kinematics, and respiratory muscle power production, whereas it does in terms of partitioning of power into pressure and velocity of shortening, and respiratory muscle contribution to respiratory effort sensation. Combinations of different patterns of flow and pressure generation made the respiratory effort sensation similar during AS and AG modes.     

Surface electrodes are not appropriate to record selective myoelectric activity of splenius capitis muscle in humans

Splenius capitis (SPL) electromyograms were recorded using conventional surface and intramuscular wire electrodes simultaneously during various head-neck movements and isometric tasks to test the selectivity of surface electrodes for SPL myoelectric signals. The insertion of bipolar wire electrodes was aided by a computerized tomographical study of each subject's neck. Surface electrodes were placed over the superficial SPL area. Head motion was recorded with an electromechanical device. The selective SPL wire recordings confirmed that SPL has two main functions: ipsilateral rotation and extension. It also plays a subordinate role in ipsilateral tilting of the head. Intramuscular and surface recording results were contradictory mainly for flexion and contralateral rotation. These discrepancies appeared to be due to cross-talk from adjacent muscles, particularly from the sternocleidomastoid muscle. We conclude the validity of electrode recordings is questionable for SPL and most dorsal neck muscles, especially during isometric tests.     

Effect of gaze direction on neck muscle activity during cervical rotation

Control of the neck muscles is coordinated with the sensory organs of vision, hearing and balance. For instance, activity of splenius capitis (SC) is modified with gaze shift. This interaction between eye movement and neck muscle activity is likely to influence the control of neck movement. The aim of this study was to investigate the effect of eye position on neck muscle activity during cervical rotation. In eleven subjects we recorded electromyographic activity (EMG) of muscles that rotate the neck to the right [right obliquus capitis inferior (OI), multifides (MF), and SC, and left sternocleidomastoid (SCM)] with intramuscular or surface electrodes. In sitting, subjects rotated the neck in each direction to specific points in range that were held statically with gaze either fixed to a guide (at three different positions) that moved with the head to maintain a constant intra-orbit eye position or to a panel in front of the subject. Although right SC and left SCM EMG increased with rotation to the right, contrary to anatomical texts, OI EMG increased with both directions and MF EMG did not change from the activity recorded at rest. During neck rotation SCM and MF EMG was less when the eyes were maintained with a constant intra-orbit position that was opposite to the direction of rotation compared to trials in which the eyes were maintained in the same direction as the head movement. The inter-relationship between eye position and neck muscle activity may affect the control of neck posture and movement.     

Radiographic control in the insertion of EMG electrodes in the lumbar part of the erector spinae muscle

Summary A method for control of the location of EMG electrodes in the multifidi, the longissimus and the iliocostalis muscles in the lumbar spine is presented. The insertion of the electrodes is guided radiographically with TV-fluoroscopy as well as electromyographically. The chosen site of the electrodes is then confirmed radiographically after injection of carbon dioxide through the electrode. The technique is discussed and the radiographic criteria for the location of the electrodes in the individual muscles is described.     

Electromyographic monitoring of respiratory muscle activity in dyspneic infants and toddlers

