Combined electromyography-31P-magnetic resonance spectroscopy study of human muscle fatigue during static contraction

Metabolic changes measured by ³¹P-magnetic resonance spectroscopy and surface electromyograms were simultaneously recorded during isometric contraction of forearm flexor muscles sustained at 60% of maximal force until exhaustion. Throughout the fatigue trial, energy in the low-frequency (L) band continuously increased whereas energy in the high-frequency (H) band first increased and then fell only prior to exhaustion. PCr content decreased linearly. Intracellular pH (pHi) transiently increased during the first 22 s of trial. The triggering of acidosis was associated with critical PCr values (35–70% of initial content) and decreased electromyogram (EMG) energy in the H band. Linear relationships were only found between energy in the L band, pHi, and PCr content. The interindividual variability of metabolic and EMG changes was high despite standardized conditions of contraction. Maximal PCr consumption was correlated with the maximal pHi decrease measured at the end of the trial. Overall, there was no correlation between H/L EMG ratio and changes in muscle metabolism. © 1996 John Wiley & Sons, Inc.     

Local venous lactate changes and spectral analysis of surface EMG during fatiguing isometric efforts in intrinsic hand muscles

In this study relationships between local venous lactate concentrations and power spectrum changes in surface EMG due to fatiguing isometric contractions were investigated. Two different isometric efforts of the opponents pollicis muscle, 25% and 50% of maximum voluntary contraction, were examined. In 25% of maximum voluntary contraction no changes in local venous lactate were observed, whereas mean power frequency values of surface EMG shifted significantly towards lower frequencies. On the contrary, in 50% of maximum voluntary contraction lactate significantly increased and was related to the mean power frequency shifts of the surface EMG. These data suggest that lactate plays a role in muscle fatigue only in a limited intensity range of isometric muscle contractions.     

Electromyography and mechanomyography of elbow agonists and antagonists in Parkinson disease

The purpose of this study was to assess the electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in Parkinson disease patients during maximal isometric elbow contraction in flexion and extension. Ten elderly females with Parkinson disease (average age 75 years) and 10 age‐matched healthy females were tested. The torque and the EMG and MMG signals from biceps brachii and triceps brachii were recorded during sustained maximal voluntary isometric contraction of the elbow flexors and extensors. There were no intergroup differences in the EMG and MMG activities of agonist and antagonist muscles or in torque. This might be because the Parkinson subjects were tested during their medication “ON” phase, or perhaps maximal isometric contraction (MVC) induced greater active muscle stiffness that affected the MMG signal. Muscle Nerve 40: 240–248, 2009     

Impaired “natural reciprocal inhibition” in patients with spasticity due to incomplete spinal cord injury

Experiments were performed to compare the ability of normal subjects and patients with spinal spasticity to suppress antagonist H reflexes during isometric ankle contractions. Soleus H reflex suppression was examined during tonic pretibial muscle contractions in which the torque levels were constant and during dynamic pretibial muscle contractions in which the torque followed a predetermined ramp. As well, subjects were instructed to alternately contract ankle plantarflexors and dorsiflexors at various frequencies to examine patterns of EMG activity during rhythmically alternating isometric contractions in antagonist muscles. Patients with incomplete spinal cord injury demonstrated reduced ability to suppress soleus H reflexes during pretibial muscle contraction. At slow speeds of alternating contraction, spinal cord injured patients retained the ability to perform alternating isometric pretibial/soleus muscle contractions. The patients demonstrated abnormal coactivation in soleus muscle during faster alternating isometric ankle muscle contractions. Furthermore, the patients who demonstrated the greatest impairment in natural reciprocal inhibition, also displayed the largest amount of coactivation. In general, the results would suggest that impairment of natural reciprocal inhibition is correlated with an increase in the amount of antagonist muscle coactivation seen during alternating isometric muscle contractions.     

