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.     

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.     

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.     

Epigenetic development of postural responses for sitting during infancy

This study examined whether postural responses emerge in children in a predetermined way before independent sitting is achieved, and in what respect postural responses in infants differ from those in adults. Children just able to sit independently and children not yet able to sit were exposed to surface perturbations (translation and rotation) while body movement and electromyographic (EMG) responses were recorded. Perturbations causing a backward sway of the body (i.e., forward translation and legs-up rotation), elicited consistent patterns of muscle activity in ventral hip, trunk, and neck muscles in the independently sitting children. A high tonic EMG background activity in trunk and neck extensor muscles was inhibited at the onset of the ventral muscle activity. Kinematic analysis revealed that backward rotation of the pelvis was the first detectable body movement, while head movements (linear and angular displacement) were irregular and occurred later than the pelvis movement. Perturbations in the opposite direction, causing a forward sway, evoked variable responses in dorsal trunk and neck muscles, suggesting that the excitability level for postural responses was set according to the stability limits of the body. Children not yet able to sit without support were tested when the support around the waist, given by the experimenter's hands, was released prior to the onset of the platform perturbation. Postural responses were elicited in ventral muscles following a backward sway in all children and in about 60% of all trials. Often, only some of the ventral muscles were activated. No distinct responses were evoked during perturbations imposing a forward sway. These results suggest that (1) backward rotation of the pelvis triggers the postural adjustments in the independently sitting children; (2) a basic form of the postural adjustment develops in a predetermined manner before children practice independent sitting; and (3) the basic structure of ventral muscle activation pattern resembles that of adults, while the activation of the dorsal muscles (inhibition) differs in several aspects. These findings are in agreement with a recent model of central pattern generators for postural responses consisting of two operative levels. At the first level, which is triggered by backward rotation of the pelvis, the basic activation pattern is generated. At the second level, the pattern is shaped and fine-tuned by multisensory interactions from all activated sensory systems. The basic pattern present in the youngest infants may be produced mainly by neural networks at the first level, while the shaping function develops during practice, the shaping function being subjected to a learning process in which appropriate responses are formed in conjunction with the establishment of an internal neural representation for sitting.     

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.

Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics

There are several controversies concerning the organization and induction of postural adjustments in standing humans. Some investigators suggest the responses are triggered by somatosensory inputs (especially from the ankle in standing subjects), while others emphasize the vestibular input induced by head acceleration. We examined postural responses in sitting subjects in order to describe the muscle activation pattern during various perturbations and to test whether somatosensory or vestibular stimulation elicited the responses. The kinematics and EMG patterns in respons to perturbations caused by movements of the support surface were studied in adults. The postural muscle activation following a backward sway was mainly the same, whether it was elicited by a forward translation or a legs-up rotation. This is remarkable, since, except for pelvis rotation, the movements of all body segments including the head differed in the two conditions. Furthermore, a second experiment showed that the direction of the initial head movement could be reversed with retainment of the same postural muscle activation pattern. The results suggest that somatosensory signals derived from the backward rotation of the pelvis, and not vestibular information from the head, trigger postural responses during sitting. There was a slight but consistent difference in the muscle activation pattern, whether the backward sway was elicited by a forward translation or legs-up rotation. The difference seemed to reflect the sensory information from head and other body parts (except the pelvis). This finding allowed us to speculate in a central pattern generator for postural adjustments containing two levels. At the first level, a simple format of the muscle activation would be generated; at the second level, the centrally generated pattern could be shaped and timed by interaction from the entire somatosensory, vestibular, and visual input.     

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     

Trunk muscle coordination in reaction to load-release in a position without vertical postural demand

The aim of this study was to investigate the coordination between the innermost muscle layer of the ventro-lateral abdominal wall, the transversus abdominis (TrA), and other trunk muscles, in reaction to a load-release without the postural demand of keeping the trunk upright. Eleven healthy male volunteers participated. Intramuscular fine-wire electromyography (EMG) was obtained bilaterally from the TrA, rectus abdominis (RA), obliquus externus (OE) and erector spinae (ES) muscles. The subjects lay on their right side on a horizontal swivel-table with immobilized pelvis and lower limbs and with the trunk strapped to a movable platform allowing for trunk flexion and extension. Subjects maintained trunk flexion or extension at different force levels against a static resistance, which was suddenly released. They were instructed to resume the start position as fast as possible. EMG signals were analysed with respect to amplitude and timing of muscle activation. Following released static flexion, TrA increased its activity in synergy with ES. Also in released static extension, TrA increased its activity, but now in synergy with RA and OE. The direction-independent activation of TrA indicates a role of this muscle in controlling inter-segmental movements of the lumbar spine. This function was not accompanied by an early activation of TrA as has been shown previously for trunk perturbations in standing, i.e. a situation with an additional demand of maintaining the trunk posture upright against gravity.     

