Effects of mechanical assistance on muscle activity and motor performance during isometric elbow flexion

Mechanical assistance on joint movement is generally beneficial; however, its effects on cooperative performance and muscle activity needs to be further explored. This study examined how motor performance and muscle activity are altered when mechanical assistance is provided during isometric force control of ramp-down and hold phases. Thirteen right-handed participants (age: 24.7 ± 1.8 years) performed trajectory tracking tasks. Participants were asked to maintain the reference magnitude of 47 N (REF) during isometric elbow flexion. The force was released to a step-down magnitude of either 75% REF or 50% REF and maintained, with and without mechanical assistance. The ramp-down durations of force release were set to 0.5, 2.5, or 5.0 s. Throughout the experiment, we measured the following: (1) the force output using load cells to compute force variability and overshoot ratio; (2) peak perturbation on the elbow movement using an accelerometer; (3) the surface electromyography (sEMG) from biceps brachii and triceps brachii muscles; and (4) EMG oscillation from the biceps brachii muscle in the bandwidth of 15–45 Hz. Our results indicated that mechanical assistance, which involved greater peak perturbation, demonstrated lower force variability than non-assistance (p < 0.01), while EMG oscillation in the biceps brachii muscle from 15 to 45 Hz was increased (p < 0.05). These findings imply that if assistive force is provided during isometric force control, the central nervous system actively tries to stabilize motor performance by controlling specific motor unit activity in the agonist muscle.     

Localized muscular fatigue duration, EMG parameters and accuracy of rapid limb movements

While much is known about the physiological basis of local muscular fatigue, little is known about the kinematic and electromyographic (EMG) consequences of brief fatiguing isometric contractions. Five male subjects performed a horizontal elbow flexion-extension reversal movement over 90° in 250 ms to reversal before and after one of five single maximal isometric elbow flexions ranging in duration from 15–120 s. Surface EMG signals were recorded from the biceps brachii, the long head of the triceps, the clavicular portion of the pectoralis major, and the posterior deltoid. Spatial and temporal errors were computed from potentiometer output. During the fatiguing bouts, maximum voluntary force dropped linearly an average of 4% in the 15 s condition and 58% in the 120 s condition relative to maximum force. The associated biceps rectified-integrated EMG signal increased from the onset of each fatigue bout for 15–30 s, then decreased over the remainder of the longer bouts. Following the fatigue bout, subjects undershot the target distance on the first movement trial in all conditions. Following short fatigue durations (i.e. 15–30 s), the peak biceps EMG amplitude was disrupted and movement velocity decreased, but both measures recovered within seconds. As fatigue duration increased, progressive decreases in peak velocity occurred with increased time to reversal, reduced EMG amplitude, and longer recovery times. However, the relative timing of the EMG pattern was maintained suggesting the temporal structure was not altered by fatigue. The findings suggest that even short single isometric contractions can disrupt certain elements of the motor control system.     

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.     

Shoulder and elbow muscle activity during fully supported trajectory tracking in neurologically intact older people

An inability to perform tasks involving reaching is a common problem for stroke patients. Knowledge of normal muscle activation patterns during these tasks is essential to the identification of abnormal patterns in post-stroke hemiplegia. Findings will provide insight into changes in muscle activation patterns associated with recovery of upper limb function.In this study with neurologically intact participants the co-ordination of shoulder and elbow muscle activity during two dimensional reaching tasks is explored. Eight participants undertook nine tracking tasks in which trajectory (orientation and length), duration, speed and resistance to movement were varied. The participants’ forearm was supported using a hinged arm-holder, which constrained their hand to move in a two dimensional plane. EMG signals were recorded from triceps, biceps, anterior deltoid, upper, middle and lower trapezius and pectoralis major.A wide variation in muscle activation patterns, in terms of timing and amplitude, was observed between participants performing the same task. EMG amplitude increased significantly with length, duration and resistance of the task for all muscles except anterior deltoid. Co-activation between biceps and triceps was significantly dependent on both task and trajectory orientation. Activation pattern of pectoralis major was dependent on trajectory. Neither trajectory orientation nor task condition affected the activation pattern of anterior deltoid. Normal ranges of timing of muscle activity during the tasks were identified.     

