Extensor Carpi Radialis Muscles Research Articles

Imaging the human motor system’s beta-band synchronization during isometric contraction

Rhythmic synchronization likely subserves interactions among neuronal groups. One of the best studied rhythmic synchronization phenomena in the human nervous system is the beta-band (15–30 Hz) synchronization in the motor system. In this study, we imaged structures across the human brain that are synchronized to the motor system’s beta rhythm. We recorded whole-head magnetoencephalograms (MEG) and electromyograms (EMG) of left/right extensor carpi radialis muscle during left/right wrist extension. We analyzed coherence, on the one hand between the EMG and neuronal sources in the brain, and on the other hand between different brain sources, using a spatial filtering approach. Cortico-muscular coherence analysis revealed a spatial maximum of coherence to the muscle in motor cortex contralateral to the muscle in accordance with earlier findings. Moreover, by applying a two-dipole source model, we unveiled significantly coherent clusters of voxels in the ipsilateral cerebellar hemisphere and ipsilateral cerebral motor regions. The spatial pattern of coherence to the right and left arm EMG was roughly mirror reversed across the midline, in agreement with known physiology. Subsequently, we analyzed the brain-wide pattern of beta-band coherence to the motor cortex contralateral to the contracting muscle. This analysis did not reveal any convincing pattern. Because the prior cortico-muscular analysis had demonstrated the expected pattern in our data, this negative finding demonstrates a current limitation of the applied method for cortico-cortical coherence analysis. We conclude that during an isometric muscle contraction, several distributed brain regions form a brain-wide beta-band network for motor control.

Investigation of paroxysmal dystonia in a patient with multiple sclerosis: A transcranial magnetic stimulation study

Objective To study the pathogenesis of paroxysmal dystonia affecting the right body side in a patient with a demyelinating lesion in the descending motor pathways, also involving the basal ganglia. Methods Single-pulse transcranial magnetic stimulation (TMS) was applied to study motor evoked potentials (MEPs) and the following silent periods (SPs) in the first dorsal interosseous muscle (FDI) of both sides and in the right extensor carpi radialis muscle (ECR) during voluntary contractions performed outside the dystonic attacks. During the dystonic paroxysms, single-pulse TMS was used to investigate the time course of MEPs and SPs in both FDI and ECR of the right side. Furthermore, paired-pulse TMS was applied at rest to investigate short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in both FDI muscles. Results At rest SICI and ICF were normal in both motor cortices. During voluntary contraction the MEP was smaller and the SP was longer in the affected FDI than in the contralateral. During the paroxysms, the MEPs and SPs were suppressed in comparison with the responses elicited during voluntary contraction. Conclusions These results fit well with the theory of ephaptic excitement of corticospinal axons for the pathogenesis of paroxysmal dystonia due to a demyelinating lesion. Significance Identification of the mechanisms underlying paroxysmal dystonia in demyelinating disorders extends our knowledge on the pathophysiology of dystonia.

Differences in Motor Unit Firing Characteristics and Motor Unit Potential Parameters in the Extensor Carpi Radialis (ECR) Muscle during Low Level Contractions