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.     

Automatic postural responses in the cat: responses of hindlimb muscles to horizontal perturbations of stance in multiple directions

Summary The effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in cats. We recorded EMG activity in eight proximal and distal muscles of the hindlimb along with vertical forces imposed by the limbs in awake behaving cats while they stood on an hydraulic platform. Postural responses to motion of the platform in 16 different horizontal directions were recorded. Vertical force changes were consistent with passive shifts of the center of mass and active correction of stance by the animals. When the perturbation was in the sagittal plane, vertical force changes began about 65 ms following initial platform movement. When the perturbation contained a component in the lateral direction, latency for vertical force changes was about 25 ms and an inflection in the vertical force trace was observed at 65 ms. No EMG responses were observed with latencies that were short enough to account for the early force component and it was concluded that this force change was due to passive shifts of the center of mass. The amplitude of the EMG responses of each muscle recorded varied systematically as perturbation direction changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. No angular range of activation was oriented strictly in the A-P or lateral directions. Most muscles displayed angular ranges of activation that encompassed a range of less than 180°. Onset latencies of EMG responses also varied systematically with perturbation direction. The amplitude and latency relationships between muscles, which made up the organization of postural responses, also varied systematically as perturbation direction was changed. This result suggests that direction of perturbation determines organizational makeup of postural responses, and for displacements in the horizontal plane, is considered a continuous variable by the nervous system.     

Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles

Summary This study contrasts the properties of compensatory postural adjustments in response to movements of the support surface with those of reaction-time voluntary movements in human subjects. Subjects stood upon a six degrees-of-freedom movable platform and performed tone and movement-triggered voluntary sways about the ankle joints both under conditions of postural stability and instability. These triggered movements could be executed as rapidly as postural adjustments to support surface perturbations (80–120 ms), but only when the former were well practiced, single-choice (direction) and were performed under conditions of postural stability. Evaluation of the properties of postural adjustments and reaction-time voluntary movements revealed a number of clear organizational differences between the two categories of movement, but most interesting was the finding that, when reaction-time movements were triggered by or at the onset of platform movement, the postural adjustment always occurred first. Only when subjects were given a tone trigger 50 ms in advance of platform movement were they able to execute the reaction-time movement first. We found that the dichotomous voluntary/reflexive classification of movements was not consistent with all of the identified properties of postural adjustments and reaction-time movements. Instead, we find a system which classifies movements by function, as either stabilizating or orientational adjustments, to be more useful. In the context of whole-body movement then, intentional focal components would be closely associated with others directed towards postural stabilization.This work was supported by NIH Grants No. NS-12661 and No. NS-00148. P.J. Cordo was supported by National Research Service Award No. 5-F32-NS-06304     

Changes in motor planning of feedforward postural responses of the trunk muscles in low back pain

Changes in trunk muscle recruitment have been identified in people with low-back pain (LBP). These differences may be due to changes in the planning of the motor response or due to delayed transmission of the descending motor command in the nervous system. These two possibilities were investigated by comparison of the effect of task complexity on the feedforward postural response of the trunk muscles associated with rapid arm movement in people with and without LBP. Task complexity was increased by variation of the expectation for a command to either abduct or flex the upper limb. The onsets of electromyographic activity (EMG) of the abdominal and deltoid muscles were measured. In control subjects, while the reaction time of deltoid and the superficial abdominal muscles increased with task complexity, the reaction time of transversus abdominis (TrA) was constant. However, in subjects with LBP, the reaction time of TrA increased along with the other muscles as task complexity was increased. While inhibition of the descending motor command cannot be excluded, it is more likely that the change in recruitment of TrA represents a more complex change in organisation of the postural response.     

