Activation among the elbow flexor muscles differs when maintaining arm position during a fatiguing contraction.

Twenty-four men (n = 11) and women (n = 13) supported an inertial load equivalent to 20% of the maximum voluntary contraction force with the elbow flexor muscles for as long as possible while maintaining a constant elbow angle at 90 degrees. Endurance time did not differ on the three occasions that the task was performed (320 +/- 149 s; P > 0.05), and there was no difference between women (360 +/- 168 s) and men (273 +/- 108 s; P = 0.11). The rate of increase in average electromyogram (EMG) for the elbow flexor muscles was similar across sessions (P > 0.05). However, average EMG during the fatiguing task increased for the long head of biceps brachii, brachioradialis, and brachialis (P < 0.05) but not for the short head of biceps brachii. Furthermore, the average EMG for the brachialis was greater at the start and end of the contraction compared with the other elbow flexor muscles. The rate of bursts in EMG activity increased during the fatiguing contraction and was greater in brachialis (1.0 +/- 0.2 bursts/min) compared with the other elbow flexor muscles (0.5 +/- 0.1 bursts/min). The changes in the standard deviation of acceleration, mean arterial pressure, and heart rate during the fatiguing contractions were similar across sessions. These findings indicate that the EMG activity, which reflects the net excitatory and inhibitory input received by the motoneurons in the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, these results contrast those from a previous study (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003) when the goal of the isometric contraction was to maintain a constant force. These results, from a series of studies on the elbow flexor muscles, indicate that the type of load supported during the fatiguing contraction influences the extent to which endurance time can change with repeat performances of the task.     

EMG and MMG of agonist and antagonist muscles as a function of age and joint angle.

The aim of this study was to determine the effect of elbow joint position on electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in young and old women. Surface EMG and MMG were recorded from the triceps and biceps brachii, and brachioradialis muscles during isometric elbow extensions in young and old women. The measurements were carried out at an optimal joint angle (A(o)), as well as at smaller (A(s) = A(o) - 30 degrees ) and larger (A(l) = A(o) + 30 degrees ) angles. The normalized to force EMG amplitude (RMS-EMG/F) was smaller in old women compared to young in all muscles. The RMS-EMG/F of the triceps brachii muscle was not affected by muscle length while that of the biceps brachii and brachioradialis muscles increased at shortest muscle length in both groups. The normalized to force MMG amplitude (RMS-MMG/F) was smaller in old than in young in the triceps brachii muscle only. There was an increase in RMS-MMG/F with triceps brachii and biceps brachii muscle shortening in both groups, and in the brachioradialis muscle -- in young only. Compared to young, older women exhibited a bigger force fluctuation during maximum voluntary contraction, but these did not contribute significantly to the RMS-MMG. Skinfold thickness accounted for the RMS-EMG/F and RMS-MMG/F differences seen between old and young women in the biceps brachii muscle only. It is concluded that, the EMG and MMG response to muscles length change in agonist and antagonist muscles is generally similar in old and young women but the optimal angle shifts toward a bigger value in older women.     

Intramuscular and surface electromyogram changes during muscle fatigue

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.     

Changes in muscle activation can prolong the endurance time of a submaximal isometric contraction in humans.

Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximal voluntary contraction (MVC) force on three occasions. Endurance time for session 3 [1,718 +/- 1,189 (SD) s] was longer than for session 1 (1,225 +/- 683 s) and session 2 (1,410 +/- 977 s). Five men and four women increased endurance time between session 1 and 3 by 60 +/- 28% (responders), whereas two men and three women did not (-3 +/- 11%; nonresponders). The MVC force was similar for the responders and nonresponders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the electromyogram (EMG) amplitude and number of bursts during the fatiguing contractions. The responders achieved a similar level of EMG at exhaustion but a reduced rate of increase in the EMG across sessions. The rate of increase in EMG across sessions declined for the nonresponders, but it remained greater than that of the responders. The increase in burst rate during the contractions declined across sessions with a negative relation between burst rate and endurance time (r = -0.42). Normalized force fluctuations increased during the fatiguing contractions, and there was a positive relation (r = 0.60) between the force fluctuations and burst rate. Changes in mean arterial pressure and heart rate during the fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who increased the endurance time of a submaximal contraction across three sessions did so by altering the level and pattern of muscle activation.     

