Influence of stimulus parameters on human postural responses. J. Neurophysiol (original) (raw)
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Influence of stimulus parameters on human postural responses
Journal of neurophysiology, 1988
1. The role of sensory information in shaping muscle activation patterns to postural perturbations in humans was investigated by varying velocity, amplitude, or duration of the perturbing stimulus. Ten normal subjects were exposed to 120 backward translations of the support surface under conditions of varying velocities (10-35 cm/s, constant amplitude), varying amplitudes (1.2-12 cm, constant velocity), or varying durations (40-800 ms). The effects of perturbation parameters on movement kinematics and EMG latencies, patterns, and integrated areas in six trunk and leg muscles were examined. Integrated EMG activity was normalized across subjects and the early (first 75 ms), middle (second 75 ms), and late (last 350 ms) components were analyzed separately. 2. Ankle, knee, and hip angle trajectories and surface reactive forces suggest that a relatively consistent movement strategy was scaled to the perturbation velocities and amplitudes applied. 3. Short-duration perturbations (75 ms) e...
Postural adjustments associated with rapid voluntary arm movements 1. Electromyographic data
Journal of Neurology, Neurosurgery & Psychiatry, 1984
Normal subjects made bilaterally symmetric rapid elbow flexions or extensions ("focal movement") while free standing or when supported by being strapped to a firm wall behind them (different "postural set"). In some trials a load opposed the movement two thirds of the way into its course. Electromyographic activity in leg and trunk muscles ("associated postural adjustments") demonstrated specific patterns for each type of movement. Activity in these muscles began prior to activity in the arm muscles and demonstrated a distal-to-proximal order of activation. The EMG patterns were characterised by alternating activity in the antagonist pairs similar to the triphasic pattern seen in the arm muscles. When the movement type was changed change of the pattern of the postural muscles occurred over several trials. It is concluded that the associated postural adjustments are pre-programmed motor activity linked to the focal movement, specific for the focal movement including anticipated events and the postural set.
Experimental brain …, 2010
We studied the evolution of stretch reXexes in relation to background electromyographic (EMG) activity in the soleus muscle preceding the onset of voluntary arm raise movements. Our objective was to investigate if changes in reXex EMG and muscle activity occur simultaneously and are similarly scaled in amplitude. Ten human subjects stood with each foot on pedals able to exert short dorsiXexor pulses during stance. Subjects were asked to product consistent voluntary arm raise movements to a target upon a visual cue. In ¼ of trials, no pulse perturbations were given, but in the remaining ¾'s of all trials pulses were given randomly during a 600-ms period, from 400 ms before until 200 ms after the onset of the movements. Perturbation trials were sorted into 20-ms bins post hoc, and the amplitude of the reXex EMG component was calculated and compared to the EMG activity obtained when no pulses were given. Results showed that despite exhibiting similar proWles over time, the background EMG consistently inhibited before the reXex EMG did. However, times of reactivation (rebound) were variable across subjects, with background EMG activating before reXex for some subjects and vice versa for others. The minimum values of inhibition, time of inhibition and time of rebound for background and reXex EMG measures did not show signiWcant linear correlations when all subjects' data were considered. These results suggest that reXex and background EMG components of anticipatory postural adjustments evolve diVerently in time and amplitude. This has implications for the independent control of reXexes and voluntary muscle activity.
EMG responses to unexpected perturbations are delayed in slower movements
Experimental Brain Research, 2009
It has previously been found that in fast point-to-point arm movements, proprioceptive feedback is centrally suppressed at the beginning of movement and is facilitated at a time that is correlated with temporal parameters of the planned movement. Here, we show that this correlation holds when subjects are explicitly instructed to move at less than maximal speed. We studied elbow flexion movements made at maximal speed and at 70% of maximal speed over a short distance against a light inertial load and over a long distance against a heavy inertial load. A small number of trials were unexpectedly perturbed by using a servo-controlled motor to decrease the movement velocity. The servo control was turned on early in the movement. The main novel finding is that responses in the surface EMG in the elbow muscles to the perturbation occurred later in the slowspeed conditions than fast-speed conditions. When viewed across all conditions, the onset of the EMG responses to the perturbation increased with the time to peak acceleration in unperturbed movements. In the inertial loaded movements, the time of peak acceleration coincides with the time of peak inertial torque, and so the observed correlation can be interpreted as reflecting the relation between either the planned movement kinematics or the planned movement dynamics. These results are compatible with a hypothesis that a descending command suppresses the proprioceptive feedback control at the movement onset and facilitates it at a time that depends on the time parameters of the planned movement.