The aim of this study was to investigate whether the changes that occurred in the clinical asthma score (CAS) correlated with the changes in the respiratory electromyographic (EMG) activity over the days during admission to hospital in dyspneic infants and toddlers. Sixteen infants and toddlers (9 males) were studied during admission and 7 days after discharge. The CAS was used to assess the severity of dyspnea and consists of five items: respiration rate, wheezing, retractions, observed dyspnea, and inspiration-to-expiration ratio. Each item was scaled 0, 1, or 2, with a maximum score of 10. Electrical activity from the diaphragm (di) and intercostal muscles (int) was derived from surface electrodes. The logarithm of the EMG-Activity-Ratio (log EMGAR; ratio of mean peak-to-bottom EMG activity during admission to the hospital, to that at baseline, 7 days after discharge) was used as EMG parameter. For assessing the association between the repeated observations of the CAS and the EMG measurements we used the quantity r2 obtained with analysis of covariance. On the day of admission the patients had a mean CAS of 5.9 +/- 1.2. On the day of discharge the mean CAS decreased significantly to 2.1 +/- 1.6, indicating that the CAS returned to normal values. In line with this observation, a significant decrease in the log EMGARdi and log EMGARint was observed during the stay in the hospital. Over all subjects the correlation coefficient (r) of log EMGARdi versus CAS was 0.71, log EMGARint versus CAS was 0.67, and the mean log EMGAR versus CAS was 0.75 (p < 0.01, for all values). The correlation coefficients of subjects of < or = 1 year seemed to be lower than those of subjects of > 1 year of age (p < 0.01) and female subjects showed higher correlation coefficients than males. This study showed a moderate, but significant, relationship between the changes that occurred in the CAS and the changes in respiratory EMG activity during admission to hospital in dyspneic infants and toddlers. Moreover, the correlation coefficients of the combined leads of the intercostals and diaphragm (mean log EMGAR) were higher than those of the separate leads. The EMG measurements would extend diagnostic possibilities and would provide an objective measure to evaluate the clinical course of the disease and the efficacy of therapy in infants and toddlers with recurrent wheezing disorders.     

Neck muscle activation patterns in humans during isometric head stabilization

Summary A musculoskeletal system with more muscles than there are motions could be programmed in alternative ways to produce a single movement. In this case, the muscles would have the potential to be maximally responsive in multiple directions rather than responding preferentially in a single direction. To determine the response patterns of muscles in the head-neck motor system, the simultaneous activation of four of the 23 neck muscles acting on the head was recorded with both surface and intramuscular electrodes. Fifteen human subjects were tested during an isometric head stabilization task. When the EMG response patterns were plotted, each muscle demonstrated a preferred direction of activation. This preferred activation direction was consistent in all of the subjects for three of the muscles tested. The fourth muscle, splenius, was preferentially activated during neck flexion in half of the subjects and during neck extension in the other half. Increasing the force parameters of the task suggested a linear relationship between force and the EMG output in the preferred response directions. Responses in the nonpreferred directions were produced by a nonlinear change in EMG activation of the muscle. This finding could have implications for theories of how reciprocal activation and cocontraction patterns of response are elicited. Results from this study, that the CNS programs neck muscles to respond in specific orientations rather than generating an infinite variety of muscle patterns, are in agreement with our findings in the cat.     

Adaptive control of stiffness to stabilize hand position with large loads

The goal of this work was to investigate stability in relation to the magnitude and direction of forces applied by the hand. The endpoint stiffness and joint stiffness of the arm were measured during a postural task in which subjects exerted up to 30% maximum voluntary force in each of four directions while controlling the position of the hand. All four coefficients of the joint stiffness matrix were found to vary linearly with both elbow and shoulder torque. This contrasts with the results of a previous study, which employed a force control task and concluded that the joint stiffness coefficients varied linearly with either shoulder or elbow torque but not both. Joint stiffness was transformed into endpoint stiffness to compare the effect on stability as endpoint force increased. When the joint stiffness coefficients were modeled as varying with the net torque at only one joint, as in the previous study, we found that hand position became unstable if endpoint force exceeded about 22 N in a specific direction. This did not occur when the joint stiffness coefficients were modeled as varying with the net torque at both joints, as in the present study. Rather, hand position became increasingly more stable as endpoint force increased for all directions of applied force. Our analysis suggests that co-contraction of biarticular muscles was primarily responsible for the increased stability. This clearly demonstrates how the central nervous system can selectively adapt the impedance of the arm in a specific direction to stabilize hand position when the force applied by the hand has a destabilizing effect in that direction.     

