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The purpose of this study was to determine whether objective electromyographical manifestations of muscle fatigue develop in the upper trapezius muscle in two assembly tasks involving contractions of different low-intensity levels (8% and 12% MVC) and whether these indications of fatigue are homogeneously distributed across different muscle parts. Ten subjects performed an assembly task for 3 h. EMG was recorded using four pairs of bipolar electrodes over the left and right trapezius muscles during the task itself and during isometric test contractions. Both recordings (during task and test) showed a significant decrease in the mean power frequency (MPF), at both intensity levels while the amplitude remained constant. A regression analysis showed significantly different temporal patterns for the MPF decrease for the two intensities. No differences in manifestations of muscle fatigue development were found between different parts of the muscle. These results indicate that in a highly repetitive low-intensity task, electromyographical manifestations of muscle fatigue can be observed from signals recorded in the task itself. Furthermore, the rate of development of fatigue manifestations was different between the two assembly tasks. This fatigue development appeared to be homogenous across the muscle. © 2008 Elsevier Ltd. All rights reserved.

A review of the literature on the pathophysiology of upper extremity muscle disorders (UEMDs) was performed. An overview is given of clinical findings and hypotheses on the pathogenesis of UEMDs. The literature indicates that disorders of muscle cells and limitations of the local circulation underlie UEMDs. However, these disorders identified do not necessarily lead to symptoms. The following mechanisms have been proposed in the literature: (1) selective recruitment and overloading of type I (Cinderella) motor units; (2) intra-cellular Ca2+ accumulation; (3) impaired blood flow; (3b) reperfusion injury; (3.3c) blood vessel-nociceptor interaction; (4a) myofascial force transmission; (4b) intramuscular shear forces; (5) trigger points; (6) impaired heat shock response. The results of the review indicate that there are multiple possible mechanisms, but none of the hypotheses forms a complete explanation and is sufficiently supported by empirical data. Overall, the literature indicates that: (1) sustained muscle activity, especially of type I motor units, may be a primary cause of UEMDs; (2) in UEMDs skeletal muscle may show changes in morphology, blood flow, and muscle activity; (3) accumulation of Ca2+ in the sarcoplasm may be the cause of muscle cell damage; (4) it seems plausible that suboptimal blood flow plays a role in pathogenesis of UEMDs; (5) since the presence of fiber disorders is not a sufficient condition for the development of UEMSDs additional mechanisms, such as sensitization, are assumed to play a role. © 2005 Elsevier Ltd. All rights reserved. Chemicals / CAS: calcium ion, 14127-61-8; cytochrome c oxidase, 72841-18-0, 9001-16-5

The objective of the present study was to gain insight into the effects of precision demands and mental pressure on the load of the upper extremity. Two computer mouse tasks were used: an aiming and a tracking task. Upper extremity loading was operationalized as the myo-electric activity of the wrist flexor and extensor and of the trapezius descendens muscles and the applied grip- and click-forces on the computer mouse. Performance measures, reflecting the accuracy in both tasks and the clicking rate in the aiming task, indicated that the levels of the independent variables resulted in distinguishable levels of accuracy and work pace. Precision demands had a small effect on upper extremity loading with a significant increase in the EMG-amplitudes (21%) of the wrist flexors during the aiming tasks. Precision had large effects on performance. Mental pressure had substantial effects on EMG-amplitudes with an increase of 22% in the trapezius when tracking and increases of 41% in the trapezius and 45% and 140% in the wrist extensors and flexors, respectively, when aiming. During aiming, grip- and click-forces increased by 51% and 40% respectively. Mental pressure had small effects on accuracy but large effects on tempo during aiming. Precision demands and mental pressure in aiming and tracking tasks with a computer mouse were found to coincide with increased muscle activity in some upper extremity muscles and increased force exertion on the computer mouse. Mental pressure caused significant effects on these parameters more often than precision demands. Precision and mental pressure were found to have effects on performance, with precision effects being significant for all performance measures studied and mental pressure effects for some of them. The results of this study suggest that precision demands and mental pressure increase upper extremity load, with mental pressure effects being larger than precision effects. The possible role of precision demands as an indirect mental stressor in working conditions is discussed.