Healthy humans use sex-specific co-ordination patterns of trunk muscles during gait

Human gait patterns differ considerably between the sexes. Therefore sex specific trunk muscle activation patterns can be expected. Healthy volunteers of both sexes (51 women, 55 men) walked on a treadmill at speeds from 2 to 6 km/h. Surface electormyography was recorded from five pairs of trunk muscles. Grand averaged root mean square (rms) curves and amplitude normalised curves were calculated. Mean amplitudes and relative amplitudes were calculated as well. Mean amplitudes as well as relative amplitude levels were not generally sex specific, but differed for single muscles. Grand averaged rms curves of all investigated muscles differed between sexes. At low walking speeds, differences mostly originated from mean amplitude level differences, alternating between sexes. At higher walking speeds, amplitude curves became more phasic, differences again alternated between sexes. Therefore, trunk muscle co-ordination during gait is sex-specific. Any interpretation of trunk muscle co-ordination patterns during gait requires sex specific normatives.     

Evaluation of nonlinear dynamics in postural steadiness time series

Fractal and correlation dimensions have been computed for time series obtained from tests of balance (postural steadiness). Although these measures appear to be reliable and differentiate subject groups, it has become clear that random (noise) time series may have finite dimensions and appear to demonstrate dynamics characteristic of nonlinear systems. Consequently, it is necessary to apply a test to distinguish a time series with putative nonlinear dynamics from random noise. A simple predictor was utilized to compare center of pressure (COP) time series with surrogate data constructed to have similar time and frequency domain characteristics. It was found that the original time series was more predictable than the surrogate data, suggesting that the COP data is derived from a nonlinear system.     

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.     

Out-of-plane trunk movements and trunk muscle activity after a trip during walking

Tripping during gait occurs frequently. A successful balance recovery implies that the forward body rotation is sufficiently reduced. In view of this, adequate control of the trunk momentum is important, as the trunk has a high inertia. The aim of this study was to establish out-of-plane trunk movements after a trip and to determine trunk muscle responses. Ten male volunteers repeatedly walked over a platform in which 21 obstacles were hidden. Each subject was tripped over one of these obstacles at mid-swing of the left foot in at least five trials. Kinematics, dynamics, and muscle activity of the main trunk muscles were measured. After a trip, an increase in trunk flexion was observed (peak flexion 37°). In addition, considerable movements outside the sagittal plane (up to 20°) occurred. Already before landing of the blocked foot, the trunk forward bending movement was reduced, while trunk torsion and lateral rotation were still increasing. Fast responses were seen in both abdominal and back muscles, indicating stiffening of the trunk. These muscle responses preceded the mechanical trunk disturbances, which implies that these responses were triggered by other mechanisms (such as afferent signals from the extremities) rather than a simple stretch reflex. A second burst of predominantly trunk muscle extensor activity was seen at landing, suggesting specific anticipation of the trunk muscles to minimize trunk movements due to landing. In conclusion, despite large movements outside the sagittal plane, it appears that trunk muscle responses to trips are aspecific and especially aimed at minimizing trunk forward bending.     

Mechanomyographic and electromyographic responses during fatigue in humans: influence of muscle length

Mechanomyography (MMG) provides a measure of muscle mechanical changes during contractions. The purpose of this study was to quantify alterations in MMG signals during fatigue at two muscle lengths. Comparisons with electromyographic (EMG) recordings were made. A group of 13 subjects performed isometric dorsiflexions (50% of maximum for 60 s) at 40° of plantarflexion (long, l _l) and 5° of dorsiflexion (short, l _s). The mean power frequency of the EMG (fˉ _EMG) and MMG (fˉ _MMG) signals and the mean rectified MMG () and EMG () were determined over each 1-s period, normalized to the respective maximal value, regressed against time, and the resulting slopes (units = %max · s⁻¹) were analyzed. The slopes were larger (P = 0.007) at l _l compared to l _s [mean l _l 0.50 (SD 0.26), mean l _s 0.27 (SD 0.16)], however there were no differences (P = 0.24) between mean fˉ _MMG slopes [l _l−0.10 (SD 0.16), l _s−0.16 (SD 0.11)]. Similarly, slopes were larger (P = 0.001) at l _l versus l _s [l _l 0.26 (SD 0.13), l _s 0.08 (SD 0.15)] and there were no differences (P=0.89) between mean fˉ _EMG slopes [l _l−0.15 (SD 0.14), l _s−0.14 (SD 0.12)]. At 5 s following the exercise to fatigue mean MVC (units = %max) were not significantly different between l _l and l _s [P = 0.08; l _l 78.8 (SD 9.1), l _s 85.2 (SD 6.0)]. These results showed that during fatiguing contractions, MMG and EMG amplitudes increased while frequency characteristics decreased at both muscle lengths. The change in and was greater at l _l but no differences in fˉ _MMG or fˉ _EMG slopes occurred between lengths. These results would suggest that larger increases in motor unit recruitment occur with time during fatigue at l _l compared to l _s. Accepted: 6 September 1999     