Movement and muscle activity during contact placing of the forelimb and their relations to other postural reactions in the cat

Summary Forelimb trajectory and the activity of eight muscles operating at the elbow, wrist and digit joints were compared during the contact placing reaction, during the swing phase of locomotion and during reactions induced by swing perturbations, to verify the hypothesis that common neural mechanisms are involved in these reactions. Both the patterns of muscle activation and forelimb kinetics during the placing reaction greatly differed from those during the swing phase of locomotion. Both similarities and differences have been found between the placing reaction and the reaction to swing perturbations. Similar latencies, patterns of muscle activation and trajectories have been found for elbow movements while considerable differences were seen in the movements of distal joints. Both reactions started with a backward and upward movement at the proximal joints which was accompanied by a locking at the elbow. At the distal joints, tactile stimuli evoked first a wrist ventroflexion during the placing reaction, whereas they induced wrist dorsiflexion to swing perturbations. A further difference between these two reactions appeared at the beginning of the extension which was highly passive during the reaction to swing perturbation and active during contact placing. These results suggest that some common, most likely spinal, reflexes are involved at the beginning of the two reactions while their extension phases are controlled in a different way.     

Development of respiratory muscle contractile fatigue in the course of hyperpnoea

To assess the development of inspiratory and expiratory muscle fatigue during normocapnic hyperpnoea, we studied fourteen healthy men performing 8 min hyperpnoea, 6 min pause, 8 min hyperpnoea, etc., until task failure. Twitch transdiaphragmatic (P_di,tw) and gastric (P_ga,tw) pressures were measured during cervical and thoracic magnetic nerve stimulation, before hyperpnoea, after every 8 min of hyperpnoea, and at task failure (i.e., at 25.3 ± 4.7 min). P_di,tw decreased during the first 16 min (−28 ± 7%, p < 0.001) and P_ga,tw during the first 8 min (−20 ± 7%, p < 0.001) of hyperpnoea without further change until task failure. During inspiration, the pressure–time-product of oesophageal pressure (PTP_oes) increased relative to PTP_di during the first 16 min (+11 ± 21%, p < 0.05). Similarly, during expiration, PTP_oes increased relative to PTP_ga during the first 8 min (+10 ± 16%, p < 0.05). Also, blood lactate concentration and respiratory sensations significantly increased during the first 8 min (+1.0 ± 0.5 mmol l⁻¹, p < 0.001) and 16 min (breathlessness +1.6 ± 1.8 points, respiratory effort +5.9 ± 2.2 points, p < 0.001), respectively. We conclude that, during hyperpnoea, contractile fatigue of the diaphragm and abdominal muscles develops long before task failure and may trigger an increased recruitment of rib cage muscles.     

Vestibulospinal and reticulospinal interactions in the activation of back muscle EMG in the rat

Summary The effects of electrical stimulation of the lateral vestibular nucleus (LVN) and medullary reticular formation (RF) on electromyographic activity in axial muscles medial longissimus (ML) and lateral longissimus (LL) in the rat were studied. Long trains (150–500 ms) at 200–330 Hz and 20–100 A were sufficient to activate ML and LL at latencies of 20–100 ms from the beginning of the train. Results of stimulation at 200–330 Hz to RF or LVN showed that muscle units were activated at a fixed latency from any effective pulse in the stimulus train. Using high frequency (1 kHz) trains of 3–6 pulses to LVN, EMG activity was detected at minimum latencies of 3.5–6 ms. When conduction times from the medulla to the spinal cord, and the spinal cord to the muscle are subtracted, this latency range is consistent with monosynaptic activation. In many cases, muscle units were recruited in order of size, with both RF and LVN stimulation. Combined stimulation of LVN and RF sites in n. gigantocellularis led to EMG activity in ML and LL at currents which were insufficient to evoke activity when presented singly. When stimulation of one site (300–400 ms train) was just sufficient to evoke a response, a shorter, overlapping train (100–150 ms) to the other site led to a higher rate of muscle activity that continued through the end of the long train, even after the short train had ended. In all cases, the effect of RF facilitating LVN was similar to the effect of LVN facilitating RF. The evidence for convergence between these two systems in the medulla and the spinal cord is discussed.