Upper trapezius muscle activity patterns during repetitive manual material handling and work with with a computer mouse.

Firstly, upper trapezius EMG activity patterns were recorded on the dominant side of 6 industrial production workers and on the side operating a computer mouse of 14 computer-aided design (CAD) operators to study differences in acute muscular response related to the repetitiveness of the exposure. The work tasks were performed with median arm movement frequencies ranging from 5 min(-1) to 13 min(-1) and were characterized by work cycle times ranging from less than 30 sec to several days. However, the static and median EMG levels and EMG gap frequencies were similar for all work tasks indicating that shoulder muscle loads may be unaffected by large variations in arm movement frequencies and work cycle times. An exposure variation analyses (EVA) showed that the EMG activity patterns recorded during production work were more repetitive than during CAD work, whereas CAD work was associated with more static muscle activity patterns, both may be associated with a risk of developing musculoskeletal symptoms. Secondly, upper trapezius EMG activity patterns recorded on the mouse side of the CAD operators were compared with those recorded on the non-mouse side to study differences in muscular responses potentially related to the risk of developing shoulder symptoms which were more prevalent on the mouse side. The number of EMG gaps on the mouse side were significantly lower than the values for the upper trapezius on the non-mouse side indicating that more continuous activity was present in the upper trapezius muscle on the mouse side and EVA analyses showed a more repetitive muscle activity pattern on the mouse side. These findings may be of importance to explain differences in the prevalence of shoulder symptoms.     

Assessment of muscle fatigue in low level monotonous task performance during car driving.

Knowledge of the muscle activation and the development of muscle fatigue may provide more inside in the effects of long-term driving in the occurrence of health problems in the neck/shoulder/back area. The basic assumption behind fatigue detection with electromyography (EMG) is an increase in the EMG amplitude and a decrease of the mean frequency (MF). This study aimed at checking this assumption in monotonous task performance with low level activity during car driving. Surface electromyography was captured from left and right trapezius and deltoid muscles, during a repetitive, non-continuous, driving task (gearing and steering) and the active parts were separated from the non-active parts. Muscle stiffness was reported by more than half of the subjects after a 1 h drive. Only for the active parts a significant decrease of the MF was seen. But also the EMG amplitude decreased significantly. Two possible mechanisms are posted in literature for this finding: no extra recruitment of motor units (MU) and potentiation of muscle fibers. Literature also hypothesizes that low-force occupational work engages only a fraction of the MU available for recruitment and that these units are selectively type I muscle fibers (Cinderella fibers). Initiators of this phenomenon are probably the time lag between activations and the stress from driving and vibration exposure.     

Active pauses induce more variable electromyographic pattern of the trapezius muscle activity during computer work

The aim of this laboratory study was to evaluate effects of active and passive pauses and investigate the distribution of the trapezius surface electromyographic (SEMG) activity during computer mouse work. Twelve healthy male subjects performed four sessions of computer work for 10 min in one day, with passive (relax) and active (30% maximum voluntary contraction of shoulder elevation) pauses given every 2 min at two different work paces (low/high). Bipolar SEMG from four parts of the trapezius muscle was recorded. The relative rest time was higher for the lower parts compared with the upper of the trapezius (p < 0.01). The centroid of exposure variation analysis (EVA) along the time axis was lower during the computer work with active pause compared with passive one (p < 0.05). The results of this study revealed (i) lower rest time for the upper parts of trapezius compared with the lower parts, in line with previous clinical findings, (ii) active pauses contributed to a more variable muscle activity pattern during computer work that might have functional implications with respect to work-related musculoskeletal disorders.     

Investigation of motor unit recruitment during stimulated contractions of tibialis anterior muscle

