increasing torque. Switzerland contractions on H reflexes of human soleus muscle.

Transcription

1 Journal of Physiology (1990), 427, pp With 6 figures Printed in Great Britain SUPERPOSITION OF H REFLEXES ON STEADY CONTRACTIONS IN MAN BY D. G. RJEGG, R. KRAUER AND H. DREWS From the Institute of Physiology, University of Fribourg, CH-1700 Fribourg, Switzerland (Received 21 November 1989) SUMMARY 1. The aim of the investigation was to study the influence of steady isometric contractions on H reflexes of human soleus muscle. 2. Stimulating and recording conditions were hardly affected by plantar flexions which subjects maintained in a force matching task. 3. If the interval between a preceding control and the test stimulus was less than 8 s the test H reflex was depressed in the relaxed subject. The depression was diminished or removed if the test reflex was superimposed on a background activity. The interval between control and test H reflex was at least 8 s in the following experiments. 4. H reflexes were nearly independent of steady plantar flexions on which they were superimposed. In some subjects, there was a slight increase with increasing torque. During dorsal flexions, H reflexes in all subjects were inhibited with increasing torque. 5. The relationship between test H reflexes, control H reflexes and background activity was evaluated by varying pseudo-randomly stimulus intensity and steady flexion torque. The surface defined by this three-dimensional relation approximated a plane suggesting linear properties of the H reflex. In some subjects threshold intensity decreased slightly with torque, in others it was constant. 6. In response to a warning signal, human subjects initiated steady plantar or dorsal flexions in both feet and, at the same time, they started to concentrate on a light at the onset of which they performed a unilateral ballistic plantar contraction as fast as possible. The relations between H reflex and maintained flexion force during the warning period of the reaction time task were identical to those during force matching, showing that the behavioural context did not modulate the relations. 7. The relations were also the same if reflexes were evoked bi- or unilaterally, illustrating the absence of a mutual modification of simultaneously evoked H reflexes. 8. The relation was the same with ipsilateral matching and relaxed contralateral muscles as with bilateral matching. If the ipsilateral side stayed flaccid contralateral matching increased H reflexes by about 20% above control values. 9. It was concluded that various factors can combine to produce an increase of MS PHY 427

2 2 D. G. RUEGG, R. KRAUER AND H. DREWS H reflexes with torque, the most important of them being the use of short intervals between H reflexes. We have various evidence from the present experiments for believing that, in the relaxed subjects, the subliminal fringe was small and that although stimulus intensities below threshold could evoke an afferent volley, the effect of this on low-threshold motor units was prevented by presynaptic inhibition at the la terminals. INTRODUCTION It has been reported repeatedly but sometimes only qualitatively that H reflexes are facilitated when they are elicited during a steady plantar flexion and depressed during a steady dorsal flexion (Hoffmann, 1922; Paillard, 1955). The most detailed quantitative study by Gottlieb & Agarwal (1971) agrees with the former reports. A careful examination of all the published reports revealed, however, that, in some studies, intervals between H reflexes were much shorter than about 10 s, which can depress the test reflexes (Toth, Solyom & Vajda, 1979; Van Boxtel, 1986). Since the depression can be reduced by a background activity (Burke, Adams & Skuse, 1989), then a progressive H reflex-background activity relation could be obtained; we have investigated this question once more in detail, being stimulated to do so by different results we obtained in a former study (Riedo & Riiegg, 1988). We have tested, in particular, if (1) stimulation and recording artifacts, (2) the stimulation intensity, (3) the behavioural context, (4) uni- or bilateral steady torque and (5) uni- or bilateral H reflexes modulate the relation between H reflex and steady torque. Some of the results have appeared in preliminary communications (Krauer & RUegg, 1986; Riiegg & Krauer, 1986). METHODS Subjects and material The experiments which have been approved by the Ethical Committee were performed on ten healthy male and three healthy female subjects, with the consent and understanding of each of them. They were sitting in a modified car chair with arm and head rests. The angle at the knee was adjusted to 120 deg, the angle at the ankle to 90 deg. The subjects were wearing shoes each of which was fixed rigidly to a plate. The plates were fixed on one side to an axis which coincided with the ankle joint. Torque applied on the plates was recorded with metal rings on which four strain gauges were mounted in a bridge configuration. The rings were spring loaded and fixed on the plates such that isometric movements of up to 20 N m could be recorded in both directions. The electromyographic (EMG) activity of the tibialis and the soleus muscle from both legs was recorded with bipolar surface electrodes 3 cm apart. H reflexes and direct muscle (M) responses of the soleus muscle were elicited with a modified Simon electrode (Simon, 1962) positioned in the popliteal fossa above the tibial nerve. Steady contractions were initiated by an acoustic signal (frequency 1 khz; duration 200 ms; intensity 60 db), and ballistic movements of the right or left foot by two light-emitting diodes of 4 mcd, 5 cm apart and at 1 m distance. Experimental data were recorded with an AD-converter (3 khz/channel) of an HP-A600- computer system. Digital data (onset of light and side of the ballistic contraction) were encoded on channel 1. The recordings on the other channels were low-pass filtered (cut-off frequency 1 khz) in order to avoid aliasing. The following recordings were made: channel 2, torque of right foot; channel 3, torque of left foot; channel 4, EMG of right soleus muscle; channel 5, EMG of left soleus muscle; channel 6, EMG of right anterior tibialis and channel 7, EMG of left anterior tibialis muscle.

