Mark B. Shapiro

The relation between posture and movement: A study of a simple synergy in a two-joint task

Abstract Human subjects performed fast, discrete elbow or wrist flexion or extension movements in a sagittal plane under the instruction to move one of the joints “as fast as possible”. We hypothesize that, when an instruction requires voluntary movement in only one joint, the muscles controlling the other, postural joint will display changes in their levels of activation tightly coupled to the primary movement, i.e. a postural synergy. Joint angles and electromyographic (EMG) signals from two flexor and two extensor muscles were recorded and analyzed. Both muscle pairs demonstrated commonly observed tri-phasic EMG patterns. The elbow flexor and the wrist flexor tended to demonstrate simultaneous EMG bursts, while the elbow extensor and the wrist extensor also showed similar patterns of activation. This was confirmed by both visual analysis and also cross-correlation analysis of the EMG pairs. The timing of the antagonist burst in postural muscles frequently coincided with shortening of the muscle and thus could not be attributed to the action of the local reflexes to muscle stretch. We consider the EMG patterns in postural muscles to be primarily of a central origin. We suggest that the formation of postural synergies (postural anticipation) may be not a separate process, but a separate peripheral pattern of a single control process that may involve a number of joints and muscles.

Time course and temporal order of changes in movement kinematics during learning of fast and accurate elbow flexions.

Abstract Learning of a motor task, such as making accurate goal-directed movements, is associated with a number of changes in limb kinematics and in the EMG activity that produces the movement. Some of these changes include increases in movement velocity, improvements in end-point accuracy, and the development of a biphasic/triphasic EMG pattern for fast movements. One question that has remained unanswered is whether the time course of the learning-related changes in movement parameters is similar for all parameters. The present paper focuses on this question and presents evidence that different parameters evolve with a specific temporal order. Neurologically normal subjects were trained to make horizontal, planar movements of the elbow that were both fast and accurate. The performance of the subjects was monitored over the course of 400 movements made during experiments lasting approximately 1.5 h. We measured time-related parameters (duration of acceleration, duration of deceleration, and movement duration) and amplitude-related parameters (peak acceleration, peak deceleration, peak velocity), as well as movement distance. In addition, each subject’s reaction time and EMG activity was monitored. We found that reaction time was the parameter that changed the fastest and that reached a steady baseline earliest. Time-related parameters decreased at a somewhat slower rate and plateaued next. Amplitude-related parameters were slowest in reaching steady-state values. In subjects making the fastest movements, a triphasic EMG patterns was observed to develop. Our findings reveal that movement parameters change with different time courses during the process of motor learning. The results are discussed in terms of the neural substrates that may be responsible for the differences in this aspect of motor learning and skill acquisition.

Velocity-dependent activation of postural muscles in a simple two-joint synergy

A simple two-joint synergy was studied over a range of movement velocities. We hypothesized that focal and postural components of the synergy are consequences of a single control process and, as such, will demonstrate similar scaling with movement velocity. Healthy subjects performed discrete elbow or wrist flexion or extension movements in a sagittal plane under the instruction to move one of the joints at different speed in different trials of a series. Joint angles and electromyographic (EMG) signals from two flexor and two extensor muscles were recorded and analyzed. Irrespective whether the focal movement took place in the elbow or in the wrist joint, and irrespective of the movement direction and velocity, the elbow flexor and the wrist flexor tended to demonstrate simultaneous EMG bursts, while the elbow extensor and the wrist extensor also showed similar patterns of activation. During flexion (extension) movements in either joint, the latencies of both elbow and wrist extensors (flexors) decreased with velocity of the focal movement. Integrals of the EMG bursts in all the muscles increased with movement speed in all the series. Typically, there was a close to linear relation between the integral EMG indices for the elbow and wrist flexors as well as for the elbow and wrist extensors. We conclude that there exists a simple, scalable synergy which is used by the central nervous system in a wide range of movement velocities to simplify control of the postural component of a motor task.

