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Biomedical subjects

M Schieppati

Publications and source records attributed to M Schieppati.

At least 55 records · Page 3Linked to original sources
PubMed1994

Short-latency neck muscle responses to vertical body tilt in normal subjects and in patients with spasmodic torticollis.

EMG responses in the sternocleidomastoid (SCM) and dorsal neck muscles (DNM) to vertical head acceleration were studied in normal subjects and in patients with spasmodic torticollis, standing on a platform that could be tilted upwards. The vertical body displacement and the induced changes in the head-neck angle (a flexion-extension sequence) were recorded. Excitatory responses, symmetrical on the two sides, were elicited in normal subjects in both muscle groups, at a latency of about 60 msec (DNM) and 90 msec (SCM). With the head initially extended, the latency of DNM response increased, leaving that of SCM unchanged. During an isometric rotatory effort, an early inhibitory period was recorded in the active muscles at a latency of about 40 msec. Downward tilt did not evoke the responses. The DNM excitatory responses appeared to be related to muscle stretch, while those in SCM, as well as the inhibitory responses in both muscles, were thought to originate in the vestibular receptors. During active head rotation the response increased in amplitude in the active SCM and decreased in the lengthened antagonist; decreased responses in the lengthened muscle persisted during passive head rotation. This was attributed to an influence from the tonic neck receptors. In the patients, SCM responses had normal latency, but were reduced in amplitude or absent in the dystonic muscle, in spite of tilt-induced head movements comparable to those recorded in normals. The diminution was even bigger if compared to normal subjects with the head actively rotated to a similar extent. It persisted when the head was returned to normal position by the "geste antagoniste." The inhibitory responses were unaffected in the active normal and dystonic muscles. The possible role of a deficit of the central vestibular connections in the decreased excitatory SCM response in dystonic patients is considered.

Adult↗
PubMed1994

The limits of equilibrium in young and elderly normal subjects and in parkinsonians.

Body sway was studied at various body inclinations, voluntarily maintained for about 1 min, in young and elderly normals and in idiopathic parkinsonians. They stood on a dynamometric platform, whose output gave the instantaneous centre of foot pressure (CFP), its mean value and body sway area, with eyes open (EO) or closed (EC). Subjects held the normal upright stance, or the maximum possible inclined posture (body straight, rotated at the ankle joints) in forward or backward direction, or intermediate postures. EMG was recorded from tibialis anterior (TA), soleus (Sol), extensor digitorum brevis (EDB) and flexor digitorum brevis (FDB). The cross-correlation function between the profile of the EMG envelope and the profile of the shift of CFP along the sagittal plane was calculated. In young subjects standing with EO, the maximum extent of antero-posterior (A-P) displacement of CFP was about 60% of foot length. EC reduced this value to about 50%. In the elderly normals, the maximum A-P displacement was about 40% (EO) and 30% (EC). In both groups, sway area was minimal during normal stance with EO and increased progressively when the subjects leant forward or backward. With EC, sway area further increased during normal stance and the rate of increase in relation to inclination augmented markedly. Sol was tonically active during normal stance. Forward leaning increased Sol EMG and induced activity in FDB. TA and EDB were active during backward leaning. The peak of the cross-correlation function between Sol EMG and instantaneous CFP was higher during normal stance than forward inclination, while the reverse was true for FDB. This suggests a role of FDB in the fine-tuning of postural adjustment during forward leaning, and a weight-supporting role of Sol. During backward inclination, TA but not EDB was cross-correlated with CFP. In the parkinsonians, maximum A-P displacement of CFP was just about 30% of foot length (EO; about 20% with EC); its extent was inversely correlated with the severity of the disease. The relationship between sway area and A-P displacement was similar to the elderly, both with EO and EC, within the common range of inclination. In the patients affected by the long-term syndrome, A-P displacement was further reduced while sway area increase at the critical postures was often absent. In all patients, the relationship between muscle activity and body inclination was comparable to normal.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1993

Central and peripheral coordination in movement sequences.