3 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS 3 Experimental procedure Stimulus duration for H reflexes and M responses was always 1 ms. In all experimental series except when otherwise noted, the intensity of the H reflex stimulus was such that it elicited a reflex of 50% of the maximal value in control conditions (2-5 ma). It was verified that, at the beginning and end of each experimental session, the maximal reflex amplitude and the stimulus intensity to obtain it were similar. In all the experimental series except the third and the fourth, a matching protocol was used. The recorded torque levels of both feet were displayed on an oscilloscope beam, the torque of the right foot with the right half and the torque of the left foot with the left half of the beam. The second beam indicated which torque level the subject should maintain by bringing both beams on the same level. Sixteen equidistant torque levels were presented covering dorsal and ventral flexions. The strongest contractions corresponded, depending on the subject, to about 10 % of the maximum voluntary contraction force of the soleus muscle and to about 40 % of maximum force of the anterior tibial muscle. In some sessions, only plantar contractions were tested. A trial started with control responses after which (7 s later) the warning signal and, at the same time, the oscilloscope was turned on, thus initiating the matching task. Test responses were elicited 1-2 s later. Six series of the experiments were performed. The first and second series of matching experiments were performed with four subjects in order to examine if stimulating conditions in the popliteal fossa and the recording conditions on the soleus muscle were modulated by a maintained background activity. Control and test stimulus within a trial were of the same modality (i.e. either eliciting H or M responses) on one side. In the first series, stimulus strength for M responses was such that it evoked a control response of 50 % of its maximum value; in the second series, it was 120% of the value which elicited just a maximum control response. Laterality of H and M responses and the different steady torque levels were arranged pseudo-randomly within a session which consisted of 128 trials. The importance of the delay between the control and the test H reflex stimulus was investigated in the third series of experiments. Trials were started with a bilateral control H reflex stimulus and a simultaneous acoustical warning signal at which the subjects did or did not initiate a steady plantar flexion of medium intensity. Test H reflexes were elicited 1-5 or 8 s later. Four different types of trials were recorded: (1) no steady contraction and control-test stimulus interval of 1-5 s, (2) no steady contraction and stimulus interval of 8 s, (3) steady contraction and stimulus interval of 1-5 s, and (4) steady contraction and stimulus interval of 8 s. The 128 trials of a session were arranged in a pseudo-random order. In the fourth series, H reflexes obtained during bilateral matching were compared with those obtained during the warning period of a reaction time task in which the subjects had to perform a ballistic plantar flexion of the right or left foot depending on which of the two lights in front of them was turned on. The lights were switched on in a pseudo-random sequence each with probability 0 5. As in the matching task, a trial started with a control H reflex on both sides. After about 6s, an acoustic warning signal started at which the subjects concentrated on the coming reaction signal or, in addition, performed a dorsal or plantar flexion of both feet depending on the previously given instruction which was to maintain either (1) strong plantar, (2) medium plantar, (3) weak plantar, (4) no, (5) weak dorsal, (6) medium dorsal, or (7) strong dorsal flexion. These different instructions were presented in a pseudo-random sequence. The light signal turned on s after the warning signal at which point the subjects performed a ballistic plantar contraction as fast as possible. Within 1 s just before light onset H reflexes were elicited bilaterally at random time. Two sessions of sixty-four trials were always recorded in a random sequence, one in the reaction time and the other one in the matching task. The modulation of H reflexes by the stimulation intensity and steady plantar flexions was elaborated in the fifth series of experiments. Before each session, the following procedures were performed separately for each side in the relaxed subject. The threshold and the intensity to evoke a maximum H reflex were determined. The first stimulator was set to the threshold, the second to an eighth of the difference between the maximum and threshold intensity, the third to a fourth, and the fourth to half of it. The constant-current stimulators were connected in parallel and could be turned on or off separately for each trial. The eight stimulation intensities which could then be obtained on right and left side and eight equidistant plantar torque levels were distributed pseudorandomly within a session of 128 trials. 1-2

4 4 4D. G. RUEGG, R. KRAUER AND H. DREWS In the sixth series, uni- and bilateral matching was combined with uni- and bilateral H reflex stimulation. All sessions were equivalent except the sequence of the trials. A trial was characterized by uni- or bilateral test H reflexes and the required plantar torque level. All combinations were tested except unilateral torque and contralateral test H reflex. Sessions thus consisted of sixty-four trials. In all trials, control responses were recorded first by the computer and recording was initiated again 200 ms before the test stimulus and continued for 1 s in the reaction time task and for 300 ms in the matching task. Analysis of data The amplitude of the H reflexes and M responses on the EMG signals was computed on-line and displayed for control purposes. The amplitude of the former was defined as the difference between the maximum and minimum value within a time window from about 30 to 50 ms after the electrical stimulus and the amplitude of the latter as the difference within a time window from 10 to 20 ms depending on the subject. H and M responses to study recording conditions were not further processed. Most other H reflexes have been normalized in relation to the preceding control value and were expressed in per cent. Reflexes which were recorded to study the relation between control H reflex, test H reflex and steady contraction (fifth series) were not normalized but a surface Y(m, n) given by the test H reflexes was computed where m represents the background torque level in multiples of the interval Ax and n the size of the control H reflex in multiples of the interval Az. The N recorded test H reflexes Pi(Xi, Zi), i = 1, 2, 3... N, where Xi represents the background torque in multiples of Ax and Zi the size of the control reflex in multiples of Az, were convoluted with a Gaussian weighting function W: N E [Pi(Xi, Zi)] W(m -Xi, n -Zi)] Y(m, n) = W(M-Xi~n-Zag) where W(j, k) = exp - [T/U Ax)]2 - [r/(k Az)]2. The following values were chosen: Ax = 0 80 N m, Az = 0-37 mv, T. i = 0 80 N m and T, = 0-37 mv. RESULTS Stimulation and recording conditions during different isometric steady contraction levels Some special experiments were performed in order to test the stability of stimulating and recording conditions. This was necessary since in most of our subjects, no direct muscle responses (M responses) in the soleus muscle were evoked with stimulating intensities which elicited control H reflexes of 50% of their maximum value. The first series of experiments was intended to study recording conditions during a matching task in which subjects maintained steady levels of motor activity in the soleus muscle. The intensity of H reflex stimuli was so adjusted that they evoked control reflexes of 50 % of the maximum value and M response stimuli were 20 % stronger than the value which evoked just a maximum response. Since recording and stimulating conditions could vary between right and left leg and from session to session, the results of three sessions were plotted separately for the right and left side (Fig. 1A). The upper cluster of curves represents average size of M responses, the lower one the average size of H reflexes. M responses consistently increased with the plantar torque level. The parallel shifts of the curves might be explained by varying electrode and interelectrode resistances from one condition to the other. In this subject the influence of the plantar torque on the size of the

