Influence of transcutaneous electrical nerve stimulation conditions on disynaptic reciprocal Ia inhibition and presynaptic inhibition in healthy adults

This study investigated the influence of stimulus conditions of transcutaneous electrical nerve stimulation (TENS) on disynaptic reciprocal Ia inhibition (RI) and presynaptic inhibition (D1 inhibition) in healthy adults. Eight healthy participants received TENS (stimulus frequencies of 50, 100, and 200?Hz) over the deep peroneal nerve and tibialis anterior (TA) muscle in the resting condition for 30?min. At pre- and post-intervention, the RI from the TA to the soleus (SOL) and D1 inhibition of the SOL alpha motor neuron were assessed by evoked electromyography. The results showed that RI was not changed by TENS at any stimulus frequency condition. Conversely, D1 inhibition was significantly changed by TENS regardless of the stimulus frequency. The present results and previous studies pertaining to RI suggest that the resting condition might strongly influence the lack of pre- vs. post-intervention change in the RI. Regarding the D1 inhibition, the present results suggest that the effect of TENS might be caused by post-tetanic potentiation. The knowledge gained from the present study might contribute to a better understanding of fundamental studies of TENS in healthy adults and its clinical application for stroke survivors.     

Time-dependent effects of neuromuscular electrical stimulation on changes in spinal excitability are dependent on stimulation frequency: A preliminary study in healthy adults

Neuromuscular electrical stimulation (NMES) can be used as treatment for spasticity. The present study examined differences in time-dependent effects of NMES depending on stimulation frequency. Forty healthy subjects were separated into four groups (no-stim, NMES of 50, 100, and 200?Hz). The un-conditioned H-reflex amplitude and the H-reflex conditioning-test paradigm were used to measure the effectiveness on monosynaptic Ia excitation of motoneurons in the soleus (SOL) muscle, disynaptic reciprocal Ia inhibition from tibialis anterior (TA) to SOL, and presynaptic inhibition of SOL Ia afferents. Each trial consisted of a 30-min period of NMES applied to the deep peroneal nerve followed by a 30-min period with no stimulation to measure prolonged effects. Measurements were performed periodically. Stimulation applied at all frequencies produced a significant reduction in monosynaptic Ia excitation of motoneurons in the SOL muscle, however, only stimulation with 50?Hz showed prolonged reduction after NMES. NMES frequency did not affect the amount of disynaptic reciprocal Ia inhibition and presynaptic inhibition of Ia afferents. The results show a frequency-dependent effect of NMES on the monosynaptic Ia excitation of motoneurons. This result has implications for selecting the optimal NMES frequency for treatment in patients with spasticity.     

The nature of facilitation of leg muscle motor evoked potentials by knee flexion

Knee flexion is a movement that initiates rising from a sitting position, which is a common therapeutic exercise for patients unable to ambulate. We investigated how voluntary isometric biceps femoris contraction affects motor evoked potential (MEP) amplitude following transcranial magnetic stimulation, background electromyographic (EMG) amplitude, and H-reflex amplitude in ipsilateral leg muscles. Subjects were seated on the edge of a bed with their hips and knees flexed at 90°, and the soles of their feet on the floor. MEP and background EMG were recorded from the tibialis anterior (TA) and soleus (SOL), and H reflexes from SOL of 30 volunteers. Background EMG and MEP also were recorded while voluntarily contracting tested muscles. Biceps femoris contraction increased MEP and background EMG for TA and SOL ( p < 0.01). Maximal background EMG and MEP increased with increasing voluntary contraction of tested muscles ( p < 0.005). Regression slope differed little between TA and SOL. Biceps femoris contraction facilitated MEP comparably for TA and SOL, while SOL background EMG exceeded that of TA ( p < 0.02). The relationship between MEP facilitation and background EMG changed to favor more efficient facilitation in TA ( p < 0.05), but not SOL ( p > 0.1). MEP recorded from TA and SOL with subthreshold stimuli using needle electrodes were more frequent with biceps femoris contraction ( p < 0.04). H-reflex amplitude of SOL decreased during biceps femoris contraction ( p < 0.001). We concluded that biceps femoris contraction affects leg muscle MEP, background EMG, and H reflexes differently.     

