EMG Gaps Research Articles

Surface Electromyography of the Trapezius and Sternocleidomastoid during Computer Work with Presbyopic Corrections.

During computer work in controlled laboratory conditions, wearing multifocal contact lenses (MFCLs) showed no lower muscle load but increased subjective perception of comfort with equivalent visual quality and comparable tolerance. Because musculoskeletal complaints are frequent among computer workers, this study used the muscle electrophysiological activity of shoulder and neck muscles in presbyopic computer workers who received either progressive addition lenses for general purpose (GP-PALs) or MFCLs. For this crossover study, 11 presbyopic computer workers aged 55 ± 4 years (mean ± standard deviation) were equipped with GP-PALs and MFCLs in a randomized order. Surface electromyography signals were recorded bilaterally from shoulder and neck muscles during short-term computer work tasks using an optimally adjusted visual display unit workplace. The amplitude probability distribution function, the number and total duration of EMG gaps, and sustained low-level muscle activity periods of the surface electromyography signals were calculated. Comfort and correction type preferences were assessed. Head inclination was objectively evaluated. Multifocal contact lenses elicited no significant lower muscle load than GP-PALs. The number of sustained low-level muscle activity periods longer than 60 seconds was similar between visual aids. The total amount of gaps was significantly higher with MFCLs (44 gaps) compared with progressive addition lenses for general purpose (15 gaps) in all analyzed periods for all participants. However, there were no significant differences for the median in the intraindividual comparisons (P = .22, dz = 0.52). Multifocal contact lenses scored statistically significant higher in comfort values with equivalent visual quality and comparable tolerance (P = .003, dz = 1.51). Although the study failed to show clear results, wearing MFCLs seems to enhance working comfort compared with GP-PALs subjectively.

Trapezius muscle EMG as predictor of mental stress

Stress is a growing problem in society and can cause musculoskeletal complaints. It would be useful to measure stress for prevention of stress-related health problems. An experiment is described in which EMG signals of the upper trapezius muscle were measured with a wireless system during three different stressful conditions: a calculation task (the Norinder test), a logical puzzle task and a memory task. The latter two tests were newly designed and aimed at creating circumstances that are similar to work stress. Amplitudes of the EMG signals were significantly higher during stress compared to rest (+2.6% of reference contraction level) and relative time with EMG gaps was lower during stress (−14.3% of time). Also, mean and median frequencies were significantly lower during stress than during rest (−8.6 and −8.8 Hz, respectively). EMG amplitude increased not only from rest to stress conditions, but also during stressful conditions and decreased during relaxation periods. EMG features correlated with subjectively indicated stress levels (correlations of 0.32 with RMS and −0.32 with relative gaptime). The results indicate that EMG is a useful parameter to detect stress. Together with other physiological sensors, EMG sensors can be included in a wireless system for ambulatory monitoring of stress levels.

Intermittently Increased Repetitive Work Intensity and Neuromuscular Function in the Cold

This study evaluated whether cold-induced deterioration in neuromuscular function can be restored by intermittently increasing the workload. We examined the level of muscular strain, agonist-antagonist co-activation, the occurrence of EMG gaps and neuromuscular efficiency in wrist flexor and extensor muscles at 21°C (TN) and 4°C (C(10)) with a 10%MVC workload. During second exposure to 4°C (C(50)) the workload was increased every fourth minute to 50%MVC. The results indicated that muscular strain and co-activation was the highest and the amount of EMG gaps and neuromuscular efficiency the lowest at C(10). By intermittently increasing the workload at C(50) we were able to reduce muscular strain and co-activation (p<0.05) and induce a trend like increase in EMG gaps and enhance neuromuscular efficiency in relation to C(10) (NS). It may be concluded that intermittently increasing the workload, i.e. breaking the monotonous work cycle was able to partially restore neuromuscular function.

Effect of local leg cooling on upper limb trajectories and muscle function and whole body dynamic balance

This study was designed to find out if local leg cooling affects muscle function and trajectories of the upper limb during repetitive light work as well as capability to maintain dynamic balance. Nine healthy female subjects performed repetitive lifting task with right hand for 60 min while standing in front of a table with six target angles (30 degrees -220 degrees ) and with the legs inside a container with 15 degrees C cold water (Cold condition, C) or without water (Normal condition, N). Muscle temperature of the medial aspect of the gastrocnemius, rectal, and skin temperatures were measured continuously. The trajectories of the right upper limb were recorded with a 3D motion analysis system. Muscular strain (averaged EMG, a-EMG) and EMG gaps in eight muscles of the right upper limb were measured. End point excursion depicting the ability to maintain dynamic balance was measured before and after each experiment. Leg cooling decreased significantly (P < 0.05) the muscle and the mean skin temperature in C compared with N (6.7 and 2.2 degrees C, respectively). No marked changes in the trajectories or EMG activity were observed between the different environmental conditions. The end point excursion was significantly (P < 0.05) reduced in C compared with N and a positive correlation between excursion and muscle temperature was found at the end of the working period in C. In conclusion, local leg cooling did not affect upper limb muscle function or trajectories, but ability to maintain dynamic balance was reduced.

