Masanori Nagaoka

Common modulation of motor unit pairs during slow wrist movement in man

1 The activity of 36 pairs of single motor units were recorded with intramuscular wire electrodes from m. extensor carpi radialis while subjects performed slow wrist extension and flexion movements. Periods of steady position holding were interposed between movements. 2 The discharge trains from pairs of motor units were analysed statistically in the time and frequency domains. During extension movements, when the muscle recorded from was the agonist, coherence between motor units was significant below 12 Hz, with a peak at 6–12 Hz in 30 of 36 pairs (83 %). The magnitude of coherence decreased during position holding compared to movements in 26 pairs, while the difference in average firing rate was small. 3 During movements, but not during position holding, coherence estimates between single motor units and acceleration showed a significant peak at 6–12 Hz in 56 out of 62 motor units, suggesting that a modulation of motor unit discharge contributed to angular acceleration at these frequencies. Common motor unit modulation was present at 3 Hz as well, although the coupling between motor unit activity was weaker than at 6–12 Hz. 4 It is concluded that a 6–12 Hz common modulation of agonist motor units is a distinguishing feature of slow voluntary wrist movements, extending the previously established notion of an 8–10 Hz rhythmic organization of slow finger movements to more proximal limb segments. It is suggested that the 6–12 Hz input is specific for movements and is normally absent or much weaker during steady maintenance of position or force.

Contribution of kinesthesia on human visuomotor elbow tracking movements

The role of peripheral information in smooth motor performance was analyzed in normal subjects and patients with deep sensory disturbance by the modified visually guided tracking method. The subjects task was to make the control cursor match the slowly moving target by elbow flexion movement. Performance of normal subjects was scarcely affected, but that of the patients was deteriorated by the withdrawal of the visual cue for handle movement. It is suggested that afferent information from the moving limb plays an important role in smooth and accurate tracking movements.

Dynamic response of human muscle spindle afferents to stretch during voluntary contraction

1 The response of twenty‐eight human muscle spindle afferents from m. extensor carpi radialis brevis to large amplitude ramp stretch and release at the wrist joint was recorded. The dynamic index was calculated as the difference in firing rate between that just before the end of stretch and that during the subsequent static phase of stretch. The value during steady voluntary contraction was compared with that during relaxation. 2 In twenty‐three primary afferents, the dynamic index increased in eleven and decreased in twelve afferents with a range of −8 to +25 impulses s−1. In five secondary afferents the change was less than 2 impulses s−1. 3 The primary afferents abruptly stopped firing when the stretch was released in the relaxed muscle. This cessation was prevented during contraction in seventeen primary afferents. 4 The results suggest the presence of dynamic and static fusimotor actions on the human muscle spindles during voluntary contraction.

Reassessment of H‐reflex recovery curve using the double stimulation procedure

We conducted two types of experiments to assess the validity of the H‐reflex recovery test, using double stimulation to test soleus motoneuron pool excitability in healthy and spastic subjects. One type dealt with the mechanical effect of the conditioning H reflex on the ankle joint; the other type with the effect of change in reflex size. The mechanical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to marked depression. We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group Ia inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical effect in the FR condition was observed despite the FX condition. The effects of systematic changes on soleus H‐reflex size were investigated at conditioning–test intervals of 80 ms, so as to avoid mechanical effects. When conditioning and test reflexes were the same size, the amount of recovery increased as the H‐reflex size increased. Comparison of the relation between amount of recovery and H‐reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity mentioned in previous literature may have resulted from the fact that relatively greater H reflexes were tested in those studies. In conclusion, the present study indicates that double stimulation is not appropriate for assessing spinal motoneuron pool “excitability increase” in spasticity.

Coupling between single muscle spindle afferent and EMG in human wrist extensor muscles: physiological evidence of skeletofusimotor (beta) innervation

OBJECTIVES This study was conducted to find physiological evidence of skeletofusimotor innervation in man. METHODS Discharges of 38 single muscle spindle afferents from m. extensor carpi radialis were recorded from 9 subjects during steady isometric contractions of wrist extension. Correlation between these afferents and rectified surface EMG was investigated by estimating cumulant density function. RESULTS In the cumulant density estimate between spindle afferent and EMG, a positive EMG peak was obtained prior to afferent firing between -30 and -10 ms in 15 afferents (39%). CONCLUSIONS The present finding indicates coupling between spindle afferents and extrafusal activity and suggests the widespread skeletofusimotor innervation in human muscle spindle.

