Voluntary and reflex control of the biceps brachii muscle in spastic-athetotic patients

Entrainment to extinction of physiological tremor by spindle afferent input

In this study the systematic modulation of wrist flexor muscle activity by imposed joint movement was examined. Ten subjects maintained a constant contraction level (25% of the maximum; trial duration: 20 s) in flexor carpi radialis while their wrists were perturbed with 50 different quasi-sinusoidal signals (frequency range: 0.5-9.5 Hz; amplitude: 0.3-4.2 degrees ). The frequency spectra of wrist position and the rectified and filtered electromyogram (EMG) were determined. The muscle activity was only weakly entrained to imposed movements of small amplitude and low frequency, as shown by a small peak in the EMG spectrum at the frequency of movement, while the most prominent peak in the spectrum was between 9 and 15 Hz, corresponding to the frequency range of physiological tremor. The entrainment of muscle activity increased markedly as the amplitude and frequency of the imposed movement increased, to the point of saturation of modulation and harmonic peaks in the spectrum. In parallel with this increase in entrainment, the 9-15 Hz tremor peak was progressively extinguished. The results are consistent with a coupled oscillator model in which the central oscillatory source(s) of tremor became fully entrained to the imposed movement at the highest amplitudes and frequencies. Such coupling depends on communication between the external forcing oscillator and the central oscillator(s), the I (a) afferent signal from the imposed movement being the most likely candidate to provide the entraining signal for the central oscillator(s).

The spasticity paradox: movement disorder or disorder of resting limbs?

BACKGROUND

Spasticity is defined/assessed in resting limbs, where increased stretch reflex activity and mechanical joint resistance are evident. Treatment with antispastic agents assumes that these features contribute to the movement disorder, although it is unclear whether they persist during voluntary contraction.

OBJECTIVES

To compare reflex amplitude and joint resistance in spastic and normal limbs over an equivalent range of background contraction.

METHODS

Thirteen normal and eight hemiparetic subjects with mild/moderate spasticity and without significant contracture were studied. Reflex and passive joint resistance were compared at rest and during six small increments of biceps voluntary contraction, up to 15% of normal maximum. A novel approach was used to match contraction levels between groups.

RESULTS

Reflex amplitude and joint mechanical resistance were linearly related to contraction in both groups. The slopes of these relations were not above normal in the spastic subjects on linear regression. Thus, reflex amplitude and joint resistance were not different between groups over a comparable range of contraction levels. Spastic subjects exhibited a smaller range of reflex modulation than normals because of decreased maximal contraction levels (weakness) and significant increases of resting contraction levels.

CONCLUSIONS

Spasticity was most evident at rest because subjects could not reduce background contraction to normal. When background contractions were matched to normal levels, no evidence of exaggerated reflex activity or mechanical resistance was found. Instead, reduced capacity to modulate reflex activity dynamically over the normal range may contribute to the movement disorder. This finding does not support the routine use of antispastic agents to treat the movement disorder.

Reduction of spasticity in cerebral palsy using feedback of the tonic stretch reflex: a controlled study

Subjects with cerebral palsy aged six to 19 years undertook a training programme to reduce spasticity and contracture of the triceps surae muscle using feedback of the gain of the tonic stretch reflex. Stretch reflex gain was significantly reduced (by 50 per cent on average) in all eight test subjects, whereas there was no significant change in control subjects, matched for age and level of spasticity, who had no reflex training. Contracture of the triceps surae muscle was not altered. Since the reduction in spasticity can be expected to slow the recurrence of muscle shortening, it is suggested that for correction of muscle contracture, muscle-lengthening procedures to promote muscle growth should be combined with reflex training to maintain the growth achieved.

Reducing spasticity to control muscle contracture of children with cerebral palsy

A biofeedback training technique to control spasticity, previously successful with adults with cerebral palsy, was adapted for three children with spastic diplegia at risk for contractures. Visual feedback of muscle stretch reflex sensitivity is provided by video games, which are played by reducing reflex sensitivity. After a 10-week training period two of the three children had significantly reduced spasticity in the gastrocnemius muscle. The technique can be used with children as young as four years, is inexpensive, and can be carried out by parents with supervision by a physiotherapist.

Stretch reflexes of triceps surae in patients with upper motor neuron syndromes

Electromyographic responses of triceps surae to dorsiflexion stretch were studied in 47 patients with a variety of lesions producing an upper motor neuron syndrome. The short latency spinal reflexes, both when the patient was at rest and when he was exerting a voluntary plantarflexion, were frequently enhanced in magnitude and the rate of increase with acceleration was also enhanced. Long-latency reflexes were uncommon at rest. With background force long-latency reflexes were present unless the short latency reflex was very large. Long latency reflexes often were normal, but in some patients they were either excessively larger or even of abnormal shape with prolonged continuous activity. The clinical assessment of the ankle jerk correlated with the magnitude of the short latency reflex. The clinical assessment of tone correlated with the magnitude of the short latency reflex, the magnitude of the long latency reflex and the duration of the long latency reflex. There appear to be multiple physiological mechanisms underlying the clinical phenomenon of spasticity.

