Intra-session and inter-day reliability of forearm surface EMG during varying hand grip forces

Surface electromyography (EMG) is widely used to evaluate forearm muscle function and predict hand grip forces; however, there is a lack of literature on its intra-session and inter-day reliability. The aim of this study was to determine reliability of surface EMG of finger and wrist flexor muscles across varying grip forces. Surface EMG was measured from six forearm flexor muscles of 23 healthy adults. Eleven of these subjects undertook inter-day test–retest. Six repetitions of five randomized isometric grip forces between 0% and 80% of maximum force (MVC) were recorded and normalized to MVC. Intra- and inter-day reliability were calculated through the intraclass correlation coefficient (ICC) and standard error of measurement (SEM).Normalized EMG produced excellent intra-session ICC of 0.90 when repeated measurements were averaged. Intra-session SEM was low at low grip forces, however, corresponding normalized SEM was high (23–45%) due to the small magnitude of EMG signals. This may limit the ability to evaluate finer forearm muscle function and hand grip forces in daily tasks. Combining EMG of functionally related muscles improved intra-session SEM, improving within-subject reliability without taking multiple measurements. Removing and replacing electrodes inter-day produced poor ICC (ICC < 0.50) but did not substantially affect SEM.     

Bit-mapped somatosensory evoked potentials and muscular reflex responses in man: comparative analysis in different experimental protocols

Bit-colour maps of somatosensory evoked potentials (SEPs) and muscular responses from forearm and hand muscles were simultaneously recorded after median nerve stimulation. Subjects were asked either to relax totally (A), or to contract the examined muscle continuously and isometrically at 10–20% (B) and 80–100% (C) of the maximal strength. Isotonic contractions ipsilateral (D) and contralateral to the stimulus (E) were also examined. Both SEPs and EMG responses were elicited by individual near-motor threshold pulses delivered at 0.2/sec to the median nerve at the elbow. SEPs were maximal in amplitude during complete relaxation, whilst all the components following the parietal N20 were depressed by muscle contraction. Such decrements affected predominantly the parietal and frontal peaks of positive polarity during condition B, whilst the frontal negative component (wave N30) dropped remarkably in conditions C and D. Early EMG responses (V1 = spinal circuitry) were usually absent in condition A; they were present together with later components (= V2 possibly long-loop, transcortical circuitry) in C and D, whilst they were alone recordable in B and E. The amplitudes of the frontal wave N30 in SEPs and of V2 in LLRs were inversely correlated. This observation is consistent with the hypothesis that a change in the reactivity of the sensorimotor brain areas to afferent impulses is coupled to LLR elicitation in forearm and hand muscles.     

Reflex control of posterior shoulder muscles from arm afferents in healthy people

In order to position the hand during functional tasks, control of the shoulder is required. Heteronymous reflexes from the upper limb to shoulder muscles are used to assist in this control. To investigate this further, the radial and ulnar nerves were stimulated at elbow level whilst surface electromyographic activity of posterior deltoid, infraspinatus and latissimus dorsi muscles were recorded. In addition, the cutaneous branch of the radial nerve and the skin of the fifth digit were stimulated in order to investigate any cutaneous contribution to reflex activity. Reflexes were evoked in all three of these shoulder muscles from hand and/or forearm afferents. However, the reflexes differed; whereas both excitatory and inhibitory reflexes were evoked in posterior deltoid and infraspinatus, the reflexes in latissimus dorsi were mainly excitatory. Cutaneomuscular reflexes were seldom evoked here, but when they were present they were generally evoked at longer latencies than the reflexes evoked by mixed nerve stimulation. The results suggest a role for reflexes originating from the forearm and/or hand in the control of the shoulder.     