Neck and Back Muscle Loading in Pilots Flying High G&lt;SUB&gt;z&lt;/SUB&gt; Sorties With and Without Lumbar Support

Fighter pilots frequently encounter neck and back pain and injuries due to high G(z) loading. A lumbar support could diminish muscle strain in the lower back under G(z) loading and might also have a positive effect on cervical muscle strain, due to a more ergonomic sitting posture. The aim of this study was to determine the effects of individually adjusted lumbar supports on fighter pilots' neck and back muscle loadings. There were 11 Finnish Air Force pilots who acted as test subjects. They flew two basic air combat maneuvering sorties with and without the LS. The EMG activity of the sternocleidomastoid (SCM), cervical (CES), thoracic (TES), and lumbar erector spinae muscles (LES) were measured during these sorties, and the number and duration of EMG gaps was analyzed. Subjective experiences about the lumbar support were evaluated using a questionnaire. In all measured muscles, mean percent maximal voluntary contraction (%MVC) was lower when the LS was used, yet these changes were not statistically significant. Individual differences in %MVC between flights diminished in the CES (9%), TES (7%), and LES (8%) with LS in use. There was also a tendency toward increased number of gaps in EMG activity when flying with the LS. According to questionnaire responses, the LS seems to relieve in-flight symptoms and reduce the fatigue in the lower back muscles. Not all subjects benefited from the LS, however. There seems to be a tendency toward a lower muscle strain with the LS, but there are considerable individual differences.

Mental stress and trapezius muscle activation under psychomotor challenge: A focus on EMG gaps during computer work

Momentary reductions in the electrical activity of working muscles (EMG gaps) contribute to the explanation for the relationship between psychosocial stress and musculoskeletal problems in computer work. EMG activity and gaps in the left and right trapezii were monitored in 23 participants under low and high mental workload (LMW and HMW) demands during computer data entry. Increases in EMG activity and decreases in EMG-gap frequencies in both left and right trapezius muscles were greater during HMW than LMW. In addition, heart period and end-tidal CO2 were lower during HMW, whereas self-reported mood states were higher during HMW. The correspondence between lower end-tidal CO2 and lower EMG-gap frequencies suggests that hyperventilation (overbreathing) may mediate trapezius muscle activation. The reduction of EMG gaps suggests that the salutary benefits of momentary rest from musculoskeletal work are diminished during mental stress.

Effects of cooling and clothing on vertical trajectories of the upper arm and muscle functions during repetitive light work

The present study was designed to find out if cooling and/or clothing affect the vertical trajectories and muscle function of the upper arm during repetitive light work. Twelve female subjects performed a one-handed lifting task for 60 min while standing in front of a table with six target angles (30 degrees to 220 degrees ). The experiment was carried out in a climatic chamber in three different conditions: at 10 degrees C (C), at 25 degrees C (TN), and at 10 degrees C dressed in cold-protective clothing (C(p)). Skin and rectal temperatures were measured continuously. The vertical trajectories of the head, shoulder, elbow, and wrist on the right side of the body were recorded. Muscular strain (averaged EMG, a-EMG) and EMG gaps in eight muscles on the right upper arm were measured. The variation of the vertical trajectory amplitude of the upper arm measured from the elbow was significantly higher (at 200 degrees ) both at C and C(p) (50 and 25% respectively) and in shoulder (at 220 degrees angle) at C (33%) compared with TN (P < 0.05). Both C and C(p) increased a-EMG and reduced the number and duration of EMG gaps significantly in all muscles studied. In conclusion, in repetitive tasks the high mean vertical trajectory and changes in the amplitude of the trajectory of the upper arm at C and C(p) compared with TN were associated with increased muscular strain and reduced number of EMG gaps (more continuous activation of given muscle fibers). The changes in trajectories may serve as indicator of a risk for local muscle fatigue.

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

Aim The purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain. Method Surface 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. Results A 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. Conclusions Number 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.