A microprocessor-based ultrasonic limb movement monitoring system

A system has been devised, using a spark-gap ultrasonic transmitter and an array of four ultrasonic receivers, to monitor the unrestrained movement of a physically handicapped child's fingertip as he attempts to perform some specific task. A microprocessor is used to measure the time taken for a pulsed ultrasonic shock-wave to travel from the fingertip-mounted spark-transmitter to each of the four ultrasonic receivers, and hence compute the Cartesian co-ordinates of the transmitter. It is hoped that this equipment will enable the voluntary components in the poorly-controlled limb movements of some of the physically handicapped children to be isolated and used to control external equipment.

Ballistic elbow flexion movements in patients with amyotrophic lateral sclerosis

Patients with amyotrophic lateral sclerosis made stereotyped 20 degrees elbow flexion movements as rapidly as possible while surface EMG was recorded from biceps and triceps. The characteristic ballistic movement EMG pattern could be recognised in almost all the patients. The first agonist burst and the first antagonist burst, which are normally limited in duration, were prolonged in patients with clinical signs of pyramidal tract disease or alpha motor neurone disease or both. Prolongation of these components permits the muscles to generate sufficient forces to accomplish the movements.

The role of action reflexes in the damping of mechanical oscillations

Reflex responses measured during voluntary contraction of the muscle being stretched are known to differ markedly from reflex responses elicited from passive muscle. The term 'action tonic stretch reflex' or 'action TSR' has been used previously to describe a reflex response to continuous stretch, separated from voluntary activity by means of a cross-correlational and spectrographic analysis. In this paper it is proposed that the action TSR play a functional role during voluntary movement by damping the transient oscillations associated with the natural resonant frequencies of the limbs. It is suggested that oscillations excite an action TSR force response with phase lead ahead of muscle stretch. This force response can be resolved into two force components, one of which has a 90 degree phase lead ahead of muscle stretch and behaves like a viscous friction reaction force causing damping of oscillations. Three experiments which support this proposition are described. (1) Analog computer model stimulation studies of a muscle supporting a mass-spring load reveal that damping only occurs when the force response has a phase lead ahead of muscle stretch. When the force response has a phage lag behind muscle stretch, the system is unstable and the amplitude of oscillation increases with time. (2) It is demonstrated that when a mass-spring load is supported as rigidly as possible by the human arm, reaction forces from the arm damp and mass-spring oscillations more rapidly than when the mass-spring is rigidly supported. Electromyogram (EMG) recordings reveal that mass-spring oscillations excite action TSR responses with phase lead ahead of muscle stretch. (3) Recordings of elbow angle and biceps EMG during rapid forearm flexion or extension movements followed by sudden stops reveal critically damped oscillations in the elbow angle signal (i.e. no more than one or two small overshoots), which are accompanied by EMG action TSR responses with phase lead ahead of muscle stretch.

Voluntary control of arm movement in athetotic patients

Visual tracking tests have been employed to provide a quantitative description of voluntary control of arm movement in a group of patients suffering from athetoid cerebral palsy. Voluntary control was impaired in all patients in a characteristic manner. Maximum velocity and acceleration of arm movement were reduced to about 30-50% of their values in normal subjects and the time lag of the response to a visual stimulus was two or three times greater than in normals. Tracking transmission characteristics indicated a degree of underdamping which was not presnet in normal or spastic patients. This underdamping could be responsible for a low frequency (0·3-0·6 Hz) transient oscillation in elbow-angle movements associated with sudden voluntary movement. The maximum frequency at which patients could produce a coherent tracking response was only 50% of that in normal subjects and the relationship between the electromyogram and muscle contraction indicated that the mechanical load on the biceps muscle was abnormal, possibly due to increased stiffness of joint movement caused by involuntary activity in agonist and antagonist muscles acting across the joint.

Measurement of involuntary arm movement in athetotic patients

Visual tracking tests have been used to obtain a quantified statistical description of the involuntary movements of the arm about the elbow joint in a group of patients suffering from athetoid cerebral palsy. Three separate components of involuntary activity can be recognized and it is possible that each may be a different physiological mechanism. First there are irregular movements which are represented by a continuous power spectrum which decreases with increasing frequency, reaching a negligible value between 2-3 Hz. The second component is a rhythmical low frequency movement which is indicated by a predominant peak in the power spectrum at a frequency of 0·3-0·6 Hz. The presence of this peak was predicted in a previous paper because of underdamping demonstrated in the closed loop voluntary control of movement in athetosis. The third component is the athetoid action tremor in which both agonist and antagonist muscle groups contract vigorously but asynchronously at a frequency of 1·5-4 Hz.

Interaction between voluntary contraction and tonic stretch reflex transmission in normal and spastic patients

Transmission characteristics of tonic stretch reflex (TSR) pathways in both normal and spastic patients have been measured during different levels of sustained voluntary contraction of the biceps brachii muscle. A cross-correlation technique of analysis was used to separate reflex responses from the total electromyographic activity. TSR transmission in normal subjects was observed to change with the level of voluntary contraction; sensitivity to stretch increased approximately three-fold as the subject stiffened the arm by simultaneously contracting flexor and extensor muscles. In contrast, TSR transmission did not alter during voluntary contraction in spastic patients. It is proposed that in spastic patients supraspinal modulation of reflex transmission is impaired because hypersensitive spinal reflexes short-circuit long loop pathways.