This work investigated motor unit (MU) recruitment during transcutaneous electrical stimulation (TES) of the tibialis anterior (TA) muscle, using experimental and simulated data. Surface electromyogram (EMG) and torque were measured during electrically-elicited contractions at different current intensities, on eight healthy subjects.EMG detected during stimulation (M-wave) was simulated selecting the elicited MUs on the basis of: (a) the simulated current density distribution in the territory of each MU and (b) the excitation threshold characteristic of the MU. Exerted force was simulated by adding the contribution of each of the elicited MUs. The effects of different fat layer thickness (between 2 and 8 mm), different distributions of excitation thresholds (random excitation threshold, higher threshold for larger MUs or smaller MUs), and different MU distributions within the muscle (random distribution, larger MU deeper in the muscle, smaller MU deeper) on EMG variables and torque were tested.Increase of the current intensity led to a first rapid increase of experimental M-wave amplitude, followed by a plateau. Further increases of the stimulation current determined an increase of the exerted force, without relevant changes of the M-wave. Similar results were obtained in simulations.Rate of change of conduction velocity (CV) and leading coefficient of the second order polynomial interpolating the force vs. stimulation level curve were estimated as a function of increasing current amplitudes. Experimental data showed an increase of estimated CV with increasing levels of the stimulation current (for all subjects) and a positive leading coefficient of force vs. stimulation current curve (for five of eight subjects). Simulations matched the experimental results only when larger MUs were preferably located deeper in the TA muscle (in line with a histochemical study). Marginal effect of MU excitation thresholds was observed, suggesting that MUs closer to the stimulation electrode are recruited first during TES regardless of their excitability.     

Assessment of time patterns of activity and rest in full-shift recordings of trapezius muscle activity – Effects of the data processing procedure

The purpose of this paper was to compare the effects of different data reduction procedures on the values of variables characterizing the time pattern of trapezius muscle activity during full work shifts. Surface electromyography (EMG) of the right and left upper trapezius muscles were obtained from 40 young subjects in different occupations, mainly electricians, hairdressers and students. The target EMG variables were gap frequency, muscle rest, and the number and duration of episodes with sustained muscle activity (from 0.13 s to 30 min as minimum duration). These variables were derived from the EMG recordings using different Root Mean Square (RMS) windows (from 0.13 to 6.38 s), and discrimination levels between “activity” and “rest” (0.5%, 1% and 2% of maximal EMG).The results give basis for practical suggestions for EMG analyses of full work shifts. For most variables, a discrimination level of 0.5% EMG_max showed to be preferable. The time proportion of muscle rest and sustained muscle activity should, in general, be preferred over the corresponding frequency measures. Sustained muscle activity should be calculated using a RMS window between 1 and 3 s, and preferably be stated in terms of variables describing time proportions of activity. Uninterrupted activity episodes longer than 10 min proved not to be a useful variable due to limited occurrence in many work shifts.     

Low-amplitude trapezius activity in work and leisure and the relation to shoulder and neck pain.

The aim of this study is to obtain evidence supporting or negating the hypothesis that muscle pain is associated with sustained activation of low-threshold motor units. Long-term surface electromyographic (EMG) recordings of trapezius activity pattern were related to subjectively reported shoulder and neck pain in work and leisure. Recordings from 118 female subjects (73 recorded both during work and leisure) were analyzed. Computer operators, secretaries, and health care and retail workers were represented in the material. The recordings were calibrated by the root-mean-square-detected response at maximal voluntary contraction (%maximum EMG). The analysis was performed by quantifying duration and amplitude of surface EMG activity exceeding 2% maximum EMG ("EMG bursts"). Three response categories were defined by duration of the burst periods during work: low- (<50%), intermediate- (50-70%), and high-response (>70%) groups. Shoulder and neck pain was assessed by hourly visual analog score throughout work and leisure and by pain score for the last 6 mo. Shoulder and neck pain was higher at work than leisure for subjects with long-term pain in both the high- and the low-response groups. Persistent pain, defined by the 6-mo score, was more prevalent in the high- than the low- and intermediate-response groups (73 vs. 37%); relative risk was 2.0. Trapezius activity was reduced from work to leisure for the high- but not the low-response group. The activity pattern is consistent with low-threshold motor unit overexertion for the high- but not the low-response group. We speculate that different mechanisms of muscle pain causation, dependent and independent of motor activity pattern, coexist.     

Ciliary muscle contraction force and trapezius muscle activity during manual tracking of a moving visual target

Previous studies have shown an association of visual demands during near work and increased activity of the trapezius muscle. Those studies were conducted under stationary postural conditions with fixed gaze and artificial visual load. The present study investigated the relationship between ciliary muscle contraction force and trapezius muscle activity across individuals during performance of a natural dynamic motor task under free gaze conditions. Participants (N = 11) tracked a moving visual target with a digital pen on a computer screen. Tracking performance, eye refraction and trapezius muscle activity were continuously measured. Ciliary muscle contraction force was computed from eye accommodative response. There was a significant Pearson correlation between ciliary muscle contraction force and trapezius muscle activity on the tracking side (0.78, p < 0.01) and passive side (0.64, p < 0.05). The study supports the hypothesis that high visual demands, leading to an increased ciliary muscle contraction during continuous eye–hand coordination, may increase trapezius muscle tension and thus contribute to the development of musculoskeletal complaints in the neck–shoulder area. Further experimental studies are required to clarify whether the relationship is valid within each individual or may represent a general personal trait, when individuals with higher eye accommodative response tend to have higher trapezius muscle activity.     