Daily muscle vibration amelioration of neural impairments of the soleus muscle during 2 weeks of immobilization

V-wave, F wave and H-reflex responses of soleus were used to determine neural adaptations to 2-week immobilization and whether muscle vibration intervention during immobilization would attenuate the negative adaptations induced by immobilization. Thirty subjects were divided into the ankle immobilization group and the immobilization with muscle vibration group. Mechanical vibrations with constant low amplitude (0.3 mm) were applied (12 × 4 min daily) with a constant frequency of 100 Hz on the soleus muscle of the subjects in vibration group during the ankle immobilization period. Soleus maximal M-wave (Mmax) and H-reflex (Hmax) were evoked at rest. F-wave was recorded by supramaximal stimulation delivered at rest and V-wave during maximum voluntary contraction (MVC). The EMG during MVC was represented by its root-mean-square (RMS) value. Each subject was examined before and after 2 weeks of immobilization. Results showed that following 2 weeks of immobilization, Mmax, Hmax and F wave all did not change with immobilization in either group (P > 0.05). After 2 weeks of immobilization, significant reductions in V/Mmax (of 30.78%) (P < 0.01) and EMG RMS (24.82%) (P < 0.001) were found in the immobilization group. However, no significant changes occurred in the immobilization with muscle vibration group. Such findings suggested that 2 weeks of immobilization resulted in neural impairments as evidenced by the reduction in EMG and V wave, and that such decrease was prevented by the intervention of muscle vibration during the immobilization period.     

Force-length, torque-angle and EMG-joint angle relationships of the human in vivo biceps brachii

The relationships of EMG and muscle force with elbow joint angle were investigated for muscle modelling purposes. Eight subjects had their arms fixed in an isometric elbow jig where the biceps brachii was electrically stimulated (30 Hz) and also in maximum voluntary contraction (MVC). Biceps EMG and elbow torque transduced at the wrist were recorded at 0.175 rad intervals through 1.75 rad of elbow extension. The results revealed that while the torque-length relationship displayed the classic inverted U pattern in both evoked and MVC conditions, the force-length relationship displayed a monotonically increasing pattern. Analyses of variance of the EMG data showed that there were no significant changes in the EMG amplitudes for the different joint angles during evoked or voluntary contractions. The result also showed that electrical stimulation can effectively isolated the torque-angle and force-length relationships of the biceps brachii and that the myoelectric signal during isometric contraction is uniform regardless of the length of the muscle or the joint angle.     

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

Percutaneous electrical nerve stimulation is a non-invasive method commonly used to evaluate neuromuscular function from brain to muscle (supra-spinal, spinal and peripheral levels). The present protocol describes how this method can be used to stimulate the posterior tibial nerve that activates plantar flexor muscles. Percutaneous electrical nerve stimulation consists of inducing an electrical stimulus to a motor nerve to evoke a muscular response. Direct (M-wave) and/or indirect (H-reflex) electrophysiological responses can be recorded at rest using surface electromyography. Mechanical (twitch torque) responses can be quantified with a force/torque ergometer. M-wave and twitch torque reflect neuromuscular transmission and excitation-contraction coupling, whereas H-reflex provides an index of spinal excitability. EMG activity and mechanical (superimposed twitch) responses can also be recorded during maximal voluntary contractions to evaluate voluntary activation level. Percutaneous nerve stimulation provides an assessment of neuromuscular function in humans, and is highly beneficial especially for studies evaluating neuromuscular plasticity following acute (fatigue) or chronic (training/detraining) exercise.     

Neuromuscular efficiency of the triceps surae in induced and voluntary contractions: morning and evening evaluations

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NMEind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (PtMmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value -10.7 +/- 5.5%, p < 0.05), whereas PtMmax was not significantly modified. NMEind (PtMmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 +/- 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (PtTmax), and the corresponding NME (NMEreflex = PtTmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 +/- 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NMEMvc was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 +/- 13.9%, p <0.05) and a trend for a lower NME25%MVC in the evening was observed, a result probably representative of a higher muscle fatigue state in the evening. Finally, to test the muscle capacities during a natural task, a NME index was calculated during a drop jump (DJ). The NMEDJ was defined as the ratio between jump height and mean amplitude of TS EMG (% of TS Mmax) between the drop and the jump. Both jump height and NMEDJ were significantly higher in the evening (mean gains of 10.9 +/- 4.5% and 15.7 +/- 7.4%, respectively, p <0.05). In conclusion, daytime changes in the efficiency of postural muscles seem to depend on both peripheral and central mechanisms. According to the experimental conditions, NME of the postural muscle could increase, remain constant, or even decrease in the evening, and this result may reflect reverse effects of better contractile capacities and higher fatigue state.     