Changes in short and long latency stretch responses during the transition from posture to movement
Brain Research, 1981
Experiments were performed in 18 normal subjects to estimate the time course of changes in the gains of pathways mediating short-and long-latency responses to muscle stretch during the transition from a maintained posture against a steady load to a rapid ballistic movement. Subjects were instructed to rapidly flex or extend their forearm in response to a tone from an initial position of 90 ° of elbow flexion. Torque pulses stretching the biceps muscle were applied to the forearm at 8 different times before and after the signal to initiate the movement, and the gains of short-and longlatency pathways were estimated from averages of rectified biceps EMG activity for 20 trials at each time interval between the onsets of the tone and torque pulse.
Experimental Brain Research, 1994
To clarify the role of somatosensory information from the lower limbs of humans in triggering and scaling the magnitude of automatic postural responses, patients with diabetic peripheral neuropathy and agematched normal controls were exposed to posterior horizontal translations of their support surface. Translation velocity and amplitude were varied to test the patients' ability to scale their postural responses to the magnitude of the translation. Postural response timing was quantified by measuring the onset latencies of three shank, thigh, and trunk muscles and response magnitude was quantified by measuring torque at the support surface. Neuropathy patients showed the same distalto-proximal muscle activation pattern as normal subjects, but the electromyogram (EMG) onsets in patients were delayed by 20-30 ms at all segments, suggesting an important role for somatosensory information from the lower limb in triggering centrally organized postural synergies. Patients showed an impaired ability to scale torque magnitude to both the velocity and amplitude of surface translations, suggesting that somatosensory information from the legs may be utilized for both direct sensory feedback and use of prior experience in scaling the magnitude of automatic postural responses.
Neuroscience Letters, 2001
The purpose of this study was to explore how the muscles which control postural body segments are activated during bilateral isometric ramp pushes exerted with the upper limbs by seated subjects. The paradigm under study presents the advantage that the subject is in a quasi-static posture, and since upper limbs are stretched out, the dynamic phenomena, which might occur, can only originate from the rest of the body, which means from the postural chain. A dynamometer was used to measure the horizontal force, Fx, exerted on the bar, and a custom-designed force-plate was used to measure global reaction forces (Rx) and displacement of the centre of pressure (Xp) along the antero-posterior axis. Electromyograms (EMGs) were picked up by bipolar surface electrodes from 14 muscles crossing the lower limb, pelvis, trunk and upper limb joints. It was shown that transient push efforts require monotonous EMG increase in postural as well as in focal muscles. The EMG sequence starts with the postural muscles and ends with the focal ones. The postural EMG sequence is anticipatory. It is concluded that the EMG sequence is programmed according to the task parameters, and that its role is to counteract in advance the perturbing effect of the effort, in order to allow the effort to be performed ef®ciently. It is suggested that excitation between the postural muscles is distributed according to their biomechanical role in relation to the supports.
2010
Movement reduces the dynamic response of muscle spindle afferents and motoneuron synaptic potentials in rat. . Among the mechanisms that may result in modulation of the stretch reflex by the recent history of muscle contraction is the history dependence observed under some conditions in the response properties of muscle spindles. The present study was designed to test one report that in successive trials of muscle stretch-release, spindle afferent firing during stretch, i.e., the dynamic response shows no history dependence beyond the initial burst of firing at stretch onset. Firing responses of spindle afferents were recorded during sets of three consecutive trials of triangular stretch-release applied to triceps surae muscles in barbiturate-anesthetized rats. All 69 spindle afferents fired more action potentials (spikes) during the dynamic response of the first trial, excluding the initial burst, than in the following two trials. The reduced dynamic response (RDR) was nearly complete after trial 1 and amounted to an average of ϳ12 fewer spikes (16 pps slower firing rate) in trial 3 than in trial 1. RDR was sensitive to the interval between stretch sets but independent of stretch velocity (4 -32 mm/s). RDR was reflected in the synaptic potentials recorded intracellularly from 16 triceps surae ␣-motoneurons: depolarization during muscle stretch was appreciably reduced after trial 1. These findings demonstrate history dependence of spindle afferent responses that extends throughout the dynamic response in successive muscle stretches and that is synaptically transmitted to motoneurons with the probable effect, unless otherwise compensated, of modulating the stretch reflex.