Experimental muscle pain increases mechanomyographic signal activity during sub-maximal isometric contractions.

This study was designed to investigate the local effect of experimental muscle pain on the MMG and the surface EMG during a range of sub-maximal isometric contractions. Muscle pain was induced by injections of hypertonic saline into the biceps brachii muscle in 12 subjects. Injections of isotonic saline served as a control. Pain intensity and location, MMG and surface EMG from the biceps brachii were assessed during static isometric (0%, 10%, 30%, 50% and, 70% of the maximal voluntary contraction) and ramp isometric (0-50% of the maximal voluntary contraction) elbow flexions. MMG and surface EMG signals were analyzed in the time and frequency domain. Experimentally induced muscle pain induced an increase in root mean square values of the MMG signal while no changes were observed in the surface EMG. Most likely this increase reflects changes in the mechanical contractile properties of the muscle and indicates compensatory mechanisms, i.e. decreased firing rate and increased twitch force to maintain a constant force output in presence of experimental muscle pain. Under well-controlled conditions, MMG recordings may be more sensitive than surface EMG recordings and clinically useful for detecting non-invasively increased muscle mechanical contributions during muscle pain conditions.     

Non-invasive characterization of single motor unit electromyographic and mechanomyographic activities in the biceps brachii muscle.

The aim of the study was to investigate amplitude and frequency content of single motor unit (MU) electromyographic (EMG) and mechanomyographic (MMG) responses. Multi-channel surface EMG and MMG signals were detected from the dominant biceps brachii muscle of 10 volunteers during isometric voluntary contractions at 20%, 50%, and 80% of the maximal voluntary contraction (MVC) force. Each contraction was performed three times in the experimental session which was repeated in three non-consecutive days. Single MU action potentials were identified from the surface EMG signals and their times of occurrence used to trigger the averaging of the MMG signal. At each contraction level, the MUs with action potentials of highest amplitude were identified. Single MU EMG and MMG amplitude and mean frequency were estimated with normalized standard error of the mean within subjects (due to repetition of the measure in different trials and experimental sessions) smaller than 15% and 7%, respectively, in all conditions. The amplitude of the action potentials of the detected MUs increased with increasing force (mean +/- SD, 244 +/- 116 microV at 20% MVC, and 1426 +/- 638 microV at 80% MVC; P < 0.001) while MU MMG amplitude increased from 20% to 50% MVC (40.5 +/- 20.9 and 150 +/- 88.4 mm/s(2), respectively; P<0.001) and did not change significantly between 50% and 80% MVC (129 +/ -82.7 mm/s(2) at 80% MVC). MU EMG mean frequency decreased with contraction level (20% MVC: 97.2 +/- 13.9 Hz; 80% MVC: 86.2 +/- 11.4 Hz; P < 0.001) while MU MMG mean frequency increased (20% MVC: 33.2 +/- 6.8 Hz; 80% MVC: 40.1 +/- 6.1 Hz; P < 0.001). EMG peak-to-peak amplitude and mean frequency of individual MUs were not correlated with the corresponding variables of MMG at any contraction level.     

Muscle synergy in isometric flexion of the elbow]

A technique was developed for calculating the torque generated by two individual muscles (biceps brachii and brachioradialis) that contribute to the isometric flexion of the elbow. The external torque is the sum of individual torques which are unknown. Each individual torque (CB or CBR) can be related to the corresponding integrated surface EMG (QB or QBR) by means of coefficients (pB or PBR). A block of several equations C = pB QB + pBR QBR is obtained by exploring several experimental conditions. In these conditions, isometric flexion efforts of the elbow were associated to isometric efforts of supination or pronation so as to vary integrated EMG by reciprocal inhibition. By means of a least square method it was possible to know the coefficients PB and PBR. With these coefficients, it was possible to calculate the individual torques generated by the biceps brachii and brachioradialis muscles in each experimental condition.     