5 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS recorded potentials was large compared to the other subjects in most of whom this effect was negligible. H reflexes slightly increased in parallel with the background activity and the different curves were, like those of the M responses, parallel to each other. There was a tendency that M (filled symbols) and H (open symbols) response 10 A 5 D 8 4 E 6 ( C D-V.. X 0 IS010 1~~~~~~~~~ Figl.Sz2fM epne (file sybos exeti)adhrfee oe ybl)a N20 0~~~~~~~~~~~~22 o z Torque (N m) Torque (N m) Fig. 1. Size of M responses (filled symbols, except in C) and H reflexes (open symbols) as a function of the plantar torque maintained in a matching task. In A and B the data of a typical subject were plotted separately for right and left side of the first three sessions of the experimental series. The intensity of the stimuli for the M responses in B and of all H reflexes was adjusted so that it evoked control responses of 50% of their maximum value. The stimuli for the M responses in A were supramaximal. Each curve in A and B was computed from thirty-two data points. The same shape of symbols for M responses (filled) and H reflexes (open) was used for each of the stimulating and recording conditions. In A and B, M and H responses from one condition tended to be in the same region within the group of curves. The maximum M response (A) increased consistently with the steady torque level. The submaximum M responses (B) tended to decrease in parallel with the steady torque level but the relation was variable from one stimulating condition to the other. The H reflex data in B have been normalized by the corresponding M responses in B and the results were plotted in C. In D the data from C were pooled (0) and the data from three subjects which were processed the same way were added. The H reflex obtained from an average subject was not consistently modulated by the level of plantar flexion. Abscissa: steady torque level in N m (negative values indicate plantar flexion). Ordinate: amplitude of M response or H reflex measured on the EMG of the soleus muscle in mv (A and B) and H reflex normalized by the submaximal M response which was obtained during the same session, on the same side, and at the same torque level (C and D). relations of the same side and session (same shape of symbol) were in the same region of the corresponding cluster of curves. The first and second series of experiments were identical except that in the second one M stimulus intensity was adjusted so that control M responses of 50 % of their

6 6 D. G. R UEGG, R. KRA UER AND H. DREWS maximum value were obtained (Fig. 1B). It must be assumed that the differences in the slope of the H reflex relation in Fig. 1 A and B were random and the level of these curves could have been modulated by slightly different settings of the stimulation intensity. M responses were generally decreasing with increasing steady torque but the rate of decrease was variable and sometimes almost zero. In some sessions of TABLE 1. Depression of H reflexes by preceding control H reflexes Subjects Subjects maintaining Subject relaxed steady torque P Mean Values are means+s.e.m. Results are H reflex sizes with confidence limits (P = 0 05) from five subjects. Reflexes obtained with a control-test reflex interval of 1-5 s were expressed as a percentage of the reflexes recorded with a control-test interval of 8 s. The depression of the H reflex was significantly larger in the relaxed subject than in the subject maintaining a steady flexion force. other subjects, we could even observe an increase of the M response with increasing plantar torque. It can be reasonably assumed that the background activity not only affected the M response but also the H reflex. In order to enable a suppression of this artifact, it was assumed that the recruitment curves for the H reflex and the M response have a similar slope at around 50 % of the maximum value (Hugon, 1973). The effect of the torque on stimulating conditions can then be eliminated by normalizing H reflexes with M responses recorded at the same side and during the same session. If this procedure is chosen the effect of changing recording conditions which was investigated in the first experimental series was eliminated automatically since it was identical for all recorded potentials. Normalized H reflexes were computed on the basis of the data in Fig. LB and they are shown in Fig. 1C. The scatter of the curves was reduced by the normalization and the excitability of the motoneuronal pool, as expressed by the H reflex, was independent of the steady plantar flexion in this subject. Attenuation of the test by the control H reflex A conditioning H reflex depresses a following test reflex at intervals up to several seconds (Hoffmann, 1922; Magladery, 1955). The aim of the following experiments was to test if this low-frequency depression (Van Boxtel, 1986; Burke et al. 1989) was influenced by a background contraction on which the test H reflex was superimposed. The subjects remained relaxed after a conditioning control reflex or initiated immediately afterwards a steady contraction of medium intensity in the soleus muscle. A test reflex was evoked 1x5 or 8 s after the conditioning stimulus. The depression of the test H reflex by the conditioning reflex was computed in the relaxed and in the subject maintaining a steady torque (Table 1). In the average relaxed subject, the test reflex obtained with the short interval of 1-5 s was 42-3 % of the

7 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS reflex obtained with the long interval of 8 s, whereas, in the subject with a preexisting steady flexion force, the reflex was only depressed to 77-3 %. These results provide evidence that the low-frequency depression of a test reflex by a preceding conditioning reflex was significantly reduced if the subject maintained a steady flexion force and that consequently the relation between steady flexion force and 7 TABLE 2. Increase of the threshold of the H reflex stimulus strength by preceding control H reflexes Increase in H reflex stimulus threshold Subject % of threshold % of range Mean Values are means+s.e.m. Results are increases of H reflex stimulus threshold with confidence limits (P = 005) from four relaxed subjects. They are expressed in per cent of the threshold intensity to elicit a control H reflex and in per cent of the difference between the stimulus strength to elicit just a reflex of maximum amplitude and the threshold strength. superimposed H reflexes can be modulated by the interval between succeeding H reflex stimuli. In order to get some hints about the physiological mechanisms involved, we measured thresholds to evoke an H reflex in the relaxed subject with control-test reflex intervals of again 1-5 and 8 s. As expected, the threshold was larger with short than with long intervals but the increase was small, i.e. 5-3 % in the average subject (Table 2). The relative increment was a bit larger (8-6%) if it was expressed in per cent of the useful range of stimulation intensities, i.e. the difference between the intensity just eliciting a maximum H reflex and the threshold. We estimated that the increase was much larger (about 30% instead of 5-3 %) if not threshold intensities were examined but intensities which evoked reflexes of 50% of a maximum control reflex. Modulation of the relation between H reflex and steady contraction force by the behavioural context and stimulation intensity In order to exclude movement or other methodological artifacts as reasons for disagreements between our recent data and reported results, the modulation of the H reflex by plantar flexions was studied in parallel with the M responses as outlined above. The H reflexes were normalized session by session and for each side separately and then the mean relation was computed for each subject (Fig. ID). There was no consistent increase or decrease of the H reflex in parallel with the level of the background activity. It rather seems that the size of the H reflex was independent of the steady plantar torque in the average subject. In the subject which had a relatively strong correlation between H reflex and plantar flexion (K), M responses decreased by far the most with increasing torque level. The slope of the curve in Fig. ID thus depended critically on the correcting algorithm.