Discrimination of different motor units by spike-triggered averaging of surface electromyograms

An objective method was sought for the discrimination of individual motor units (MUs). Discrimination using the conventional needle electrode recording is difficult because the waveforms are easily changed by slight shifts of the electrode. A surface EMG recording was combined with the needle recording and MU activities were obtained by MU-triggered averaging of the surface EMGs. Waveforms of surface MU potentials were stable, irrelevant to changes in recording conditions of needle electrodes and thus were found useful for the purpose of discrimination. Efficacy of this method was tested in the MUs with known axonal conduction velocities.

Development of robotic upper limb orthosis with tremor suppressiblity and elbow joint movability

Essential Tremor (ET) refers to involuntary movements of a part of the body. ET patients have serious difficulties in performing daily living activities. We have been developing a myoelectric controlled exoskeletal robot to suppress tremor. However, the EMG signal of ET patients contains not only signals from voluntary movements but also noise from involuntary tremors. Then, we have developed a signal processing method to suppress tremor noise present in the surface EMG signal. The proposed filter is based on the hypothesis that tremor noise could be approximate to powered sine wave. In this paper, we have integrated tremor canceling filter and neural network (NN) to recognize the tremor patients movement. According to the result, it was confirmed that the proposal filter increased accuracy of recognition, especially stable phase on elbow flexed position as “OFF”.

Analysis of EMG signals of patients with essential tremor focusing on the change of tremor frequency

Essential tremor is a disorder that causes involuntary oscillations in patients while engaging in actions or while maintaining a posture. ET patients have serious difficulties in performing activities of daily living such as eating food, drinking water, and writing. We have thus been developing an EMG-controlled exoskeletal robot to suppress tremors. The EMG signal of ET patients involves a mix of voluntary movement and tremor signals. To control the exoskeletal robot accurately, tremor signals must be removed from the patients EMG signal. To date, we have been developing a filter to remove tremor signals from the patients EMG. The design of this filter was based on the hypothesis that the rectified tremor signals are able to be approximated by a powered sine wave. This filter was found to have a large effect on removing tremor signals. However, tremor signals are generated both while performing voluntary movement and while maintaining a posture, and the filter was attenuating both signals. To control this robot accurately, the signal generated while performing voluntary movement is expected not to be attenuated. To accomplish this, we try to use a parameter that reflects a state of the patients movement, performing a voluntary movement or maintaining a posture, as a switch to activate the powered sine filter. This paper provides an analysis of the favorable parameters. We focus on two parameters: the peak-to-peak interval of the rectified EMG signal, and the interval of the flat and low amplitude area of the rectified EMG signal. Through evaluation, it is affirmed that both parameters change with the state of the patients movement. However, the latter parameter is superior to the former in terms of variability, which indicates that the interval of the flat and low amplitude area of the rectified EMG signal is a more favorable parameter to promote control of the exoskeletal robot. As a future work, we will mount the parameter to the algorithm and evaluate the robotic system.

The weight load inconsistency effect on voluntary movement recognition of essential tremor patient

Essential Tremor (ET) refers to involuntary movements of a part of the body. ET patients have serious difficulties in performing their daily living activities. Our ultimate goal is to develop a system that can enable ET patients to perform their daily living activities. We are in the process of developing an exoskeletal robot for ET patients. This robot is controlled by estimation of voluntary movement using surface electromyogram (EMG) signal input and a Neural Network (NN) learning algorithm. However, the EMG signal of ET patients contains not only signals from voluntary movements but also noise from involuntary tremors. We have therefore developed a signal processing method to suppress tremor noise present in the surface EMG signal. The proposed filter is based on the hypothesis that tremor noise can be approximated to powered sine wave. It have been confirmed that the proposed filter increases the accuracy of recognition. In this paper, we have focused on the effect of inconsistency of weight load between instruction signal and input signal. When the instruction signal comprised unloaded motion, our voluntary movement estimation method worked stably with the loaded motions EMG input.

The effect of l-threo-DOPS on synaptic transmission to soleus motoneuron in normal subjects and patients with Parkinson's disease

The effect of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) on synaptic transmission to the soleus motoneurons was studied using the H-reflex method in 12 normal subjects and in 4 patients with Parkinsons disease. A long-lasting depression of the test H-reflex, which was evoked by a conditioning group I volley in the homonymous tibial nerve with an onset latency of ca. 30 ms and a duration of more than 1 s and was regarded to represent the transmitter depletion caused by a preceding volley, diminished significantly after the oral administration of L-threo-DOPS both in the normal subjects and in the patients. On the other hand, no change was recognized in the slow depression which was caused by a group I volley of the heteronymous common peroneal nerve and was regarded as the presynaptic inhibition. The findings suggest that L-threo-DOPS influences the reduction of transmitter depletion.