Laboratory evaluation of belt usage with apple buckets

BACKGROUND: Migrant and seasonal orchard harvest workers experience musculoskeletal strain caused by carrying heavy loads and holding awkward postures. An ergonomic hip belt designed to redistribute weight from the upper back, neck, and shoulders to the hips was evaluated. METHODS: A comparison of muscle exertion (using electromyography [EMG]) of four key back and shoulder muscles and shoulder surface pressure (using a surface pressure sensor [PS]) was made among 10 laboratory volunteers under intervention and control conditions, in two postures (standing erect [0 degrees ]) and flexed [45 degrees ]). RESULTS: All but one measure showed a significant reduction in the flexed posture with the intervention. Two of eight measures showed a significant reduction in the erect posture. Correlations with shoulder discomfort were found for deep shoulder EMG (r=0.71, P=0.021) and shoulder surface pressure (r=0.58, P=0.082). CONCLUSIONS: The hip belt intervention produces significant reductions in muscle activity while in a posture commonly assumed during orchard work.     

Shoulder strain in keyboard workers and its alleviation by arm supports

Summary Keyboard work consists mostly of dynamic contractions of the small muscles of the forearms and hands. This is accompanied by continuous activity in the arm, shoulder and neck muscles keeping the head and hand in the correct position. Eliminating the weight from the arm by means of support and the position of the arms influences the electrical activity of shoulder muscles when working at a keyboard. We studied the influence of elbow angle, as well as that of different arm supports, on electrical activity of upper trapezius muscle during keyboard work in healthy workers and persons suffering from shoulder pains. The measurements were carried out in the laboratory. EMG activities, which where measured as mean square root (RMS)-values at every 100-millisecond period in trapezius muscle when working, were lower, the greater the elbow angle. Furthermore electrical activity decreased when subjects used arm supports while working. It is evident that the static load to shoulder muscles can be lowered significantly in keyboard work, when the forearms are at an angle of at least 100 degrees and by using arm supports. The most convienient and ergonomic working position can also be found individually be the method used here.     

Activity analysis: a case study utilising electromyography

The therapeutic skateboard in a case study, utilising electromyography (EMG) in the analysis of an assistive activity, is presented. The EMG levels of eight shoulder muscles during horizontal adduction and abduction were studied. Three conditions were analysed: the arm supported by a therapeutic skate, a five-pound weight belt on the supported arm and the non-assistive condition where the arm was unsupported. It was found that:

• 1 The therapeutic skate activity demanded little movement (3.8–48%) from the shoulder muscles and is suitable for early mobilisation.
• 2 The skateboard assists primarily the stabilisers in both horizontal abduction and adduction.
• 3 Addition of weights was a gentle upgrading activity creating a small increase in the eight muscles being monitored. Copyright © 1996 Whurr Publishers Ltd.

     

Electromyographic signs of shoulder muscle fatigue in repetitive arm work paced by the Methods-Time Measurement system.

Surface electromyographic (EMG) recordings from the lateral and cervical portions of the descending trapezius muscle and from the infraspinatus muscle were assessed for six female subjects performing a standardized repetitive work simulation task for 1 h. The work pace was determined according to the Methods-Time Measurement system to 2466 cycles.h-1. The variation in load levels were high and indicated dynamic muscle activity. A decrease in the EMG mean power frequency and an increase in the root-mean-square amplitudes, both indicating muscle fatigue, developed in all muscles of most of the subjects. EMG signs of fatigue, analyzed separately for each 5-min period, showed variable fatigue patterns both between muscles and between subjects. It is therefore suggested that dynamic work movements do not protect the muscles of the shoulder and neck from fatiguing processes in highly repetitive work with short cycle times.     