Spinal reflexes and coactivation of ankle muscles during a submaximal fatiguing contraction.

This study examined the involvement of spinal mechanisms in the control of coactivation during a sustained contraction of the ankle dorsiflexors at 50% of maximal voluntary contraction. Changes in the surface electromyogram (EMG) of the tibialis anterior and of two antagonist muscles, the soleus and lateral gastrocnemius, were investigated during and after the fatigue task. Concurrently, the compound action potential (M-wave) and the Hoffmann reflex of the soleus and lateral gastrocnemius were recorded. The results showed that the torque of the ankle dorsiflexors and the average EMG of the tibialis anterior during maximal voluntary contraction declined by 40.9 +/- 17.7% (mean +/- SD; P < 0.01) and 37.0 +/- 19.9% (P < 0.01), respectively, at task failure. During the submaximal fatiguing contraction, the average EMG of both the agonist and antagonist muscles increased, leading to a nearly constant ratio at the end of the contraction when normalized to postfatigue values. In contrast to the monotonic increase in average EMG of the antagonist muscles, the excitability of their spinal reflex pathways exhibited a biphasic modulation. The amplitude of the Hoffman reflexes in the soleus and lateral gastrocnemius increased to 147.5 +/- 52.9% (P < 0.05) and 166.7 +/- 74.9% (P < 0.01), respectively, during the first 20% of the contraction and then subsequently declined to 66.3 +/- 44.8 and 74.4 +/- 44.2% of their initial values. In conclusion, the results show that antagonist coactivation did not contribute to task failure. The different changes in voluntary EMG activity and spinal reflex excitability in the antagonist muscles during the fatiguing contraction support the concept that the level of coactivation is controlled by supraspinal rather than spinal mechanisms. The findings indicate, however, that antagonist coactivation cannot simply be mediated by a central descending "common drive" to the motor neuron pools of the agonist-antagonist muscle pairs. Rather, they suggest a more subtle regulation of the drive, possibly through presynaptic mechanisms, to the motoneurons that innervate the antagonist muscles.     

Increase in electromyogram low-frequency power in nonfatigued contracting skeletal muscle

We studied the relationship between changing elbow joint angle and the power spectral density of the biceps brachii muscle electromyogram (EMG) during submaximal isometric contractions. For this purpose, we recorded the EMG of the biceps brachii muscle with surface electrodes in 13 subjects. Each subject held a 2.8-kg weight and contracted the biceps isometrically for 30 s at one of two lengths. The length of the muscle was changed by flexing the forearm toward the upper arm to form an angle of 135 degrees (L1) or 45 degrees (L2). We found that the mean centroid frequency (fc) of the EMG power spectral density was 26% lower at L1 than at L2 (P less than 0.01). For each subject there was no significant change in fc during the isometric contraction at either angle. In addition, in nine subjects who sustained fatiguing contractions of the biceps with a 6-kg load, fc decreased by 15% (P less than 0.025). These data suggest that a change in the length at which a muscle contracts isometrically can alter or induce indirectly an alteration in the frequency content of its EMG. This finding may have important implications for the assessment of respiratory muscle EMG especially during loaded breathing.     

Activation varies among the knee extensor muscles during a submaximal fatiguing contraction in the seated and supine postures.

