Studies of the human stretch reflex

189

Journal of the neurological Sciences

Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

Short Report

Studies of the Human Stretch Reflex INTRODUCTION It has been stated that resistance to slow passive stretch in animal and human postural muscles is dependent on rheologic factors (STOLOV 1966), while rapid stretch elicits stretch receptor activity adding "reflex tension" to this passive resistance (LIDDELL AND SrIEm~tNGTON1924; LONG et al. 1964). We find that this is true for a large segment of the human population. But, in addition, there are individuals whose muscle tone is determined largely by rheologic properties irrespective of the velocity of stretch; and there are others whose tone is greatly influenced by reflex activity, even at slow velocities of stretch. In this latter group rapid velocities of stretch elicit a large "reflex tension" similar to that in patients with hemiplegic spasticity. MATERIALSAND METHOD Twelve normal college students were studied. Each subject was placed in a prone position on an adjustable supporting frame. The foot was secured in the boot of a Rotational Joint Apparatus (RJA) (REINER et al. 1967) which was secured to the frame. The knee was maintained in an extended position. The RJA rotated the foot (thus stretching the calf musculature) through preselected ranges of motion, 30 ° plantarflexion to maximum dorsiflexion, and through preset rates of movement, from 4°/see to 70°/sec. The 0 ° position was the same for all individuals and was taken with the foot at 90 ° with the long axis of the body. The RJA unit measured torque generated about the ankle joint via strain gauge transducers, and the degree or amplitude of displacement by an angular potentiometer. Measurements of torque were made at uniform points of displacement from the rest position during lengthening and shortening of the muscle. Electromyographic recordings were made from the triceps surae muscle (soleus and medial gastrocnemius) with wire electrodes. The electrical signals were displayed through a high gain amplification system into a Tektronix Rm 561 oscilloscope and a multichannel galvanometer recording oscillograph using 7 inch ultraviolet sensitive paper. Galvanometer recordings were made on 4 channels: (1) 2 channels for electromyographic recordings; (2) 1 for torque; (3) 1 for angular displacement. The muscle force of the Achilles tendon reflex was determined while the foot was placed in the RJA at the 0 ° position. The tendon was struck in a uniform manner by one of the authors. The response was recorded as torque in dyne-cm by the RJA. Percutaneous stimulating electrodes were placed on the tibial nerve low in the popJ, neurol. Sci. 0968) 7:189-192

190

SHORT REPORT

liteal fossa, and on the sciatic nerve in the sciatic notch (Fig. 1). After control studies of length-tension relationships were made, procaine (2% USP) was infiltrated around the tibial nerve (between the stimulating electrodes) until neural transmission was completely blocked. Criteria for complete blockage were: anesthesia in the distribution of the tibial nerve, loss of voluntary power, and inability to drive a maximal twitch and tetanus impulse from the sciatic notch past the area of block. Repeated stimulation via the electrode low in the popliteal fossa ensured that the procaine did not diffuse down into the muscle substance itself. Each maneuver was done a minimum of 5 times at a given velocity. The individual torque values represent a mean value of the 5 determinations. The range of variation about the mean never exceeded 3.106 dyne-cm. Procaine Block Gastrocnemius

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Fig. 1. The EMG recording electrodes are shown in the gastrocnemius and soleus muscles, and the stimulating electrodes are in the sciatic notch ($1) and the popliteal fossa (Sz). Torque-tension is measured at the ankle joint as the foot is moved in dorsiftexionor plantar flexion. RESULTS Torque varied during stretch in response to displacement in all subjects and in response to velocity in some subjects. The results in 7 of the subjects (Group A) show a moderate velocity sensitivity in the torque generated on passive stretch i.e. the more rapid the stretch, the greater the torque during the stretch and the greater the shift of the curve to the left (Fig. 2). After procaine block this group showed no change in torque generated at a slow velocity (6°/sec) when compared to preblock levels. But at rapid velocities (30--60°/see) there was a moderate decrease in torque during stretch, and a shift of the torque-length curve to the right. This decrease presumably corresponds to the "reflex tension", and agrees with the findings of previous workers. All the individuals in this group had an average amplitude (20" 106 dyne-cm) of Achilles tendon reflexes on percussion with a rubber hammer. In 3 of the subjects (Group B) torque elicited on both slow and rapid stretch showed only minimal gain with increasing velocity (Fig. 3). In these subjects block of the tibial nerve produced only a slight change from the preblock records at any velocity. Therefore in this group reflex activity appeared to play a negligible role in resistance to stretch. In all of these the tendon reflex was barely (2.106 dyne-cm) perceptible, and in one case could be elicited via the Jendrassik maneuver. In 2 of the subjects (Group C) (Fig. 4) there was marked velocity sensitivity at even J. neurol. Sci. (1968) 7:189-192

