Mechanism of monosynaptic reflex reinforcement during Jendrassik manoeuvre in man.

The facilitating effect of the Jendrassik manoeuvre on monosynaptic reflexes in man has been studied by comparing H and T reflexes of the soleus and by blocking the effects of spindle activation by ischaemia of the leg. The Jendrassik manoeuvre equally enhances H and T reflexes provided that the test reflexes are small. The H reflex remains facilitated when the spindle activation cannot affect the soleus alpha-motoneurones--that is, when the Ia afferent nerve fibres from the soleus are blocked by ischaemia. It is concluded that the facilitating effect of the Jendrassik manoeuvre on the alpha-motoneurones is not predominantly routed via the gamma-loop.

The facilitating effect of the Jendrassik manoeuvre on tendon reflexes in man is well-known but the mechanism for this reinforcement is not yet clear. It is not known whether this enhancement is predominantly due to a direct facilitation of a-motoneurones or to an increase in spindle activity induced by y-activation. Controversial results have been obtained by experimenters using either reflex or microneuronographic methods. Reflex studies compared the effects of the Jendrassik manoeuvre on the Achilles tendon reflex and on the H reflex of the soleus. This electrically evoked H reflex is also a monosynaptic reflex of the soleus, but it bypasses the solcus spindles, and, therefore, does not depend directly on y-fibre activity. Sommer (1940), Paillard (1955), and Buller and Dornhorst (1957) claimed that the H reflex was less facilitated than the Achilles tendon (T) reflex. Different results have been obtained by Landau and Clare (1964) who found that the Jendrassik manoeuvre affected both H and T reflexes equally. The observed discrepancies are certainly due to technical differences regarding the strength of the reinforcing manoeuvre, the temporal relationship between the beginning of the handgrip and the test reflex, and the amplitude of the H and T test reflexes before the Jendrassik manoeuvre. Clare and Landau (1964) argued strongly against the gamma hypothesis persisted after fusimotor block with procaine. However, the use of microneuronographic techniques, which permit monitoring of the y activity by recording from the la afferent fibres from the spindles has raised the problem once more. Burg et al. (1974) found an increase of spindle activity in relaxed muscles during contraction of remote muscles and thought, therefore, that y-fibre activity was responsible for the enhancement of tendon reflexes during the Jendrassik manoeuvre. On the contrary, Hagbarth et al. (1975) have shown that the spindle activation is always accompanied by a very slight contraction of skeletomotor fibres which implies a-activation. Therefore, for them, y-activation during the Jendrassik manoeuvre is never isolated.
In this study we have observed the effect of the Jendrassik manoeuvre in man upon the T and H reflexes of the soleus. We have studied the effects observed at the beginning of the handgrip and during a sustained handgrip; the latter situation was first described by Jendrassik (1883). In addition, we have compared the effect of the Jendrassik manoeuvre upon the soleus H reflex before and during an ischaemic block of large diameter afferent nerve fibres from the leg; the disappearance of the Achilles tendon reflex during ischaemia proved the block of Ia afferent fibres from the soleus.

Methods
This investigation was performed in five healthy adult volunteers (26-41 years old). The subjects were comfortably seated in an armchair with hip and knee flexed at 1200. We studied the variations of the soleus H reflex and Achilles tendon reflex during the Jendrassik manoeuvre before and during ischaemia of the leg. The reflex responses were measured in terms of muscle action potentials recorded by two surface electrodes placed 15 mm apart on the soleus muscle below the gastrocnemius. The EMG responses were recorded on a magnetic tape and analysed by a Nova 1200 computer. The soleus H reflex was obtained by stimulation of the posterior tibial nerve at the popliteal fossa with single rectangular electrical pulses of 1 ms duration delivered through a monopolar electrode. The tendon reflex was elicited with an electromagnetic hammer (Bruel and Kjaer 4809), which produced a stretch of the soleus of 5 ms duration and 8 mm amplitude. The strength of the tap delivered by the hammer was adjusted so that the amplitude of the control T reflex was half its maximum value. The intensity of the H reflex stimulation was fixed so that the resulting H reflex had approximately the same amplitude as the control T reflex. The Jendrassik manoeuvre was performed in two ways.
(1) The subjects were instructed to perform a At 4 B D -_~~~~~~~-.I n brisk and strong handgrip of both hands (duration of one second, in response to a visual stimulus. This stimulus was delivered every three seconds. The test reflex was evoked 400 ms after the onset of the handgrip. This test reflex was alternatively an H reflex or a tendon reflex. Each series of 20 conditioned reflexes was followed by 20 additional reflexes evoked at rest. These control reflexes were preceded by the same visual stimulus. (2) The subjects were asked to perform a sustained handgrip of both hands and to control the force of their handgrip, which was possible by using a dynamometer. Three levels of force (maximal, half maximal, quarter maximal) were used. Each series of 20 (or 10) unconditioned reflexes was followed by three groups of conditioned reflexes, each group being obtained with a given strength of handgrip. In both situations, the mean amplitude of the H and T reflexes was calculated for each period of 30 s, a time during which five T reflexes and five H reflexes were evoked. This mean amplitude was expressed as a percentage of the maximum M direct response of the soleus. Ischaemia of the leg was obtained by placing a sphygmomanometer cuff at the high part of the  leg, but below the site of the H reflex stimulation (Fig. 1A). The sphygmomanometer cuff was inflated at 300 mm Hg. Time elapsed since the onset of ischoemia (min) Fig. 2 Effect of ischaemia of the leg on soleus monosynaptic reflex facilitation obtained by a strong and brisk handgrip. Each symbol represents the mean value of five H reflexes (upper graph) or five Achilles tendon reflexes (lower graph). These reflexes were obtained at rest (o) and during a brisk and strong handgrip (a). Abscissa: time elapsed since the onset of ischaemia; ordinate: size of the monosynaptic reflex expressed as a percentage of maximum direct motor response. The horizontal bar indicates the time interval during which the tendon reflex is reduced and then abolished. of the soleus. Figure 2 shows that there was an increase of about 50% in the amplitude of both H and T reflexes with the Jendrassik manoeuvre, and that the facilitation of the Achilles reflex was no stronger than that of the H reflex. Similar results were obtained in all subjects. The amplitude of the facilitated reflexes varied among the subjects between 122% and 185% of their control value for the H reflex ( Fig. lB and D), and between 140% and 196% for the tendon reflex ( Fig. IC and D). As claimed by Paillard (1955), the effect of a sustained handgrip upon H and T reflexes is much less marked than that of a brisk handgrip. For this reason, with subjects exhibiting a relatively high variability of monosynaptic reflexes, one would have to increase the number of reflexes in order to obtain significant results. However, as the period of ischaemia which can be explored (see below) is necessarily short, we have been able to study the effects of a sustained handgrip only on subjects in whom variability of the H and T reflexes was low. The results obtained in one subject are shown in Fig. 3. Throughout the control period the H and T reflexes were usually more facilitated when obtained during a stronger handgrip. The mean value of the facilitation obtained when the handgrip was 25%, 50%, and 100% of the maximal force was 131%, 152%, and 150% respectively of the corresponding H reflex control or 135%, 140%, and 180% of the corresponding T reflex control.