Effects of homosynaptic depression on spectral properties of H-reflex recordings

The purpose of this study was to determine the effects of homosynaptic depression (HD) on spectral properties of the soleus (SOL) H-reflex. Paired stimulations, separated by 100?ms, were used to elicit an unconditioned and conditioned H-reflex in the SOL muscle of 20 participants during quiet standing. Wavelet and principal component analyses were used to analyze features of the time-varying spectral properties of the unconditioned and conditioned H-reflex. The effects of HD on spectral properties of the H-reflex signal were quantified by comparing extracted principal component scores. The analysis extracted two principal components: one associated with the intensity of the spectra and one associated with its frequency. The scores for both principal components were smaller for the conditioned H-reflex. HD decreases the spectral intensity and changes the spectral frequency of H-reflexes. These results suggest that HD changes the recruitment pattern of the motor units evoked during H-reflex stimulations, in that it not only decreases the intensity, but also changes the types of motor units that contribute to the H-reflex signal.     

Changes in the excitability of soleus muscle short latency stretch reflexes during human hopping after 4 weeks of hopping training

Changes in the excitability of the human triceps surae muscle short latency stretch reflexes were investigated in six male subjects before and after 4 weeks of progressive two-legged hopping training. During the measurements the subjects performed 2-Hz hopping with: preferred contact time (PCT) and short contact time. The following reflex parameters were examined before and after the training period: the soleus muscle (SOL) Hoffmann-reflex (H-reflex) at rest and during hopping, the short latency electromyogram (EMG) components of the movement induced stretch reflex (MSR) in SOL and medial gastrocnemius muscle (MG), and the EMG amplitude of the SOL and MG tendon reflexes (T-reflexes) elicited at rest. The main results can be summarized as follows: the SOL T-reflex had increased by about 28% (P < 0.05) after training while the MG T-reflex was unchanged; the SOL MSR (always evident) and the MG MSR (when observable) did not change in amplitude with training, and before training the SOL H-reflex in both hopping situations was significantly depressed to about 40% of the reference value at standing rest (P < 0.05). After training the H-reflex during PCT hopping was no longer depressed. As the value of the measured mechanical parameters (the total work rate, joint angular velocity and the ankle joint work rate) was unchanged after training in both hopping situations, the reflex changes observed could not be ascribed to changes in the movement pattern. To explain the observed changes, hypotheses of changes in the excitability of the stretch reflex caused by the training were taken into consideration and discussed. Accepted: 22 May 1998     

The relationship between the soleus H-reflex amplitude and vibratory inhibition in controls and spastic subjects. II. Computer model

A computer model is presented that describes soleus H-reflex recruitment as a function of electric stimulus intensity. The model consists of two coupled non-linear transfer functions. The first transfer function describes the activation of muscle spindle (Ia) afferent terminals as a function of the electric stimulus intensity; whereas the second describes the activation of a number of motoneurons as a function of the number of active Ia afferent terminals. The effect of change in these transfer functions on the H-reflex recruitment curve is simulated. In spastic patients, a higher average maximal H-response amplitude is observed in combination with a decreased H-reflex threshold. Vibration of the Achilles tendon reduces the H-reflex amplitude, presumably by reducing the excitatory afferent input. Vibratory inhibition is diminished in spasticity. In the model, the afferent-motoneuron transfer function was modified to represent the possible alterations occurring in spasticity. The simulations show that vibratory suppression of the H-reflex is determined only in part by the inhibition level of the afferent input. With a constant level of presynaptic inhibition, the suppression of reflexes of different sizes may vary. A lowering of the motoneuron activation thresholds in spastic patients will directly contribute to a decrease of vibratory inhibition in spasticity.     

Contraction level-related modulation of corticomuscular coherence differs between the tibialis anterior and soleus muscles in humans