Relation between muscle response onset and body segmental movements during postural perturbations in humans

Summary This study has examined the individual movements of the body segments of a group of 10 standing adults during anterior and posterior platform displacements (3 and 6 cm amplitudes), and compared body movements to neck and ankle muscle response onset times. Differences in the kinematics of movement were observed for anterior vs. posterior platform displacements: hip, shoulder, and head began to move much earlier for posterior compared to anterior platform movements. This could explain differences in postural muscle temporal response organization for the two directions of body movement. Though anterior/posterior neck and head displacements were late in comparison to neck flexor muscle response onset, small vertical movements of the shoulder and head occurred early (40 and 67 ms after platform movement onset). These movements were consistently directed upward for anterior platform displacements and downward for posterior platform displacements. In order to determine whether neck proprioceptors were responsible for response activation in the neck we repeated the experiment using a neck stabilization device, on one of our subjects. In this condition, we found normal neck muscle response latencies. This suggests that neck proprioceptors are not the primary contributors to the early neck muscle responses seen during horizontal support surface displacements. In studying the effect of repeated exposure to horizontal platform displacements we found a diminution in the amplitude and an increase in onset latencies in neck and antagonist ankle muscle responses over the sequence of 16 trials, in many of the subjects tested. This corresponded to smaller head accelerations, and smaller displacements of the head and shoulder in later trials in the experimental sequence. The result implies that these subjects changed their postural set during the course of the experiment, possibly by relaxing the muscles of the body to allow the viscoelastic properties of the lower body segments to absorb more of the impact of platform displacement.     

Postural muscle activity during bilateral and unilateral arm movements at different speeds

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.     

Loss of set in muscle responses to limb perturbations during cerebellar dysfunction

The properties of electromyograph (EMG) responses that enabled the arm to return accurately to target following limb perturbations were investigated in five Cebus monkeys. In particular, factors that affected the timing and magnitude of an early antagonist response that occurred prior to stretch of the antagonist muscle were examined. The early antagonist response was large and early (latency, 60 ms) when the perturbation was brief and a constant force assisted the return movement. In this situation, early contraction of the antagonist muscle was required to prevent the return movement from overshooting the target. To determine whether this early antagonist response was influenced by prior instruction (which in this case was the type of perturbation the monkey had previously received), two types of perturbations requiring different EMG responses were studied. When torque steps (duration, 2,000 ms) were expected and were applied, monkeys generated M1, M2, and M3 responses and later activity only in the agonist (the initially stretched) muscle. When torque pulses (duration, 40 ms) were expected and were applied, monkeys generated M1 and M2 responses in the agonist and an early antagonist response. EMG responses to torque pulses and steps were then compared when the type of perturbation was expected and when it was unexpected. These comparisons revealed that the early antagonist response only occurred when the monkey expected a torque pulse. Therefore, this response was dependent on set. Expectation of a torque step caused enhancement of the agonist M2 and M3 responses. These agonist and antagonist EMG responses that were dependent on set were also influenced by changes in afferent drive. Cerebellar nuclear cooling through probes implanted lateral and medial to the dentate abolished that component of EMG responses attributed to set. The residual EMG responses in agonists and antagonists appeared to be driven by stretch of their respective muscles. The results suggest that when the nature of an arm perturbation is correctly predicted, the cerebellum provides accuracy in repositioning the limb a) by adjusting the magnitude of the M2 agonist response and b) by enabling activity after a latency of 60 ms (e.g., the M3 and early antagonist response) to be switched to the agonist or antagonist as appropriate, irrespective of which muscle is being stretched. This latter mechanism provides the motor system with predictive ability.