A motion of forearm supination with maintenance of elbow flexion produced by electrical stimulation to two elbow flexors in humans.

Motions of the forearm induced by electrical stimulation to two elbow flexors (brachioradialis: BR, biceps brachii: BB) were examined in five healthy human subjects. Stainless steel wire electrodes were implanted percutaneously into each motor point of the muscles. The muscles were stimulated separately with a computer-controlled multi-channel stimulator. The motions were taken with a digital video system. Angular changes of the motions in elbow flexion/extension and forearm pronation/supination were measured. Electromyograms (EMG) of BR, BB, and the triceps brachii (TB) were recorded. Electrical stimulation to BR induced a motion of flexion and that to BB motions of flexion and supination. The stimulation to BR with an adequate intensity provided holding of flexion with the prone forearm in all the subjects. In this situation, additional stimulation to BB resulted in motions of flexion and supination. However, the additional stimulation accompanied with a decrease of the stimulation intensity for BR provided a motion of supination with maintenance of the flexion in all the subjects. Since during the stimulation BR, BB, and TB showed no voluntary contraction in EMG, it is suggested that modulation of contraction between BR and BB by the stimulation can produce force in supination with keeping constant force in flexion to support the weight below the elbow.     

Nerve,spinal cord and brain somatosensory evoked responses: a comparative study during electrical and magnetic peripheral nerve stimulation

Somatosensory evoked potentials (SEPs) and compound nerve action potentials (cNAPs) have been recorded in 15 subjects during electrical and magnetic nerve stimulation. Peripheral records were gathered at Erb's point and on nerve trunks at the elbow during median and ulnar nerve stimulation at the wrist. Erb responses to electrical stimulation were larger in amplitude and shorter in duration than the magnetic ones when ‘electrical’ and ‘magnetic’ compound muscle action potentials (cMAPs) of comparable amplitudes were elicited. SEPs were recorded respectively at Cv7 and on the somatosensory scalp areas contra- and ipsilateral to the stimulated side. SEPs showed a statistically significant difference in amplitude only for the brachial plexus response and for the ‘cortical’ N20-P25 complex; differences were not found between the magnetic and electrical central conduction times (CCTs) or for the peripheral nerve response latencies. Magnetic stimulation preferentially excited the motor and proprioceptive fibres when the nerve trunks were stimulated at motor threshold intensities.     

Biomechanical outcomes and neural correlates of cutaneous reflexes evoked during rhythmic arm cycling

During walking cutaneous stimulation of the foot yields neural and mechanical reflexes that serve a functional purpose to correct or assist the ongoing movement. Concurrently, while cutaneous stimulation of the hand during rhythmic arm movement parallel the neural responses observed in the legs, studies of rhythmic arm movement have only limited mechanical measurements. Therefore it is difficult to determine whether reflex responses in the arms during rhythmic arm movement serve a functional purpose similar to those seen in the lower limbs. The purpose of this study was to explore the mechanical outcomes of stimulating a cutaneous nerve innervating the hand during arm cycling. We hypothesized that there would be measurable mechanical effects to cutaneous stimulation during arm cycling that function to correct or assist the task of arm cycling. Specifically, kinetic responses measured at the handle would be considered assistive if they were tangential to the arm cycling path in the direction of forward progression. Also, limb kinematic responses would be considered corrective if they allowed limb movement that would result in removal of the limb from stimulus while not altering the kinetic profile at the handle necessary for arm cycling progression. Participants performed seated arm cycling while EMG was recorded from the arm and trunk muscles, kinematic data was recorded from the right arm, and kinetic data was recorded from the handle. Cutaneous reflexes were evoked by stimulating the superficial radial nerve. The results show that there are observable mechanical responses to cutaneous stimulation of the hand during arm cycling. Subjects responded to cutaneous stimulation of the hand during arm cycling with significant changes in backward and lateral forces at the handle as well as wrist abduction/adduction and wrist flexion/extension kinematics. These responses, related to the task and phase of movement, are consistent with the anatomical location of the stimulus and are correlated to the neural responses. Therefore, these responses are comparable to functionally relevant responses in the legs during rhythmic movement. However, while there is a single observation of a kinematic corrective strategy, the kinetics measured at the handle are not tangential to the arm cycling path and therefore not considered an assistive response. Therefore, unlike the observations in the lower limbs, the mechanical responses during arm cycling are not clearly related to the functional context of the ongoing task.     