Motor coordination has been too poorly defined to be a useful construct in studying the control of movement. In general, motor coordination involves controlling both the timing and the kinematics of movement. Yet the motor behaviors typically used for the study of coordination have required controlling only the timing or the spatial aspects of a movement. To understand better the basis of motor behavior, this study examined movement sequences, a class of movement in which both the timing and the kinematics must be controlled. In one experiment we studied a reaching and grasping movement sequence to characterize the central coordination of movement sequences. In another experiment we studied a throwing movement sequence to characterize the peripheral (kinesthetic) coordination of movement sequences. An heuristic model is presented to explain how central and peripheral mechanisms of coordination might interact to produce accurate movement.

Adolescent↗
PubMed1992

Activation of the neck muscles from the ipsi- or contralateral hemisphere during voluntary head movements in humans. A reaction-time study.

Reaction times (RTs) of EMG onset of agonist right and left sternocleidomastoid (SCM) and splenius (SPL) muscles in response to acoustic (AC) or unilateral somatosensory (SS) stimulation were measured in normal subjects, during head rotation (to the right or to the left) and flexion. No significant difference was present in the AC-RT of SCM muscles of the two sides between rotations or flexion. The same was true for the SPL. Under the tactile condition, in which the stimulus was delivered to one index finger, the RTs of both agonist muscles were shorter during head rotation toward the stimulus than away from it: the SCM contralateral to the stimulated finger was faster than the ipsilateral SCM, while the reverse was true for the SPL. During flexion, the SS-RTs of the SCM of both sides were similar, and similar in turn to the SCM-RT during rotation away from the stimulus. When the stimulus was delivered to the shoulder, the RT difference between the agonist SCMs disappeared. The delay in the activation of the SCM ipsilateral to the stimulated finger is compatible with the interhemispheric transmission time in the absence of callosal connection between the hand areas of the primary somatosensory cortex. The data favour the hypothesis that SCM activation during voluntary head rotation is controlled by the ipsilateral hemisphere.

Adult↗
PubMed1991

Free and supported stance in Parkinson's disease. The effect of posture and 'postural set' on leg muscle responses to perturbation, and its relation to the severity of the disease.

Upright stance and its reflex control were studied in parkinsonian patients and in age-matched normal subjects. They stood unperturbed on a force-measuring surface (static conditions), or were displaced by movement of a supporting platform (dynamic conditions). During quiet stance the following variables were analysed, with eyes open or closed: position of the centre of foot pressure (CFP), average sway area, length of sway path, amplitude and distribution of tonic leg muscle EMG activity. Perturbations of stance were induced by toe-up or toe-down rotations, and by backward or forward translations of the platform. Amplitude of short, medium and long-latency EMG responses to displacement were measured in the tibialis anterior (TA) and in the three heads of the triceps surae (TS) muscle. The perturbations were produced during both free and supported stance (holding onto a stable structure), under which condition normal subjects suppress medium and long-latency responses. Under static conditions, the only significant finding in parkinsonians was represented by a shift in the position of the CFP. This was correlated with the severity of the disease (Webster scale), the less affected patients being shifted backwards and the more affected patients forwards, with respect to normals. Under dynamic conditions, the reflex responses to perturbations of free stance were similar in both groups. Only the medium-latency burst of gastrocnemius lateralis and the long-latency burst TA evoked by TS stretch were larger in parkinsonians. The amplitude of these responses, as well as of all the others, was not related to the Webster score. Within the patients' group, a relationship between position of CFP and area of EMG burst was found for both medium and long-latency TA responses evoked by forward translation and toe-up rotation, respectively. Under supported conditions, the capability to suppress all medium and long-latency muscle responses to any perturbation was lost or impaired in the parkinsonians. The degree of impairment was unrelated to the position of the CFP, but was significantly related to the severity of the disease. The suppression to 40% (supported/nonsupported), of TA response to toe-down rotation is proposed as the point of separation between normals and parkinsonians. The forward projection of the CFP, occurring in the severe stages of the disease, and the increase in amplitude of some responses to perturbations of free stance might be a compensatory adaptation to the anomalous upright posture.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1991

Recurrent and reciprocal inhibition of the human monosynaptic reflex shows opposite changes following intravenous administration of acetylcarnitine.