Ten young men sustained an isometric contraction of the knee extensor muscles at 20% of the maximum voluntary contraction (MVC) torque on three separate occasions in a seated posture. Subjects performed an isometric knee extension contraction on a fourth occasion in a supine posture. The time to task failure for the seated posture was similar across sessions (291 +/- 84 s; P > 0.05), and the MVC torque was similarly reduced across sessions after the fatiguing contraction (42 +/- 12%). The rate of increase in electromyograph (EMG) activity (%MVC) and torque fluctuations during the fatiguing contractions were similar across sessions. However, the rate of increase in EMG differed among the knee extensor muscles: the rectus femoris began at a greater amplitude (31.5 +/- 11.0%) compared with the vastus lateralis and vastus medialis muscles (18.8 +/- 5.3%), but it ended at a similar value (45.4 +/- 3.1%). The time to task failure and increase in EMG activity were similar for the seated and supine tasks; however, the reduction in MVC torque was greater for the seated posture. These findings indicate that the time to task failure for the knee extensor muscles that have a common tendon insertion did not alter over repeat sessions as had been observed for the elbow flexor muscles (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003).     

Fatigue and muscle activation during submaximal elbow flexion in children with cerebral palsy

The purpose of this study was to investigate whether children with cerebral palsy (CP), like typically developing peers, would compensate for muscle fatigue by recruiting additional motor units during a sustained low force contraction until task failure.Twelve children with CP and 17 typically developing peers performed one submaximal isometric elbow flexion contraction until the task could no longer be sustained at on average 25% (range 10–35%) of their maximal voluntary torque. Meanwhile surface electromyography (EMG) was measured from the biceps brachii and triceps brachii, and acceleration variations of the forearm were detected by an accelerometer. Slopes of the change in EMG amplitude and median frequency and accelerometer variation during time normalised to their initial values were calculated.Strength and time to task failure were similar in both groups. Children with CP exhibited a lower increase in EMG amplitude of the biceps brachii and triceps brachii during the course of the sustained elbow flexion task, while there were no significant group differences in median frequency decrease or acceleration variation increase. This indicates that children with CP do not compensate muscle fatigue with recruitment of additional motor units during sustained low force contractions.     

Effect of elbow joint angle on force–EMG relationships in human elbow flexor and extensor muscles

The purpose of this study was to examine the effect of joint angle on the relationship between force and electromyogram (EMG) amplitude and median frequency, in the biceps, brachioradialis and triceps muscles. Surface EMG were measured at eight elbow angles, during isometric flexion and extension at force levels from 10% to 100% of maximum voluntary contraction (MVC). Joint angle had a significant effect on MVC force, but not on MVC EMG amplitude in all of the muscles examined. The median frequency of the biceps and triceps EMG decreased with increasing muscle length, possibly due to relative changes in electrode position or a decrease in muscle fibre diameter. The relationship between EMG amplitude and force, normalised with respect to its maximum force at each angle, did not vary with joint angle in the biceps or brachioradialis muscles over all angles, or in the triceps between 45° and 120° of flexion. These results suggest that the neural excitation level to each muscle is determined by the required percentage of available force rather than the absolute force required. It is, therefore, recommended that when using surface EMG to estimate muscle excitation, force should be normalised with respect to its maximum value at each angle.     

Influence of aging on sex differences in muscle fatigability.

The purpose of this study was to compare time to task failure for a sustained isometric contraction performed at a submaximal intensity with elbow flexor muscles by young and old men and women. Twenty-seven young (14 men and 13 women, 18-35 yr) and 18 old (10 men and 8 women, 65-80 yr) adults sustained an isometric contraction at 20% of maximal voluntary contraction torque until target torque could no longer be achieved for > or = 5 s. Young adults were stronger than old adults (66.8 +/- 17.9 vs. 47.7 +/- 18.1 N x m, P < 0.05), and men were stronger than women (69.8 +/- 17.9 vs. 47.1 +/- 15.3 N x m, P < 0.05), with no interaction between age and sex (P > 0.05). Time to task failure was longer for old than for young adults (22.8 +/- 9.1 vs. 14.4 +/- 7.6 min, P < 0.05) and for young women than for young men (18.3 +/- 8.0 vs. 10.8 +/- 5.2, P < 0.05), but there was no difference between old women and men (21.3 +/- 10.7 and 24.1 +/- 8.0 min, respectively, P > 0.05) or between young women and old adults (P > 0.05). Mean arterial pressure, heart rate, average electromyographic (EMG) activity, and torque fluctuations of elbow flexor muscles increased during the fatiguing contraction (P < 0.05) for all subjects. Rates of increase in mean arterial pressure, heart rate, and torque fluctuations were greater for young men and old adults, with no differences between old men and women (P > 0.05). Similarly, the rate of increase in EMG activity was greater for young men than for the other three groups. EMG bursts were less frequent for old adults (P < 0.05) at the end of the fatiguing contraction, and this was accompanied by reduced fluctuations in torque. Consequently, time to task failure was related to target torque for young, but not old, adults, and differences in task duration were accompanied by parallel changes in the pressor response.     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Fatigability of the elbow flexor muscles for a sustained submaximal contraction is similar in men and women matched for strength.