With preferential use of one limb over another, measurable differences in motor unit potential (MUP) morphological and firing rate characteristics might exist between dominant and non-dominant upper limbs, as limb dominance might lead to changes in recruitment characteristics and/or fiber-type changes. PURPOSE: This study was designed to determine (i) if firing rate behavior and MUP parameters in the ECR muscle are different between dominant and non-dominant ECR muscles in a sample of healthy volunteers, and (ii) if there are detectable changes in motor unit (MU) firing rate and MUP morphology features between recruitment threshold and a low-level contractions in the wrist extensor muscles. METHODS: Changes in mean MU firing rates and MUP parameters were investigated in the ECR muscles of eight healthy volunteers (mean age = 30.9 yr, SD ± 4.5) during isometric contractions held at threshold and 10% MVC. Handedness was assessed using the Edinburgh Handedness Inventory questionnaire. In random order and on each ECR muscle, threshold contractions corresponding to 1–2 detected MUs (3–5% MVC) were recorded followed by three to five repetitions of a sustained wrist extension contraction (30s) at 10% maximum voluntary contraction. Signal decomposition was achieved using DQEMGTM software, resulting in surface (SMUP) and needle detected MUP size (amplitude, duration, area), and MU firing rates. RESULTS: An independent t-test showed no significant differences between dominant and non-dominant ECR muscles for any of the MUP or SMUP size or firing rate statistics. The data from both arms were therefore pooled to compare the parameters at threshold and at 10%MVC. Significant increases with contraction level were found in MU firing rate, MUP amplitude, SMUP amplitude and negative peak area. CONCLUSIONS: No limb-dominance effect was noted in MUP size of MU firing rate parameters. Larger MUP amplitude and area features at 10%MVC as compared to threshold indicate recruitment, while higher firing rates indicate rate-coding. The ability to detect these differences might be of clinical value in the investigation of certain neuromuscular conditions.

Time- and frequency-domain monitoring of the myoelectric signal during a long-duration, cyclic, force-varying, fatiguing hand-grip task

The amplitude and mean power spectral frequency (MNF) of the electromyogram (EMG) of flexor digitorum superficialis and extensor carpi radialis muscles were monitored during cyclic, force-varying, constant-posture, submaximal, grip-force contractions until endurance. These contractions are reminiscent of work tasks associated with the risk of repetitive stress injuries. Based on recommendations from a prior cross-comparison study of these data, the cyclic grip-force contractions were temporally aligned cycle-by-cycle to the achieved grip force profile, then EMG amplitude was computed using signal whitening (500 ms window) and MNF was computed using the short-time Fourier transform (500 ms window). In addition, brief (8 s) constant-force (static) contractions were interspersed within the cyclic contractions every 5 min. MNF was tracked during these periods. All subjects reported a marked increase in pain/discomfort/fatigue during the contraction trials, until self-selecting to discontinue contractions after 30–90 min. Discomfort returned to near-baseline levels during the ensuing 45 min recovery (rest) period. No statistical trend was found in either EMG amplitude or MNF during the cyclic contractions or the recovery period. Initial MNF and MNF slope were monitored during the 8 s interspersed static contractions. These parameters also did not follow any consistent trend. These results indicate limitations in the use of these EMG descriptors for assessment of fatigue during long-duration, force-varying contractions.

Sexual dimorphism of extensor carpi radialis muscle morphology, isometric stress, kinetics and fatigue resistance during the breeding season of the frog Rana temporaria

Sexual dimorphism of extensor carpi radialis muscle morphology, isometric stress, kinetics and fatigue resistance during the breeding season of the frog Rana temporaria

Evaluation of Distal Upper Extremity Musculoskeletal Load During Routine Colonoscopy