A long-latency, long-lasting increase in the recurrent inhibitory effect on the soleus monosynaptic (Hoffmann, H) reflex was induced after intravenous administration of L-acetylcarnitine, a substance known to process central cholinergic activity. This effect was paralleled by disappearance of the H reflex inhibition (functionally disinhibition) induced by stimulation of Ia afferent fibres from the tibialis anterior (reciprocal inhibition) and gastrocnemius medialis muscle. Magnitude and time course of the L-acetylcarnitine-induced effects were significantly correlated. The data suggest that (1) the L-acetylcarnitine depression of the reciprocal inhibition is mediated by excitation of Renshaw cells impinging on Ia interneurones (INs), and (2) the inhibitory effect of GM Ia afferents onto Sol is mediated by INs subjected to Renshaw inhibition. The results point to the similarity in the wiring of the 'output stage' circuit between cat and humans, and provide a method for testing this network in man.

Acetylcarnitine↗
PubMed1990

Different activations of the soleus and gastrocnemii muscles in response to various types of stance perturbation in man.

The soleus (Sol) and medial and lateral gastrocnemii (GM and GL) behave differently during various movements, but no attempt has been made to disclose any distinct activation of these muscles during perturbations of upright stance. Therefore the pattern of activation of the three triceps surae (TS) muscles and of the tibialis anterior (TA) was studied in normal subjects following rotational and linear displacements of a movable platform. The effect of "postural set" on the responses was also studied while holding onto a frame. In free-standing subjects, TS stretches (upward tilt, UT and backward translation, BT) evoked a large short latency response (SLR) in Sol; smaller SLRs were sometimes induced in GM or GL. A medium latency response (MLR) was consistently present in both or only one gastrocnemii. On the average, the amplitude and the frequency of occurrence of the responses were distributed as follows: SLR, Sol greater than GL greater than GM; MLR, GM greater than GL greater than Sol. The type of perturbation did not affect the latency of all TS muscle responses, but the duration, amplitude and frequency of MLRs were larger during BT than UT. MLRs were followed by an antagonistic reaction (AR) in the TA, larger and more frequent during UT than BT. TA stretches (downward tilt, DT and forward translation, FT) induced a TA MLR, with duration and area larger in FT than DT. ARs occurred in one or more muscles of TS, being larger and more frequent in Sol. Under the conditions of stabilized stance, SLRs were not affected, but all the MLRs and ARs were much reduced in amplitude. The analogies between TS and TA MLRs (frequency of occurrence, latency and suppression under stabilized condition) suggest a common underlying mechanism and a similar postural role. On the other hand, all the TS responses are unequally distributed in the individual muscles and in the various subjects. This recommends caution in drawing conclusions in their absence or from their susceptibility to postural set in patients, when only one muscle of TS is being recorded.

Adult↗
PubMed1990

Responses of leg muscles in humans displaced while standing. Effects of types of perturbation and of postural set.

