Antagonist muscle activity during rapid arm movements: central versus proprioceptive influences.

Seven normal subjects were instructed to adduct the arm as fast as possible. On some trials the movement was prevented mechanically. For each movement or attempted movement, the EMG response in the antagonist muscle was measured. On trials in which no movement occurred, the respones were significantly smaller than those obtained during free, ballistic movement. The results demonstrate the role of proprioceptive influences on the contraction of the antagonist muscle.     

Postural adjustments associated with rapid voluntary arm movements 1. Electromyographic data.

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.     

Postural adjustments associated with rapid voluntary arm movements. II. Biomechanical analysis.

Normal subjects performed bilaterally symmetric rapid elbow flexions or extensions ("focal movements") while standing. Specific patterns of electromyographic activity in leg and trunk muscles ("associated postural adjustments") were seen for each type of movement. The biomechanical significance of these postural adjustments was analysed by means of the ground reaction forces and motion of the various body segments. Experimental data were compared with that from a theoretical model of the body consisting of a six segment kinetic chain with rigid links. Distinct patterns of the ground reaction forces with elbow flexion were opposite in direction to those seen with elbow extension. Movements of the various body segments were small and specific for a certain focal movement. Dynamic perturbations arising from the arm movement in an anteroposterior direction were found to be compensated by postural adjustments, whereas vertical perturbations were not compensated. The muscular activity acting about different joints in the different movements was found to correlate with the predictions of activity needed to compensate for net joint reaction moments arising from the focal movement. Motion of the various body segments could be understood as resulting from the interplay of the net reaction moments and the net muscular moments at the different joints. Dynamic postural requirements are accomplished by a precise active compensation initiated before the focal movement.     

Lack of presaccadic positivity before rapid eye movements in human REM sleep

Differences between oculomotor control of rapid eye movements (REMs) in REM sleep and that of saccades in wakefulness were examined electrophysiologically in human adults. Fourteen healthy young volunteers participated in the study. Brain potentials were recorded from the scalp and time-locked to the onsets of saccades and REMs during a visually triggered saccade task and natural nocturnal sleep. In wakefulness, presaccadic positivity (PSP) appeared at centro-parietal sites starting about 150 ms before saccades. In REM sleep, no PSP was found but a slow negative potential (pre-REM negativity: PRN) appeared at the prefrontal sites. The findings suggest that the generation of REMs does not involve the cortical process reflected in the PSP but is associated with a different neural process reflected in the PRN.     

Firing of human hippocampal units in relation to voluntary movements.

During certain movements (termed "type I," "instrumental," or "voluntary"), the rodent hippocampal EEG is dominated by regular 7-10 Hz waves. This "theta rhythm" is accompanied by increased firing of hippocampal interneurons and dentate gyrus granule cells. No obvious theta rhythm is present in comparable situations in humans or other primates. However, a widespread scalp negativity (the "readiness potential") starts approximately 1 second before spontaneous key presses at long intervals. The readiness potential has been recorded in the monkey hippocampus. In this study, action potentials were recorded in the human hippocampus in relation to various movements. During a broadly ranging interview including various movements and memory tests, hippocampal units were found that fired during movements of the tongue and/or hands. Only movements that required a high degree of effort were effective. Other hippocampal units appeared to be correlated with either the transitions between tasks or the interruptions within tasks. In a second experiment, hippocampal units were found to change their firing in the seconds preceding spontaneous key presses. These data indicate that, like the rodent hippocampus, human hippocampal neuronal activity is strongly influenced by movement.     

Intersensory facilitation in rapid single-joint voluntary activation and cancellation of arm movements