Effect of mental and physical demands on muscular activity during the use of a computer mouse and a keyboard

OBJECTIVES: This study explores the effects of mental and physical demands on muscular activity in the use of the mouse and keyboard in computer work in a time-pressed situation. METHODS: Twelve female subjects performed a computer task with four combinations of two forms of input device (computer mouse and keyboard) and two levels of mental demand (with and without a color word test). The time available for response was limited. Performance and electromyography (EMG) were recorded for the forearm, shoulder, and neck muscles. RESULTS: The mean static muscular activity was 2-7% of the maximum EMG for the forearm and neck extensor muscles. All the muscles for which recordings were made showed increased muscular activity in the mentally demanding condition. The increase in the static muscular activity level varied from 0.3% of the maximum EMG for the right trapezius muscle to 2.6% for the extensor carpi ulnaris muscle. Significant differences in muscular activity between the use of a computer mouse and a keyboard were only found for the neck extensors; in the computer mouse condition there was a static EMG level of 3.5% for the maximum EMG, compared with 2.8% for the keyboard. CONCLUSIONS: Mental demands during computer work increased muscular activity in all of the recorded muscles. Increased muscular activity was found in the neck during the use of the mouse in comparison with the use of the keyboard; this phenomenon may be related to higher visual demands during the use of a mouse than with a keyboard. These findings may help explain the adverse effects of psychosocial work factors on the musculoskeletal system.     

短时和快速傅里叶变换用于动态活动下表面肌电频谱分析的比较研究

目的短时傅里叶变换(short time Fourier transform,STFT)作为频谱分析的新方法比快速傅里叶变换(fast Fourier transform,FFT)更适合于非稳定信号,探讨这两种方法在分析评价动态活动肌肉疲劳的差异。方法9名男性大学生志愿者在实验室从事模拟的搬举活动(变换搬举重量和距离),测定搬举时左右两侧胸10和腰3水平竖脊肌的表面肌电信号。另10名志愿者在功率车上以3种速度跑步,记录此间双腿股内侧肌和股外侧肌的肌电。分别用STFT和FFT求得活动期间每秒钟肌电信号的中位频率(median frequency,MF),计算MF值依时间变化回归的斜率,比较两种算法所得结果的差异。结果搬举试验,STFT所得MF值多小于FFT,左侧胸10(LT10)、右侧腰3(RL3)竖脊肌比较,差异有统计学意义(P<0.05);右侧胸10(RT10)、左侧腰3(LL3)比较,差异无统计学意义(P>0.05)。STFT变异性多大于FFT。两种MF斜率比较,差异无统计学意义(P>0.05),然而二者间呈高相关性(r>0.5)。STFT与FFT疲劳检出率比较,差异无统计学意义(P>0.05)。跑步试验,两种方法计算所得4块肌肉MF比较,差异均无统计学意义(P>0.05)。STFT变异性多大于FFT。MF斜率和疲劳检出率比较,差异均无统计学意义(P>0.05)。两种MF斜率仍然呈高相关性(r>0.7)。结论至少在低负荷情况下,FFT依然可用于动态活动下肌电信号的频谱分析。仍值得进一步研究这两种频率变换方法以及EMG信号在不同活动和肌肉的特性。     

男性受试者搬举MAWL和RWL时的劳动负荷评价

目的对手工搬举方程在我国应用的适合性进行研究,为我国制订手工搬举作业劳动负荷的标准提供依据。方法用表面肌电信号技术和Borg量表对男性受试者搬举最大可接受搬举重量(MAWL)和推荐搬举重量(RWL)时的劳动负荷进行了评价。结果受试者对称搬举MAWL时竖脊肌肌电幅度在50%~70%MVE(最大自主收缩时肌电幅度)范围内波动,对称搬举RWL时在41%~71%MVE范围内波动。在水平距离25 cm处,从地面到可触及高度,搬举RWL时竖脊肌肌电幅度比搬举MAWL时高7.51%。从指节到可触及高度,搬举RWL时竖脊肌肌电幅度比搬举MAWL时高13.08%。非对称搬举MAWL时的竖脊肌肌电幅度明显高于搬举RWL时,差异有统计学意义(P<0.01)。身体各部位搬举MAWL时的自我用力感受(RPE)评分都明显高于搬举RWL时的RPE。结论美国职业安全卫生研究所(NIOSH)搬举方程对于水平距离和不对称的校正是充分的,但对于高度的校正是不足的,尤其是对于过肩的搬举作业是不适合的,对于过肩搬举作业的推荐搬举重量限值还应降低。     