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Figs. 2, 3, 4. The graphs show the torque-tension curve of a typical individualfrom each of the groups. The ordinate shows the torque-tension in dyne-cm, the abscissa is the degree of extension of the foot. The numbers on the individual curves indicate the velocity of stretch in degrees/sec. the slower speeds. O n rapid stretch there was a large increase in the amplitude of t o r q u e a n d a p r o n o u n c e d shift to the left of the t o r q u e - l e n g t h curve. After procaine block there was a large decrease in the resistance to stretch at slow speeds as c o m p a r e d to the preblock level. The postblock resistance to passive stretch showed m i n i m a l velocity sensitivity, a n d at high stretch velocity the c o m p a r i s o n with preblock records was even more striking t h a n at slow speeds. I n addition, all 3 individuals i n this group had very brisk t e n d o n reflexes (57.106 dyne-cm.).

J. neurol. Sci. (1968) 7:189-192

192

SHORT REPORT

In 5 individuals the studies were repeated on a different day without procaine infiltration. On the repeat experiments the torque-length relationship, velocity, sensitivity, and tendon reflexes were noted to be identical with previous measurements.

CONCLUSIONS From this study we conclude that normal muscle tone is comprised of both rheologic and reflex elements. In the largest group (Group A, 7 subjects) rheologic factors predominated at slow rates of stretch with some reflex activity at higher rates; in the second group (Group B, 3 subjects) theologic factors were virtually the only determinants of torque, and almost no reflex activity was present at high speeds of stretch; and in the smallest group (Group C, 2 subjects) there was a large reflex component at slow speeds, which was still greater at high velocities. There was no correlation of these phenomena with body habitus, muscle bulk, occupation, or overt anxiety. The fact that the group with the most brisk tendon reflexes showed both the greatest velocity sensitivity and decrease in torque after procaine is further evidence that reflex factors were more important in these individuals. ACKNOWLEDGEMENTS The authors wish to thank S. Katz and H. Rosenberg for their assistance. This work was supported by U. S. Public Health Service Grant 5 TI-MH6413 and Vocational Rehabilitation Administration Grant RD1863-M.

SUMMARY Measurements of the response to stretch in the triceps surae muscle of 12 normal human subjects are described. Three types of response were observed. One response indicated rheologic factors to be virtually the only determinants of tension, another response indicated rheologic factors predominant at slow speeds with reflex activity at high velocities, and the remaining response indicated a large reflex response at all velocities. Department of Neurology, Department of Rehabilitation Medicine, Albert Einstein College of Medicine, Bronx, New York, N.Y. (U.S.A.)

H. SCHAUMBURG R. HERMAN

FOLEY,J. (1961) The stiffness of spastic muscle, J. Neurol. Neurosurg. Psychiat., 24: 125-131. LIDDELL,E. G. T. ANDSm C. SHERRINOTOr4(1924) Reflexes in response to stretch, Proc. roy. Soc. B, 96: 212-242. LONG, C., D. THOMASAND W. S. CROCHEXIERE(1964) Objective measurement of muscle tone in the hand, Clin. PharmacoL Ther., 5: 909-917. R~INER, S., H. SCrIAt~mORGAND R. HERMAN(1967) The rotational joint apparatus, Med. Res. Engin., (In Press). STOLOV,W. C. (1966) The concept of normal muscle tone, hypotonia and hypertonia, Arch. phys. Med., 47: 156-168. (Received 12 December, 1967) J. neurol. Sci. (1968) 7:189-192