Fatigue‐related changes in fascicle–tendon geometry over repeated contractions: Difference between synergist muscles

Fatigue‐induced changes in force production of synergist muscles were evaluated through observation of fascicle‐tendon geometry and electromyography (EMG). Seven subjects performed 60 maximal isometric plantar flexions intermittently. No statistically significant intermuscle difference was observed in the decrease of mean EMG amplitudes or mean power frequency for the medial gastrocnemius (MG) and soleus (SOL) muscles. The tendon elongation of MG significantly decreased after the 19th contraction, and MG fascicle length increased after the 29th contraction, while SOL fascicle and tendon length did not change except for the last contraction. The declines in torques were highly correlated with the increase of MG fascicle length and decrease in tendon elongation in each subject, while no consistent relationship was found for SOL. These results suggest that changes in force‐production of MG and SOL over repeated contractions differ, which is reflected in fascicle–tendon geometry. Muscle Nerve 40: 395–401, 2009     

Electromyographic and mechanical characteristics of human skeletal muscle during fatigue under voluntary and reflex conditions

Effects of fatigue produced by a maintained 50% isometric loading on electromyographic (EMG) and mechanical characteristics of voluntary and reflex contractions of human skeletal muscles were studied in 14 males. A continuous isometric loading of the knee extensors caused significant (P less than 0.001) increase in integrated EMG (IEMG) and decrease (P less than 0.001) in mean power frequency (MPF) of the EMG spectrum. The decrease in MPF was related (P less than 0.05) to percentage fast twitch (FT) fibre area of the vastus lateralis muscle. In reflexly induced contractions no changes were observed during fatigue in reflex latency (LAT) but electromechanical delay (EMD) increased significantly (P less than 0.01). The decrease in voluntary MPF and increase in reflex EMD were interrelated (P less than 0.05). Fatigue loading increased (P less than 0.05) the peak-to-peak amplitude of EMG of the reflex contraction but decreased (P less than 0.01) the corresponding force amplitude. The increase (P less than 0.01) in this reflex EMG/force ratio was related (P less than 0.05) to the corresponding increase observed in IEMG/force ratio of the voluntary contraction. The present findings support the differential fatiguing properties of fast twitch and slow twitch muscle fibres. In addition the fatigue effects on reflexly induced contractions show that fatigue took place primarily in the contractile processes and that muscle spindle sensitivity was increased during fatigue loading.     

Firing rates of motor units during strong dynamic contractions

Muscle behavior is usually studied during isometric contractions but many tasks include contractions that involve changes in muscle length. Our aim was to record motor unit action potentials and surface electromyograms (EMGs) from triceps brachii muscles during rhythmic dynamic contractions (3-s concentric, 3-s eccentric; 40 degrees/s; four subjects) performed at the highest voluntary forces subjects could exert (maximal concentric contraction) and at various submaximal intensities. Mean unit firing rates and surface EMG increased significantly with contraction intensity in both concentric and eccentric contractions, but at each intensity mean concentric values were significantly higher than eccentric values. In contrast, mean unit firing rates and surface EMGs were similar during maximal concentric and maximal isometric contractions. These data suggest muscles were activated maximally during the strongest concentric contractions but submaximally during the strongest eccentric efforts. After estimated eccentric contraction intensity was adjusted using surface EMG data, mean unit firing rates during eccentric contractions were still lower than the concentric values. Thus, protective mechanisms may limit motor unit firing rates during forceful lengthening contractions to minimize damage.     

Power spectral analysis of surface electromyography (EMG) at matched contraction levels of the first dorsal interosseous muscle in stroke survivors

ObjectiveThe objective of this study was to help assess complex neural and muscular changes induced by stroke using power spectral analysis of surface electromyogram (EMG) signals.MethodsFourteen stroke subjects participated in the study. They were instructed to perform isometric voluntary contractions by abducting the index finger. Surface EMG signals were collected from the paretic and contralateral first dorsal interosseous (FDI) muscles with forces ranging from 30% to 70% maximum voluntary contraction (MVC) of the paretic muscle. Power spectral analysis was performed to characterize features of the surface EMG in paretic and contralateral muscles at matched forces. A Linear Mixed Model was applied to identify the spectral changes in the hemiparetic muscle and to examine the relation between spectral parameters and contraction levels. Regression analysis was performed to examine the correlations between spectral characteristics and clinical features.ResultsDifferences in power spectrum distribution patterns were observed in paretic muscles when compared with their contralateral pairs. Nine subjects showed increased mean power frequency (MPF) in the contralateral side (>15 Hz). No evident spectrum difference was observed in 3 subjects. Only 2 subjects had higher MPF in the paretic muscle than the contralateral muscle. Pooling all subjects’ data, there was a significant reduction of MPF in the paretic muscle compared with the contralateral muscle (paretic: 168.7 ± 7.6 Hz, contralateral: 186.1 ± 8.7 Hz, mean ± standard error, F = 36.56, p < 0.001). Examination of force factor on the surface EMG power spectrum did not confirm a significant correlation between the MPF and contraction force in either hand (F = 0.7, p > 0.5). There was no correlation between spectrum difference and Fugl–Meyer or Chedoke scores, or ratio of paretic and contralateral MVC (p > 0.2).ConclusionsThere appears to be complex muscular and neural processes at work post stroke that may impact the surface EMG power spectrum. The majority of the tested stroke subjects had lower MPF in the paretic muscle than in the contralateral muscle at matched isometric contraction force. The reduced MPF of paretic muscles can be attributed to different factors such as increased motor unit synchronization, impairments in motor unit control properties, loss of large motor units, and atrophy of muscle fibers.SignificanceSurface EMG power spectral analysis can serve as a useful tool to indicate complex neural and muscular changes after stroke.     