Trapezius muscle load as a risk indicator for occupational shoulder-neck complaints

Summary Upper trapezius muscle activity was quantified by electromyogräphic (EMG) recordings using surface electrodes to study occupational muscle load as a risk indicator for the development of shoulder-neck complaints. Thirty-nine female' production workers and thirty-two female office workers showed much larger interindividual differences than the mean difference in muscle activity between the two groups. By comparison with the production workers, the muscle activity patterns of the office workers were characterized by more short pauses and a lower static load. The median load level was similar for the two groups. For the office workers, but not for the production workers, weak correlations were found between symptoms of pain in the shoulder-neck region and some of the EMG parameters (static level and frequency of micropauses > 0.6 s. Current techniques for measuring shoulder muscle load by EMG recordings seem inadequate as screening methods to predict future risk of development of muscle pain symptoms.     

Stress reactions to cognitively demanding tasks and open-plan office noise

Objectives  To investigate the effects of cognitively demanding work tasks and office noise on heart rate variability (HRV), cardiovascular responses and electromyography (EMG) activity in the trapezius muscles. Methods  Ten female volunteers were exposed to simulated open-plan office noise for 35 min (Leq 65 dBA), while engaged in cognitively demanding tasks. Task performance, self-rated stress and energy, affective state, perceived exertion in the shoulders and in the head, EMG in the left and right trapezius muscle, blood pressure, heart period length, HRV, and salivary cortisol were measured. Results  Cognitively demanding work tasks were associated with changes in HRV, systolic blood pressure and EMG that reflects increased sympathetic activity in the autonomic nervous system. No effect of noise was observed, except for a higher rating of perceived exertion in the head and, contrary to expectations, a 4% lower diastolic blood pressure in the noise conditions. Conclusions  Psychophysiological measures reflected the mental load imposed by cognitive work tasks. Short-term exposure to office noise resulted in increased ratings of perceived exertion in the head, but not in physiological stress reactions.     

Prospective study of the relationship between musculoskeletal and psychological complaints and electromyographic activity during isometric muscular contractions in a working population

OBJECTIVES: This study focused on determining whether musculoskeletal and psychological complaints reported monthly over a 4-month period predicted muscular activity during and immediately after standardized worktasks and whether muscular activity during and immediately after these tasks predicted changes in complaint severity in the following 12 months. METHODS: Surface electromyography (EMG) was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles of 45 postal workers (30 women) during sustained submaximal (25% of peak force) isometric contractions (wrist extension and shoulder abduction). Self-reported health complaints were recorded monthly. Musculoskeletal and psychological complaint-severity indices (MSI and PI, respectively) were computed from complaint-severity scores (intensity score x duration score). The history of complaints over the previous 4 months was included in adjusted regression models to predict muscular activity during and immediately after submaximal contractions. Muscular activity was included in adjusted models to predict changes in the complaint severity over the subsequent 12-month period. RESULTS: A higher MSI predicted a lower EMG level in the trapezius muscle during submaximal contractions (P<0.014), whereas the PI did not predict the level of EMG in any of the muscles studied (P > 0.194). The EMG activity did not predict changes in the complaint severity over the subsequent 12 months. CONCLUSIONS: These findings may support the hypotheses of pain adaptation or the dysfunction of synergistic muscular control in relation to musculoskeletal complaints. However, the findings did not indicate that increased or decreased muscular activity is a risk factor for heightened levels of complaints in the subsequent 12 months.     

Dose response effect of cement dust on respiratory muscles competence in cement mill workers

Electromyography (EMG) of respiratory muscles is a reliable method of assessing the ventilatory muscle function, but still its use has not been fully utilized to determine the occupational and environmental hazards on respiratory muscles. Therefore, EMG of intercostal muscles was performed to determine the dose response effect of cement dust on respiratory muscles competence. Matched cross-sectional study of EMG in 50 non-smoking cement mill workers with an age range of 20 - 60 years, who worked without the benefit of cement dust control ventilation or respiratory protective devices. EMG was performed by using surface electrodes and chart recorder. Significant reduction was observed in number of peaks (p < 0.0005), maximum peak amplitude (p < 0.0005), peak-to-peak amplitude (p < 0.0005) and duration of response (p < 0.0005) in cement mill workers compared to their matched control. Cement dust impairs the intercostal muscle competence and stratification of results shows a dose-effect of years of exposure in cement mill.     

