Influence of basal ganglia on upper limb locomotor synergies. Evidence from deep brain stimulation and L-DOPA treatment in Parkinson's disease.

Clinical evidence of impaired arm swing while walking in patients with Parkinson's disease suggests that basal ganglia and related systems play an important part in the control of upper limb locomotor automatism. To gain more information on this supraspinal influence, we measured arm and thigh kinematics during walking in 10 Parkinson's disease patients, under four conditions: (i) baseline (no treatment), (ii) therapeutic stimulation of the subthalamic nucleus (STN), (iii)L-DOPA medication and (iv) combined STN stimulation and L-DOPA. Ten age-matched controls provided reference data. Under baseline conditions the range of patients' arm motion was severely restricted, with no correlation with the excursion of the thigh. In addition, the arm swing was abnormally coupled in time with oscillation of the ipsilateral thigh. STN stimulation significantly increased the gait speed and improved the spatio-temporal parameters of arm and thigh motion. The kinematic changes as a function of gait speed changes, however, were significantly smaller for the upper than the lower limb, in contrast to healthy controls. Arm motion was also less responsive after L-DOPA. Simultaneous deep brain stimulation and L-DOPA had additive effects on thigh motion, but not on arm motion and arm-thigh coupling. The evidence that locomotor automatisms of the upper and lower limbs display uncorrelated impairment upon dysfunction of the basal ganglia, as well as different susceptibility to electrophysiological and pharmacological interventions, points to the presence of heterogeneously distributed, possibly partially independent, supraspinal control channels, whereby STN and dopaminergic systems have relatively weaker influence on the executive structures involved in the arm swing and preferential action on those for lower limb movements. These findings might be considered in the light of phylogenetic changes in supraspinal control of limb motion related to primate bipedalism.

Effect of L-dopa and subthalamic nucleus stimulation on arm and leg swing during gait in Parkinson's Disease.

The effects of subthalamic nucleus (STN) stimulation and L-dopa administration on the arm and leg swing movements associated with overground walking were studied in a group of patients with idiopathic Parkinson's disease (PD). Ten patients undergoing deep brain stimulation and twenty controls were tested using 3D kinematic motion analysis. Parkinsonian patients under basal conditions walked more slowly and with reduced arm and leg swing compared to controls. Moreover, they displayed significant impairments of the normal interlimb coordination. Both STN stimulation and L-dopa increased the walking speed and the amplitude of arm and leg swing movements. Additional improvements of the coordination between upper and lower limb were documented by reductions of the phase-shift between arm and ipsilateral leg motion, with displacement toward the control range (perfect counterphase). STN stimulation alone and L-dopa alone produced similar effects on the variables analyzed. The combination of the two treatments, instead, yielded additive effects on the gait speed and a slight increase of the upper and lower limb range of motion, in the absence of further improvements in the inter-segmental coordination. Moreover, whereas the increased arm swing could be accounted by the sole adoption of a higher gait speed, both the increment of the leg movement amplitude and the decreased interlimb phase shift appeared to imply an additional effect, possibly related to the treatment. These results may suggest that differential supraspinal controls operate on the neural networks subserving upper and lower limb motion during human walking.

The association between impaired turning and normal straight walking in Parkinson's disease.

Turning whilst walking was investigated by gait analysis in a group of Parkinson's Disease (PD) patients with mild clinical impairment and no significant abnormalities in stride parameters and kinematics of steady-state, linear walking. Comparison with age-matched controls demonstrated that patients approached turns with a slower step and completed turning with a greater number of steps. Moreover, the normal cranio-caudal sequence, whereby rotation of the head toward the intended direction of travel is followed by rotation of the trunk, was replaced by nearly simultaneous rotation of head and trunk and decreased relative head excursion after the second turning step. The evidence of abnormal inter-segmental coordination during turning in mildly affected, normally walking patients suggests that task-specific pathophysiological mechanisms, not necessary related to basic locomotor deficits, underlie disturbed directional changes in PD. Furthermore, turning-related neural systems may be more vulnerable to functional impairments associated with PD, as compared with linear walking. Hierarchically higher control levels involved in the turning ability may explain the observed unexpected association.

Impact of subthalamic nucleus stimulation on the initiation of gait in Parkinson's disease.