Background: Physicians who perform endoscopies are at increased risk for developing recurrent thumb, hand, elbow and low back pain. Musculoskeletal disorders of the upper extremities are related to repeated, forceful movements of the hands and arms. As the demand for colonoscopy increases, so will the physical loads on the hands and arms of endoscopists. The risk factors of upper extremity overuse injuries during routine endoscopy, such as applied pinch force and muscle loads, have not been evaluated during routine endoscopy. We performed a pilot study to determine right thumb pinch force and hand and wrist muscle activities during colonoscopy. Methods: Three gastroenterologists were studied while each performed three colonoscopies. Electromyographic (EMG) activity of the right and left forearm extensor carpi radialis (ECR), right and left flexor digitorum superficialis (FDR), and left abductor pollicis longus (APL) was measured with surface electrodes. Summary measures of EMG data were calculated using the amplitude probability distribution function (APDF), which is a method to quantify muscular activity by normalizing it to maximum voluntary contraction (MVC). Prior studies have shown median levels of muscle activity (APDF 50%tile) of >7-20% MVC and low levels of muscle activity (APDF 10%ile) of >2-5% MVC are associated with increased risk of injury. Right thumb pinch force was measured using a thin profile, custom designed pressure sensor. Prior studies have shown peak pinch force levels (APDF 90%tile) of >10N are associated with an increased risk of injury. Results: The mean peak pinch force across the 9 patients (APDF 90) was 8.6N (SD ± 2.1). The highest pinch forces occurred during insertion: APDF 90 = 10.4N in the left colon and APDF 90 = 10.1N in the right colon. The highest muscle activities were observed in the left ECR (APDF 10 = 10% MVC and APDF 50 = 20% MVC). Left ECR activity was greatest during insertion into the left colon (APDF 10 = 9.0%, APDF 50 = 25.1%) and right colon (APDF 10 = 11.4%, APDF 50 = 23.9%). APDF 10 values of the other muscle groups studied did not exceed 5% of MVC, and APDF 50 values did not exceed 20% of MVC. Conclusions: Based on this pilot data, colonoscopy is associated with a borderline risk of musculoskeletal disorders of the upper extremities, specifically right thumb and left wrist extensor injury. Repeated, high thumb pinch forces can lead to DeQuervain's tendinitis and carpal tunnel syndrome. Repeated, high left ECR muscle loads can lead to second and third extensor compartment tenosynovitis at the wrist and epicondylitis. Further study is needed to identify modifiable risk factors and evaluate methods to reduce this risk.

Sexual dimorphism of extensor carpi radialis muscle size, isometric force, relaxation rate and stamina during the breeding season of the frogRana temporariaLinnaeus 1758

Mating success of individual male frogs within explosive breeding species can depend on their ability to compete for a mate and to hold onto that mate during amplexus. Such importance of amplexus has resulted in the evolution of sexual dimorphism in the morphology and contractile characteristics of the anuran forelimb muscles used during amplexus. The aims of our study were to use an explosive breeding frog (Rana temporaria) during the breeding season to compare extensor carpi radialis (ECR) muscle length, mass, isometric activation times, relaxation times, absolute force, relative force (stress) and fatigue between male and female frogs. We found that ECR muscle mass and length were greater (tenfold and 1.4-fold, respectively), absolute tetanic muscle force and relative tetanic force (stress) were greater (16-fold and 2.2-fold, respectively) and relaxation times were slower in males than in females. Male ECR muscles incompletely relaxed during fatigue tests and showed less fatigue than female muscles. These sex differences are likely to be beneficial to the male frogs in allowing them to produce relatively high absolute muscle forces for prolonged periods of time to hold onto their mate during amplexus.

An additional radial wrist extensor and its clinical significance

The knowledge of anatomical variations in the antebrachial and dorsal regions of the arm and hand are useful in hand surgery. The extensor carpi radialis intermedius and extensor carpi radialis accessorius are two classic variants described for the radial wrist extensors, in the antebrachial region. We report an additional extensor carpi radialis muscle taking origin from the common extensor origin, between the extensor carpi radialis longus and extensor digitorum communis. The tendon of the variant muscle divides below the abductor pollicis longus and becomes attached to the base of the second and third metacarpal bone. Due to its considerable size and independent origin from the lateral epicondyle, we suggest the present variation should be named extensor carpi radialis tertius. The clinical significance of the present variation is discussed.