The sensorimotor cortex activity measured by scalp EEG shows coherence with electromyogram (EMG) activity within the 15- to 35-Hz frequency band (β-band) during weak to moderate intensity of isometric voluntary contraction. This coupling is known to change its frequency band to the 35- to 60-Hz band (γ-band) during strong contraction. This study aimed to examine whether such contraction level-related modulation of corticomuscular coupling differs between muscles with different muscle compositions and functions. In 11 healthy young adults, we quantified the coherence between EEG over the sensorimotor cortex and rectified EMG during tonic isometric voluntary contraction at 10-70% of maximal voluntary contraction of the tibialis anterior (TA) and soleus (SOL) muscles, respectively. In the TA, the EEG-EMG coherence shifted from the β-band to the γ-band with increasing contraction level. Indeed, the magnitude of β-band EEG-EMG coherence was significantly decreased, whereas that of γ-band coherence was significantly increased, when the contraction level was above 60% of maximal voluntary contraction. In contrast to the TA, the SOL showed no such frequency changes of EEG-EMG coherence with alterations in the contraction levels. In other words, the maximal peak of EEG-EMG coherence in the SOL existed within the β-band, irrespective of the contraction levels. These findings suggest that the central nervous system regulates the frequency of corticomuscular coupling to exert the desired levels of muscle force and, notably, that the applicable rhythmicity of the coupling for performing strong contractions differs between muscles, depending on the physiological muscle compositions and functions of the contracting muscle.     

Age-related changes of the stretch reflex excitability in human ankle muscles

The purpose of this study was to characterize the effects of aging on the stretch reflex in the ankle muscles, and in particular to compare the effects on the ankle dorsi-flexor (tibialis anterior: TA) and the plantar-flexor (soleus: SOL). Stretch reflex responses were elicited in the TA and SOL at rest and during weak voluntary contractions in 20 elderly and 23 young volunteers. The results indicated that, in the TA muscle, the elderly group had a remarkably larger long-latency reflex (LLR), whereas no aging effect was found in the short latency reflex (SLR). These results were very different from those in the SOL muscle, which showed significant aging effects in the SLR and medium latency reflex (MLR), but not in the LLR. Given the fact that the LLR of the TA stretch reflex includes the cortical pathway, it is probable that the effects of aging on the TA stretch reflex involve alterations not only at the spinal level but also at the cortical level. The present results indicate that the stretch reflexes of each of the ankle antagonistic muscles are affected differently by aging, which might have relevance to the neural properties of each muscle.     

Startle stimuli exert opposite effects on human cortical and spinal motor system excitability in leg muscles

Increased excitability of the spinal motor system has been observed after loud and unexpected acoustic stimuli (AS) preceding H-reflexes. The paradigm has been proposed as an electrophysiological marker of reticulospinal tract activity in humans. The brainstem reticular formation also maintains dense anatomical interconnections with the cortical motor system. When a startling AS is delivered, prior to transcranial magnetic stimulation (TMS), the AS produces a suppression of motor evoked potential (MEP) amplitude in hand and arm muscles of healthy subjects. Here we analyzed the conditioning effect of a startling AS on MEP amplitude evoked by TMS to the primary motor leg area. Ten healthy volunteers participated in two experiments that used a conditioning-test paradigm. In the first experiment, a startling AS preceded a suprathreshold transcranial test stimulus. The interstimulus interval (ISI) varied between 20 to 160 ms. When given alone, the test stimulus evoked a MEP amplitude of approximately 0.5 mV in the slightly preinervated soleus muscle (SOL). In the second experiment, the startling AS was used to condition the size of the H-reflex in SOL muscle. Mean MEP amplitude was calculated for each ISI. The conditioning AS suppressed MEP amplitude at ISIs of 30-80 ms. By contrast, H-reflex amplitude was augmented at ISIs of 100-200 ms. In conclusions, acoustic stimulation exerts opposite and ISI-specific effects on the amplitude of MEPs and H-reflex in the SOL muscle, indicating different mechanism of auditory-to-motor interactions at cortical and spinal level of motor system.     

Enhanced D1 and D2 Inhibitions Induced by Low-Frequency Trains of Conditioning Stimuli: Differential Effects on H- and T-Reflexes and Possible Mechanisms