The effects of subthalamic nucleus (STN) stimulation on the anticipatory postural actions associated with the initiation of gait were studied in ten patients with idiopathic Parkinson's disease undergoing therapeutic deep brain stimulation. Kinematic, dynamic and electromyographic analysis was performed before and while subjects were starting gait in response to an external cue. Effects of STN stimulation on the standing posture preceding the go signal included significant improvement of the vertical alignment of the trunk and shank, decrease of the hip joint moment, backward shift of the center of pressure (CoP) and reduction of abnormal tonic and/or rhythmic activity in the thigh and leg muscles. Responses to bilateral STN stimulation were more consistent than those evoked by unilateral stimulation. Moreover, comparison between postural changes induced by STN stimulation applied prior to the gait initiation cue and during simple quiet standing revealed more significant responses in the former condition. Effects on the actual gait initiation process included shortening of the imbalance phase, larger backward/lateral displacement of CoP and more physiological expression of the underlying anticipatory muscular synergy. Additional changes were shortening of the unloading phase, shortening of the first-swing phase and increase in the length of the first step. Results demonstrate substantial influence of STN stimulation on functionally basic motor control mechanisms. In particular, the evidence of more significant responses upon attention-demanding conditions and the remarkable effects on postural programmes sub-serving feed-forward regulation of the onset of complex multijoint movements, suggests a consistent action on postural sub-systems relying on cognitive data processing and internal models of body mechanics.

The heel-contact gait pattern of habitual toe walkers.

We used kinematic, kinetic and EMG analysis to compare the spontaneous heel-contact gait patterns of 13 children classified as habitual toe walkers (HTWs) and age-matched controls. In the HTWs, the incidence of spontaneous heel-contact strides during a single recording session ranged from 15% to 92%, with no correlation with age, passive ankle joint excursion, walking speed and trial order. Hallmarks of the heel-contact strides were premature heel-rise, reversal of the second rocker, relative shortening of the loading response and anticipation and enhancement of the electromyographic (EMG) activity normally observed in the triceps surae (TS) during the first half of the stance phase. This variant of the locomotor program is different from the walking patterns observed in normally developing toddlers and children with cerebral palsy (CP). It does not necessarily reflect a functional adaptation to changes in the rheological properties of the muscle-tendon complex.

Motor programmes for the termination of gait in humans: organisation and velocity-dependent adaptation.

1. The organisation of the muscular activities responsible for the termination of gait, their modulation as a function of the rate of progression and the associated mechanical effects were investigated in normal adults, using EMG, force plate and kinematic recordings. In particular, the braking actions in reaction to a visual cue presented at the instant of heel-strike were analysed quantitatively, with a focus on representative leg and thigh muscles of the weight-supporting (stance) and oscillating (swing) limb, during walk-and-stop trials performed at three different velocities. 2. In the stance limb, the EMG associated with braking started approximately 150 ms after the stop signal and, on average, displayed a distal-to-proximal activation sequence that primarily involved the posterior muscle groups (soleus, SOL, and hamstring, HAM). With the exception of SOL, which showed a single EMG burst, EMG patterns consisted of two or three progressively larger components occurring reciprocally in antagonistic muscles. Increasing walking speed yielded a significant reduction of the activity in distal muscles, and a simultaneous increment in proximal muscles. The mechanical effect of the earlier braking actions, estimated from the backward-directed wave of the horizontal ground reaction force, decreased in a velocity-dependent manner. 3. In the swing limb the braking activities began approximately 330 ms after the stop signal and, on average, revealed a proximal-to-distal activation sequence with the extensor groups (quadriceps, QUAD, and SOL) playing a prominent role. They always consisted of single EMG bursts, largely co-activated in the antagonist muscles. The onset latencies of the individual components showed a close correlation, and the spatio-temporal parameters were always scaled in parallel. Unlike the stance limb, the mechanical braking action associated with the final contact of the swing limb increased with walking speed. 4. The results indicate that the muscle synergies responsible for the rapid termination of gait in response to a ground-contact visual cue are produced by a relatively flexible set of motor commands modulated according to different velocity-dependent strategies in the weight-bearing limb, and by a single, fairly robust motor programme in the swing limb. Mechanical constraints related to the relative position of the centre of foot pressure and centre of body mass at the time the braking commands begin to affect external forces, may condition the difference between the two sides of the body.

Spasticity and 'spastic' gait in children with cerebral palsy.