Neuromuscular Efficiency of the Triceps Surae in Induced and Voluntary Contractions: Morning and Evening Evaluations

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NME_ind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (Pt_Mmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value ?10.7 ± 5.5%, p < 0.05), whereas Pt_Mmax was not significantly modified. NME_ind (Pt_Mmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 ± 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (Pt_Tmax), and the corresponding NME (NME_reflex = Pt_Tmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 ± 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NME_MVC was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 ± 13.9%, p < 0.05) and a trend for a lower NME_25%MVC in the evening was observed, a result probably representative of a higher muscle fatigue state in the evening. Finally, to test the muscle capacities during a natural task, a NME index was calculated during a drop jump (DJ). The NME_DJ was defined as the ratio between jump height and mean amplitude of TS EMG (% of TS Mmax) between the drop and the jump. Both jump height and NME_DJ were significantly higher in the evening (mean gains of 10.9 ± 4.5% and 15.7 ± 7.4%, respectively, p < 0.05). In conclusion, daytime changes in the efficiency of postural muscles seem to depend on both peripheral and central mechanisms. According to the experimental conditions, NME of the postural muscle could increase, remain constant, or even decrease in the evening, and this result may reflect reverse effects of better contractile capacities and higher fatigue state.     

Difference in aftereffects following prolonged Achilles tendon vibration on muscle activity during maximal voluntary contraction among plantar flexor synergists.

It has been suggested that a suppression of maximal voluntary contraction (MVC) induced by prolonged vibration is due to an attenuation of Ia afferent activity. The purpose of the present study was to test the hypothesis that aftereffects following prolonged vibration on muscle activity during MVC differ among plantar flexor synergists owing to a supposed difference in muscle fiber composition. The plantar flexion MVC torque and surface electromyogram (EMG) of the medial head of gastrocnemius (MG), the lateral head of gastrocnemius (LG), and the soleus (Sol) were recorded in 13 subjects before and after prolonged vibration applied to the Achilles tendon at 100 Hz for 30 min. The maximal H reflexes and M waves were also determined from the three muscles, and the ratio between H reflexes and M waves (H/Mmax) was calculated before and after the vibration. The MVC torque was decreased by 16.6 +/- 3.7% after the vibration (P < 0.05; ANOVA). The H/Mmax also decreased for all three muscles, indicating that Ia afferent activity was successfully attenuated by the vibration in all plantar flexors. However, a reduction of EMG during MVC was observed only in MG (12.7 +/- 4.0%) and LG (11.4 +/- 3.9%) (P < 0.05; ANOVA), not in Sol (3.4 +/- 3.0%). These results demonstrated that prolonged vibration-induced MVC suppression was attributable mainly to the reduction of muscle activity in MG and LG, both of which have a larger proportion of fast-twitch muscle fibers than Sol. This finding suggests that Ia-afferent activity that reinforces the recruitment of high-threshold motor units is necessary to enhance force exertion during MVC.     

Co-contraction of the pronator teres and extensor carpi radialis during wrist extension movements in humans.