Motor strategies and excitability changes of human hand motor area are dependent on different voluntary drives

The present study examined whether there were different voluntary drives between intended and non-intended muscle contractions. In experiment 1, during intended and non-intended muscle contractions, electromyograms (EMGs) were recorded from the first dorsal interosseous (FDI) and extensor carpi radialis (ECR) muscles when force levels were varied from 10% to 50% maximal voluntary contraction (MVC) in 10% MVC steps. In experiment 2, using transcranial magnetic stimulation, motor-evoked potentials (MEPs) were recorded from the FDI muscle when EMGs were varied from 10% to 40% EMGmax (EMG activities during MVC) in 10% EMGmax steps during intended and non-intended muscle contractions. In experiment 3, at 10% MVC force level MEPs were recorded before and after practice. The results showed that, in the FDI muscle, EMGs during intended muscle contractions were larger than those during non-intended ones at higher force levels (30-50% MVC). In the ECR muscle, reverse results were observed. At comparable EMG levels of the FDI muscle MEPs were the same during intended and non-intended muscle contractions. After practice, MEPs during intended muscle contraction became larger than those during non-intended at 10% MVC force level, while EMGs were the same between two muscle contractions. It is concluded that motor strategies and excitability changes of hand motor area are different during intended and non-intended muscle contractions, and these differences are due to the different voluntary drives of intended and non-intended. The present findings may contribute to the understanding of rehabilitation for patients suffering from damages of the central motor system.

Firing Characteristics of Motor Units during the First 90 Seconds of a Fatiguing Low-Level Contraction

The mechanisms responsible for the changes in surface electromyographic (EMG) signal amplitude and frequency content during prolonged fatiguing low-level contractions are not well understood. In addition, the normal behaviour of motor units (MU) during these contractions also remains elusive. PURPOSE: This study was designed (i) to determine decomposition-based quantitative electromyography (DQEMG), could detect changes in motor unit firing statistics that accompany fatigue in the brachioradialis (BR) and extensor carpi radialis (ECR) muscles over the first 90 seconds of sustained, low-level isometric contractions, (ii) to determine if time-dependent changes in surface EMG parameters are correlated with changes in MU firing rates, and (iii) to determine if time-dependant changes seen in the MU firing characteristics are repeatable within individuals. METHODS: Changes in mean MU firing rates were investigated in the BR (n=1 0) and ECR (n=10) muscles in healthy volunteers (mean age = 28.6 yr, SD ± 3.9) during constant-torque isometric contractions held until exhaustion. Three BR contractions were sustained at 25% of the maximum voluntary contraction (MVC) and two ECR contractions were sustained at 20% MVC. Mean MU firing rates were estimated following the decomposition of 30-second EMG data windows. Time dependant changes in firing rates were investigated using an analysis of variance with time (first, second and third 30 second epochs) included as a main effect. Pearson product-moment correlation coefficients were computed to determine if changes in MU firing rates were related to changes in surface EMG amplitude and frequency parameters (α=0.05). Intra-class correlation coefficients (ICC) were used to determine the within-subject repeatability of the MU firing rate changes, and SEMG parameters. RESULTS: Mean MU firing rate decreased within the first 90 seconds in both muscle groups, and was repeatable across trials using the DQEMG method. Significant correlations between changes in mean firing rates and surface EMG amplitude and frequency content were found in the ECR muscle but not the BR muscle. The reliability of firing rate changes within subjects was moderate in both the ECR (ICC=0.72) and BR muscles (ICC=0.59). The SEMG mean power frequency significantly decreased over contraction time in the ECR muscle, and was found to be a highly reliable measure in both muscles (ICC=0.99 and 0.98, respectively). CONCLUSIONS: Changes in MU characteristics were observed as early as 90 seconds into a low level contraction. Differences found between the two muscles may be due to contraction level or fiber type distributions.

Rapid Plastic Changes of Human Primary Motor Cortex with Repetitive Motor Practice and Transcranial Magnetic Stimulation

Excitability changes of human primary motor cortex are assumed to be associated with motor learning processes. To examine motor behavioral and neural mechanisms in these processes, the adaptive motor learning processes of the index finger abduction were investigated using motor evoked potential (MEP) elicited from the first dorsal interosseous and extensor carpi radialis muscles. Practice effects were examined on changes of MEP amplitudes elicited from these muscles during motor imagery. Given general consensus that the MEP amplitude change during motor imagery is a useful parameter reflecting changes in excitability of the human primary motor cortex, the present results, that MEP amplitudes of both muscles increased with repeated practice by the index finger abduction and that magnitudes of MEP amplitudes of both muscles (motor learning curves) were clearly different, suggested that participation of the muscles performing the index finger abduction gradually changed with practice. Short-term plastic changes of human primary motor cortex occur with repetitive practice and such adaptive change in human primary motor cortex is expressed in human voluntary movement that becomes more automatic.