Control of the Elbow Extensor Muscles in Slow Targeted Extensions of the Arm in Humans

Relations between the kinematic parameters of slow (non-ballistic) targeted extension movements in the elbow joint of humans and characteristics of the movement-related EMG activity in the two heads of the m. triceps brachii were analyzed. Test movements were performed under conditions of application of non-inertional external loadings directed toward flexion. It was shown that the movement-related EMG activity of the elbow extensors, similarly to what was observed in the flexors at flexion movements with the same parameters, demonstrates a complex structure and includes dynamic and stationary phases. In the former phase, in turn, initial and main components can be differentiated. The rising edge and decay of the main component of the dynamic extensor EMG phase could be approximated by exponential functions; this component was never split into a few subcomponents. Dependences between the amplitudes of m. triceps brachii EMG phases and the amplitude of the movement (or external loading) were, as a rule, nonlinear but monotonic. An increase in the test movement velocity led to an increase in the rate of rise of the rising edge of the dynamic EMG phase, while an increment in the amplitude was less significant. Under the used test conditions, the activity of the elbow extensors was usually accompanied by some coactivation of the antagonists (m. biceps brachii). It is concluded that motor commands coming to the elbow extensors at performance of the extension test movements differ from motor commands to the flexors at analogous flexion test movements by a simpler structure and more tonic pattern. Biomechanical specificities of fixation of the mentioned muscle groups to the arm bones (stability of the moment for application of the extensor force under conditions of changing the joint angle vs variable moment of the flexor force) are considered one of the main reasons for such specificity of the patterns of the extensor and flexor motor commands.     

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.     

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.     

Electromyographic activity in the Rhesus monkey forelimb muscles during a goal directed movement and locomotion before, during and after spaceflight

The aim of the present study was to analyse the effects of microgravity on i) the achievement of goal-directed arm movements and ii) the quadrupedal non-human primate locomotion. A reaching movement in weightlessness would require less muscle contraction since there is no need to oppose gravity. Consequently the electromyographic (EMG) activity of the monkey forelimb muscles should be changed during or after spaceflight. EMG activity of the biceps and triceps muscles during goal-directed arm movements were studied in Rhesus monkeys before, during and after 14 days of spaceflight and flight simulation at normal gravity. The EMG activity was also recorded during treadmill locomotion before and after spaceflight. When performing arm motor tasks, the delay values of the EMG bursts were unchanged during the flight. On the contrary, mean EMG was significantly decreased during the flight comparatively to the pre- and post-flight values, which were very similar. Compared with flight animals, the control ground monkey showed no change in the burst durations and mean EMG. After spaceflight, quadrupedal locomotion was modified. The animals had some difficulty in moving, and abnormal steps were numerous. The integrated area of triceps bursts was increased for the stance phase during locomotion. Taken together these data showed that spaceflight induces a dual adaptative process: first, the discharge of the motor pools of the forelimb musculature was modified during exposure to microgravity, and then upon return to Earth, monkeys changed their new motor strategy and re-adapt to normal gravity.     

Transition from slow to ballistic movement: development of triphasic electromyogram patterns.

The purpose of this investigation was to determine how the triphasic electromyogram (EMG) pattern of muscle activation developed from the agonist muscle only pattern as movement time (tmov) decreased. Six adult women produced a series of 30 degrees elbow extension movements in the horizontal plane at speeds ranging from ballistic (less than 400-ms tmov) to very slow (greater than 800-ms tmov). Surface EMG from triceps brachii (agonist) and biceps brachii (antagonist) muscles were recorded, together with elbow angle, on a microcomputer. The results showed that triphasic EMG patterns developed systematically as tmov decreased from 1000 ms to less than 200 ms. In trials with very long tmov, many elbow extension movements were produced by a single continuous activation of the agonist triceps brachii muscle. As tmov decreased however, agonist activation became predominantly burst-like and other components of the triphasic EMG pattern [activation of the antagonist (Ant) and second agonist activation (Ag2)] began to appear. At the fastest movement speeds, triphasic EMG patterns (Ag1-Ant-Ag2, Ag1 being first activation of agonist muscle) were always present. This data indicated that the triphasic pattern of muscle activation was not switched on when a particular tmov was achieved. Rather, each component systematically developed until all were present, as distinctive bursts of activity, in most trials with tmov less than 400 ms.     

In vivo assessment of elbow flexor work and activation during stretch-shortening cycle tasks.

The purpose of this study was to use an electromyography (EMG) based muscle model to investigate the performance enhancement of stretch-shortening cycle (SSC) tasks at different elbow flexion-extension velocities. A torque motor was used to oscillate the forearms of seven healthy male subjects (23-40 years) during SSC and non-SSC contractions at four frequencies of movement (.58, 1.5, 2.4 and 3.3Hz) over a range of 105 degrees -162 degrees of elbow extension. The torque was integrated as a function of joint angle to yield the work produced by the elbow flexors. The elbow flexors were transcutaneously stimulated with a voltage equivalent to 60% maximum voluntary isometric contraction torque for 4s at 50Hz. EMG of the elbow flexors and extensors was recorded from the biceps and triceps respectively. The processed EMG was used to drive a Hill based model to predict the torque of the elbow flexors. Results indicate that muscle work increases from non-SSC to SSC trials. Work decreases for SSC and non-SSC trials with increasing velocity. The simulated constant activation muscle model predicted work well for all trials and conditions, indicating muscle model accuracy. The EMG driven model predicted well for all non-SSC trials, but significantly underestimated the work for SSC tasks, suggesting that the contractile component is directly involved in optimising muscle work during SSC tasks.     