Mechanically evoked reflexes have been postulated to be less sensitive to presynaptic inhibition (PSI) than the H-reflex. This has implications on investigations of spinal cord neurophysiology that are based on the T-reflex. Preceding studies have shown an enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1 Hz was applied to the nerve of the antagonist muscle. The main questions to be addressed in the present study are if indeed T-reflexes are less sensitive to PSI and whether (and to what extent and by what possible mechanisms) the effect of low frequency conditioning, found previously for the H-reflex, can be reproduced on T-reflexes from the soleus muscle. We explored two different conditioning-to-test (C-T) intervals: 15 and 100 ms (corresponding to D1 and D2 inhibitions, respectively). Test stimuli consisted of either electrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanical percussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioning electrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSI as compared to a single conditioning pulse, for both reflexes (T and H), regardless of C-T-intervals. Moreover, the conditioning train of pulses (with respect to a single conditioning pulse) was proportionally more effective for T-reflexes as compared to H-reflexes (irrespective of the C-T interval), which might be associated with the differential contingent of Ia afferents activated by mechanical and electrical test stimuli. A conceivable explanation for the enhanced PSI effect in response to a train of stimuli is the occurrence of homosynaptic depression at synapses on inhibitory interneurons interposed within the PSI pathway. The present results add to the discussion of the sensitivity of the stretch reflex pathway to PSI and its functional role.     

Activation timing of soleus and tibialis anterior muscles during sit-to-stand and stand-to-sit in post-stroke vs. healthy subjects

Introduction. Sit-to-stand (SitTS) and stand-to-sit (StandTS) are very important functional tasks that become compromised in stroke patients. As in other voluntary movements, they require an adequate postural control (PC) involving the generation of anticipatory postural adjustments (APAs). In order to give clues for more efficient and directed rehabilitation programs, a deeper knowledge about APAs during challenging and daily life movements is essential.

Purpose. To analyze the activation timing of tibialis anterior (TA) and soleus (SOL) muscles during SitTS and StandTS in healthy subjects and in post-stroke patients.

Methods. Two groups participated in this study: one composed of ten healthy subjects and the other by ten subjects with a history of stroke and increased H-reflex. Electromyographic activity (EMGa) of SOL and TA was analyzed during SitTS and StandTS in the ipsilateral (IPSI) and the contralateral (CONTRA) limb to the side lesion in stroke subjects, and in one limb in healthy subjects. A force plate was used to identify the movement onset.

Results. In both sequences, in the stroke group SOL activation timing occurred prior to movement onset, contrary to the pattern observed in the healthy subjects. Statistically significant differences were found in SOL activation timings between each lower limb of the stroke and healthy groups, but no significant differences were found between the IPSI and the CONTRA limb. The TA activation timing seems to be delayed in the CONTRA limb when compared to the healthy subjects and showed a better organization of TA timing activation in StandTS when compared to SitTS.

Conclusion. Compared to healthy subjects, APAs seem to be altered in both limbs of the post-stroke subjects, with the SOL activation timing being anticipated in both SitTS and StandTS.     

Somatosensory graviception inhibits the soleus H-reflex in standing man - a parabolic flight experiment-

The purpose of this study was to investigate how gravity level affects the excitability of the soleus muscle (SOL) motoneuron pool to la afferent input while erect posture is maintained in humans. Three healthy male subjects participated in an experiment whereby three different gravity conditions (micro gravity (MG), normal gravity (NG), and hyper gravity) were imposed using a parabolic flight procedure. The SOL H-reflex was evoked every 2 seconds while the subjects kept an erect posture. The background electromyographic activity (BGA) of the SOL was almost absent during MG. The SOL H-reflex amplitude was significantly larger during MG than during NG. These results suggest that the somatosensory systems detecting a load at the lower limbs and/or vertebral column play a role in reducing the excitability of the SOL motoneuron pool to la afferent inputs by presynaptic inhibition.     

Presynaptic inhibition in humans: a comparison between normal and spastic patients.

During the last 40 years, several studies in man have been devoted to the pathophysiological mechanisms underlying spasticity. Spasticity is characterised by a velocity dependent increase in muscle tone. Many spinal pathways control stretch reflex excitability and a malfunction in any one of them could theoretically produce the exaggeration of the stretch reflex. Delwaide showed that the vibration-induced inhibition of Ia fibres is reduced in spastic patients. However, the relation between a decrease in presynaptic Ia inhibition and the pathophysiology of spasticity has been recently questioned since it was argued that homosynaptic depression (or post-activation depression) also contributes to the vibratory-induced depression of monosynaptic reflexes. This paper is thus devoted to a review of the methods recently developed to study selectively presynaptic Ia inhibition in man and to a reevaluation of the relations between modifications in presynaptic Ia inhibition and spasticity in hemiplegic and spinal spastic patients.     