Sensorimotor integration to cutaneous afferents in humans: the effect of the size of the receptive field

Transcranial magnetic stimulation (TMS) can be used to study sensorimotor integration in humans non-invasively. Motor excitability has been found to be inhibited when afferent stimuli are given to a peripheral nerve and precede TMS at interstimulus intervals (ISIs) of 20-50 ms. This phenomenon has been referred to as short-latency afferent inhibition (SAI). To better understand the functional meaning of these phenomena, we examined the effect of the size of the receptive field on SAI to cutaneous afferents in upper-limb sensorimotor areas in humans. We examined the effect of the stimulation of the isolated right first (D1), second (D2) and third finger (D3), the right second and third finger together (D23) and the right first three fingers together (D123) on the amplitude of motor evoked potentials (MEPs) to TMS in hand and forearm muscles. We examined the right abductor pollicis brevis (APB), first dorsal interosseous (FDI), extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles. Digital stimulation preceded TMS at ISIs of 20-50 ms. The effect of D2 stimulation was MEP inhibition (SAI), which was more marked and consistent in APB and FDI muscles than in ECR and FCR muscles. Similarly, D1 and D3 stimulation caused MEP reduction, while no MEP enhancement could be found to single finger stimulation. In contrast, D123 stimulation induced less effective SAI in upper-limb muscles. MEP potentiation was recorded in some muscles to D123 stimulation. A significant difference between D2 and D123 stimulation was found in APB (ISIs = 30-50 ms) and FDI (ISIs = 40-50 ms) muscles, but not in forearm muscles. The effect to D23stimulation on MEP amplitude was intermediate between those to D2 and D123 stimulation. Our data suggest that motor excitability to cutaneous afferents may be influenced by the size of the receptive fields, this effect being the result of increasing convergence between hand afferents in the somatosensory system. These phenomena appear to be topographically arranged across the representation of upper-limb muscles. These findings may help to understand the functional significance of SAI in normal physiology and pathophysiology.

Electromyographic Analyses of Global Synkinesis in the Paretic Upper Limb After Stroke

Global synkinesis (GS), or motor irradiation, is an involuntary movement associated with the coactivation of numerous muscles in one limb when the opposite limb is active. The electromyographic (EMG) patterns of people with stroke and people who were healthy were analyzed to characterize GS development in relation to joint involvement and to attempt to relate these findings to clinical observations. Twenty patients with stroke, divided into 2 groups with either greater levels of irradiation (SG, n = 10) or lesser levels of irradiation (SL, n = 10), and 20 subjects in a control group were studied. A dynamometer was used to provide resistance for voluntary isometric muscle contractions of the flexor muscle groups of the shoulder, elbow, and wrist. The summated and standardized net EMG amplitudes of 8 principal muscles of the unexercised (paretic) upper extremity were used to characterize intensity and spatial representation of GS. Clinical measurements included the Fugl-Meyer Assessment Scale (FMA), Barthel Index of Activities of Daily Living (BI), and the stage on the Brunnström Stages of Motor Recovery Scale (BR). In the SG and control groups, a more substantial GS intensity was associated with muscle contractions of the flexor muscles of the opposite proximal joint than was the case for contractions of the flexor muscles of the distal joint, whereas such a gradient change was absent in the SL group. The corresponding spatial patterns of GS exhibited a predominant cross-excitation over the unexercised pectoralis major and extensor carpi radialis muscles in the control group, contrary to the enhanced activation of the brachioradialis and biceps brachii muscles noted in patients with stroke. The SG group had a better FMA score and a more satisfactory BR stage than did the SL group, and the 2 neurological scores were related to GS intensity for patients with stroke, depending on joint involvement. Intensity of GS provided an affiliation with motor deficits and a promising window for poststroke recovery mechanisms.