Prevalence of musculoskeletal disorders among Indian railway sahayaks

Railway sahayak or also known as coolie is one of the oldest occupations in India. This occupational group is continuously subjected to various musculoskeletal problems due to repetitive load exertion in unnatural postures. The present research aims to examine the prevalence of musculoskeletal disorders (MSDs) among Indian railway sahayaks. A cross-sectional study has been conducted on a sample of 51 male sahayaks. With a modified Nordic questionnaire, data were collected on their demographics, work, and musculoskeletal pain history. Electromyography (EMG) of the upper trapezius muscle of selected 12 healthy sahayaks having no history of neck–shoulder complaints were analyzed under different load lifting and carrying conditions. Additionally, ergonomic risk levels were recorded using Rapid Entire Body Assessment method. The questionnaire revealed that for the past 1 year prevalence, the most affected body regions of railway sahayaks were neck (47%), shoulder (51%), lower back (43%), and knee (47%). The association between age group and MSDs in different body regions were found significant (p = 0.012 for neck, p = 0.017 for shoulder, etc.). Work experience also had a significant relation to the MSDs in different body regions (p = 0.013 for shoulder,p = 0.023 for calf, etc.). Analysis of variance reveals high significance level between EMG values and tasks performed by the sahayaks. In conclusion, railway sahayaks were found to have high rate of MSDs which may be associated with lifting/carrying heavy loads in awkward postures. Ergonomic interventions must be implemented to reduce the risk level on this group of workers.     

三种计算机工作台的工效学评价

为评价三种计算机操作的工效学特点，我们在实验室采用ＥＭＧ技术对使用三种操作台时腕部及肩部肌肉的负荷及疲劳的发生发展过程进行了研究。结果表明，使用新型台时桡侧腕屈肌的ＩＥＭＧ值比使用普通台和抽屈台时低，提示使用新型台时腕部负荷较小。随时间延长，使用普通台，抽屉台，新型台时桡侧腕屈肌的ＩＥＭＧ值分别在第１０分，３０分和６０分时达到最高值，提示使用新型台可延缓腕部肌肉疲劳的发生，而对肩部肌肉无明显影响。     

Effects of wearing the wrong glove size on shoulder and forearm muscle activities during simulated assembly work

This study was performed to determine the changes in electromyographic activities in the shoulder and forearm muscles when using the bare hands, well-fitting gloves, and gloves that are one size smaller or one size larger for simulated assembly operations. Sixteen asymptomatic seated workers with normal hands and no obvious deformities, skin diseases, or allergies were recruited. The subjects were asked to simulate assembly operations using their bare hands, well-fitting gloves, and one size smaller or one size larger. This study showed that wearing the wrong glove size led to a decrease in forceful activation of the forearm muscle and a compensatory increase in shoulder movement. In contrast, use of the bare hands or wearing well-fitting gloves led to effective forearm muscle activation, which decreased inefficient shoulder movement. These data indicate that wearing the wrong glove size will lead to continuous inefficient use of the forearm and shoulder muscles, and result in overuse of the shoulder.     

Changes in neuromuscular function due to intermittently increased workload during repetitive work in cold conditions