Surface EMG mapping of the human trapezius muscle: the topography of monopolar and bipolar surface EMG amplitude and spectrum parameters at varied forces and in fatigue.

OBJECTIVES: To investigate the factors affecting the topography of trapezius muscle EMG, multichannel recordings were made at different forces of isometric shoulder elevation and during fatiguing exercise. METHODS: Twenty-eight channels of monopolar EMG were recorded from an array of 4 x 7 electrodes placed on the upper trapezius muscle. From the monopolar EMG and the bipolar derivations the root mean square (RMS(monopolar), RMS(bipolar)) and power spectrum median frequency (MF(monopolar), MF(bipolar)) were calculated.RESULTS: The maximum RMS(monopolar) was located above the middle part of the trapezius muscle, where a minimum was found for RMS(bipolar). The cranial-caudal RMS distribution shifted upwards when the force was increased from 50 to 100% MVC and during fatigue. MF(bipolar) showed a peak above the endplate region, where the MF(monopolar) was low. During fatigue the normalized MF slope was independent of the cranial-caudal electrode position, but MF(monopolar) decreased most strongly at positions above the endplate region, where MF(bipolar) decreased less.CONCLUSIONS: While the changes in MF reflected metabolic properties and volume conduction phenomena in the muscle, changes in RMS reflected a compensation for the fatigue processes within the muscle. The RMS changes in fatigue can be explained by the direction of the fibres involved in shoulder elevation.     

Differential distribution of coherence between beta-band subthalamic oscillations and forearm muscles in Parkinson’s disease during isometric contraction

ObjectiveUnder rest condition, beta-band (13–30 Hz) activity in patients with Parkinson’s disease (PD) is prominent in the subthalamic nucleus (STN). However, the beta-band coupling between STN and muscle activity, its distribution and relation to motor symptoms remains unclear.MethodsUsing up to five electrodes, we recorded local field potentials (LFPs) above (zona incerta, ZI) and within the STN at different recording heights in 20 PD patients during isometric contraction. Simultaneously, we registered activity of the contralateral flexor and extensor muscle. We analysed LFP–EMG coherence to estimate coupling in the frequency domain.ResultsCoherence analysis showed beta–associated coupling in the ZI and STN with more significant LFP–EMG coherences in the STN. Coherence varied depending on the localisation of the LFP and muscles. We found significant difference between coherence of the extensor and the flexor muscle to the same LFP (p = 0.045).ConclusionsWe demonstrated that coherence between beta-band oscillations and forearm muscles are differentially distributed in the subthalamic region and between the forearm muscles in Parkinson’s disease during isometric contraction. However, the significant LFP–EMG coupling did not associate with motor deficits in PD patients.SignificanceThe differential distribution of beta-band activity in the STN highlights the importance of a topographically distinct therapeutic modulation.     

Effect of antagonistic voluntary contraction on motor responses in the forearm.