Proprioceptive feedback during point-to-point arm movements is tuned to the expected dynamics of the task

It has previously been found that in point-to-point movements against inertial loads, proprioceptive feedback is centrally suppressed in the beginning of movement and is facilitated at a time that is correlated with the expected time of peak velocity. This suggests that the modulation of proprioceptive feedback is governed by the desired movement kinematics. Here we show that in movements against inertial and viscous loads, the correlation of the time when the feedback is facilitated is strongest with the time when the joint torque is expected to be maximal. This suggests that the modulation of proprioceptive feedback is governed by the desired movement dynamics. We applied unexpected perturbations in point-to-point elbow flexion movements against known light and heavy inertial and viscous loads and determined the time and magnitude of responses in the electromyogram (EMG) of the biceps and triceps muscles. In movements against the inertial and viscous loads, the time of the EMG responses was better predicted by the time of the peak joint torque in the unperturbed movement than by the time of peak velocity or the time of peak acceleration or by measures related to the agonist EMG. Moreover, the EMG response changed from a reciprocal pattern in the inertial load conditions to a co-contraction pattern in the viscous load conditions. Our results suggest that during movements against known stable dynamic loads, proprioceptive feedback is tuned to the expected task dynamics and is facilitated so as to maintain muscle stiffness at a time when the muscles are expected to generate maximal force.

Effect of short and long term STN stimulation periods on parkinsonian signs

Currently, no study of subthalamic nucleus (STN) stimulation has compared continuous stimulation with a period of short‐term stimulation, which is frequently employed in the clinic and in research studies. Therefore, this study examined the effects of STN stimulation over 90 min (short) and greater than 3 months (long) on the cardinal signs of Parkinsons disease. The 90 min time period immediately followed a 12 hour withdrawal from both STN stimulation and medication. Ten PD patients who received STN stimulation were studied. Bradykinesia, rigidity, and tremor were evaluated using the UPDRS and motor control measures which included peak velocity (bradykinesia), work (rigidity), and amplitude (tremor). Results showed no difference between 90 min and greater than 3 months of STN stimulation for the UPDRS or motor control measures. This finding confirms that the treatment efficacy that is derived from a relatively short time course of stimulation generalizes to longer time periods of high frequency STN stimulation that patients experience in their daily lives. As such, it is reasonable to evaluate the effect of DBS after 90 min of stimulation in clinical trials and research studies.

Muscle activation is different when the same muscle acts as an agonist or an antagonist during voluntary movement

During movement, the intrinsic muscle force-velocity property decreases the net force for the shortening muscle (agonist) and increases it for the lengthening muscle (antagonist). The authors present a quantitative analysis of the effect of that muscle property on activation and force output of the same muscle acting as agonist and antagonist in fast and medium speed goaloriented movements. They compared biceps activation and force output when that muscle was the agonist in a series of elbow flexions and when it was the antagonist in a series of elbow extensions. They performed the same analysis for the lateral, long, and medial heads of the triceps muscle. Muscle EMG was about 2 times larger and the angular impulse developed by the modeled contractile torque was up to 3 times larger when the muscle or muscles acted as the agonist than when the same muscle or muscles acted as the antagonist in movements with similar kinematics. The large effect of the muscle force-velocity property strongly suggests that the neural controller must account for intrinsic muscle properties to generate movements with a commonly observed bell-shaped velocity profile.

EMG responses to unexpected perturbations are delayed in slower movements

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 slow-speed 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.

Reaction time is not impaired by stimulation of the ventral-intermediate nucleus of the thalamus (Vim) in patients with tremor

We studied the effect of high‐frequency electrical stimulation of the ventral‐intermediate nucleus of the thalamus (Vim) in four patients implanted with chronic stimulators to determine whether this procedure adversely affects reaction time to a proprioceptive stimulus. Two patients had undergone this surgery for treatment of tremor resulting from Parkinsons disease insufficiently responsive to levodopa therapy and two patients for treatment of essential tremor. Reaction times to auditory, visual, cutaneous, and proprioceptive stimuli were tested in a simple motor task requiring flexion of the elbow joint to a visual target in response to each stimulus. Reaction times were tested postoperatively with and without the stimulator turned on. We found that reaction time for all stimulus modalities was not increased when the stimulator was turned on; in fact, reaction times were, on average, slightly shorter during stimulation, but this difference was not statistically significant. We conclude that transmission of somatosensory inputs, necessary for initiating voluntary movement, from the periphery to the cortex is not significantly impaired by stimulation of the ventral‐intermediate nucleus of the thalamus in patients with pathological tremor.