The influence of task and angle on torque production and muscle activity at the elbow

This study investigated the effect of changing internal mechanical variables and task demands on muscle activity and torque production during high effort isometric contractions of the elbow flexors. The effect of adding a 50% maximal voluntary contraction (MVC) of supination to an MVC of elbow flexion was studied over a range of angles from 30° to 110° of elbow flexion. Surface EMGs were recorded from the biceps brachii (BIC), brachioradialis (BRAD) and triceps brachii (TRI) of 10 healthy subjects. BIC was the only muscle to show a consistent trend of increasing root mean square (rms) EMG with increasing elbow flexion angle. BIC activity also remained constant or increased with the addition of the supination task at all angles. In contrast, BRAD showed decreased activity when supination was added at several angular positions. Maximal flexion torque was reduced when the second task of submaximal supination was added. This torque reduction was statistically significant at all angles except 70° and appeared related to the decreased contribution from BRAD. In a small subset of subjects, however, BRAD activity did not decrease when the second degrees of freedom (df) task was added. These subjects exhibited higher flexion torques averaged over task than the majority, at all angles except 30°. These data support the view that internal mechanical considerations influence the manner in which the central nervous system (CNS) distributes activity to muscular synergists in response to altered task demands. Further, subject-specific patterns exist which must be recognized if these findings are to be incorporated in training or rehabilitation programmes.     

Effects of Local and Remote Muscle Pain on Stretch ReflexActivities in the Elbow Joint Flexors and Extensors of Unanesthetized Cats

In experiments on unanesthetized cats, we compared the effects of experimentally induced pain in the m. biceps brachii or in the neck muscles on EMG activity of the flexors and extensors of the elbow joint (mm. biceps et triceps brachii, respectively) evoked by a passive extension-flexion of the above joint. Muscle pain was induced by injections of 0.5 ml of a hypertonic (7%) NaCl solution into the above-mentioned muscles. In the case of pain in the biceps, i.e., in the muscle directly involved in realization of the reflex, we observed an increase in the amplitude and significant shortening of the latency of EMG responses of this muscle. The amplitude of a short-latency (supposedly monosynaptic) component of the biceps reflex (М1 response) increased by 65%, while an increment of the latter (supposedly polysynaptic) М2 component was 117%. When pain was induced in anatomically remote neck muscles, the stretch reflex in the biceps was considerably suppressed. The maximum amplitudes of the М1 and М2 components decreased by 25 and 30%, respectively, but the latencies of these components decreased significantly, similarly to what was observed in the case of induction of experimental pain in the biceps. Under both conditions of experimental pain, changes in the parameters of EMG responses of the forearm extensor (m. triceps brachii) demonstrated similarity with those of the biceps responses. The maximum effect of pain induction was observed within the first 5 min after injections of the hypertonic solution; full recovery of the stretch reflex parameters was observed on the 20th to 30th min. We conclude that the effects of pain induction on the reflex under study are not generalized. They depend on the site of such induction with respect to the muscle where the stretch reflex is elicited. Unidirectional effects of both types of pain on the antagonist muscles allow us to suppose that modulation of the reflex reactions upon pain induction is mediated by influences from the supraspinal CNS structures. Induction of pain in the biceps increased the amplitude of EMG manifestations of the stretch reflex, while such induction in the neck muscles decreased such responses; nonetheless, in both cases the latency of the reflexes decreased. This fact allows us to believe that the sensitivity of muscle spindles increased under both conditions of the pain influence.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.     

Electromyographic patterns of upper extremity muscles during sitting pivot transfers performed by individuals with spinal cord injury

Although substantial upper extremity (U/E) muscular efforts are required when individuals with spinal cord injury (SCI) perform sitting pivot transfers, little is known about the electromyographic (EMG) activation patterns of key shoulder and elbow muscles solicited during the performance of this functional task. The aims of this study were to examine the EMG activation patterns of U/E muscles in 10 males with SCI, and to compare them across sitting pivot transfers performed toward seats of different heights (low, same, high). EMG data from the biceps, triceps, deltoid, pectoralis major and latissimus dorsi were recorded bilaterally. Transfers were divided into pre-lift, lift, and post-lift phases. Each phase was time- and amplitude-normalized using a mean dynamic EMG approach. Similar EMG activation patterns were found across the different transfers for all muscles (r_mean = 0.942–0.991), whereas moderate to high inter-subject variability (CV: 20.9–70.6%) was reported for the different muscles and transfers. Peak EMG occurred earlier at the trailing U/E compared to the leading one, and was observed around seat-off for most of the muscles. When transfer to a high target seat was compared to transfer to one of the same height, significantly higher relative EMG values were observed at the biceps (mean: 1.64 vs. 1.00) of the leading U/E as well as the deltoid (mean: 1.20 vs. 1.00) and pectoralis major (mean: 1.20 vs. 1.00; peak: 2.27 vs. 1.79) of the trailing U/E. Transferring to a low target seat did not lead to lower muscular demand than transferring to one of the same height (P > 0.05). These results indicate that coordinated and higher muscular efforts were generated at the trailing deltoid and pectoralis major when transferring to a high target seat compared to one of similar height. Higher muscular efforts were also developed at the leading biceps when transferring to a high target seat compared to a leveled one. Lowering the target seat with respect to the initial seat had no favorable effect on muscular demand.     