The purpose of this study was to compare the time to task failure for a submaximal fatiguing contraction sustained with the elbow flexor muscles by men and women who were matched for strength (n = 20, 18-35 yr). The maximal torque exerted at the wrist was similar for the men and women [64.5 +/- 8.7 (SD) vs. 64.5 +/- 8.3 N.m; P > 0.05], which meant that the average torque exerted during the fatiguing contraction [20% of maximum voluntary contraction (MVC)] was similar for the two sexes. The time to task failure was similar for these strength-matched men and women (819 +/- 306 vs. 864 +/- 391 s; P > 0.05). The mean arterial pressure was similar at the beginning of the contraction for men (97 +/- 12 mmHg) and women (96 +/- 15 mmHg; P > 0.05) and at task failure (134 +/- 18 vs. 126 +/- 26 mmHg; P > 0.05, respectively). Furthermore, the increases in heart rate, torque fluctuations, and rating of perceived exertion during the fatiguing contraction were similar for the two sexes. However, the electromyogram (EMG) activity differed for the men and women: the rate of increase in the average of the rectified EMG (% peak MVC) for all the elbow flexor muscles was less for the women compared with the men (P < 0.05). Furthermore, the bursts of EMG activity for the elbow flexor muscles increased toward exhaustion for all subjects but at a greater rate for the women compared with the men (P < 0.05). The results indicate that strength-matched men and women experienced similar levels of muscle fatigue and cardiovascular adjustments during a sustained low-force isometric contraction, despite differences in the EMG activity for the two groups of subjects.     

Electromyographic instantaneous amplitude and instantaneous mean power frequency patterns across a range of motion during a concentric isokinetic muscle action of the biceps brachii.

The purpose of this study was to examine the electromyographic (EMG) instantaneous amplitude (IA) and instantaneous mean power frequency (IMPF) patterns for the biceps brachii muscle across a range of motion during maximal and submaximal concentric isokinetic muscle actions of the forearm flexors. Ten adults (mean +/- SD age = 22.0 +/- 3.4 years) performed a maximal and a submaximal [20% peak torque (PT)] concentric isokinetic forearm flexion muscle action at a velocity of 30 degrees s(-1). The surface EMG signal was detected from the biceps brachii muscle with a bipolar electrode arrangement, and the EMG IA and IMPF versus time relationships were examined for each subject using first- and second-order polynomial regression models. The results indicated that there were no consistent patterns between subjects for EMG IA or IMPF with increases in torque across the range of motion. Some of the potential nonphysiological factors that could influence the amplitude and/or frequency contents of the surface EMG signal during a dynamic muscle action include movement of the muscle fibers and innervation zone beneath the skin surface, as well as changes in muscle fiber length and the thickness of the tissue layer between the muscle and the recording electrodes. These factors may affect the EMG IA and IMPF patterns differently for each subject, thereby increasing the difficulty of drawing any general conclusions regarding the motor control strategies that increase torque across a range of motion.     