8 8 D. G. RUEGG, R. KRAUER AND H. DREWS Since our first data, which were obtained in a reaction time task (Riedo & Rfiegg, 1988), were different to most other reported data, we studied whether the H reflex is modulated by the behavioural context during which steady contractions were maintained. Reflexes were elicited when the subject was either concentrating on a A B C D Time (ms) Fig. 2. Plantar flexion of the right foot in a visual choice reaction task. The subject was instructed to initiate a steady flexion force at the onset of a warning signal. H reflexes were elicited bilaterally during the warning period (200 ms before light onset). At light onset, the subject performed as fast as possible a ballistic plantar contraction on the right side whereas he released the torque on the left side. A, torque recording on the right foot. B, torque recording of the left foot. On both torque recordings, the contraction due to the H reflexes are superimposed on the steady torque. C, EMG recording of the right soleus muscle. D, EMG recording of the left soleus muscle. Abscissa: time in ms (time 0 corresponds to the light onset). Ordinate: 4 N m (A and B), 500 1sV (C and D). matching task or when he was maintaining a steady contraction and, in addition, waiting for a light signal at which he had to perform a ballistic contraction as fast as possible. An example of a trial in the reaction time situation is shown in Fig. 2. Although the voluntary contraction was on one side, H reflexes from both sides were equivalent and could be pooled since the subject was in a choice reaction time situation and did not know in advance on which side the 'go' signal would turn on. The relation between H reflexes and steady contraction is shown for a representative subject (Fig. 3). By comparing the relation obtained during the matching task (Fig. 3A) with that obtained during the warning period of the reaction time task (Fig. 3B), it becomes obvious that the two relations did not differ from each other. This finding that H reflexes were not modulated by the behavioural context was confirmed by all other subjects. The shape of the H reflex-steady contraction relation itself was alike in all but one subject in which reflexes were more and more depressed with increasing plantar flexions. Since results from this and the following experiments were normalized by preceding control H reflexes and not by corresponding M responses as above, we

9 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS could not exclude that, in this subject, the H reflex stimulus efficacy diminished with the steady torque level. All the above results were obtained with a stimulation intensity which evoked H reflexes of about 50 % of the maximum H reflex which can be obtained in control A B x Torque (N m) Fig. 3. H reflexes during steady contractions. They were elicited either during a bilateral matching task (A) or during the warning period of a reaction time task while the subjects were maintaining a steady torque in both feet (B). Relations of a typical subject in which H reflexes were independent of steady torque. Abscissa: torque in N m (negative values indicate plantar flexion; positive values indicate dorsal flexion). Ordinate: size of H reflexes in percentage of control values at rest. conditions. Four subjects varied not only the background activity from 0 to 20 N m which corresponded to about 10 % of the maximum contraction force but also the intensity of the H reflex stimulus was changed pseudo-randomly from trial to trial. All the data were pooled and a surface was computed which shows how test H reflexes depended on the maintained steady flexion force and the strength of the stimulus as reflected by preceding control H reflexes (Fig. 4). Peaks and depressions are randomly distributed which we verified by comparing the surfaces obtained from the different subjects (it is not possible to compute mathematically confidence limits for this surface). The threshold intensity to evoke an H reflex decreased slightly with increasing plantar flexion. This reduction in threshold was always small or even absent in other subjects. In all of the subjects there was a smooth transition in H reflex size from relaxed to slightly contracted muscles suggesting that the subliminal fringe of the motoneuronal pool was small in the relaxed subjects. The relation between test and control H reflexes was approximately linear for all steady torque levels. However, if test reflexes were plotted as a function of the stimulation intensity (not illustrated) reflexes increased only slightly at high stimulation intensities and a flattening of the H reflex-intensity relation occurred. Summarizing, the surface defined by the relationship between test H reflex, control H reflex and background activity approximated a plane in all the subjects. The causes of this linearity are

10 10 D. G. RUEGG, R. KRAUER AND H. DREWS E x 0) I en 0) I- 0 Fig. 4. Influence of background activity (torque in N m) and size of control H reflex on test H reflexes elicited during the matching task. The relation was obtained by twodimensional filtering of 728 data points which were obtained from one subject. Close to the threshold of control reflexes, test reflexes increased slightly with increasing background activity, a feature not present in all subjects. Otherwise H reflexes were almost insensitive to background activity. Regional variation of test reflexes proved to be random. x 0) A B E.) 0 0) N.co 0) r.5 (0 I I I I I I I, Il l I I. l. Il l. I i Torque (N m) Fig. 5. Mutual influences of H reflexes which were elicited during a bilateral matching task (plantar flexion). H reflexes were elicited bilaterally (A) or unilaterally (B). Pooled results from four subjects. Abscissa and ordinate as in Fig. 3. difficult to understand since it was the result of the interaction of various highly nonlinear processes as will be exposed in the Discussion. During dorsal flexion, the H reflexes were progressively depressed in all the subjects (Fig. 3). The inhibition was more or less pronounced in the different subjects. Recent findings indicate that this depression may not be due to reciprocal I a inhibition by the antagonists (Crone, Hultborn & Jespersen, 1985; Iles, 1986) but to