The current notion of spasticity as a velocity-dependent increase of muscle response to imposed stretch was mainly derived from studies performed under stationary experimental conditions. To address the issue of a spastic muscle behaviour under dynamic conditions, we conceived a novel approach, aimed at quantitatively assessing motor output over the lengthening periods which take place during unperturbed functional movements. Application to the analysis of overground walking in children with spastic cerebral palsy (CP) revealed that, for representative lower limb muscles, the relationship between EMG levels and estimated muscle lengthening rate displays either increased gain or reduced velocity threshold. Parallel measurement of gait kinetics frequently showed congruent increase of the mechanical resistance to joint rotation. Abnormalities preferentially targeted the lengthening contractions occurring around the ground contact period of the stride. The pathophysiological profile of what is clinically defined as 'spastic' gait in CP children did not only consist of dynamic spasticity, as described above. Most often it resulted from the simultaneous contribution of other factors, including paresis, co-contraction, immature and non-neural components.

Moment-angle relationship at lower limb joints during human walking at different velocities.

The coupling between joint kinematics and kinetics during level walking was analysed by plotting joint angles vs. joint moments about the hip, knee and ankle in nine normal male subjects walking at three different velocities. The curves obtained were reproducible, and variability among subjects was relatively low. Counterclockwise loops corresponded to energy produced, and clockwise loops to energy absorbed at the joint; both loops are described in different phases of the stride cycle. At increasing walking velocity some of the loops narrowed, thus revealing the possibility of energy recovery. Analysis of individual diagrams revealed that consistent portions of the moment-angle loops can be described as a sequence of quasi-constant slope phases, separated by transition periods where quasi-isometric changes in joint moment occur. This figure, which was particularly evident of the hip and ankle joints, is reminiscent of a mechanical system with elastic components, which, in different phases of the rhythmic locomotion activity, moves along discrete status levels characterized by specific length-tension relationships. Implications of the above results in terms of the neurol control of joint properties during active movement are discussed.

Modelling the triceps surae muscle-tendon complex for the estimation of length changes during walking.

The triceps surae muscle-tendon complex has been modelled by many authors seeking to estimate the change in muscle length that occurs in locomotion. The objective of the present study is to assess to what extent the commonly adopted assumptions of foot rigidity and pure sagittal motion are acceptable. A model of the triceps surae muscle-tendon complex was implemented by taking into account all possible movements between forefoot and rear foot. Length and velocity curves from a 3-dimensional gait analysis were obtained from six normal subjects. The angle between forefoot and rear foot proved to be changeable with stride (11.8 degrees +/- 4.7 SE). The effect on the length and velocity estimation was analysed by comparing the curves obtained by our model to those obtained by a model in which the foot is considered to be a rigid body. Significant differences were found for the soleus muscle length at late stance/early swing and late swing phases, and for the soleus muscle velocity at early stance phase. The length and velocity curves were also compared to curves calculated on a pure sagittal projection. No changes were observed, except for an offset of 1-3 mm caused by the general external rotation of the foot (which is also present in standing). The curves appeared superimposable when referred to the standing upright position. Care needs to be taken, however, when extending the above results to the clinical application, where foot deformity and deviation from a normal pattern of motion can occur.

A motor programme for the initiation of forward-oriented movements in humans.

1. The EMG sequence activated before the initiation of a number of fast forward-oriented voluntary movements was analysed quantitatively in normal subjects. 2. The sequence consisted of an initial inhibitory component directed to the soleus motor nucleus, followed by a second excitatory one directed to the tibialis anterior (TA). 3. The spectrum of functional utilization included motor tasks in which the prime movers are leg and thigh muscles (initiation of gait, rising on tip-toes), thigh and trunk muscles (fast-forward bending of the trunk, standing up) and upper-limb muscles (forward throw or catch). 4. In a same motor task and across the different motor tasks, performed at various speeds, the latency of soleus inhibition and TA activation with respect to the onset of movement co-varied according to a linear function, indicating a close temporal correlation between the two components. 5. In all the movements investigated, the earliest mechanical effect was a backward displacement of the centre of foot pressure in the sagittal plane. 6. Soleus inhibition alone and TA burst alone were each able to produce a backward displacement of the centre of foot pressure, but the effect was significantly slower after soleus inhibition. 7. The spatio-temporal parameters of the sequence were modulated according to the pre-existing postural conditions. For the gait initiation protocol, increasing initial forward leaning led to a decrease in the amplitude of soleus inhibition and the TA burst, and to a change in their relative time delays. Modulation was different on the two sides. We could define a postural boundary as the degree of forward leaning beyond which the full sequence is no longer called into action. 8. The spatio-temporal parameters of the sequence were pre-set according to the requirements of the forthcoming movement. In the gait initiation protocol, the amplitude and synchronization of the TA burst were directly correlated with velocity of movement, while the relative delay between soleus inhibition and TA activation was inversely correlated. Modulation on the two sides differed. We could define a velocity boundary as the velocity of movement below which the full sequence is no longer called into action. 9. We suggest that the EMG sequence described can be considered a motor programme that, through direct action on the position of the centre of foot pressure (the variable primarily controlled), will precisely adjust the configuration of forces external to the body, allowing the contraction of the prime mover(s) to interact appropriately with them for the production of a specific, forward-oriented movement.