剪切波弹性成像技术对脑卒中偏瘫患者肌张力和肌肉硬度的评估价值研究

摘要 目的：研究应用剪切波弹性成像技术对脑卒中偏瘫患者肌张力、肌硬度进行评估的临床价值。方法：选取2019年3月到2021年2月在我院进行治疗的79例脑卒中偏瘫患者作为研究对象,应用超声仪检测所有研究对象健康侧（健侧）和患病侧（患侧）肱二头肌、肱肌和肱桡肌放松位和拉伸位下杨氏模量值,进行对比分析。结果：在放松位下,脑卒中偏瘫患者患侧肱二头肌和肱桡肌杨氏模量与健康侧肌肉相比无显著差异（P>0.05）,而患侧肱肌杨氏模量显著低于健侧（P<0.05）。在拉伸位下,脑卒中偏瘫患者患侧肱二头肌、肱肌和肱桡肌杨氏模量均显著高于健康侧肌肉（P<0.05）;脑卒中偏瘫患者放松位与拉伸位肱二头肌、肱肌和肱桡肌杨氏模量差值也均显著高于健康侧肌肉（P<0.05）。此外,不同改良Ashworth肌张力分级的脑卒中偏瘫患者患侧肱二头肌、肱肌和肱桡肌杨氏模量均存在显著差异（P<0.05）,并且患侧肱二头肌、肱肌和肱桡肌杨氏模量值随改良Ashworth肌张力分级升高而增加。结论：剪切波弹性成像技术可用于评估脑卒中偏瘫患者肌张力、肌硬度,以指导临床康复。     

Magnetic resonance imaging and electromyography as indexes of muscle function.

Electromyography (EMG) is commonly used to determine the electrical activity of skeletal muscle during contraction. To date, independent verification of the relationship between muscle use and EMG has not been provided. It has recently been shown that relaxation- (e.g., T2) weighted magnetic resonance images (MRI) of skeletal muscle demonstrate exercise-induced contrast enhancement that is graded with exercise intensity. This study was conducted to test the hypothesis that exercise-induced magnetic resonance (MR) contrast shifts would relate to EMG amplitude if both measures reflect muscle use during exercise. Both MRI and EMG data were collected for separate eccentric (ECC) and concentric (CON) exercise of increasing intensity to take advantage of the fact that the rate of increase and amplitude of EMG activity are markedly greater for CON muscle actions. Seven subjects 30 +/- 2 (SE) yr old performed five sets of 10 CON or ECC arm curls with each of four resistances representing 40, 60, 80, and 100% of their 10 repetition maximum for CON curls. There was 1.5 min between sets and 30 min between bouts (5 sets of 10 actions at each relative resistance). Multiple echo, transaxial T2-weighted MR images (1.5 T, TR/TE 2,000/30) were collected from a 7-cm region in the middle of the arm before exercise and immediately after each bout. Surface EMG signals were collected from both heads of the biceps brachii and the long head of the triceps brachii muscles. CON and ECC actions resulted in increased integrated EMG (IEMG) and T2 values that were strongly related (r = 0.99, P < 0.05) with relative resistance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationships between individual isometric muscle forces, EMG activity and joint torque in monkeys

The aim of the present work was to determine the EMG activity and the moment of force developed by the main elbow flexor muscles, and to establish on this basis the degree of their participation in isometric contractions performed at various positions of the elbow. This was achieved by recording the following biomechanical parameters: EMG and tensile stress (or force) from biceps brachii (BB) and brachioradialis (BR); EMG from brachialis; external resultant force (FE). There was: a linear or quadratic relationship between the integrated EMG from each muscle and FE; a linear relationship between the force produced by BB or BR and FE. The slope of these relationships depended on the elbow angle, except for that between BB force and FE. It is proposed that iEMG changes compensate for those of the force lever arm. It has been calculated that the contribution of BR to external torque decreased from the extension to flexion while that of BB increased from 70 degrees to 90 degrees and then decreased. How far these data can be extrapolated to man is a matter of discussion based on iEMG and anthropometrical data.     

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study. II. Motor unit firing rate and recruitment

The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%-30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%-30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.