11 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS 11 presynaptic inhibition of I a terminals (Hultborn, Meunier, Pierrot-Deseilligny & Shindo, 1987b). Mutual modification of bilaterally evoked H reflexes It is conceivable that H reflexes which are elicited simultaneously on both sides might influence each other since the distance between motoneuronal pools of the I^AA_ luu 50 - T 19r 0O a I I, I I * Ị I I I I Id I - I A wi, r I I v I I I I I I I I I I I I I I I I I I r-t-ti B x 0) 0) I._ N en._) 0) 50 (U 0* I I I I I I I I I I I I I I I I I I. I I'~~~~~~~~~~~~~~~~~ 0 i Torque (N m) v I I 1 TI I.,, I I I I Fig. 6. Influence of steady plantar torque on H reflexes during a matching task. H reflexes were elicited bilaterally. Steady contraction was maintained on both (A and B), or on one side (C-F). H reflexes which were ipsilateral (C and D) and contralateral (E and F) to the steady torque were plotted separately. Pooled data from three subjects on the left side (A, C and E); data from a further subject on the right side (B, D and F). Abscissa and ordinate as in Fig. 3. F right and left side is short. In addition, EPSPs giving rise to the H reflex are relatively long-lasting and time might be sufficient for mutual influences via interneurones, as it has been shown recently that the H reflex could also have polysynaptic components (Burke, Gandevia & McKeon, 1984). In order to study mutual influences, H reflexes were tested bi- or unilaterally while the subjects maintained bilaterally a steady level of voluntary drive in a matching

12 12 D. G. R UEGG, R. KRA UER AND H. DREWS task. The experiments were performed on four subjects (different subjects than in the preceding section). Since the relationship between the H reflex and the level of steady plantar flexion was similar in all of them, data were averaged across subjects (Fig. 5). In accordance with Fig. ID, the H reflex-torque relationship was flat, i.e. a maintained steady flexion force had no influence on the size of H reflexes. A comparison of Fig. 5A with Fig. 5B shows distinctly that bilateral H reflex testing (Fig. 5A) provides exactly the same results as unilateral testing (Fig. 5B), implying that simultaneously evoked H reflexes do not influence each other. Relation between H reflex and bi-, ipsi- and contralateral steady contractions H reflexes were elicited bilaterally in all following experiments since, as illustrated in the preceding section, reflexes did not influence each other and twice as many data points were obtained with bilateral than with unilateral stimulation. Average responses of three subjects are shown on the left side of Fig. 6. The relation in Fig. 6A was obtained with bilateral steady flexion forces and corresponds thus to Fig. 5 except that the former was obtained with three, the latter with four subjects. The H reflexes obtained during ipsilateral force matching (Fig. 6C) did not differ at any torque level from the H reflexes obtained during bilateral matching (Fig. 6A). These results indicate that a contralateral steady torque did not influence H reflexes if an equal ipsilateral steady torque was maintained. If the ipsilateral ankle was relaxed and a motor discharge was maintained on the contralateral side, the relation between H reflex and torque level (Fig. 6E) was different from the previous conditions. Instead of a slight depression of the H reflexes with increasing torques there was a facilitation. Although facilitation and depression were not strong, the two relations were significantly different from each other as indicated by the confidence limits. Thus a steady flexion force on the contralateral side only had an influence on H reflexes if no ipsilateral steady force was maintained. The results of one subject which were different from the others are shown on the right side of Fig. 6. Bilateral and ipsilateral steady flexions did not slightly inhibit as in the other three subjects but facilitated H reflexes (Fig. 6B and D) as did only contralateral flexion (Fig. 6F). Presumably, this subject reacted as the other subjects to contralateral flexions but slightly differently to bi- and ipsilateral flexions and, by chance, this was the same way as to contralateral flexion alone. DISCUSSION Methods and experimental design Our first preliminary results (Riedo & Ruiegg, 1988) which were obtained in a reaction time situation suggested that background activity did influence H reflexes in the soleus muscle to a much lesser extent than published in most other reports (Hoffmann, 1922; Paillard, 1955; Hagbarth, 1962; Gottlieb & Agarwal, 1971). We have therefore tested if recording conditions stayed constant during different levels of background activity. Special care was taken to test this possibility since our recordings were from the distal portion of the soleus muscle and it had been demonstrated in cats that tendon length can change significantly during isometric contractions (Hoffer, Caputi & Pose, 1988). We recorded a small increase of the maximum M response with increasing plantar torque level in most subjects. But in

13 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS none of the subjects was it such that it could explain the facilitation observed by other authors. By eliciting M responses in parallel with the H reflexes, we tested whether the efficacy of the electrical stimulus depended on the level of the plantar flexion. There was a tendency that responses decreased with increasing torque but the effect varied from one stimulating condition to the next. In order to enable a correction of this effect, we assumed that-the slopes of the recruitment curves for the H reflex and the M response were similar at 50 % of their maximum value (Hugon, 1973). The reflexes were normalized by the M response for each torque level and stimulating condition, which eliminated not only the effect of varying stimulating conditions but also of varying recording conditions since M and H responses were affected the same way. Only the two-dimensional relation between steady plantar flexion and H reflex during the matching task was computed in this way because (1) the experimental protocol was very much complicated by this procedure, (2) the effect of recording and stimulation conditions tended to be in opposite directions and cancel, (3) normalization by a preceding H reflex decreased the variability of the data since H reflexes tended to fluctuate spontaneously and (4) because, in most of the further experimental series, differences between experimental conditions and sides were investigated which were insensitive to normalization. Having excluded methodological reasons for the insensitivity of H reflexes on the steady plantar torque level, we examined if differences in the experimental protocol could modulate the reflexes. BQth intertrial intervals and the intervals between control and test reflexes were about 8 s in our experimental series. But some authors studying the H reflex facilitation in the soleus muscle during contraction used intervals shorter than 5 s. Our experimental results revealed that these intervals are of crucial importance for the steady torque-h reflex relation. With short intervals of 1-5 s, reflexes were depressed in the relaxed subject to about 40% of the control value. This depression in the soleus muscle (Van Boxtel, 1986) as well as in forearm muscles (Burke et al. 1989) decreases continuously with longer intervals. If control H reflexes are reduced by short intertrial intervals and, as in most of our experimental series, test reflexes are normalized by the preceding control, all normalized test reflexes are affected the same way by the attenuated controls and the shape of H reflex relations is not changed. The situation is, however, different if the control-test H reflex intervals are so short that a depression of the test reflexes can occur. Of importance then is the finding that the depression was reduced if the subject maintained a background activity in the soleus muscle. This is in line with results obtained in human forearm muscles (Rothwell, Day, Berardelli & Marsden, 1986; Burke et al. 1989). Results which have not been yet published (D. G. Riiegg & H. Drews) offer evidence that the decline of the depression is positively correlated with the intensity of the background activity. Taking into consideration these results, the various published H reflex-steady torque relations can partly be explained. Large (Hagbarth, 1962; Gottlieb & Agarwal, 1971; Hultborn & Pierrot-Deseilligny 1979), medium-sized (Hultborn & Pierrot-Deseilligny, 1979; Schieppatti & Crenna, 1984), and no or small (Gottlieb & Agarwal, 1978; Capaday & Stein, 1986; our results) facilitations have been reported. In some of the papers, H reflex intervals considerably shorter than 10 s have been used which implies that reference H reflexes were depressed which produced an apparent facilitation. This interpretation is further supported by the finding that, with stimulus intervals of 4 s, the regular 13