Spectral properties of H-reflex recordings after an acute bout of whole-body vibration

Although research supports the use of whole-body vibration (WBV) to improve neuromuscular performance, the mechanisms for these improvements remain unclear. The purpose of this study was to identify the effect of WBV on the spectral properties of electrically evoked H-reflex recordings in the soleus (SOL) muscle. The H-reflex recordings were measured in the SOL muscle of 20 participants before and after a bout of WBV. The H-reflexes were evoked every 15 seconds for 150 seconds after WBV. A wavelet procedure was used to extract spectral data, which were then quantified with a principle components analysis. Resultant principle component scores were used for statistical analysis. The analysis extracted 1 principle component associated with the intensity of the myoelectric spectra and 1 principle component associated with the frequency. The scores of the principle component that were related to the myoelectric intensity were smaller at 30 and 60 milliseconds after WBV than before WBV. The WBV transiently decreased the intensity of myoelectric spectra during electrically evoked contractions, but it did not influence the frequency of the spectra. The decrease in intensity likely indicates a smaller electrically evoked muscle twitch response, whereas the lack of change in frequency would indicate a similar recruitment pattern of motor units before and after WBV.     

Effect of angular velocity on soleus and medial gastrocnemius H-reflex during maximal concentric and eccentric muscle contraction

At rest, the H-reflex is lower during lengthening than shortening actions. During passive lengthening, both soleus (SOL) and medial gastrocnemius (MG) H-reflex amplitudes decrease with increasing angular velocity. This study was designed to investigate whether H-reflex amplitude is affected by angular velocity during concentric and eccentric maximal voluntary contraction (MVC). Experiments were performed on nine healthy men. At a constant angular velocity of 60°/s and 20°/s, maximal H-reflex and M-wave potentials were evoked at rest (i.e., H_max and M_max, respectively) and during concentric and eccentric MVC (i.e., H_sup and M_sup, respectively). Regardless of the muscle, H_max/M_max was lower during lengthening than shortening actions and the H_sup/M_sup ratio was higher than H_max/M_max during lengthening actions. Whereas no action type and angular velocity effects on the MG H_sup/M_sup were found, the SOL H_sup/M_sup was lower during eccentric than concentric MVC and this depression was increased with higher angular velocity. Our findings indicate that the depression of the H-reflex amplitude during eccentric compared to concentric MVC depends mainly on the amount of inhibition induced by lengthening action. In conclusion, H-reflex should be evoked during both passive and active dynamic trials to evaluate the plasticity of the spinal loop.     

Experimental Evidence of the Tonic Vibration Reflex during Whole-Body Vibration of the Loaded and Unloaded Leg

Increased muscle activation during whole-body vibration (WBV) is mainly ascribed to a complex spinal and supraspinal neurophysiological mechanism termed the tonic vibration reflex (TVR). However, TVR has not been experimentally demonstrated during low-frequency WBV, therefore this investigation aimed to determine the expression of TVR during WBV.  Whilst seated, eight healthy males were exposed to either vertical WBV applied to the leg via the plantar-surface of the foot, or Achilles tendon vibration (ATV) at 25Hz and 50Hzfor 70s. Ankle plantar-flexion force, tri-axial accelerations at the shank and vibration source, and surface EMG activity of m. soleus (SOL) and m. tibialis anterior (TA) were recorded from the unloaded and passively loaded leg to simulate body mass supported during standing.  Plantar flexion force was similarly augmented by WBV and ATV and increased over time in a load- and frequency dependent fashion. SOL and TA EMG amplitudes increased over time in all conditions independently of vibration mode. 50Hz WBV and ATV resulted in greater muscle activation than 25Hz in SOL when the shank was loaded and in TA when the shank was unloaded despite the greater transmission of vertical acceleration from source to shank with 25Hz and WBV, especially during loading. Low-amplitude WBV of the unloaded and passively loaded leg produced slow tonic muscle contraction and plantar-flexion force increase of similar magnitudes to those induced by Achilles tendon vibration at the same frequencies. This study provides the first experimental evidence supporting the TVR as a plausible mechanism underlying the neuromuscular response to whole-body vibration.     