OBJECTIVES: This study attempted to determine whether repetitive work in the cold affects the number of electromyographic (EMG) gaps and whether cold-induced deterioration in neuromuscular function can be restored by altering work intensity. METHODS: During 2 hours of simulated sausage packing, the occurrence of EMG gaps was studied in eight upper-extremity muscles at 19 degrees C and 4 degrees C. The additional effect of continuous, light repetitive work [wrist flexion-extension 10% of the maximal voluntary contraction (MVC), Cwork] and intermittently increased workload (10% or 30% MVC) (Iwork) on forearm muscle EMG gaps, the median frequency of the EMG spectrum, EMG activity, and muscle fatigue (as indicated by changes in the MVC of wrist flexion) at 4 degrees C was studied. RESULTS: During work simulation, the mean skin temperature decreased from 33.3 (SD 0.1) degrees C to 30.6 (SD 0.2) degrees C (P<0.05), and the number of EMG gaps diminished in seven muscles by 1.4-68.2% (P<0.05). During Iwork there were 44% and 37% more EMG gaps in the forearm flexors and extensors, respectively, and the median frequency shifted to higher frequencies (P<0.05) as compared with the situation in Cwork. The average EMG activity of the forearm flexors was 13% lower (P<0.05) during Iwork than during Cwork. Wrist flexion MVC decreased 18% and 15% in Cwork and Iwork, respectively. CONCLUSIONS: The cold-induced decrease in the number of EMG gaps and increased muscle strain and fatigue can be, at least partially, restored by intermittently increasing the workload (ie, breaking the monotonous work cycle).     

Electromyographic study of respiratory muscles during human diving at 46 ATA

Electromyographic (EMG) recordings of the 6th to 7th intercostal space (thoracic EMG) and abdominal muscles, ventilatory pattern, and the work of breathing were studied in 4 human subjects exposed for 12 days to 46 ATA of helium-oxygen (density = 8.7 g.liter-1) then of nitrogen-helium-oxygen gas mixture (ternary mixture) (density = 11.1 g.liter-1). We found that the respiratory muscle work necessary for eupneic ventilation was multiplied by 4 at 46 ATA. During quiet breathing as well as during forced inspiratory maneuvers, the power spectrum of thoracic EMG shifted to the left in three individuals during the sojourn at maximal pressure, whichever gas mixture was inhaled. This was corroborated by the decreased ratio of EMG power in a high to that in a low band of frequencies. These alterations disappeared at the end of the decompression period, suggesting the existence of inspiratory muscle fatigue at high pressure. Hyperbaric tremor was recorded on the thoracic EMG and was maximal with He-O2 inhalation. It disappeared at the end of the period at 46 ATA (He-N2-O2).     

Acute response to precision, time pressure and mental demand during simulated computer work

OBJECTIVES: The electromyographic (EMG) activity of shoulder and forearm muscles was recorded during a standardized computer task with different combinations of time pressure, precision demands, and mental demands to study the interaction of these factors and their effect on muscular response during simulated computer work. METHODS: The computer task lasted 5 minutes, and it was performed by 14 female computer-aided design (CAD) operators during 8 exposure combinations that differed with respect to time pressure, precision demand, and mental demand. Performance (number of produced drawings, mouse clicks, and errors) were recorded. The EMG activity was recorded from the trapezius, infraspinatus, deltoid, and extensor digitorum muscles. An electrogoniometer was used to measure wrist postures and movements. RESULTS: High time pressure (combined with low precision and low mental demands) resulted in higher EMG activity for all the muscles and in a small increase in the number of produced drawings. High precision demands caused a large reduction in the number of produced drawings, but not always a change in EMG activity. High precision demands and high mental demands led to no change or a reduction in muscle activity because the number of drawings was greatly reduced. CONCLUSIONS: The interaction between work pace and other exposure factors must be taken into account when the effects of changes in exposure demands on muscular response are predicted. Only then can it be predicted whether changing demands will constitute a risk of developing musculoskeletal disorders.     

Bootstrap exploration of the duration of surface electromyography sampling in relation to the precision of exposure estimation