14 14 D. G. R UEGG, R. KRA UER AND H. DREWS alternation of control with test stimuli can lead to biased results (Fournier, Katz & Pierrot-Deseilligny, 1984). In addition to the timing of the experimental protocol, the following factors should be considered, although we assume that they are not the main causes for the differences between the published results. The size of H reflexes depends on the foot angle, namely dorsal flexion reduces reflexes and plantar flexion potentiates them (Gottlieb & Agarwal, 1978; Iles & Roberts, 1987). Hultborn & Pierrot-Deseilligny (1979) observed that contractions were not strictly isometric and that up to 20% plantar flexion of the ankle occurred during the strongest activations of the soleus muscle. In the experiment of Nardone & Schieppati (1988) foot angle and torque changed concomitantly and the facilitation of the H reflex could be thus increased by angle changes. The type of isometric contractions could also modulate the H reflex-steady torque relation. The subject can either perform a muscle contraction against a rigid pedal as in the present experiments or he might be required to keep the ankle angle constant against a constant force of a torque motor (Verrier, 1985; Iles & Roberts, 1987). The latter task favours co-activation in order to stabilize the limb and suppress any oscillatory movements (Akazawa, Milner & Stein, 1983). Even a small activation of the tibialis muscle might then excite Ia afferents by an a-y-co-activation (Vallbo, 1970) and Ib afferents. Ia (Eccles, Magni & Willis, 1962; Barnes & Pompeiano, 1970) and Ib discharges (Devanandan, Eccles & Yokoto, 1965) of flexor muscles can inhibit H reflexes by presynaptic inhibition (Lance, de Gail & Neilson, 1966; Hagbarth & Eklund, 1966) and depress the H reflexes at small torque levels. H reflexes in the upper limb might correlate more strongly with the pre-existing level of motor discharge than those in the lower limb (Upton, McComas & Sica, 1971; Day, Marsden, Obeso & Rothwell, 1984; Verrier, 1985). There are clear differences of excitability between motoneuronal pools of large (lower limb) and small (upper limb) muscles. In hand muscles, as in the first dorsal interosseus muscle, all motor units are recruited at relatively low force levels and then force is modulated by firing rate changes (Milner-Brown, Stein & Yemm, 1973b; Monster & Chan, 1977; Kukulka & Clamann, 1981), whereas in large muscles, as in the brachialis muscle motor unit, recruitment occurs over the whole force range of voluntary movements and rate changes play only a minor role (Kanosue, Yoshida, Akazawa & Fujii, 1979; Kukulka & Clamann, 1981). The behavioural context during which H reflexes are superimposed on a background activity might also have an effect on the H reflex-steady torque relation. One aspect of this question has been tested by comparing the relations obtained during the warning period of a reaction time task with those obtained during force matching. The results proved to be indistinguishable in both conditions; the differences between conditions were much smaller than between subjects. Influence of background activity on theh reflex Having minimized methodological artifacts, we wanted to test if the behavioural context could modify the steady torque-h reflex relation. One task was chosen during which the subject attended to the torque in a matching protocol whereas, during the second task, the subject was concentrating on a coming reaction signal and by the way maintaining a background activity. The relations in the two situations were very similar. Interindividual differences were much larger. These

15 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS results suggest that the steady torque-h reflex relation is fairly immutable within a subject. Although interindividual differences were significant it turned out that a typical average subject has a flat steady torque-h reflex relation, i.e. with constant stimulation parameters, H reflexes always have the same size, irrespective of the torque on which they are superimposed. Such properties might facilitate the supraspinal programming of movements which are superimposed on already contracting muscles (Riiegg & Bongioanni, 1989). A further analysis of the features of the H reflex-steady torque relation is difficult to make because of the complexity of the factors which determine the size of H reflexes. As a starting point it can be expected that, for H reflexes (Henneman, Somjen & Carpenter, 1965; Burke, 1968; Henneman, 1974) and for voluntary contractions (Milner-Brown, Stein & Yemm, 1973a), motor units are recruited following the size principle. The following motor units can then contribute to an H reflex superimposed on a contracting muscle: (1) motor units which are not tonically firing but are discharged by the H reflex stimulus and (2) motor units which are tonically active but are not refractory at the arrival of the afferent volley. The size of the latter group of motor units mainly depends on presynaptic inhibition which decreases the EPSPs in the motoneurones (Frank, 1959; Eccles, Schmidt & Willis, 1962) and both groups can be modulated by expanding or contracting the useful range of inputs recruiting motor units, i.e. changing the slope of the relation between firing threshold and motor unit size. This could be accomplished by, for example, inhibitory inputs (Kanda, Burke & Walmsley, 1977) without violating the size principle. The correlation of these factors with background activity is not known. No straightforward interpretation can be found for the facilitatory effect of a contralateral steady voluntary drive on H reflexes if the ipsilateral limb was relaxed. Presumably this facilitatory influence originating from the contralateral side is inhibited by steady contraction of the ipsilateral leg. The results we have observed are in line with experimental results from acute spinal cats, where monosynaptic reflexes are potentiated by tetanic stimulation of the motor nerve of the corresponding contralateral muscle (Perl, 1959). It has been suggested that tendon organ activation is the origin of these crossed reflexes (Baxendale & Rosenberg, 1977). Low-frequency H reflex depression In the relaxed subject, H reflexes which follow a conditioning H reflex up to about 10 s later are inhibited in leg (Toth et al. 1979; Van Boxtel, 1986) as well as in forearm muscles (Rothwell et al. 1986; Burke et al. 1989). In the present investigation, H reflexes of 50% of their maximum amplitude decreased by 60% with an interval of 1X5 s. In contrast to this marked depression, the threshold to elicit an H reflex increased only by about 5%. With the same stimuli which elicit a liminal control reflex, large reflexes can be obtained by reduction of presynaptic inhibition if the reflexes are elicited during the reaction time before the onset of a conditioned movement (Riedo & RUegg, 1988). A significant proportion of I a afferents is thus excited at threshold stimulus intensity. The mechanisms leading to the ineffectiveness of such a volley, both in the relaxed subject and also in the subject maintaining a background activity (cf. Fig. 4), are not clear and further experiments are needed to elucidate them. 15