Changes in spinal reflex excitability in brain-dead humans.

The excitability of proprio- and exteroceptive spinal reflexes was monitored electrophysiologically and clinically during the occurrence of brain death (BD) in 8 patients. After a period of total reflex unresponsiveness, the soleus H reflex attained a steady-state excitability level in 2-6 h. The recovery cycle of this response regained its normal shape at 10-20 h. The threshold of the cutaneous reflex evoked in the biceps femoris by electrical stimulation of the sural nerve had become normal in 4-13 h, although the response displayed an abnormal multi-component pattern. Digital responses to mechanical stimulation of the foot sole were evident after 6-8 h. Knee and ankle jerks were never evoked during the time of monitoring. The time-courses of the changes in excitability were not directly correlated with the fall in the blood pressure which may occur during BD. It is concluded that the human spinal cord reacts to BD with a spinal shock, characterized by sequential recovery of reflex transmission. The overall timing of this process appears to be much shorter than that previously described for the spinal shock following traumatic transection of the cord, but the latter was never studied in the earliest phases.

Postural synergies in axial movements: short and long-term adaptation.

Fast backward trunk movements are accompanied by hip, knee and ankle rotation which compensate for the backward shift of the center of gravity. The electromyographic pattern associated with the performance of these movements and the associated synergies consists of a fairly synchronous activation of the prime mover (erectores spinae) and the muscles situated at the back of the leg (hamstring, calf muscles). This pattern is called the "non anticipated pattern". The effect of training on the EMG pattern and on the subjects' mechanical performances was investigated by comparing a population of untrained subjects with one of highly trained gymnasts. A new EMG pattern was observed in the highly trained gymnasts, the "distally anticipated pattern" consisting of an early activation of the gastrocnemius, and in some subjects also of the hamstring, indicating that a long term adaptation had taken place. Performances expressed as a ratio between the displacement of the center of gravity projection onto the ground and the velocity of the movement were clearly better in the gymnasts. Short term adaptation was found to occur in the gymnasts and not in the untrained group when the movement was performed while standing on a narrow support. A suppression of the distal gastrocnemius burst occurred in the gymnasts from the first trial under the constrained standing condition whereas no change occurred in the untrained group. The flexibility of the EMG patterns associated with axial movements occurring either spontaneously or as a result of long or short term training is discussed.

Forward and backward axial synergies in man.

Upper trunk and head forward and backward movements were analyzed in human subjects standing on a force platform. EMG of several flexor and extensor muscles was recorded together with the kinematics of the movement (EL.I.TE. system). It was found that upper trunk movements are accompanied by movements of hip and knees in the opposite direction, resulting in a slight displacement of the center of gravity projection on the ground. In fast movements, all the body segments were displaced at the same time, which suggests a feedforward control, whereas in slow movements, onset of displacement of the body segments was found to take place sequentially in a cranio-caudal direction. EMG analysis during fast movements revealed two different types of control, utilized in forward and backward movements. With forward bending movements the action of two sets of muscles could be recognized: the prime mover (R. Abd.), the activation of which was not correlated with that of the other muscles and preceded the onset of movement with a fairly constant lead, and a group of postural muscles, the activation (VM, TA) and inhibition (Sol) of which were closely correlated. By contrast, with backward movements, the prime mover (Er.S.) and the postural leg muscles (Hamstrings, Sol) were activated simultaneously. In both cases, a feedforward type of control is evident. Performance of the fast forward movements was accompanied by an initial forward displacement of the knee. The function of this phenomenon is discussed in term of a destabilizing action favouring the forward bending of the body or a prestretching of the knee extensor muscles increasing the strength of their subsequent contraction.

Excitability of the soleus H-reflex arc during walking and stepping in man.