In order to elucidate the functional significance of excitatory spinal reflex arcs (facilitation) between musculus (M.) pronator teres (PT) and M. extensor carpi radialis (ECR, longus: ECRL, brevis: ECRB) in humans, activities of the muscles were studied with electromyography (EMG) and electrical neuromuscular stimulation (ENS). In EMG study, activities of PT, ECRL, ECRB, and M. flexor carpi radialis during repetitive static (isometric) wrist extension and a series of a dynamic motion of wrist flexion/extension in the prone, semiprone, and supine positions of the forearm were recorded in 12 healthy human subjects. In the prone, semiprone, and supine positions, PT and ECR showed parallel activities during the static extension in all, eight, and eight subjects, respectively, and at the extension phase during the dynamic motion in all, eight and five subjects, respectively. These findings suggest that co-contraction of PT and ECR occurs during wrist extension movements at least with the prone forearm. The facilitation must be active during the co-contraction. In ENS study, ENS to PT was examined in 11 out of the 12 and that to ECRL was in the 12 subjects. Before ENS, the forearm was in the prone, semiprone, and supine positions. In all the subjects, ENS to PT induced a motion of forearm pronation to the maximum pronation. ENS to ECRL induced motions of wrist extension to the maximum extension and abduction (radial flexion) to 5-20 degrees of abduction regardless of the positions of the forearm. Moreover, it induced 30-80 degrees supination of the forearm from the prone position. Consequently, combined ENS to PT and ECRL resulted in motions of the extension and abduction while keeping the maximum pronation. These findings suggest that the co-contraction of PT and ECR during wrist extension movements occurs to prevent supinating the forearm. Forearm supination from the prone position should be added to one of the actions of ECRL.     

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise.

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.     

Similar response of agonist and antagonist muscles after eccentric exercise revealed by electromyography and mechanomyography.

The purpose of this study was to investigate the influence of eccentric contractions (ECC) on the biceps (BB) and triceps brachii (TB) muscles during maximal voluntary contraction (MVC) of elbow flexors using electrical (EMG) and mechanomyographical activities (MMG). Each of 18 male students performed 25 submaximal contractions (50% MVC) of the elbow flexors. Root mean square amplitude (RMS) and median frequency (MDF) were calculated for the EMG and MMG signals recorded during MVC. All measurements were taken before, immediately after, 24, 48, 72, and 120 h post-ECC from the BB and TB muscles. MVC was reduced by 34% immediately after exercise and did not return to the resting value within 120 h (P0.05). The EMG MDF decreased significantly (P< or =0.05) in both muscles after ECC. The MMG RMS at 24h, 48, 72 and 120 h post-ECC was significantly lower compared to that recorded immediately after ECC in both muscles (P< or =0.05). The present research showed that (i) there were similar changes in electrical and mechanical activities during MVC after submaximal ECC in agonist and antagonist muscles suggesting a common drive controlling the agonist and antagonist motoneuron pool, (ii) the ECC induced different changes in EMG than in MMG immediately after ECC and during 120 h of recovery that suggested an increased tremor and contractile impairments, i.e., reduced rate of calcium release from the sarcoplasmic reticulum (acute effect), and changes in motor control mechanisms of agonist and antagonist muscles, and increased muscle stiffness (chronic effect).     

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.     

Soleus H-reflex dynamics during fast plantarflexion in humans.

The relationship between the size of the soleus (Sol) Hoffmann (H-) reflex and the level of background (BG) electromyographic (EMG) activity was examined during plantarflexing at different force levels. The experiments were carried out on seven healthy male subjects aged 20-37 years. The subjects were asked to perform fast plantarflexion under a reaction-time condition. The amounts of contraction force were 10, 20, 50 and 80% of maximum voluntary contraction (MVC). Since the maximum size of the M-wave (Mmax) changed systematically during the plantarflexion, we tried to maintain the size of the reference M-wave, an indicator of the efficiency of the electrical stimulation, at a constant value (20% of Mmax) throughout the experiment. The size of the H-reflex was rapidly increased at the very beginning of the movement, and then it tended to decrease in the later phase of the movement. Consequently, even with the same level of BG EMG, the size of the H-reflex was always larger in the early rising phase of the EMG activity than in the later falling phase. The maximum size of the H-reflex was poorly correlated with the force exerted. In contrast, the size of the F-response was proportional to the force exerted. The non-linear relationship between the size of the H-reflex and the BG EMG suggests that the level of the presynaptic inhibition onto Ia terminals was modified depending on the required force level and during the course of the movement.     