16 16 D. G. R UEGG, R. KRA UER AND H. DREWS In spite of some evidence that the H reflex depression was caused by presynaptic inhibition, its further origin remains unclear since the depression was abolished by a background activity and since presynaptic inhibition provoked by vibration lasts only about 300 ms (Morin, Pierrot-Deseilligny & Hultborn, 1984; Hultborn, Meunier, Morin & Pierrot-Deseilligny, 1987a). Mutual influences between H reflexes The H reflex has been considered for a long time as monosynaptic but recently evidence accumulated that there can also be oligosynaptic components (Burke et al. 1984). Composite EPSPs of motoneurones have a rising phase of about 4-5 ms and the neurones discharge only during the last half of this phase. Theoretically, there is enough time for simultaneously evoked H reflexes to influence each other on the level of the spinal cord. In acute spinalized cats, a volley confined to group I muscle afferents inhibited motoneurones supplying the corresponding muscle of the contralateral limb if the delay between conditioning and test stimulus was less than 2 ms (Perl, 1959). Holmquist (1961) was able to show this effect only in chronic low spinal cats. In accordance with Holmquist (1961), we could not detect the slightest mutual influence between simultaneously evoked H reflexes in man. Research funding was provided by the Swiss National Science Foundation (grant No ). We wish to express our gratitude to Dr B. Hyland for reading the manuscript and to the referee whose comments enabled us to improve the manuscript considerably. The technical assistance of Ms. S. Rossier, Ms M. J. Ventura and J. Corpataux is gratefully noted. REFERENCES AKAZAWA, K., MILNER, T. E. & STEIN, R. B. (1983). Modulation of reflex EMG and stiffness in response to stretch of human finger muscle. Journal of Neurophysiology 49, BARNES, C. D. & POMPEIANO, 0. (1970). Inhibition of monosynaptic extensor reflex attributable to presynaptic depolarization of the group I a afferent fibers produced by vibration of flexor muscle. Archives italiennes de biologie 108, BAXENDALE, R. H. & ROSENBERG, J. R. (1977). Crossed reflexes evoked by selective activation of tendon organ afferent axons in the decerebrate cat. Brain Research 127, BURKE, D., ADAMS, R. W. & SKUSE, N. F. (1989). The effects of voluntary contraction on the H reflex of human limb muscles. Brain 112, BURKE, D., GANDEVIA, S. A. & MCKEON, B (1984). Monosynaptic and oligo-synaptic contributions to human ankle jerk and H-reflex. Journal of Neurophysiology 52, BURKE, R. E. (1968). Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae. Journal of Physiology 196, CAPADAY, C. & STEIN, R. B. (1986). Amplitude modulation of the soleus H-reflex in the human during walking and standing. Journal of Neuroscience 6, CRONE, C., HULTBORN, H. & JESPERSEN, B. (1985). Reciprocal Ia inhibition from the peroneal nerve to soleus motoneurones with special reference to the size of the test reflex. Experimental Brain Research 59, DAY, B. L., MARSDEN, C. D., OBESO, J. A. & ROTHWELL, J. C. (1984). Reciprocal inhibition between the muscles of the human forearm. Journal of Physiology 349, DEVANANDAN, M. S., ECCLES, R. M. & YOKOTO, T. (1965). Muscle stretch and the presynaptic inhibition of the group Ia pathway to motoneurones. Journal of Physiology 179, ECCLES, J. C., MAGNI, F. & WILLIS, W. D. (1962). Depolarization of central terminals of group Ia afferent fibers from muscle. Journal of Physiology 160, ECCLES, J. C., SCHMIDT, R. F. & WILLIS, W. D. (1962). Presynaptic inhibition of the spinal monosynaptic reflex pathway. Journal of Physiology 161, FOURNIER, E., KATZ, R. & PIERROT-DESEILLIGNY, E. (1984). A re-evaluation of the pattern of