In eight normal subjects, the excitability of the soleus (Sol) H-reflex was tested in parallel with Sol length changes, EMGs of leg and thigh muscles and ground contact phases, during three different pacing movements: bipedal treadmill walking, single limb treadmill walking, and single-limb stepping on one spot. A computerized procedure was used which compensated for changes in stimulus effectiveness that occurred during free motion. In the three paradigms examined, significant excitability modulations were observed with respect to a control level determined in standing weight-bearing position. During bipedal treadmill walking, excitability was decreased in the early stance, maximally enhanced in the second half of the stance, and again decreased during the end-stance and the whole swing phase, with a minimum value around the toe off period. The main modulation pattern was retained during single-limb treadmill walking. During single-limb stepping on one spot, the stance-phase increase in excitability and the swing phase depression were still present. However, in the second half of the swing phase, reflex responsiveness returned to reference level, which was maintained during the subsequent contact period. Moreover, a decrease in reflex excitability was detected around the mid-stance. The time course of the described modulations was only partly correlated with the EMG and length changes of the Sol muscle. Furthermore, in the three movements tested, during the early stance phase, the excitability of the H-reflex arc did not correspond to the one expected on the basis of the available H-reflex studies performed under static conditions. It is suggested that, at least in certain stride phases (e.g. around the early contact period), an active regulation affects the transmission in the Sol myotatic arc during the pacing movements investigated.

Hindered muscle relaxation in spasticity: experimental evidence suggesting a possible pathophysiological mechanism.

In ten spastic patients and in an equal number of healthy controls, the relaxation phase occurring at the end of an isotonic voluntary contraction has been studied on soleus muscle using EMG and H-reflex methods. In the spastic group, the duration of motor unit-decruitment was consistently prolonged (from two to six times the control values). Moreover, the decrease in the excitability of the H-reflex arc, which normally accompanies the end of muscle contraction was delayed in time, reduced in amplitude or in some cases even absent. As a rule, patients in whom such release-associated inhibition (RAI) was lacking or severely reduced exhibited the longest motor unit-decruitment times, due to the interference of sustained clonic sequences. It is proposed that a lowered effectiveness of RAI might explain the clonic activity which frequently hinders voluntary muscle relaxation in spasticity.

Excitability of reciprocal and recurrent inhibitory pathways after voluntary muscle relaxation in man.

We studied the potential contribution of postsynaptic mechanisms to the depression of reflex excitability which occurs immediately after a voluntary release from tonic muscle contraction. The excitability of the Soleus (Sol) motor pool was tested at rest and after voluntary muscle relaxation. In both cases the Sol H-reflex was conditioned by a single shock to the peroneal nerve, in order to activate the Ia interneurones (INs) mediating the reciprocal inhibition via a peripheral input, or by a short-lasting voluntary contraction of the Tibialis Anterior (TA) muscle, to activate the Ia INs via a central command. Changes in excitability of Renshaw cells were also tested at rest and after release, to assess the role of recurrent inhibition in the release-induced inhibition of the Sol H-reflex. It was demonstrated that: the excitability of the INs mediating the reciprocal inhibition was only slightly enhanced in comparison with resting conditions; the H-reflex of the antagonist muscle (TA) evoked after Sol release was not consistently facilitated with respect to rest; the command to contract the TA muscle reduced the H-reflex of the Sol muscle during rest but not after Sol release; recurrent inhibition did not increase its effect in the post-release period. Such features suggest that recurrent and reciprocal post-synaptic inhibitions do not play a major role in reducing the reflex excitability of a relaxing muscle; rather, the command to release prevents the reciprocal inhibitory effect which accompanies the contraction of the antagonist muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of phase-dependent nociceptive reflexes during locomotion in man.