OBJECTIVES: This study examined the effect of sampling duration, in units of work cycles, on the precision of estimates of exposure to forceful exertion obtained with surface electromyography (EMG). METHODS: Recordings of the activity of the flexor digitorum superficialis, extensor digitorum, and upper trapezius muscles over 30 consecutive work cycles were obtained for a random sample of 25 manufacturing workers, each of whom was performing a unique production task representing a portion of the whole job. The mean root-mean-square amplitude and the 10th, 50th, and 90th percentiles of the distribution function of the amplitude probability were calculated for each cycle. Bootstrap analyses were used to examine the precision of the summary measures as the sampling duration increased incrementally from 1 to 30 work cycles. Precision was estimated by calculating the coefficient of variation (CV) of the bootstrap distributions at each sampling duration increment. RESULTS: The average minimum sampling duration for a bootstrap distribution CV of 15% ranged from 2.0 (SD 1.5) cycles to 7.5 (SD 9.6) cycles, depending on muscle and summary measure. For a 5% CV, the average minimum sampling duration ranged from 11.9 (SD 9.0) to 20.9 (SD 10.5) cycles. CONCLUSIONS: The results suggest that sampling as few as three work cycles was sufficient to obtain a bootstrap distribution CV of 15% for some of the muscles and summary measures examined in this study. While limited to machine-paced, cyclic manufacturing work, these results will assist the development of exposure assessment strategies in future epidemiologic studies of physical risk factors and musculoskeletal disorders.     

重复性搬举作业所致背部肌肉疲劳的表面肌电信号分析

目的　应用表面肌电信号 (SEMG)技术对重复性搬举所致的竖脊肌疲劳进行评价。方法　通过对 13名自愿者 10 0次搬举 8kg负荷至腰部高度的EMG波幅分析和搬举前后 2次静态实验EMG信号快速傅立叶转换 (FFT)后的频谱变化分析对竖脊肌疲劳进行评估。结果　 10 0次动态搬举过程中 ,随着时间的延长 ,EMG平均波幅逐渐上升 ,T10右竖脊肌和L3左竖脊肌平均波幅的差异有显著性 (P <0 .0 5、P <0 .0 1) ;而T10左竖脊肌和L3右竖脊肌的差异无显著性 (P >0 .0 5 )。中频截距在T10右竖脊肌、T10左竖脊肌、L3右竖脊肌、L3左竖脊肌均有下降。T10右竖脊肌中频截距下降了2 .0 % ,与搬举前的差异无显著性 (P >0 .0 5 ) ,T10左竖脊肌、L3右竖脊肌和L3左竖脊肌分别下降了10 .9%、2 9.9%、2 7.9% ,与搬举前的差异均有显著性 (P <0 .0 5、P <0 .0 1)。 10 0次搬举后平均功率频率截距在L3左竖脊肌下降了 9% ,与搬举前比较 ,差异有显著性 (P <0 .0 5 ) ,其他部位下降不明显。结论　重复性的搬举可导致竖脊肌疲劳 ,L3部位比T10部位更容易疲劳 ;用中频截距的下降评价肌肉疲劳比用平均功率频率截距下降更敏感。     

Surface EMG of shoulder and back muscles and posture analysis in secretaries typing at visual display units

Objective: A study was carried out to investigate temporal changes of activation of shoulder and back muscles in workers at visual display units by means of surface EMG. Moreover, postural parameters were recorded to distinguish fatigue-related from posture-related changes of the myoelectrical activity. Methods: Nine healthy female office workers typed texts spoken from tape during three 1-h-long sessions. After the first and again after the second hour there was a break of 15 min. Sixteen-channel surface EMG was bipolarly recorded from the erector spinae, trapezius, deltoid and sternocleidomastoid muscles. Root mean square (RMS) and power spectrum median frequency of the EMG were calculated. Sitting posture was assessed using an eight-channel movement analysis system with ultrasound markers. The position of the seventh cervical spinous process and the left and the right acromion were analysed synchronously with the EMG characteristics using regression analysis. Results: The normalised RMS of the left and right trapezius muscle increased, while the median frequency did not change. The increase of the normalised RMS was significantly lower when the linear influence of posture was excluded. On average, the distance between C7 and the left and right acromion decreased within each working an hour. C7 became lower on average by 5.5 mm within an hour, whereas the acromions became lower by only 1.7 mm (left) and 3.3 mm (right). Conclusion: The increase in trapezius muscle activity was partly related to a lifting of the shoulders to compensate a slight slumping of the back. Another part of the EMG activity increase has to be attributed to fatigue, to attention-related activity or to the combination of both. Therefore, training of the back muscles and a varied organisation of work might have a preventive effect with respect to musculoskeletal complaints in VDU workers. Received: 18 September 1998 / Accepted: 9 May 1999