Isotonic and isoelectric endurance tests for the upper trapezius muscle

As an alternative to a conventional endurance electromyogram (EMG) test for assessment of muscle capacity in the upper trapezius muscle (isotonic test, IT), an isoelectric test (IE) has been investigated. Nine subjects performed the two endurance tests. The tests were performed with a straight horizontal arm in the plane of the scapula. In IE, EMG amplitude was fed back to the subject and the subject was instructed to maintain a constant EMG activity equal to the level with the arm unsupported. Subjective ratings of local fatigue were obtained during the experiments. The EMG recordings from both tests were analysed for the root mean square value as well as according to the mean power frequency (MPF) technique. All the subjects endured maximal 15 min IE while in IT the median endurance was 11.9 min. Average subjective ratings of perceived fatigue increased more in IT compared to IE. The average normalised MPF curve from IT increased while that from IE was unaffected. There was a significant difference between the MPF results at 6 and 8 min. It is noteworthy that the MPF was higher in IT in spite of a higher accumulated biomechanical load. It was concluded from these studies that the interpretation of decreased MPF as an indicator of increased local muscle fatigue is doubtful at low contraction levels. It is suggested that an IE is a more appropriate method for the functional evaluation of low threshold motor units of the upper trapezius muscle in research into occupational disorders. Accepted: 15 October 1996     

EMG biofeedback effectiveness to alter muscle activity pattern and scapular kinematics in subjects with and without shoulder impingement

BackgroundMuscle imbalance between serratus anterior (SA), upper trapezius (UA), middle trapezius (MT), and lower trapezius (LT) muscles has been observed in subjects with subacromial impingement syndrome (SAIS).Objective(1) To investigate the effect of electromyography (EMG) biofeedback training on muscle balance ratios and scapular kinematics in healthy adults and subjects with SAIS. (2) To investigate whether the effects of EMG biofeedback on muscle balance ratios are different between groups.DesignTwelve healthy adults and 13 subjects with SAIS were recruited in this study. EMG was used to record the activity of scapular muscles. The ratios (UT/SA, UT/MT, and UT/LT) during exercises with/without EMG biofeedback were calculated. Scapular kinematics were recorded before and after exercises with/without EMG biofeedback.ResultsFor the subjects with SAIS, muscle balance ratios were lower during forward flexion with EMG biofeedback than during exercise only (UT/SA: 70.3–45.2; UT/LT: 124.8–94.6). Additionally, similar results were found during side-lying external rotation (UT/MT: 58.5–36.4). For the scapular upward rotation and tipping in both groups, there were no significant differences with and without EMG biofeedback.ConclusionEMG biofeedback improved the scapular muscular balance during training exercises in both groups. Further clinical trials should investigate the long-term effects of EMG biofeedback.     

Myofascial trigger point development from visual and postural stressors during computer work.

The mechanism of musculoskeletal pain underlying low level static exertions, such as those experienced during computer work, is poorly understood. It was hypothesized that static postural and visual stress experienced during computer work might contribute to trigger point development in the trapezius muscles, resulting in myofascial pain. A study was conducted to observe the development of myofascial trigger points while 16 female subjects used a computer under conditions of high and low postural and visual stress. Trigger point development was monitored via expert opinion, subject self-report, and electromyographic activity. Only the high visual stress conditions resulted in greater trigger point sensitivity as reported by subjects and the myofascial specialist. Cyclic trends in median frequency of the EMG signal were assessed for the trapezius muscle. When high visual stress was combined with low postural stress condition there were significantly fewer cycles (1.6 cycles) as compared to the condition of low visual and low postural stress (2.8 cycles), and the condition of high visual and high postural stress (3.5 cycles). These significant differences between conditions were found for the right trapezius but not for the left. The findings suggest that high visual stress may be involved in the development of the myofascial pain response.     