Toe-up or toe-down tilts of a platform on which a subject stands induce early EMG responses in the leg muscles initially stretched by the perturbation and late responses in the antagonist muscles. Early responses are thought to be connected with the stretch of the leg muscle in which they appear. Disagreement exists as to the origin of the late responses occurring in the antagonist muscle. The aims of this study were to assess (1) whether the late responses are induced by afferent volleys from the spindles of the muscle stretched by the initial perturbation, or (2) whether they are connected with the induced overall postural imbalance, and (3) whether the postural set may influence the occurrence of the late responses. Subjects standing on a platform underwent randomized perturbations stretching the soleus (Sol) muscle (upward tilts and backward translations) and tibialis anterior (TA) muscles (downward tilts and forward translations). The platform movement was regulated in order to yield changes in ankle angle of similar extent and velocity during both tilt and translation. Surface EMGs of Sol and TA were recorded bilaterally. An optoelectronic device detected the movements of markers fixed on the body. From these data, movements of the head, and changes in hip, knee and ankle angles, along with variations in the length of Sol, gastrocnemii (Gas) and TA were computed. Both tilts and translations, equally stretching Sol or TA, induced similar early responses in the stretched muscle. Consistent late responses in the antagonist muscle (antagonist reactions, ARs) were induced only by tilts. In spite of similar changes in ankle angles, the most striking differences in body movements between tilts and translations stretching the same leg muscle concerned changes in knee angles and Gas length. Slight differences were also seen in vertical head movements. Standing and holding onto a frame strongly decreased the amplitude and the frequency of occurrence of both early responses and ARs only in the TA muscle, while all Sol responses were not affected. This modulation of TA responses occurred in spite of changes in ankle angle and head movements similar to those occurring under the free-standing condition. It was concluded that early EMG responses are connected with the stretch of the muscle induced by the platform movement. The ARs, on the other hand, appear to be related to the type of overall postural imbalance. The absence of ARs during translations suggests a role in these responses of the afferences from the joint and muscles of the lower limb.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Convergence of Ia fibres from synergistic and antagonistic muscles onto interneurones inhibitory to soleus in humans.

1. The possibility that Ia afferent fibres from the gastrocnemius medialis (GM) and from the tibialis anterior (TA) muscle could converge on to a single interneuronal pool inhibitory to the soleus motoneurones was investigated. 2. The soleus H reflex, evoked by tibial nerve stimulation in the popliteal fossa, was conditioned by separate or combined stimulation of the nerves to the GM or TA muscles. Stimulus intensity was below the motor threshold (MTh), and the conditioning-test intervals were such as to evoke short-latency inhibition of the soleus H reflex. Care was taken to avoid current spread and artifacts connected with the closeness in time and space of the conditioning and test stimuli. 3. Separate stimulation of both GM and TA nerves was able to induce significant inhibitory effects on the H reflex amplitude at stimulus strengths larger than 0.75 x MTh, on the average. Combined stimulation of the two nerves was able to reduce the H reflex at lower stimulus strengths, at which either nerve was ineffective alone. 4. Conditioning stimulus strengths close to the MTh reduced the H reflex to approximately 80% of the control value, both on single and combined stimulation, i.e. saturation of the inhibitory effect was found. 5. By extrapolating the regression line through the normalized data from all subjects, it was assumed that the smallest stimulus strength necessary to drive the inhibitory interneurones to threshold was, on the average, 0.5 and 0.6 x MTh, on combined and separate nerve stimulation, respectively. 6. Tonic voluntary activation of the soleus abolished the inhibitory effects of both separate and combined stimulations. This was tested on the H reflex, on the rectified and averaged EMG, and on the peristimulus histogram of single motor unit discharge. 7. The findings strongly suggest the existence of spatial summation of the effects from GM and TA muscle at the level of a single interneuronal pool. Most probably, the responsible afferent fibres are group I spindle afferents, and the interneurones those mediating the reciprocal inhibition. The data do not support the notion of parallel pathways, exclusive to each nerve.

Adult↗
PubMed1990

Enhancement of recurrent inhibition by intravenous administration of L-acetylcarnitine in spastic patients.

The recurrent inhibition of the soleus alpha-motoneurons at rest, evaluated by a specially designed method of paired H reflexes, was estimated in 10 patients with spastic paraparesis. In three of these patients, Renshaw cell activity produced inhibition of the corresponding alpha-motor neurons comparable to that obtained in normal subjects, while the inhibition was reduced in five and absent in two. The effects of intravenously administered L-acetylcarnitine on the activity of Renshaw cells were studied in these patients. In all patients except the two with no evidence of recurrent inhibition, L-acetylcarnitine was found to significantly increase the amount of recurrent inhibition. Its effect became evident at about 30 minutes, reached a maximum around 50 minutes and vanished about 70 minutes from the onset of administration. A significant correlation was found between Renshaw cell activity and the ability of the drug to increase it. L-acetylcarnitine appeared to act specifically by enhancing recurrent inhibition, since no variation in the excitability of the monosynaptic reflex arc was observed.