In 10 healthy subjects freely walking along a straightline, the effects of painful sural nerve stimulation, applied in different phases of the step cycle, were investigated on two antagonistic muscles of the ipsilateral lower limb acting on the knee joint: vastus lateralis (VL) and biceps femoris caput breve (BF). A clear-cut modulation in the amplitude (area) of the net reflex responses was consistently observed in both the motor nuclei explored. The extensor muscle, VL, exhibited a long-latency (mean 122 ms) reflex response, which was maximally increased by stimuli applied toward the end of the swing and in the first half of the stance phase of the stride, whereas the response appeared to be gated during the transition from the foot-flat to forefoot-contact phase. A second facilitation period was brought about by stimuli delivered in the early swing. When the response occurred superimposed on the VL locomotor activity, suppression of the ongoing EMG preceded the reflex discharge. In the flexor, BF, the same stimulus elicited a short-latency (mean 57 ms) and a long-latency (mean 132 ms) reflex response. The former was maximal after stimulation around the toe-off phase and the latter was strikingly enhanced in the late swing, where it was preceded by suppression of the background locomotor EMG activity. Responses with intermediate features (latency 70 to 80 ms, duration 90 to 120 ms), probably resulting from the merging of the early and late components, might be evoked in addition, being greatest in the last swing and in the period preceding toe-off. The findings show that in man the reflex pattern evoked by a painful cutaneous stimulus during locomotion is determined by the phase of the step cycle during which the stimulus is delivered. A functional role in maintenance of postural balance during destabilizing withdrawal reactions is conceivable.

Natural cutaneous stimulation induces late and long-lasting facilitation of extensor motoneurons in the cat.

An investigation was made of the effects of physiological cutaneous stimulation on the excitability of extensor motoneurons in spinal unanesthetized cats. The time course of changes in the monosynaptic reflex (MSR) amplitude of the soleus (Sol) and gastrocnemius medialis (GM) and lateralis (GL) was studied after conditioning stimulation with air jets (delivered to different regions of the skin of the ipsilateral hind limb), pinpricks, or stretching of the skin of the heel induced by passive rotation of the tibio-tarsal joint. Low-intensity electrical stimulation of the sural or saphenous nerves was also employed in order to condition the MSRs of the triceps surae muscles. Hair bending, skin indentation or stretching, as well as electrical nerve stimulation, can induce a similar biphasic excitability cycle of the extensor MSRs, characterized by an early inhibition followed by a late facilitatory period (LFP). The LFP started approximately 20 ms after the arrival of the cutaneous afferent volley, and lasted about 80 ms. Conditioned MSRs could attain values corresponding to 200% or more of controls. The receptive field of the LFP evoked by the air jet proved to be as large as the whole leg and foot skin surface. No significant differences were found in the extent of the late facilitation in the MSRs of Sol, GM and GL, conditioned by electrical stimulation. The LFP was also present, after conditioning stimulation of the same types as above, in intact (and spinal) chloralose-anesthetized cats.

Cutaneous nerve stimulation and motoneuronal excitability. II: Evidence for non-segmental influences.

After stimulation of a sensory nerve, even distant motor nuclei undergo excitability changes which are evidenced by recovery curves. In all, two unequal peaks of facilitation can be identified. They appear when a given motor nucleus is tested after stimulation of various sensory nerves or when the same stimulation (of the sural nerve) is studied in various motor nuclei. The first facilitation is seen earlier in the masseter, then in the arm muscles and finally in lower limb muscles. The existence of a supraspinal centre activated by low threshold exteroceptive afferents and facilitating motor nuclei in a rostro-caudal sequence is postulated to account for certain features of the first and second peaks of facilitation.

From activity to rest: gating of excitatory autogenetic afferences from the relaxing muscle in man.

The changes in reflex excitability of the motoneurones to the soleus (Sol) muscle occurring during and after voluntary releases of various duration from a constant plantar-torque level in isometric or isotonic conditions have been investigated in normal humans by means of the H-reflex and T-response. The amplitude of both reflexes during the release phase attains values lower than control (obtained in resting conditions) even in the presence of force and EMG activity; the maximal inhibition is reached at the end of the release; the reflexes recover gradually to control values over several seconds. The more abrupt is the release, the more inhibited is the reflex and the shorter is the time to recovery and vv. These results apply both in isometric and isotonic conditions. Activation of the antagonist muscles, sometimes occurring at the end of the fastest release, does not contribute to the H-reflex inhibition. Tonic isometric contractions and relative releases have also been evoked by the tonic vibration reflex (TVR). The H-reflex during the TVR-induced contractions were lower than control values, at variance with those obtained during the voluntary contractions, but their amplitudes during the releases had similar values and time-courses in both conditions, pointing to a common involved inhibitory mechanism. Any voluntary ballistic or ramp contraction taking place after a preceding release, in the period in which the H-reflexes were still inhibited, was not apparently influenced, despite the fact that H-reflexes evoked during the release-conditioned ramp contractions were significantly lower than when evoked during control ramps of similar characteristics. The results are discussed in terms of a premotoneuronal, possibly presynaptic, inhibitory mechanism.