Physical workload during use of speech recognition and traditional computer input devices

Musculoskeletal symptoms among computer users are frequently found. The aim was to investigate the musculoskeletal workload during computer work using speech recognition and traditional computer input devices (keyboard/mouse). Ten experienced computer users (nine female, one male) participated. They performed three different computer tasks: (1) text entry and (2) text editing of a standard text and (3) a self-selected work task. These tasks were performed twice using speech recognition and traditional computer input devices (keyboard/mouse). Additionally, a task consisting of reading aloud of the standard text was performed. Surface EMG from the forearm (m. extensor carpi ulnaris, m. extensor carpi radialis), the shoulder (m. trapezius) and the neck extensor muscles was recorded, in addition to the voice-related muscles (m. scalenii, m. cricothyroideus). Using speech recognition during text entry and text editing reduced the static muscle activity of the forearm, neck and to some extent the shoulder muscles. Furthermore, tendencies to longer periods of muscle activity pause (relative time with EMG gaps) in the forearm and shoulder muscles were found. This was seen at the expense of a tendency to an increased static activity and a decreased relative time with EMG gaps in m. cricothyroideus. Finally, during use of speech recognition the hand was tied to the keyboard/mouse for a shorter period of time, while the eyes were viewing the screen for a longer period of time compared to the condition with traditional computer input devices. It is recommended to use speech recognition as a supplementary tool to traditional computer input devices.

The influence of psychological stressors on myoelectrical signal activity in the shoulder region during a data entry task

Psychological stress is suspected to play a key role in the development of work-related musculoskeletal disorders (WMSDs), and work pace may be a primary contributor. The purpose of this study was to observe how psychological stress affects muscle activity recorded from key muscles used to support the shoulder (the trapezius and levator scapulae muscles) during keying tasks. We hypothesized that muscle activity [amplitude of the myoelectric signal (MES)] would increase and the number of periodic rests in muscle activity ('EMG gaps') would decrease when computer work was performed at an increased pace, and with distractions in the environment. Ten healthy volunteers performed three typing tasks (N = no stress; I = stress induced by a work pace reward; IS = stress induced by a work pace reward plus irritation/distraction). Both muscles showed significant increases ( p < .05) in MES amplitude when comparing IS to the N and I conditions, whereas N and I were not different. There was significant reduction in the number of gaps observed during the I and IS conditions (essentially no gaps were observed) as compared to the N condition. Interestingly, MES amplitude measures were capable of preferentially detecting changes in activation due to the distraction stress, whereas the number of EMG gaps preferentially detected the stress of the increase in work pace. The effect of these stressors on heart rate and productivity are also discussed.

Trapezius muscle activity as a risk indicator for shoulder and neck pain in female service workers with low biomechanical exposure

Electromyographic activity of the upper trapezius muscles was recorded over the workday for two groups of service workers, shopping centre (n = 22) and healthcare workers (n = 44), both with low observed biomechanical exposure. Static and median EMG activity level, number of EMG gaps and gap time were determined. The variability of these variables over the workday was examined by calculating the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) of 1-h consecutive recording periods. All variables except gap time showed acceptable reliability (ICC = 0.69-0.78), i.e. the largest fraction of variance in the data set was due to intersubject variance, despite relatively large hour-to-hour variation (CV = 0.21-0.62). The EMG activity level in the trapezius muscles was low (static activity level < 1% EMGmax), despite the high prevalence of shoulder and neck pain for both groups of workers. In addition to the work recordings, tests were performed to determine intersubject variation in muscle activity when adopting a standardized resting posture, and in a dynamic muscle activity pattern during paced arm movement. Neither the EMG variables from the work recordings nor the tests with EMG recording indicated higher trapezius EMG activity levels for workers with pain in the shoulders and neck in this study. The low EMG levels are interpreted to indicate a low risk of developing shoulder and neck complaints due to biomechanical exposure for both groups of workers. The possibility of pain-initiating mechanisms, associated with stress and not mediated through muscle activity, is considered in the discussion.