Acetylcarnitine↗
PubMed1989

Patterns of activity of perioral facial muscles during mastication in man.

The activity of buccinator, orbicularis oris superior and inferior, quadratus and triangularis muscles was recorded by surface and needle electrodes during spontaneous unilateral mastication of small or large boluses. Their activity was compared with that of the masticatory muscles masseter and digastric, and with the vertical movement of the jaw. The facial muscles were active during mastication, displaying both tonic and phasic activity. Although the cyclic activity was broadly linked to lowering of the jaw, there was no strict time correspondence between the activity of the various facial muscles themselves, or between the activity of the facial muscles and the digastric. The timing and amplitude of facial muscle activity were affected by the duration of the masticatory cycle, the side on which chewing took place, the size of the bolus, and whether or not lip to lip contact was made.

Adult↗
PubMed1989

Selective recruitment of high-threshold human motor units during voluntary isotonic lengthening of active muscles.

1. We have investigated the possibility that voluntary muscle lengthening contractions can be performed by selective recruitment of fast-twitch motor units, accompanied by derecruitment of slow-twitch motor units. 2. The behaviour of motor units in soleus, gastrocnemius lateralis and gastrocnemius medialis muscles was studied during (a) controlled isotonic plantar flexion against a constant load (shortening contraction, S), maintained plantar flexion, or dorsal flexion resisting the load and gradually yielding to it (lengthening contraction, L), (b) isometric increasing or decreasing plantar torque accomplished by graded contraction or relaxation of the triceps surae muscles, (c) isometric or isotonic ballistic contractions, and (d) periodic, quasi-sinusoidal isotonic contractions at different velocities. The above tasks were performed under visual control of foot position, without activation of antagonist muscles. The motor units discharging during foot rotation were grouped on the basis of the phase(s) during which they were active as S, S + L and L. The units were also characterized according to both the level of isometric ramp plantar torque at which they were first recruited and the amplitude of their action potential. 3. S units were never active during dorsal flexion; some of them were active during the sustained contraction between plantar and dorsal flexion. Most S + L units were active also during the maintenance phase and were slowly derecruited during lengthening; their behaviour during foot rotations was similar to that during isometric contractions or relaxations. L units were never active during either plantar or maintained flexion, but discharged during lengthening contraction in a given range of rotation velocities; the velocity of lengthening consistently influenced the firing frequency of these units. Such dependence on velocity was not observed in S + L units. 4. A correlation was found between the amplitude of the action potential and the threshold torque of recruitment among all the units. In addition, the amplitudes of both the action potential and the threshold torque were higher in the case of L units than in the case of S and S + L units. Most L units could be voluntarily recruited only in the case of ballistic isometric or isotonic contraction. 5. Occasionally, L units were directly activated by electrical stimulation of motor fibres and their conduction velocity was in the higher range for alpha-axons. In contrast, nerve stimulation could induce a reflex activation of S and S + L units.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1989

Short-latency inhibition of soleus motoneurones by impulses in Ia afferents from the gastrocnemius muscle in humans.

1. The possibility that the Ia afferent fibres from the gastrocnemius medialis muscle could be responsible for a decrease in excitability of the soleus motor pool was investigated. 2. The soleus H reflex, evoked by tibial nerve stimulation in the popliteal fossa, was conditioned by a single stimulus to the gastrocnemius medialis nerve at various stimulus intensities and conditioning-test intervals. Care was taken to avoid spread of current from the conditioning stimulus to the tibial nerve, and the results obtained by surface stimulation were compared with those obtained by stimulation through a needle whose tip was positioned closer to the nerve. 3. Stimulation of the gastrocnemius medialis nerve induced two short-lasting periods of inhibition in the soleus H reflex, peaking at about 0 and 5 ms of conditioning-test delay. The early inhibition could begin at a stimulus strength as low as 0.5 x MTh (the Motor Threshold). The later inhibition appeared on greater stimulus strength than the earlier. 4. Prolonged vibration of the Achilles tendon abolished the capability of the conditioning stimulus to induce the short-latency inhibition of the soleus H reflex. 5. By stimulating the gastrocnemius medialis nerve at two points separated by a known distance, the conduction velocity of the fibres responsible for the early inhibition was estimated, and found to be around 100 m s-1. 6. Isometric leg flexion, accomplished by tonic activation of gastrocnemius medialis and lateralis but not soleus, was able to induce an inhibition of the soleus H reflex even at very low levels of gastrocnemius electromyographic activity. 7. These findings strongly suggest the existence of an inhibitory effect of primary spindle afferent fibres from the gastrocnemius medialis muscle onto the soleus motor pool. This is not unexpected, since the gastrocnemius medialis muscle can be either agonist or antagonist to the soleus muscle in the performance of different movements.