Electromyographic activity during shivering of muscles acting at the human elbow

• 1.1.|Surface electromyograms have been recorded from biceps and triceps brachii during cold induced shivering in normal human subjects.
• 2.2.|Biceps was commonly found to be co-contracting with triceps when the shivering subject was voluntarily producing an extension force at the elbow; when the subject was warm only triceps contracted.
• 3.3.|During shivering the EMG spectra of both biceps and triceps normally showed a pronounced peak in the range 7–12 Hz. The cross-spectrum of the EMGs for the two muscles showed a similar peak, with their linked activity organised reciprocally (i.e. approximately 180 out of phase).

     

Effects of repetitive dynamic contractions upon electromechanical delay

The effect of repeated maximal effort isotonic contractions on electromechanical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow flexion trials. The kinematics and surface electromyographic (EMG) activity of the biceps brachii of these subjects were recorded. The period from the onset of the EMG until the beginning of movement was defined as the electromechanical delay. The period from the beginning of movement until the end of the EMG was defined as the second component of the contraction. Over the 4 day period there was an increase in the speed of limb movement. The mean power frequency and the duration of the EMG during the electromechanical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction remained stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.     

Strategies used to stabilize the elbow joint challenged by inverted pendulum loading

The stiffness of activated muscles may stabilize a loaded joint by preventing perturbations from causing large displacements and injuring the joint. Here the elbow muscle recruitment patterns were compared with the forearm loaded vertically (a potentially unstable inverted pendulum configuration) and with horizontal loading. Eighteen healthy subjects were studied with the forearm vertical and supinated and the elbow flexed approximately 90 degrees. In the first experiment EMG electrodes recorded activity of biceps, triceps, and brachioradialis muscles for joint torques produced (a) by voluntarily exerting a horizontal force isometrically (b) by voluntarily flexing and extending the elbow while the forearm was loaded vertically with 135N. The relationship between the EMG and the torque generated was quantified by the linear regression slope and zero-torque intercept. In a second experiment a vertical load increasing linearly with time up to 300N was applied.In experiment 1 the EMG-torque relationships for biceps and triceps had an intercept about 10% of maximum voluntary effort greater with the vertical compared to the horizontal force, the inverse was found for Brachioradialis, but the EMG-torque slopes for both agonist and antagonistic muscles were not different. In experiment 2 there were 29 trials with minimal elbow displacement and all the three muscles activated on the order of 11% of maximum activation to stabilize the elbow; 19 trials had small elbow extension and 14 trials small flexion requiring altered muscle forces for equilibrium; 7 trials ended in large unstable displacement or early termination of the test. An analysis indicate that the observed levels of muscle activation would only provide stability if the muscles' short-range stiffness was at the high end of the published range, hence the elbow was marginally stable. The stability analysis also indicated that the small elbow extension increased stability and flexion decreased stability.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

Force generation performance and motor unit recruitment strategy in muscles of contralateral limbs.

The purpose of the present study was to determine whether the motor unit (MU) recruitment strategy of the agonist and antagonist muscles in the dominant arm differs from that in the non-dominant arm. The median frequency (MF) of the power density spectrum (PDS) of the electromyogram (EMG) was used as a tracking parameter to describe the MU recruitment. In 8 subjects the EMG was recorded from the biceps brachii and triceps brachii of each limb during isometric elbow flexion performed in a ramp fashion. Force was increased from 0 to 100% of the maximum voluntary contraction (MVC) in 3 s following a track displayed on an oscilloscope. When comparing the dominant versus non-dominant arm we found no statistical difference in the MU recruitment pattern of the biceps brachii and the triceps. Because the dominant arm was not always the better performing arm, we grouped the data according to the ability of the subjects to track the ramp signal. In this case we found a statistically significant difference between the better and worse performing arm in the full MU recruitment of the biceps. A more precise and accurate control of the increase in force was obtained when the central nervous system selected a slower and prolonged recruitment of MUs in the agonist muscle.