Variations in the relationship between the frequency content of EMG signals and the rate of torque development in voluntary and elicited contractions.

Our purpose was to characterize the relationship between EMG mean power frequency (MPF) or median frequency (MF) and rate of torque development in voluntary ballistic and electrically elicited isometric contractions. Twenty-three healthy adults participated in two sets of experiments performed on elbow flexor muscles. For Experiment 1, subjects were asked to generate voluntary ballistic contractions by reaching four different target torque levels (20, 40, 60 and 100% of the maximal voluntary contraction (MVC)) as fast as they could. For Experiment 2, electrical (M-waves) and mechanical (twitches) responses to electrical stimulation of the nerves supplying the biceps brachii and brachioradialis muscles were recorded with the subjects at rest and with a background isometric contraction of 15% MVC. MPF, MF and rate of torque development (% MVC/s) were calculated for both voluntary and elicited contractions. Significant positive correlations were observed between MPF and rate of torque development for the voluntary contractions, whereas significant negative correlations were observed between the two variables for elicited contractions. This suggests that factors other than muscle fiber composition influence the frequency content of EMG signals and/or the rate of torque development, and that the effect of these factors will vary between voluntary and elicited contractions.     

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography.

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC10 min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC10 min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC10 min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067+/-0.028 mV before 10%MVC10 min to 0.107+/-0.049 and 0.087+/-0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054+/-0.039 ms(-2) to 0.133+/-0.104 and 0.127+/-0.099 ms(-2), respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.     

Neural Mechanisms Influencing Interlimb Coordination during Locomotion in Humans: Presynaptic Modulation of Forearm H-Reflexes during Leg Cycling

Presynaptic inhibition of transmission between Ia afferent terminals and alpha motoneurons (Ia PSI) is a major control mechanism associated with soleus H-reflex modulation during human locomotion. Rhythmic arm cycling suppresses soleus H-reflex amplitude by increasing segmental Ia PSI. There is a reciprocal organization in the human nervous system such that arm cycling modulates H-reflexes in leg muscles and leg cycling modulates H-reflexes in forearm muscles. However, comparatively little is known about mechanisms subserving the effects from leg to arm. Using a conditioning-test (C-T) stimulation paradigm, the purpose of this study was to test the hypothesis that changes in Ia PSI underlie the modulation of H-reflexes in forearm flexor muscles during leg cycling. Subjects performed leg cycling and static activation while H-reflexes were evoked in forearm flexor muscles. H-reflexes were conditioned with either electrical stimuli to the radial nerve (to increase Ia PSI; C-T interval  = 20 ms) or to the superficial radial (SR) nerve (to reduce Ia PSI; C-T interval  = 37–47 ms). While stationary, H-reflex amplitudes were significantly suppressed by radial nerve conditioning and facilitated by SR nerve conditioning. Leg cycling suppressed H-reflex amplitudes and the amount of this suppression was increased with radial nerve conditioning. SR conditioning stimulation removed the suppression of H-reflex amplitude resulting from leg cycling. Interestingly, these effects and interactions on H-reflex amplitudes were observed with subthreshold conditioning stimulus intensities (radial n., ∼0.6×MT; SR n., ∼ perceptual threshold) that did not have clear post synaptic effects. That is, did not evoke reflexes in the surface EMG of forearm flexor muscles. We conclude that the interaction between leg cycling and somatosensory conditioning of forearm H-reflex amplitudes is mediated by modulation of Ia PSI pathways. Overall our results support a conservation of neural control mechanisms between the arms and legs during locomotor behaviors in humans.