Colour of muscle from 18-month-old steers given long-term daily exercise

Darker beef from pasture-fed compared with grain-fed cattle may result from differences in physical activity rather than differences in nutrition. The objective was to determine if steers that were exercised produced darker meat than non-exercised steers and whether any effect was muscle-related. Exercised steers were walked 4.41 km daily in a single bout, six days per week for 18 weeks at an average speed of 5.2 km h −1. All steers were fed grass silage on an ad libitum basis plus 6 kg concentrates. Following slaughter, muscle colour coordinates (‘ L’ (lightness), ‘ a’ (redness) and ‘ b’ (yellowness) values) of M. longissimus dorsi (LD), M. semimembranosus (SM) and M. extensor carpi radialis (ECR) were recorded at 48 h postmortem and redness and yellowness were used to calculate muscle hue (‘ H’) and colour intensity/saturation (‘ C’). The pH of all muscles was measured at 1.5, 3, 6, 22 and 48 h postmortem and LD samples were recovered (90 min postmortem) for glycolytic potential (GP) assessment. Exercise did not affect muscle lightness, yellowness, hue or colour intensity. However, LD was the darkest ( P < 0.001) and SM the most saturated ( P < 0.001) muscle. Exercise affected muscle redness in a muscle-dependent manner (muscle × exercise, P = 0.038) whereby ECR became more red with exercise but LD and SM were unaffected. There were muscle × time ( P < 0.001) and time × exercise ( P = 0.045) interactions for muscle pH. The ECR muscle had the highest pH at all times. The exercised steers had higher ( P < 0.05) LD muscle pH than control steers at 3 and 6 h postmortem. Exercise did not affect myoglobin concentration, which was muscle dependent, decreasing in the order: SM (6.72 mg/g) > ECR (6.33 mg/g) > LD (5.48 mg/g), which were all different ( P < 0.001). Exercise had no effect on GP in LD muscle (111 vs. 99 μmol/g for control and exercised steers, respectively; SED = 6.6 μmol/g). It was concluded that although application of exercise did not affect muscle lightness and thus, did not cause ‘darker’ meat, it did affect muscle redness in a muscle-dependent manner.

Mechanomyographic response to transcranial magnetic stimulation from biceps brachii and during transcutaneous electrical nerve stimulation on extensor carpi radialis

Transcranial magnetic stimulation (TMS) elicits short latency excitatory responses in the target muscles, termed motor evoked potential (MEP). When TMS is delivered during a voluntary contraction, the MEP is followed by a period of silence called silent period (SP). These MEP parameters are in general recordable by electromyography (EMG). Mechanomyography (MMG) on the other hand is the mechanical counterpart of EMG. Thus, this study has been conducted to observe whether the MEP parameters from MMG signals showed similar trait of EMG recordings. Five normal healthy male subjects were included in this study. The subjects were required to perform right biceps brachii muscles contraction at diverse graded of load level at 5, 10, 20, 30, 40, 60, and 100% maximum voluntary contraction (MVC). MEPs by single pulse TMS on left hemisphere were obtained from both EMG electrode and MMG accelerometer at rest and at different levels of predetermined load level. MEP amplitude and area obtained both from EMG and MMG record were increased with the increase of muscle contraction with a maximum of 60% MVC. With increasing the level of contraction there was a shortening of onset latency and decreasing in the length of silent period in both EMG and MMG signals. We also recorded the EMG- and MMG-MEP from the right extensor carpi radialis muscle during transcutaneous electric nerve stimulation in order to observe neural changes in sensory stimulation from both EMG and MMG responses. The EMG-MEP was not visible in electrical artifact whereas it was obvious in MMG responses. In accordance with other study, this study showed that the voluntary contraction of biceps brachii muscle influenced the MEP parameter which are moreover obtainable by MMG even in electrical noise may provide insight for future study.

Effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense

The present study investigated the effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense in the upper extremity. Twenty-four healthy subjects (12 males and 12 females) performed a 45-min standardized mouse-operated computer task on two occasions. The task consisted of painting rectangles that were presented on the screen. On one occasion, time pressure and precision demands were imposed (more demanding task, MDT), whereas, on the other occasion, no such restraints were added (less demanding task, LDT). The order of the two task versions was randomized. Tissue oxygen saturation in the trapezius and extensor carpi radialis muscles was recorded throughout, and the position-matching ability of the wrist was measured before and after the tasks. In addition, measurements of autonomic nervous system reactivity and subjective ratings of tenseness and physical fatigue were obtained. Performance was measured in terms of the number of rectangles that were painted during the task. During MDT, oxygen saturation in extensor carpi radialis decreased (P < 0.05) compared to LDT. These data were paralleled by increased electrodermal activity (P < 0.05), skin blood flow (P < 0.05), ratings of tenseness and fatigue (P < 0.01), and increased performance (P < 0.01) during MDT. Females exhibited lower oxygen saturation than males, during rest as well as during the computer tasks (P < 0.01). Wrist repositioning error increased following LDT as compared to MDT (P < 0.05). In conclusion, computer mouse work under time pressure and precision demands caused a decrease in forearm muscle oxygenation, but did not affect wrist position sense accuracy. We attribute our changes in oxygenation more to increased oxygen consumption as a result of enhanced performance, than to vasoconstriction.

Activity patterns of wrist extensor muscles during wrist extensions and deviations

Wrist extensor muscles are prone to certain focal musculoskeletal disorders for which the activation pattern of the extensor carpi radialis (ECR) and ulnaris (ECU) muscles may be important risk factors. Surface and intramuscular EMG of these muscles were recorded during isometric low-force wrist extension in semipronation and pronation as well as for ulnar/radial deviation, and were analyzed using root mean square (RMS) and decomposition methods. Despite shorter ECR length at semipronation, higher amplitudes of intramuscular EMG and of motor unit action potentials (MUAPs) were found in pronation than in semipronation. However, these changes were not detectable in the surface EMG. Higher ECR activity levels were also found during wrist extension compared to ulnar/radial deviation, and differences in the motor unit (MU) properties were found during ulnar deviation compared to radial deviation and extension. Remarkably, the MUAPs of ECR were almost twice as large as those of the ECU. Overall, the ECR muscle did not respond as predicted from biomechanical considerations, and in general activity level was higher than expected. This may partly explain why the tendon of the ECR often is associated with lateral epicondylitis.

Proprioceptive control of human wrist extensor motor units during an attention-demanding task

The responsiveness of the tonically firing single motor units (SMU) to Ia afferent volleys elicited by either mechanical (T-reflex) or electrical nerve stimulation (H-reflex) was tested in the extensor carpi radialis muscle (ECR) while the subjects were maintaining a steady wrist extension force using visual feedback set either at low or high gain. The aim was to determine whether the proprioceptive control of tonic motoneuronal activity depends on the level of attentiveness required by the behavioural context. The response probability of the SMUs to tendon taps was significantly higher ( p<0.0001) and that to electrical nerve stimulation was lower ( p<0.001) during the more demanding task. Since these changes in SMU responsiveness were not accompanied by any differences in either the motor unit firing patterns or the mean levels of EMG muscle activity, it can be concluded that there were no attention-related changes in the net excitatory drive to the ECR motoneurons. These results are consistent with the idea that fusimotor sensitization of the muscle spindle may have occurred in the more demanding task: an increase in the mechanical sensitivity of the muscle spindles would certainly account for both the T-reflex facilitation and the H-reflex depression observed. The attention-demanding task therefore seemed to involve an independent fusimotor drive activation process. The results of this study suggest that an adaptation of the fusimotor system occurs in humans, depending on the levels of attention and accuracy required to perform the ongoing motor task, as previously reported to occur in animals.