Scapulothoracic motion and muscle activity during the raising and lowering phases of an overhead reaching task

Scapulothoracic muscle activity is essential for normal scapulothoracic motion. While previous research has furthered the understanding of scapulothoracic motion and muscle activity during the raising phase of motion, a gap exists with respect to the lowering phase. The purpose of this study was to compare scapulothoracic motion and scapulothoracic muscle activity between the raising and lowering phases of an overhead reaching task. Twenty healthy subjects volunteered to participate in the study. Three-dimensional scapulothoracic motion was collected using an electromagnetic device. Surface electromyography (EMG) was used to assess muscle activity from the upper trapezius, lower trapezius, and serratus anterior muscles. Overall scapulothoracic motion was similar for the raising and lowering phases of the overhead reaching task. However, significantly lower EMG amplitude values existed during the lowering phase across all muscles. Less muscle activity during the lowering phase may reflect differing neuromuscular control strategies between arm raising and lowering. These findings suggest that scapulothoracic muscle activation levels during eccentric contractions may be closer to an activation threshold below which their ability to control scapulothoracic motion may be compromised subsequently leading to altered scapulothoracic motion (scapular dyskinesis). This provides a possible explanation for why scapular dyskinesis is more notable during the lowering phase of motion.     

Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis.

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.     

A method to quantify frequency and duration of sustained low-level muscle activity as a risk factor for musculoskeletal discomfort

AimThe purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain.MethodSurface electromyography (sEMG) of the right upper trapezius and the right extensor digitorum muscles was conducted continuously during one working day in 19 male forest machine operators driving harvesters, 20 driving forwarders and 20 researchers at the Forest Research Institute.Perceived discomfort/pain in the right side of the neck and the right forearm was rated morning, noon and afternoon with Borg’s CR-10 scale. Static, median and peak levels of muscle activity were analyzed and the number and total duration of EMG gaps (muscular rest) were calculated. Sustained low-level muscle activity (SULMA) was defined as continuous muscle activity above 0.5% of the maximal EMG activity quantified into 10 periods of predetermined duration intervals from 1.6 to 5 s up to above 20 min. The number of SULMA periods is presented within each interval and as cumulative periods above the already determined levels. The operators handled control levers seated in a fixed position while the researchers performed mainly PC work and other varied tasks.ResultsA positive correlation was found between discomfort/pain in the right upper trapezius muscle region in the afternoon and cumulative SULMA periods above 10 min duration, and a negative correlation to cumulative SULMA periods also including the short durations. No specified patterns were found for discomfort/pain in the right extensor digitorum or for the other EMG measurements. All EMG measurements distinguished to some extent between the occupational groups, especially between machine operators driving harvesters and researchers.ConclusionsNumber of SULMA periods longer than 10 min per hour was positively correlated, and predominantly short periods were negatively correlated, to complaints in the neck region. This seems promising in order to find duration limits for sustained low-level muscle activity as a risk factor for musculoskeletal disorders.     

A study of the electromyogram using a population stochastic model of skeletal muscle

The features of the electromyogram (EMG) are studied using a population model of skeletal muscle based on the differing properties and the independent activation of motor units (MUs). It is shown, both analytically and by computer simulation, that: (a) The power spectrum of the EMG is determined by the distribution of filtering and firing properties of the active MUs. (b) A tendency towards a rhythmical grouping of action potentials is to be expected from a set of asynchronous MUs firing semiregularly at similar rates; the grouped electrical activity has a phase-lead over the force output of the set of about 180°. A unified explanation of the properties of the muscle force waveform and the electromyogram, in terms of asynchronous activity of MUs, is proposed. The explanation covers the relationship and the differences between the two signals.     

Functional subdivision of the upper trapezius muscle during maximal isometric contractions

The electromyographic (EMG) activity pattern across the upper trapezius of 22 healthy subjects was investigated during maximal isometric contractions. Eight bipolar surface electrodes with 10 mm distance between adjacent electrode pairs were placed on a line from the clavicle to the scapula. At the region near the clavicle the highest EMG amplitudes were recorded during 90 ° arm abduction. At the more posterior parts the highest amplitudes were found both during arm abduction and shoulder elevation. A double differential recording technique which reduced the EMG cross-talk contribution supported the finding that the upper trapezius was differently activated when the arm posture was changed. The normalized EMG amplitude-force relationship during the shoulder elevation showed a curvilinear relationship on the anterior part of the upper trapezius with a slower increase in EMG amplitude than force at low force. The slope of the curve, at low force, increased gradually in the posterior direction on the upper trapezius. The EMG activity patterns across the upper trapezius indicate a flexibility in motor activation which maybe reflects a functional optimization of the contractions performed by this muscle.     