EFFECTS OF ISCHAEMIA OF THE LEG ON THE H REFLEX FACILITATION OBSERVED DURING REINFORCEMENT MANOEUVRES
Twenty to 27 minutes after the onset of ischaemia, the amplitude of the Achilles tendon reflex began to diminish; it then decreased more and more. About 30 min after the inflation of the cuff, the test H reflex and the M response were no smaller than at the onset of ischaemia, the Achilles tendon reflex being completely abolished (Fig. 1G). The disappearance of the tendon reflex while the H and M responses persist is due to the blockade of group I afferent nerve fibres below the site of the H reflex stimulation. This is in agreement with the results obtained by Magladery et al. (1950) who found that ischaemia of the leg blocks the group I afferent fibres before blocking the motor axons. Under these conditions, both kinds of reinforcement manoeuvre still result in a facilitated H reflex which varied among the subjects between 135% and 197% ( Fig. lF; right

Discussion
We have studied the effects of t manoeuvre on H and T reflexes of man. The H and T test reflexes we] and relatively small amplitude. T manoeuvre increased both reflexes facilitation was much more marked; a brisk handgrip than during susta tions, as found by Paillard (195 sustained handgrip the amount c depended on the strength of the observed by Bishop et al. (1968). shown by Landau and Clare (1964), difference between the increase ( reflexes provided that the size of bot was low. This suggests that the spindle acLivity does not play any critical role in the facilitating ;he Jendrassik effect of the Jendrassik manoeuvre, and this is the soleus in confirmed by the persistence of facilitation of the re of identical H reflex during ischaemia of the leg. This persisthe Jendrassik ing facilitation is certainly not due to the painful equally. This effects of ischaemia since these effects would at the onset of modify the H reflex in the same way whether it tined contracwas reinforced or not. Our results, therefore, sug-5). During a gest that blocking the Ia afferent nerve fibres Af facilitation which discharge after y-fibre activity does not handgrip, as modify the H reflex facilitation observed during As previously the Jendrassik manoeuvre. Similar conclusions there was no have been reached by Clare and Landau (1964) )f H and T who blocked y-fibres with lidocaine, a technique h test reflexes without objective proof of the efficiency of the block. In our experimental design, the block of the Ia fibres coming from the soleus was confirmed by the disappearance of the Achilles tendon reflex. Since the M response was unchanged, it is likely that group II fibres were not blocked, since they are smaller than a-fibres. The discharge O P K0' of these fibres is also controlled by y-activation > 9, but they are generally considered as inhibiting * * . .. soleus motoneurones (Eccles and Lundberg, 1959) and can, therefore, not be responsible for the persistence of the H reflex facilitation. The persistence of the facilitating effects of a Jendrassik manoeuvre, whichever way it is per-25 30 formed, when the fusimotor effects are eliminated by ischaeinia does not exclude the possibility that * y-motoneurones are normally activated during the v max manoeuvre. Our results in fact demonstrate that 0 V2 max the Jendrassik manoeuvre does not exert its effects A /4 max by predominantly facilitating .y-motoneurones.