OBJECTIVE: We investigated the effects of voluntary contraction of agonist and antagonist muscles on motor evoked potentials (MEP) and on myoelectric activities in the target (agonist) muscle following transcranial magnetic stimulation (TMS). METHODS: The left extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles were studied in 16 healthy subjects. H reflexes, MEP induced by TMS, and background electromyographic (EMG) activity were recorded using surface electrodes at rest and during voluntary contraction of either agonist or antagonist muscles. RESULTS: Voluntary contraction of antagonist muscles (at 10% of maximum contraction) enhanced the amplitudes of MEP for both muscles. The H reflex of the FCR muscle was inhibited by contraction (10% of maximum) of the ECR muscle. Background EMG activity did not differ between H-reflex trials and TMS trials. Enhancement of MEP amplitudes and background EMG activity during voluntary antagonist contraction was comparable in the two muscles. Appearance rate of MEP recorded by needle electrodes in response to subthreshold TMS was increased by antagonistic voluntary contraction. CONCLUSION: Facilitation occurs during voluntary contraction of antagonist muscles. Differences between the effects of voluntary contraction of the ECR muscle for the MEP and the H reflex of the FCR suggest that cortical facilitatory spread occurs between agonist and antagonist muscles.     

Measurements of muscle stiffness, the electromyogram and activity in single muscle spindles of human flexor muscles following conditioning by passive stretch or contraction

In experiments on adult human subjects we examined the effect on passive mechanical properties of a muscle by conditioning it with either an isometric contraction or passive muscle extension. The test measurement was the amount of muscle displacement (stiffness) and the accompanying EMG in response to a brief torque pulse. Two muscles were tested, flexor digitorum profundus (FDP) and brachialis. In FDP the discharge of single muscle spindles was recorded as well. After muscle extension and return to the initial length, passive stiffness was less than after an isometric contraction. The changes in stiffness were accompanied by changes in pattern of EMG and in the responses of muscle spindles. It is suggested that in resting muscle there are stable cross bridges between actin and myosin filaments of muscle fibres which largely determine the passive stiffness. Muscle extension leads to detachment of these cross bridges which then re-form at the longer length. Return of the muscle to its starting length leads to development of slack in muscle fibres because, stiffened by the presence of the stable cross bridges, they are unable to shorten. Slack in muscle fibres lowers their measured stiffness. Muscle contraction, on the other hand, will result in any preexisting slack being taken up by the actively shortening muscle fibres, thereby raising muscle stiffness. Stiffness in intrafusal fibres is likely to follow a similar pattern to that in extrafusal fibres, leading to changes in stretch responsiveness of muscle spindles and consequently in the reflex EMG. It is concluded that the changes in stiffness and accompanying reflexes observed in this study are likely to be seen, at least under some conditions, in normal movements.     

Vascular and electromyographic responses evoked in forearm muscle by isometric contraction of the contralateral forearm

There is controversy over whether isometric contraction of the forearm evokes vasoconstriction or vasodilatation in the muscles of the contralateral forearm. In the present study we have investigated in normal man, the effects of isometric contraction of one arm at 75, 50 and 25% maximum voluntary contraction (MVC) on arterial pressure, heart rate, blood flow and vascular resistance of the contralateral forearm and on electromyographic (EMG) activity recorded from that same arm with sensitive, surface electrodes.When EMG activity was not being recorded from the resting arm, isometric contraction of the contralateral arm for 2 min evoked increases in arterial pressure and heart rate whose magnitudes were graded with % MVC and an increase in forearm blood flow and a decrease in forearm vascular resistance at 75, 50 and 25% MVC, indicating vasodilatation. Further experiments in which EMG activity was recorded from the resting arm demonstrated that the decrease in forearm vascular resistance evoked by 75% MVC was associated with a substantial increase in EMG activity of the extensor and flexor muscles of that arm. By contrast, when forearm contraction was performed at 75% MVC whilst subjects viewed the EMG activity in the resting arm on an oscilloscope and kept EMG activity minimal, vascular resistance increased in that arm, indicating vasoconstriction. Further, when subjects performed contraction at 25% MVC whilst showing minimal EMG activity in the contralateral arm, vascular resistance in that same arm increased (from 78 ± 16 to 124 ± 29 mmHg/ml/min/100 ml tissue). These results are discussed in relation to those of previous studies. We propose, that in normal man, isometric contraction of the forearm evokes primary vasoconstriction in the muscles of the contralateral forearm, but that this response may be overcome by muscle vasodilatation occurring secondary to unintended muscle contraction or as part of the alerting response to acute stress.     