Long-term electromyographic activity in upper trapezius and low back muscles of women with moderate physical activity.

The habitual activity patterns of trapezius and postural back muscles (multifidus, iliocostalis, longissimus) of 23 female subjects with moderate physical activity were studied. Bilateral surface electromyographic (sEMG) recordings from start of work until bedtime were analyzed. The activity level was calibrated as percentage of root mean square-detected muscle activity at maximal voluntary contraction (EMG(max)). Sixty-six previous trapezius recordings of women with moderate physical activity were included in some analyses to pursue the full range of variation in trapezius activity. Twenty-six of these were recorded twice, separated by 16-28 mo. Median activity level and duration of periods with sEMG activity of <0.5% EMG(max) ("rest time"; only trapezius) and exceeding 2 ("burst time"), 10, 30, and 50% EMG(max) was determined. The trapezius median activity level ranged from 0.6 to 8.8% EMG(max), burst time from 9 to 84%, and rest time from 2 to 84%. The activity patterns of the back muscles showed similar large interindividual variation. Repeated trapezius recordings of the same subject showed high consistency; intraclass correlation coefficients ranged from 0.62 to 0.79 for different sEMG variables. Periods with high sEMG amplitude were of short duration; 7% of the trapezius recordings did not present time intervals (0.2-s duration) above 50% EMG(max). The activity patterns of the postural muscles, despite large interindividual variability, were distinctly different from activity patterns of upper and lower limb muscles reported by others (e.g., mean burst time 40-50 vs. 10-20%). We conclude that postural trunk muscles show idiosyncratic activity patterns with large interindividual variation. High-threshold motor units are activated to a very minor extent.     

Effects of whole body vibration on motor unit recruitment and threshold

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.     

Reliability of surface EMG matrix in locating the innervation zone of upper trapezius muscle

The identification of the motor unit (MU) innervation zone (IZ) using surface electromyographic (sEMG) signals detected on the skin with a linear array or a matrix of electrodes has been recently proposed in the literature. However, an analysis of the reliability of this procedure and, therefore, of the suitability of the sEMG signals for this purpose has not been reported.The purpose of this work is to describe the intra and inter-rater reliability and the suitability of surface EMG in locating the innervation zone of the upper trapezius muscle.Two operators were trained on electrode matrix positioning and sEMG signal analysis. Ten healthy subjects, instructed to perform a series of isometric contractions of the upper trapezius muscle participated in the study. The two operators collected sEMG signals and then independently estimated the IZ location through visual analysis.Results showed an almost perfect agreement for intra-rater and inter-rater reliability. The constancy of IZ location could be affected by the factors reflecting the population of active MUs and their IZs, including: the contraction intensity, the acquisition period analyzed, the contraction repetition. In almost all cases the IZ location shift due to these factors did not exceed 4 mm. Results generalization to other muscles should be made with caution.     

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)     

Trapezius muscle rest time during standardised computer work--a comparison of female computer users with and without self-reported neck/shoulder complaints.

Work related musculoskeletal disorders (WMSDs) in the shoulder/neck area are a common and increasing problem among European computer workers, especially women. Long-term low-level workloads with low degree of muscle rest are a potential risk factor for developing WMSDs. The purpose of the present study of female computer users (age 45-65 years) in Denmark and Sweden was to investigate if subjects with self-reported neck/shoulder complaints (cases, N=35) show less trapezius muscle relative rest time (RRT) than controls (N=44) when performing standardised short-term computer work tasks in controlled laboratory conditions. Surface electromyography (EMG) signals were recorded bilaterally from the upper trapezius muscles during a type, edit, precision and colour word stress task. Besides RRT, 10th percentile RMS values were calculated. On the average, 15 of the cases and 18 of the controls showed analysable EMG files per task. For the colour word stress task, the results showed lower RRT values and higher 10th percentile RMS amplitude levels among cases compared to controls. No such signs could be found for the other tasks performed. The present results indicate an increased motor response to a psychological stressor among subjects with self-reported neck/shoulder complaints.