Reduced Muscular Oxygenation During Computer Mouse Use with Time Pressure and Precision Demands

1509 Monitoring oxidative metabolic states may provide information on the mechanism behind computer use related musculoskeletal disorders. PURPOSE: To compare tissue oxygen saturation (Sat-O2) profiles in the forearm extensor carpi radialis (ECR) during computer mouse use with and without time pressure and precision demands. METHODS: Twenty-four healthy, right-handed subjects (12 females, 12 males; age 19–28 years) participated in the study. Subjects performed a 45-min mouse operated computer task on two occasions, separated by 3–5 days. The task consisted of painting squares that were presented on the screen. On one occasion, time pressure and precision demands were imposed by limiting the time available for painting a square and introducing a scoring system based on precision of painting (STRESS). On the other occasion, no such restraints were added (NON-STRESS). The order of the two task versions was randomized. During the task, Sat-O2 in the right ECR muscle was measured by near-infrared spectroscopy – NIRS (Inspectra, Hutchinson Technology). In addition, subjective ratings of tenseness and strain, and painting performance measures were recorded. RESULTS: A repeated measures ANOVA revealed a significant decline in Sat-O2 during the STRESS condition (p < 0.05), but no change for the NON-STRESS condition. A gender difference was apparent as females exhibited an overall lower Sat-O2 than males (p < 0.05); however, no interaction was found. Subjects' ratings of tenseness and strain were significantly higher during the STRESS as compared to the NON-STRESS condition (p < 0.001). These data were paralleled by work pace (i.e. squares painted during STRESS = 119; NON-STRESS = 84, p < 0.001). Furthermore, accuracy of painting was greater for the STRESS as compared to the NON-STRESS condition (p < 0.01), i.e., number of times outside the square, STRESS = 2, NON-STRESS = 4. No gender differences in subjective ratings or performance variables were detected. CONCLUSIONS: Our finding of a change in local metabolic states under stressful conditions may shed light on the mechanism behind computer mouse related forearm muscular disorders. Furthermore, that females demonstrate a lower ECR Sat-O2 could give precedence to their higher incidence of disorders than males. Supported in part by the Swedish Council for Work Life Research, and technically by Kerstin Nilsson and Majken Rahm.

Proposed cortical and sub-cortical contributions to the long-latency stretch reflex in the forearm.

The short- and long-latency (SLSR, LLSR) components of the stretch reflex response were investigated in the forearm using a paired transcranial magnetic stimulation (TMS)-stretch reflex protocol. Responses to TMS were recorded in the flexor and extensor carpi radialis muscles (FCR, ECR) after conditioning with a rapid wrist extension movement. The cortical stimuli were timed to elicit a motor-evoked potential (MEP) at either the SLSR or LLSR onset in the FCR muscle. Responses were also collected in TMS-alone and stretch reflex-alone conditions. Six intensities of magnetic stimulation were applied in all conditions. In the FCR muscle, MEP amplitude when timed to arrive at the LLSR onset was significantly greater than the sum of the MEP and stretch reflex responses when given separately. MEP amplitudes at the SLSR onset in the FCR muscle and in the ECR muscle at both SLSR and LLSR onset were not significantly different from that expected from the sum of the two stimuli given separately. This indicates heightened corticospinal excitability at a time corresponding to the passage of an afferent volley induced by the stretch, and raises the possibility of a transcortical loop of the LLSR in the forearm. The extent of MEP facilitation was generally consistent across all stimulus intensities tested. A reduced component of the LLSR was evident when the stretch response was timed to arrive during the silent period following the cortical stimulus, suggesting both cortical and sub-cortical components to the reflex response.