The ability to initiate and cancel actions is a basic requirement for motor control in humans. Rapid movements to stationary targets over single joints are characterized by triphasic bursts of electromyographic (EMG) activity. While analysis of reaction time in motor activation tasks, in relation to different modalities of sensory inputs, has studied, its diametrically opposite task of motor cancellation has not been adequately addressed. We studied 9 normal right-handed subjects using biceps (agonist) and triceps (antagonist) EMG recordings. Each underwent 3 motor activation and 3 motor cancellation tasks to light, sound and dual stimuli (6 blocks). The former consisted of ballistic elbow flexion over 45 degrees, while the latter involved dropping of the forearm from a 45-degree elbow flexion angle. For motor activation, onset latencies and duration of agonist (Lat1, Dur1) and antagonist (Lat2, Dur2) muscles were recorded. For motor cancellation, onset latencies and duration of agonist (Lat1 only) and antagonist (Lat2, Dur2) were noted. Motor cancellation showed significantly shorter Dur2 EMG bursts (p < .0005) for all 3 stimuli conditions. Lat1 and Lat2 demonstrated significant correlation (p < 0.0005 for all), with the exception of dual stimulus condition during motor cancellation (p = 0.089). While dual stimulus during motor cancellation resulted in significantly shorter Lat2 (p = .013) in comparison with light and sound stimuli, this was not evident for motor activation tasks. The findings suggest that while a common central program exists for executing motor activation and cancellation, generation of antagonist activity in the latter may involve distinct neural pathways specifically robust to the effects of intersensory facilitation. This is discussed in relation to reciprocal motor oscillatory activity manifestations at the level of single joint movements.     

Concurrent stable and unstable cortical correlates of human wrist movements

Cortical activity has been shown to correlate with different parameters of movement. However, the dynamic properties of cortico‐motor mappings still remain unexplored in humans. Here, we show that during the repetition of simple stereotyped wrist movements both stable and unstable correlates simultaneously emerge in human sensorimotor cortex. Using visual feedback of wrist movement target inferred online from MEG, we assessed the dynamics of the tuning properties of two neuronal signals: the MEG signal below 1.6 Hz and within the 4 to 6 Hz range. We found that both components are modulated by wrist movement allowing for closed‐loop inference of movement targets. Interestingly, while tuning of 4 to 6 Hz signals remained stable over time leading to stable inference of movement target using a static classifier, the tuning of cortical signals below 1.6 Hz significantly changed resulting in steadily decreasing inference accuracy. Our findings demonstrate that non‐invasive neuronal population signals in human sensorimotor cortex can reflect a stable correlate of voluntary movements. Hence, we provide first evidence for a stable control signal in non‐invasive human brain‐machine interface research. However, as not all neuronal signals initially tuned to movement were stable across days, a careful selection of features for real‐life applications seems to be mandatory. Hum Brain Mapp 35:3867–3879, 2014. © 2014 Wiley Periodicals, Inc .     

Mechanisms of voluntary movements

Voluntary movements constitute a mixture of drive related, motivational deep brain mechanisms and cortical goal representations. Some recent studies led to a better understanding of these aspects of voluntary motor behaviour. These data are discussed with reference to the pathophysiology of Parkinson's disease.     

Responses to force perturbations preceding voluntary human arm movements

Brief force perturbations were applied 30–120 ms prior to onset of step-tracking forearm movements by normal humans. The perturbations altered the first agonist burst of the movement-related triphasic EMG pattern. Perturbations opposing the movement resulted in an increase in the magnitude of the late part of the first agonist burst, the early part being unchanged. Conversely, in movements which would be assisted by the perturbation, EMG magnitude decreased during the late part of the burst. No reflex EMG responses were elicited during the period following the perturbation and preceding onset of the first agonist burst.     

Scaling of the size of the first agonist EMG burst during rapid wrist movements in patients with Parkinson''s disease.

Rapid wrist flexion movements were studied in a group of 10 patients with Parkinson's disease both on and off their normal drug therapy, and were compared with the same movements made by a group of eight normal individuals. When normal subjects made movements through 60 degrees, the first agonist burst of EMG activity in the wrist flexor muscles was longer and larger than that seen in movements of 15 degrees. If a large opposing load of 2.2 Nm was added, this also increased the size and duration of the first agonist EMG burst. Although the movements made by the patients were slower than those of normals, the size and duration of the first agonist EMG burst changed with movement size and added load in the normal way. This shows that patients can produce large, long bursts of EMG activity, but that there is a failure to match these parameters appropriately to the size of movement required. The effect of levodopa therapy on the movements was not dramatic. Although patients produced faster wrist movements when on medication than when off, the change was relatively small compared with the change seen in their overall clinical rating. Changes in the velocity of movements at a single joint are not a good reflection of the overall clinical state of patients with Parkinson's disease.     