Achilles Tendon↗
PubMed1989

Motoneurone recurrent inhibition is enhanced by L-acetylcarnitine in humans.

Renshaw cells, which mediate the recurrent inhibition of spinal a-motoneurones, are activated by acetylcholine, both through motoneurone collaterals and the reticulo-spinal system. Since it is known that L-acetylcarnitine (L-AC) has central cholinergic effects, we tested in ten normal subjects and three spastic patients the ability of L-AC to induce changes in excitability of the Renshaw cells. These were activated by a conditioning monosynaptic reflex of the soleus muscle, evoked by electrical stimulation of the tibial nerve, and the resulting recurrent inhibition of the motoneurones was assessed by a subsequent monosynaptic reflex (H'). Recurrent inhibition was tested prior to, during and after an intravenous administration of a solution containing 2000 mg of L-AC. L-AC administration proved to be able to induce in all subjects a decrease in the H'-reflex. This effect ensued approximately 30 min after onset of L-AC administration, reached the peak after 40 min and vanished in about one hour. The extent of the decrease in H' varied among subjects, being on the average 22% of the control values. A relationship was found between duration of L-AC administration and time for reaching the maximal effect. These results show that L-AC is able to decrease a-motoneurone excitability by increasing Renshaw cell activity, both in normal and spastic subjects.

Acetylcarnitine↗
PubMed1988

Postural adjustments associated with voluntary contraction of leg muscles in standing man.

The postural adjustments associated with a voluntary contraction of the postural muscles themselves have been studied in the legs of normal standing men. We focussed on the following questions. Do postural adjustments precede the focal movement as in the case of movements of the upper limb? Which muscle(s) are involved in the task of stabilizing posture? Can the same postural muscle be activated in postural stabilization and in voluntary movement at the same time, in spite of the opposite changes in activity possibly required by these conditions? Six subjects standing on a dynamometric platform were asked to rise onto the tips their toes by contracting their soleus muscles, or to rock on their heels by contracting their tibialis anterior muscles. The tasks were made in a reaction time (RT) situation or in a self-paced mode, standing either freely or holding onto a stable structure. Surface EMGs of leg and thigh muscles, and the foot-floor reaction forces were recorded. The following results were obtained in the RT mode, standing freely. 1. Rising onto toe tips: a striking silent period in soleus preceded its voluntary activation; during this silent period, a tibialis anterior burst could be observed in three subjects; these anticipatory activities induced a forward sway, as monitored by a change in the force exerted along the x axis of the platform. 2. Rocking on heels: an enhancement in tonic EMG of soleus was observed before tibialis anterior voluntary burst, at a mean latency from the go-signal similar to that of the silent period; this anticipatory activity induced a backward body sway. 3. Choice RT conditions showed that the above anticipatory patterns in muscle activity were pre-programmed, specific for the intended tasks, and closely associated with the focal movement. When both tasks were performed in a self-paced mode, all the above EMG and mechanical features were more pronounced and unfolded in time. If the subjects held onto the frame, the early features in the soleus or tibialis anterior EMG were absent, and the corresponding changes in the foot-floor reaction forces were lacking. The anticipatory phenomena observed are considered postural adjustments because they appear only in the free-standing situation, and induce a body sway in the appropriate direction to counteract the destabilizing thrust due to the voluntary contraction of soleus or tibialis anterior. The central organization and descending control of posture and movements are briefly discussed in the light of the short latency of the anticipatory phenomena and of their close association with the focal movement.