Trapezius muscle activity as a risk indicator for shoulder and neck pain in female service workers with low biomechanical exposure

Electromyographic activity of the upper trapezius muscles was recorded over the workday for two groups of service workers, shopping centre (n = 22) and healthcare workers (n = 44), both with low observed biomechanical exposure. Static and median EMG activity level, number of EMG gaps and gap time were determined. The variability of these variables over the workday was examined by calculating the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) of 1-h consecutive recording periods. All variables except gap time showed acceptable reliability (ICC = 0.69-0.78), i.e. the largest fraction of variance in the data set was due to intersubject variance, despite relatively large hour-to-hour variation (CV = 0.21-0.62). The EMG activity level in the trapezius muscles was low (static activity level < 1% EMGmax), despite the high prevalence of shoulder and neck pain for both groups of workers. In addition to the work recordings, tests were performed to determine intersubject variation in muscle activity when adopting a standardized resting posture, and in a dynamic muscle activity pattern during paced arm movement. Neither the EMG variables from the work recordings nor the tests with EMG recording indicated higher trapezius EMG activity levels for workers with pain in the shoulders and neck in this study. The low EMG levels are interpreted to indicate a low risk of developing shoulder and neck complaints due to biomechanical exposure for both groups of workers. The possibility of pain-initiating mechanisms, associated with stress and not mediated through muscle activity, is considered in the discussion.

Methodologies for evaluating electromyographic field data in ergonomics

Surface electromyography (SEMG) is an important tool for work load assessment in ergonomics. Several different approaches using amplitude as well as frequency parameters give fruitful information depending on question at issue in the laboratory as well as in field studies. One basic factor determining the choice of analysis method is whether the SEMG is interpreted as an indicator of forces/torques or pure muscular activation. Two methods for occupational SEMG data reduction representing two different approaches to SEMG applications in ergonomics, Exposure Variation Analysis (EVA), and Joint Analysis of EMG Spectrum and Amplitude (JASA), applied on the same SEMG recording from three muscles during urology surgeon work, have been compared. The EVA method categorised the three muscle recordings as too static with no EMG gaps while the JASA method identifies fatigue in two of the three recordings. The practical relevance of these findings is discussed.

Musculoskeletal Workload during Non-Linear Video Editing

Today, video editing in TV production is mainly performed using digital computer systems. The aim was to evaluate the musculoskeletal workload among the technicians performing digital (non-linear) video editing. Six technicians volunteered in the study and they performed non-linear video editing during a normal day. As a reference they performed a multidirectional computer mouse task. Muscle activity (EMG) was recorded from the forearm muscles (m. extensor carpi radialis, m. extensor digitorum and the forearm flexors) and from the shoulder/neck region (right and left m. trapezius, neck muscles (right side)). Perceived exertion was rated according to a CR-10 scale. The perceived exertion located to hand/wrist, forearm/elbow and shoulder and neck increased during the working day. Very few or none EMG gaps were found for the forearm extensor muscles during work. A static loading of the neck and the forearm extensor muscles was found during non-linear video editing despite the low number of input device operations per time (mouse clicks and keystrokes) and a well adjusted work place where the forearms were supported most of the working time. This may be related to high demands on precision in time and constrained working postures with the hands alert on keyboard and mouse and the eyes focused on monitors for the major part of the working time. Finally, eye blink frequency was found to be very low during non-linear video editing.

Stress responses in low-status jobs and their relationship to health risks: musculoskeletal disorders.

Conditions typical of many low-status jobs are known to induce elevated stress. In keeping with this, blue-collar workers show elevated psychophysiological stress levels both during and after work compared with workers in more stimulating and flexible jobs. Health-related behaviors, such as cigarette smoking and drug abuse, that are known to contribute to the social gradient in health, can be seen as ways of coping with a stressful work situation in order to get short-term relief. Negative emotional states associated with low-status jobs, combined with a lack of economic resources, are also likely to reduce the individual's motivation to seek proper medical treatment and, thus, increase the risk that transient symptoms develop into chronic illness. With regard to musculoskeletal disorders, it is well documented that physically monotonous or repetitive work is associated with an increase in neck, shoulder, and low back pain problems. However, recent studies also report an association between psychosocial factors and muscle pain syndromes. Possible mechanisms explaining these findings involve the assumption that psychological stress may induce sustained activation of small, low-threshold motor units that may lead to degenerative processes, damage, and pain. Analysis of short periods of very low muscular electrical activity (EMG gaps) shows that female workers with a high frequency of EMG gaps seem to have less risk of developing myalgia problems than do workers with fewer gaps. Stress induced by psychosocial conditions at work, which is usually more lasting than that resulting from physical demands, may prevent the individual from shutting off their physiological activation and reduces the time for rest and recovery. In the modern work environment, with strong emphasis on a high work pace, competitiveness, and efficiency, it is possible that lack of relaxation is an even more important health problem than is the absolute level of contraction or the frequency of muscular activation.

Upper trapezius muscle activity patterns during repetitive manual material handling and work 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 continous 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.