Aging effects on posture-related modulation of stretch reflex excitability in the ankle muscles in humans

The purpose of this study was to examine the effects of aging on posture-related changes of the stretch reflex excitability in the ankle extensor, soleus (SOL), and flexor, tibialis anterior (TA) muscles. Fourteen neurologically normal elderly (mean 68 ± 6 years) and 12 young (mean 27 ± 3 years) subjects participated. Under two postural conditions, upright standing (STD) and sitting (SIT), stretch reflex electromyographic (EMG) responses in the SOL/TA muscle were elicited by imposing rapid ankle dorsi-/plantar-flexion. Under the SIT condition, subjects were asked to keep the SOL background EMG level, which is identical to that under the STD condition. In the SOL muscle, both groups showed significant enhancement of the short-latency stretch reflex (SLR) response when the posture changed from SIT to STD. In the TA muscle, the young group showed significant enhancement of the middle- (MLR) and long-latency stretch reflex (LLR) when the posture changed from SIT to STD; no such modulation was observed in the elderly group. Since the TA stretch reflex responses under the STD condition were comparable in the young and elderly groups, the lack of posture-related modulation of the TA muscle in the elderly group might be explained by augmented stretch reflex excitability under the SIT condition. The present results suggest that the (1) SOL SLR responses are modulated both in the young and elderly subjects when the posture is changed from SIT to STD, (2) TA MLR and LLR responses are not modulated in the elderly subjects when the posture is changed from SIT to STD, while each response is same between the young and elderly in STD, and (3) the effect of aging on the posture-related stretch reflex differs in the SOL and TA muscles.     

Spinal excitation and inhibition decrease as humans age

Although changes in the soleus H-reflex (an electrical analog of the tendon jerk) with age have been examined in a number of studies, some controversy remains. Also, the effect of age on inhibitory reflexes has received little attention. The purpose of this paper was to examine some excitatory and inhibitory reflexes systematically in healthy human subjects having a wide range of ages. We confirmed that both the maximum H-reflex (Hmax) and the maximum M-wave (Mmax) (from direct stimulation of motor axons) decrease gradually with age. The decrease in Hmax was larger so the Hmax/Mmax ratio decreased dramatically with age. Interestingly, the modulation of the H-reflex during walking was essentially the same at all ages, suggesting that the pathways that modulate the H-reflex amplitude during walking are relatively well preserved during the aging process. We showed for the first time that the short-latency, reciprocal inhibitory pathways from the common peroneal nerve to soleus muscle and from the tibial nerve to the tibialis anterior muscle also decreased with age, when measured as a depression of ongoing voluntary activity. These results suggest that there may be a general decrease in excitability of spinal pathways with age. Thus, the use of age-matched controls is particularly important in assessing abnormalities resulting from disorders that occur primarily in the elderly.     

The influence of increased muscle spindle sensitivity on Achilles tendon jerk and H-reflex in relaxed human subjects

Whether the fusimotor system contributes to reflex gain changes during reinforcement maneuvers is re-examined in the light of new data. Recently, from direct recordings of spindle afferent activity originating from ankle flexor muscles, we showed that mental computation increased the muscle spindle mechanical sensitivity in completely relaxed human subjects without concomitant alpha-motoneuron activation, providing evidence for selective fusimotor drive activation. In the present study, the effects of mental computation were investigated on monosynaptic reflexes elicited in non-contracting soleus muscle either by direct nerve stimulation (Hoffmann reflex, H) or by tendon tap (Tendinous reflex, T). The aim was to relate the time course of the changes in reflex size to the increase in spindle sensitivity during mental task in order to explore whether fusimotor activation can influence the size of the monosynaptic reflex. The results show changes in reflex amplitude that parallel the increase in muscle spindle sensitivity. When T-reflex is consistently facilitated during mental effort, the H-reflex is either depressed or facilitated, depending on the subjects. These findings suggest that the increased activity in muscle spindle primary endings may account for mental computation-induced changes in both tendon jerk and H-reflex. The facilitation of T-reflex is attributed to the enhanced spindle mechanical sensitivity and the inhibition of H-reflex is attributed to post-activation depression following the increased Ia ongoing discharge. This study supports the view that the fusimotor sensitization of muscle spindles is responsible for changes in both the mechanically and electrically elicited reflexes. It is concluded that the fusimotor drive contributed to adjustment of the size of tendon jerk and H-reflex during mental effort. The possibility that a mental computation task may also operate by reducing the level of presynaptic inhibition is discussed on the basis of H-reflex facilitation.