Step-tracking movements of the wrist in humans. II. EMG analysis

We asked human subjects to make accurate step-tracking movements of the wrist to targets that required 5 degrees-30 degrees of radial or ulnar deviation. Speed instructions were given prior to each trial. Muscle activity was recorded from extensor carpi radialis longus (ECRL) and extensor carpi ulnaris (ECU) using surface electrodes. The agonist muscle initiated each movement with a brief burst of activity which began approximately 45 msec before movement onset. Then, the antagonist muscle displayed a brief burst of activity which began approximately 10 msec after movement onset. The magnitude, but not the timing, of these bursts was modulated by changes in the task requirements. The area of the initial agonist burst varied with changes in both displacement and intended speed. This burst was most highly correlated with the initial peaks of acceleration and jerk. In contrast, the area of the initial antagonist burst varied with changes in intended speed and was less well modulated by changes in displacement. This burst was highly correlated with the reciprocal of movement duration. Some small, fast movements had the same agonist bursts as some large, slow movements. However, the antagonist bursts for these movements differed greatly. This observation provides clear evidence that the magnitudes of the agonist and antagonist bursts are independently controlled. In a prior paper (Hoffman and Strick, 1986b), we proposed that step-tracking movements of different amplitudes and intended speeds are centrally generated by adjusting 2 kinematic variables: (1) the peak value and (2) the duration of a derivative of displacement. The present results suggest that these 2 kinematic parameters are separately generated by independently modulating the magnitudes of the agonist and antagonist bursts. Thus, the peak displacement of a step-tracking movement must be determined by the appropriate adjustment of both bursts of muscle activity.     

Modification of motor output to compensate for unanticipated load conditions during rapid voluntary movements

Mechanisms responsible for load compensation during fast voluntary movements were investigated in 20 normal subjects trained to carry out rapid wrist flexions against a standard load. When an unanticipated increase in load occurred, there was a compensatory increase in agonist EMG and decrease in antagonist EMG. Unanticipated decreases in load produced reciprocal changes with a decrease in agonist EMG and an increase in antagonist EMG. The latency of these EMG changes was quite short and compatible with a spinal reflex mechanism rather than a long loop response. The results suggest that mechanisms exist at the spinal level to allow rapid modification of motor programs when unanticipated load conditions are encountered on initiation of movement.     

Step-tracking movements of the wrist in humans. I. Kinematic analysis

We have examined the kinematics of the initial trajectory of step-tracking movements performed by human subjects. Each subject tracked a target that required 5-30 degrees of radial or ulnar deviation of the wrist. All movements were to be performed as accurately as possible. Speed instructions were given before each trial. When subjects performed different amplitude movements following the same speed instruction, the peaks of velocity, acceleration, and jerk were linearly related to peak displacement. The peaks of velocity, acceleration, and jerk also changed when the speed instruction was altered. Thus, for any given movement, the peak values of the derivatives of displacement were dependent on both movement amplitude and intended speed. As a result, the peak values of the derivatives cannot be used by themselves to control or monitor peak displacement. When subjects performed different amplitude movements following the same speed instruction, movement duration tended to remain constant. In contrast, movement duration changed when the speed instruction was altered. Movements performed when subjects intended to move slowly had longer durations than when subjects intended to move quickly. These results suggest that subjects volitionally alter intended speed by selecting different movement durations. When both movement amplitude and intended speed were varied, the peak displacement of a step-tracking movement was linearly related to the product of 2 kinematic variables: the initial peak of a derivative of displacement (either velocity, acceleration, or jerk) and movement duration. On the basis of our observations, we propose that central commands generate step-tracking movements of different amplitudes and intended speeds by adjusting both the magnitude and duration of a derivative of displacement.     

Antagonist inhibition during rest and precontraction.