17 H REFLEXES SUPERPOSED ON STEADY CONTRACTIONS group I fibre projections in the human lower limb on using randomly alternated stimulations. Experimental Brain Research 56, FRANK, K. (1959). Basic mechanisms of synaptic transmission in the central nervous system. IRE. Transactions on Medical Electronics ME-6, GOTTLIEB, G. L. & AGARWAL, G. C. (1971). Effects of initial conditions on the Hoffmann reflex. Journal of Neurology, Neurosurgery and Psychiatry 34, GOTTLIEB, G. L. & AGARWAL, G. C. (1978). Stretch and Hoffmann reflexes during phasic voluntary contractions of the human soleus muscle. Electroencephalography and Clinical Neurophysiology 44, HAGBARTH, K. E. (1962). Post-tetanic potentiation of myotatic reflexes in man. Journal of Neurology, Neurosurgery and Psychiatry 25, HAGBARTH, K. E. & EKLUND, G. (1966). Motor effects of vibratory muscle stimuli in man. In Muscular Afferents and Motor Control, ed. GRANIT, R., pp Almqvist and Wiksell, Stockholm. HENNEMAN, E. (1974). Principles governing distribution of sensory input to motor neurons. In The Neurosciences: 3rd Study Program, ed. SCHMIDT, F. 0. & WORDEN, F. G., pp MIT Press, Cambridge, MA, USA. HENNEMAN, E., SOMJEN, G. & CARPENTER, D. 0. (1965). Functional significance of cell size in spinal motoneurons. Journal of Neurophysiology 28, HOFFER, J. A., CAPUTI, A. A. & POSE, I. E. (1988). Role of muscle activity on the relationship between muscle spindle length and whole muscle length in the freely walking cat. In Afferent Control of Posture and Locomotion, Satellite Symposium of the 11th Annual Meeting of the European Neuroscience Association, p. 40. WWB Drucherei, Basel. HOFFMANN, P. (1922). Untersuchungen iiber die Eigenreflexe (Sehnenreflexe) menschlicher Muskeln. Springer, Berlin. HOLMQUIST, B. (1961). Crossed spinal reflex actions evoked by volleys in somatic afferents. Acta physiologica scandinavica 52, supply. 181, HUGON, M. (1973). Methodology of Hoffmann reflex in man. In New Developments in Electromyography and Clinical Neurophysiology, ed. DESMEDT, J. E., vol. 3, pp Karger, Basel. HULTBORN, H., MEUNIER, B., MORIN, C. & PIERROT-DESEILLIGNY, E. (1987a). Assessing changes in presynaptic inhibition of Ia fibres: a study in man and the cat. Journal of Physiology 389, HULTBORN, H., MEUNIER, B., PIERROT-DESEILLIGNY, E. & SHINDO, M. (1987b). Changes in presynaptic inhibition of Ia fibers at the onset of voluntary contraction in man. Journal of Physiology 389, HULTBORN, H. & PIERROT-DESEILLIGNY, E. (1979). Changes in recurrent inhibition during voluntary soleus contractions in man studied by an H-reflex technique. Journal of Physiology 297, ILES, J. F. (1986). Reciprocal inhibition during agonist and antagonist contraction. Experimental Brain Research 62, ILES, J. F. & Roberts, R. C. (1987). Inhibition of monosynaptic reflexes in the human lower limb. Journal of Physiology 385, KANDA, K., BURKE, R. E. & WALMSLEY, B. (1977). Differential control of fast and slow twitch motor units in the decerebrate cat. Experimental Brain Research 29, KANOSUE, K., YOSHIDA, M., AKAZAWA, K. & FuJiI, K. (1979). The number of active motor units and their firing rates in voluntary contraction of human brachialis muscle. Japanese Journal of Physiology 29, KRAUER, R. & RUEGG, D. G. (1986). Motor unit activation by superimposed voluntary and reflex contractions. Experientia 41, 707. KUKULKA, C. G. & CLAMANN, P. (1981). Comparison of the recruitment and discharge properties of motor units in human brachial biceps and abductor pollicis during isometric contractions. Brain Research 219, LANCE, J. W., DE GAIL, P. & NEILSON, P. D. (1966). Tonic and phasic spinal cord mechanisms in man. Journal of Neurology, Neurosurgery and Psychiatry 29, MAGLADERY, J. W. (1955). Some observations on spinal reflexes in man. Pflugers Archiv 261,

18 18 D. G. R UEGG, R. KRA UER AND H. DREWS MILNER-BROWN, H. S., STEIN, R. B. & YEMM, R. (1973a). The orderly recruitment of human motor units during voluntary isometric contractions. Journal of Physiology 230, MILNER-BROWN, H. S., STEIN, R. B. & YEMM, R. (1973b). Changes in firing rate of human motor units during linearly changing voluntary contractions. Journal of Physiology 230, MONSTER, W. & CHAN, H. (1977). Isometric force production by motor units of extensor digitorum communis muscle in man. Journal of Neurophysiology 40, MORIN, C., PIERROT-DESEILLIGNY, E. & HULTBORN, H. (1984). Evidence for presynaptic inhibition of muscle spindle Ia afferents in man. Neuroscience Letters44, NARDONE, A. & SCHIEPPATI, M. (1988). Shift of activity from slow to fast muscle during voluntary lengthening contractions of the triceps surae muscles in humans. Journal of Physiology 395, PAILLARD, J. (1955). Rapports entre les durees de la period de silence et du myogramme dans le triceps sural chez l'homme. Journal de physiologie 47, PERL, E. R. (1959). Effects of muscle stretch on excitability of contralateral motoneurones. Journal of Physiology 145, RIEDO, R. & RtEGG, D. G. (1988). Origin of the specific H reflex facilitation preceding a voluntary movement in man. Journal of Physiology 397, ROTHWELL, J. C., DAY, B. L., BERARDELLI, A. & MARSDEN, C. D. (1986). Habituation and conditioning of the human long latency stretch reflex. Experimental Brain Research 63, RtEGG, D. G. & BONGIOANNI, F. (1989). Superposition of ballistic on steady contractions in man. Experimental Brain Research 77, RtEGG, D. G. & KRAUER, R. (1986). Activation of a motoneuronal pool by voluntary and reflex contractions. Proceedings of the International Union of Physiological Sciences 16, 257. SCHIEPPATI, M. & CRENNA, P. (1984). From activity to rest: gating of excitatory autogenetic afferences from the relaxing muscle in man. Experimental Brain Research 56, SIMON, J. N. (1962). Dispositif de contention des electrodes de stimulation pour l'etude du reflexe de Hoffmann chez l'homme. Electroencephalography and Clinical Neurophysiology, suppl. 22, TOTH, S., SOLYOM, A. & VAJDA, J. (1979). Frequency resonance investigation of the H reflex. Journal of Neurology, Neurosurgery and Psychiatry 42, UPTON, A. R. M., MCCOMAS, A. J. & SICA, R. E. P. (1971). Potentiation of 'late' responses evoked in muscles during effort. Journal of Neurology, Neurosurgery and Psychiatry 34, VALLBO, A. B. (1970). Discharge pattern in human muscle spindle afferents during isometric voluntary contractions. Acta physiologica scandinavica 80, VAN BOXTEL, A. (1986). Differential effects of low-frequency depression, vibration-induced inhibition, and posttetanic potentiation on H-reflexes and tendon jerks in the human soleus muscle. Journal of Neurophysiology 55, VERRIER, M. C. (1985). Alterations in H reflex magnitude by variations in baseline excitability. Electroencephalography and Clinical Neurophysiology 60,