Measurement of muscle contraction with ultrasound imaging

To investigate the ability of ultrasonography to estimate muscle activity, we measured architectural parameters (pennation angles, fascicle lengths, and muscle thickness) of several human muscles (tibialis anterior, biceps brachii, brachialis, transversus abdominis, obliquus internus abdominis, and obliquus externus abdominis) during isometric contractions of from 0 to 100% maximal voluntary contraction (MVC). Concurrently, electromyographic (EMG) activity was measured with surface (tibialis anterior only) or fine-wire electrodes. Most architectural parameters changed markedly with contractions up to 30% MVC but changed little at higher levels of contraction. Thus, ultrasound imaging can be used to detect low levels of muscle activity but cannot discriminate between moderate and strong contractions. Ultrasound measures could reliably detect changes in EMG of as little as 4% MVC (biceps muscle thickness), 5% MVC (brachialis muscle thickness), or 9% MVC (tibialis anterior pennation angle). They were generally less sensitive to changes in abdominal muscle activity, but it was possible to reliably detect contractions of 12% MVC in transversus abdominis (muscle length) and 22% MVC in obliquus internus (muscle thickness). Obliquus externus abdominis thickness did not change consistently with muscle contraction, so ultrasound measures of thickness cannot be used to detect activity of this muscle. Ultrasound imaging can thus provide a noninvasive method of detecting isometric muscle contractions of certain individual muscles.     

Activation linearity and parallelism of the superficial quadriceps across the isometric intensity spectrum

The purpose of this study was to assess neuromuscular activation of the three superficial portions of the quadriceps femoris muscles during linearly increasing isometric contraction intensities. Thirty healthy volunteers were assessed for isometric electromyographic (EMG) activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees of flexion. For 5 s, subjects performed isometric contractions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the average of three maximal voluntary contractions (MVC), in random order. Full-wave rectified and integrated EMG signals over the middle 3 s of each contraction were expressed as a percentage of the activity recorded during the three averaged MVCs. One sample t-tests and 95% confidence intervals were calculated at each relative torque level. A two-factor analysis of variance (muscle by intensity) with repeated measures was performed to evaluate parallel activation across the intensity levels. Activation linearity was assessed via regression analysis for each muscle. VM activation was shown to be significantly lower than expected at 20-70% MVC. VL and RF activations were significantly higher than expected at 10% MVC, and RF EMG was less than expected at 40-70% MVC. EMG of VM was shown to increase significantly more than VL and RF from 80% to 90% MVC. Significant linear and quadratic relations were also demonstrated for all three muscles. Parallel activation of the superficial quadriceps muscles occurred from low to moderate intensities, whereas convergence was noted at near maximal intensities.     

Simultaneous measurement of muscle conduction velocity and EMG power spectrum changes during fatigue

Muscle conduction velocity and mean frequency of the electromyogram (EMG) spectrum were measured simultaneously in the biceps brachii muscles of nine healthy subjects during sustained isometric contraction. Recordings were made at 60% and 70% of the maximum voluntary contraction. In all subjects, both conduction velocity and mean frequency decreased during the time period of the fatiguing contractions. Also, the muscle conduction velocity was found to vary linearly with the mean frequency. This demonstrates that the EMG spectral shifts observed during muscle fatigue are due to slowing of the conduction velocity.     

Effects of immobilizing a single muscle on the morphology and the activation of its muscle fibers

A single muscle of Wistar female rats, either soleus or peroneus longus, was immobilized by fixing its cut distal tendon to the bone during 8 weeks. We observed a transitory weight loss in both muscles; the mean fiber cross-sectional area (CSA) showed a reduction at day 30, followed by an increase at day 60. The time course of the activation of the immobilized muscle was evaluated by recording the chronic electromyographic (EMG) activity during short periods (1 min every other day) of treadmill locomotion. During immobilization, the integrated EMG amplitude of the soleus increased, reaching a maximum at 4 weeks, but remained close to control values during 8 weeks for the peroneus. The median frequency (MF) of the power density spectrum of the soleus EMG was not statistically different between immobilized and control muscles, while MF of the immobilized peroneus EMG was permanently higher than that of control muscles. This suggests two different modes of adaptation in motor unit command, depending on the muscle profile, which could be concomitant with the restoration of muscle fibers CSA after 8 weeks.     

Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function

The relation between muscle pain, muscle activity, and muscle co-ordination is still controversial. The present human study investigates the influence of experimental muscle pain on resting, static, and dynamic muscle activity. In the resting and static experiments, the electromyography (EMG) activity and the contraction force of m. tibialis anterior were assessed before and after injection of 0.5 ml hypertonic saline (5%) into the same muscle. In the dynamic experiment, injections of 0.5 ml hypertonic saline (5%) were performed into either m. tibialis anterior (TA) or m. gastrocnemius (GA) and the muscle activity and co-ordination were investigated during gait on a treadmill by EMG recordings from m. TA and m. GA. At rest no evidence of EMG hyperactivity was found during muscle pain. The maximal voluntary contraction (MVC) during muscle pain was significantly lower than the control condition (P < 0.05). During a static contraction at 80% of the pre-pain MVC muscle pain caused a significant reduction in endurance time (P < 0.043). During dynamic contractions, muscle pain resulted in a significant decrease of the EMG activity in the muscle, agonistic to the painful muscle (P < 0.05), and a significant increase of the EMG activity of the muscle, antagonistic to the painful muscle (P < 0.05). Muscle pain seems to cause a general protection of painful muscles during both static and dynamic contractions. The increased EMG activity of the muscle antagonistic to the painful muscle is probably a functional adaptation of muscle co-ordination in order to limit movements. Modulation of muscle activity by muscle pain could be controlled via inhibition of muscles agonistic to the movement and/or excitation of muscles antagonistic to the movement. The present results are in accordance with the pain-adaptation model (Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Værøy and H. Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain. Elsevier, Amsterdam, 1993, pp. 311–327.) which predicts increased activity of antagonistic muscle and decreased activity of agonistic muscle during experimental and clinical muscle pain.     

Pulmonary function and electromyographic study of respiratory muscles in myotonic dystrophy

Ten adult myotonic dystrophy patients underwent measurements of lung function, maximal dynamic and static ventilatory efforts, and respiratory muscle electromyography (EMG). EMG studies were performed during spontaneous breathing or when subjects breathed through high inspiratory or expiratory resistive loads. Present results show that (1) a moderate restriction of lung volumes with hypoxemia plus normocapnia is often observed; (2) patients sustain dynamic ventilatory efforts more easily than static work; and (3) abnormalities in respiratory muscle EMG exist with spontaneous expiratory and inspiratory intercostal activities during quiet breathing and changes in muscular response to resistive loads. Inspiratory loading evokes contraction of expiratory muscles, with a marked decrease in inspiratory activities. Expiratory resistive loads prolong the diaphragmatic contraction throughout the expiratory time, and in some patients, relaxation of the diaphragm does not occur during the loaded run. These EMG data suggest that the reciprocal inhibition among respiratory neurons is enhanced in myotonic dystrophy and that myotonia also occurs in the diaphragm when loads oppose its relaxation.     

Fusimotor outflow to pretibial flexors during fatiguing contractions of the triceps surae in decerebrate cats

Changes in discharge rate of fusimotor neurones to pretibial flexor muscles were recorded during and after long-lasting fatiguing isometric and/or isotonic contractions of triceps surae in decerebrate cats. The contractions were elicited by electrical stimulation of the nerves to triceps. Fusimotor spikes were recorded from nerve filaments dissected free from the peroneal nerve. Responses of the fusimotor neurones were diverse. In isometric regime, 22 out of 40 units recorded exhibited an initial increase at the onset of muscle contraction, different in amplitude and duration among the units. In seven of these units an additional brisk burst of spike discharges, of different duration, occurred at the end of the contraction. In 15 fusimotor neurones (14 units with the initial response and an additional unit without it) a slow increase in discharge rate developed, starting during the contraction and outlasting it. In another eleven units the intial response was a decrease in discharge rate, lasting in six of them throughout the contraction. Another six units exhibited a sustained increase in discharge rate throughout the contraction, as well as, at a lower level, but still above the spontaneous one, thereafter. Similar patterns of changes in discharge rate, recorded in 31 of the units, were encountered during isotonic triceps contractions. It should be mentioned that many (about 20) additional silent neurones, responding to manipulating the skin and paw and/or stroking the fur, but not to triceps contractions were encountered. The majority of changes in discharge rate of fusimotor neurones to pretibial flexors differed markedly from those found previously in fusimotor neurones to triceps and hamstring muscles. According to their timing, most of them seemed to be related to tension and/or length changes in the contracting triceps muscles. Only the slowly developing late increase in firing rate, encountered in approximately one third of the responding units (38% in isometric regime and 32% in isotonic regime) could be related to muscle fatigue. Differences in response patterns of fusimotor neurones to different muscle groups might be in accordance with function of muscles of their destination.