The excitability of antagonist soleus motoneurons was tested during fast voluntary contractions of tibialis anterior (TA). Contractions of TA started either from zero or higher levels of tonic contraction of ankle extensors. Results showed that H reflex depression under 3 different conditions: 25% (R25) or 50% (R50) of the maximal isometric dorsiflexion from the resting state and the sequential isometric response (SW) appeared about 20-40 msec prior to the EMG onset of TA. In particular, H reflex depression was clear in the SW response, and the stronger the prior contraction of extensors the greater the H reflex depression. There was a significant difference in the integrated EMG between R25 and SW but none between R50 and SW. The different amounts of inhibition found for dorsiflexion from the resting state and sequential movement is most probably explained by presynaptic inhibition. If presynaptic inhibition were increased throughout the sequential movement, in comparison with the resting state, change in the H reflex gain would occur.     

Inhibition in the human motor cortex is reduced just before a voluntary contraction.

OBJECTIVE: To examine inhibition in the human motor cortex before and during voluntary movements. METHODS: The balance between the excitation and inhibition of corticospinal neurons in the human motor cortex was tested by conditioning the motor evoked potentials (MEP) evoked in forearm muscles by transcranial magnetic stimulation with a preceding subthreshold stimulus delivered through the same coil. RESULTS: When normal individuals (n = 9) made a tonic wrist extension, inhibition of the forearm extensor MEP decreased, whereas that of the forearm flexors was unchanged. When these individuals made a tonic wrist flexion, inhibition of the forearm flexor MEP diminished, whereas that of the forearm extensors was unchanged. When normal individuals (n = 10) made a phasic wrist extension in response to an auditory signal, inhibition of the extensor MEP began to decline about 95 msec before the onset of the agonist EMG activity. CONCLUSIONS: The changes in balance of excitation and inhibition of corticospinal neurons associated with a voluntary movement precede the movement and are directed at the corticospinal neurons projecting to the agonists. These changes may help to select the population of cortical neurons responsible for the movement.     

Low-threshold afferent signalling of viscous loads during voluntary movements of the human digits

Humans can discriminate changes in load viscosity during voluntary contractions. The afferent signal origin is unknown. Microneurographic recordings from 83 single low-threshold afferents were made while participants performed triangular ramps either unloaded or with a viscous load. The neural discharges for each cycle were compared across load and velocity. Fifty-eight afferents did not respond. Afferents with sufficient activity were classified as ambiguous--discharges correlated to velocity and load (n=4), infinite viscosity--strong load and weak velocity signal (n=6), no viscosity--strong velocity and weak load signal (n=10) and those with neither (n=5). No single class of afferent provides a coherent signal of viscosity. These data suggest that the central nervous system compares the population response of different inputs to discriminate viscosity.     

Hemispherical asymmetry in human SMA during voluntary simple unilateral movements. An fMRI study

Functional magnetic resonance imaging (fMRI) was used to test the hypothesis of a prevailing role of left supplementary motor area (SMA) during voluntary right and left finger movements, in line with subjects' right hand preference. fMRI responses were quantified using task-related percent increase of the signal from statistically activated voxels in primary somatosensory (S1), primary motor (M1), and SMA cortical regions. Regional analysis comprised both extension and intensity of statistically activated groups of voxels. Results replicated previous fMRI evidence. Right M1 and S1 were much more activated during left rather than right movements, whereas such a difference was less evident in left M1 and S1. A novel finding consisted in an analogous functional hemispherical asymmetry in left and right SMA. Strikingly, left SMA activation did not differ statistically during right (contralateral) vs. left (ipsilateral) movements. It was concluded that, in right-handed subjects, left SMA plays a prevailing role in the control of voluntary movements.     

EMG analysis of stereotyped voluntary movements in man.

EMG activity was recorded in biceps and triceps while subjects voluntarily flexed their elbows during a visual matching task. With fast flexion, the initial EMG was characterized by a triphasic pattern with a burst of activity first in biceps, then in triceps with a silent period in biceps, and finally in biceps again; these components were analysed quantitatively. Smooth flexion was characterized by continuous activity in biceps. Inhibition of tonic activity of triceps in relation to a fast flexion occurred in the 50 ms before the initiation of biceps activity. A patients with a severe pansensory neuropathy performed normally on these tasks. Physiological mechanisms underlying these patterns are analysed; an important conclusion is that the triphasic activity with fast flexion is 'centrally programmed'.