Adult↗
PubMed1988

Shift of activity from slow to fast muscle during voluntary lengthening contractions of the triceps surae muscles in humans.

1. Raw or rectified and integrated electromyograms (integrated EMGs) of the leg muscles were recorded during (a) isotonic ramp shortening or lengthening contractions consisting of foot plantar flexions against a constant load, or dorsal flexions accomplished by braking the load and yielding to it, respectively, and (b) isometric increasing or decreasing plantar torques accomplished by graded contractions or relaxations of the triceps muscles. 2. During plantar flexions or increasing torques, the EMG of soleus, gastrocnemius lateralis, medialis, and peroneus increased in parallel. During decreasing torques, motor unit derecruitment took place gradually and simultaneously. The tibialis anterior was silent. During dorsal flexions, one of two characteristic patterns was observed in different subjects: (a) soleus was abruptly derecruited at the beginning of the task, while gastrocnemius lateralis (or medialis) exhibited a large recruitment lasting throughout the lengthening contraction; (b) soleus remained active during the task, showing large motor unit potentials, while the gastrocnemius lateralis recruitment was of a lesser extent than in (a). Peroneus derecruitment was gradual and tibialis anterior activity was absent in both cases. 3. The EMG patterns observed during plantar flexions or in increasing and decreasing torques, and the two patterns observed during shortening or lengthening contractions, were closely reproduced during sinusoidal oscillations of the foot or in isometric contractions and relaxations. 4. When recruitment of the gastrocnemius lateralis was present during dorsal flexion, the slope of its integrated EMG envelope was steeper, the higher the velocity of lengthening contraction. The most rapid and the slowest tasks, however, did not require its activation. Gastrocnemius lateralis integrated EMGs of an amplitude similar to those occurring during lengthening contractions were observed only during ballistic plantar flexions. 5. The two patterns of triceps activation occurring during lengthening contraction could be traced to different mechanical characteristics of the soleus muscles, the gastrocnemius lateralis being activated preferentially in subjects with long soleus half-relaxation times, and the soleus in subjects with short soleus half-relaxation times. 6. The soleus and gastrocnemius lateralis H reflexes were tested during shortening and lengthening contractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1988

The H reflex recovery curve reinvestigated: low-intensity conditioning stimulation and nerve compression disclose differential effects of presumed group Ia fibres in man.

The recovery curve of the soleus H reflex, evoked by stimulation of the tibial nerve in the popliteal fossa, was studied by applying an electrical conditioning stimulus to the inferior soleus nerve. Under these conditions a long-latency facilitatory phase could be superimposed on a long-lasting inhibition, the excitability cycle being therefore similar to that obtained by means of a paired shock to the tibial nerve. In order to identify the afferent fibres responsible for the effects observed, various conditioning stimulus strengths and nerve compression were used. A low-intensity stimulus induced only a facilitatory phase, while the inhibition promptly ensued on increasing stimulus strength, which remained however subliminal for activation of group II fibres. During calf compression exerted by a sphygmomanometer cuff placed between the conditioning and test stimuli, the facilitatory effects disappeared within 10 to 15 min, and the inhibitory ones disappeared within 25 to 30 min from the onset of compression. Tonic voluntary contraction enhanced both the inhibition and the facilitation. In a subject with complete spinal section, both inhibitory and facilitatory phases could be demonstrated on low-intensity stimulation. The present data, and previous results of ours, allow the following conclusions. (1) Facilitation and inhibition are produced by fibres likely belonging to group Ia spindle afferents. (2) Both effects are of spinal origin. (3) The spinal circuits mediating the effects may be modulated by descending commands. (4) The facilitation is sustained by tonic supraspinal influences while the inhibition is independent of it. Arguments are proposed against the hypotheses that the inhibition be due to transmitter depletion or to presynaptic mechanisms.

Adult↗