The aim of the study was to investigate the effectiveness of a resistance-training program on muscle strength of the back and neck/shoulder muscles, relative physical workload, muscle fatigue and musculoskeletal discomfort during a simulated assembly and lifting task. Twenty-two workers were randomized over an 8-week resistance-training group, and a control group. Isokinetic muscle strength was assessed using the Cybex dynamometer, muscle fatigue was measured using EMG, and perceived discomfort was measured using a 10-point scale. At the follow-up, we found no effect of the resistance-training program on isokinetic muscle strength of the back and shoulder muscles. Furthermore, we did not find any effect on EMG data, nor on musculoskeletal discomfort during the simulated work tasks. However, trained workers performed the lifting tasks for a longer time before reporting considerable discomfort than those in the control group. © 2008 Elsevier Ltd. All rights reserved.

This paper presents an overview of theories concerning the development of RSI (repetitive strain injury), related to muscle disorders. Movement is a noisy process. The level of noise is affected by factors such as fatigue and psychosocial stress. In order for precision movements to be made in such situations, an increased level of muscle activity is required. Positive feedback loops through γ-motoneurones may be responsible for these increases in muscle activity. The Cinderella hypothesis suggests that damage may take place even when muscle activity is of low intensity, due to locally high levels of muscle load. An increase in calcium concentration in muscle cells during long periods of muscle activity may act as a mechanism of this damage. Optimisation of task demands and the prevention of continuous activation of the same groups of muscles appear to be effective measures in the prevention of RSI.

In combined cultures of chick muscle cells and ciliary ganglia, the effect of indirect, long-term stimulation on the number of neuromuscular contacts on individual muscle cells was studied. Seven days of phasic stimulation resulted in a marked reduction in the number of neuromuscular contacts, the majority of the muscle cells becoming innervated by only one neuron. Neuromuscular blockade by D-tubocurarine prevented this loss of innervation. Chemicals/CAS: tubocurarine chloride, 57-94-3, 57-95-4, 8006-51-7

This study was designed to determine whether trunk extensor fatigue occurs during low-level activity and whether this is associated with a drop in muscle tissue oxygenation. Electromyography (EMG) feedback was used to impose constant activity in a part of the trunk extensor muscles. We hypothesized that electromyographic manifestations of fatigue and decreased oxygenation would be observed at the feedback site and that EMG activity at other sites would be more variable without fatigue manifestations. Twelve volunteers performed 30-min contractions at 2% and 5% of the maximum EMG amplitude (EMGmax) at the feedback site. EMG was recorded from six sites over the lumbar extensor muscles and near-infrared spectroscopy was used to measure changes in oxygenation at the feedback site (left L3 level, 3 cm paravertebral). In both conditions, mean EMG activity was not significantly different between electrode sites, whereas the coefficient of variation was lower at the feedback site compared to other recording sites. The EMG mean power frequency (MPF) decreased consistently at the feedback site only. At 5% EMGmax, the decrease in MPF was significant at the group level at all sites ipsilateral to the feedback site. These results suggest that the limited variability of muscle activity at the EMG feedback site and at ipsilateral locations enhances fatigue development. No decreases in tissue oxygenation were detected. In conclusion, even at mean activity levels as low as 2% EMGmax, fatigue manifestations were found in the trunk extensors. These occured in absence of changes in oxygenation of the muscle tissue. © 2007 Elsevier Ltd. All rights reserved.

Previously tropic effects of extracts from whole chick embryos and from innervated muscles on cultured muscle cells were described. The present study demonstrated similar effects of extracts from 10-days denervated chick muscles. Extracts from innervated as well as from denervated muscles exsanguinated in vivo with saline prior to dissection showed only marginal trophic activity, suggesting a major contribution of serum components to the trophic effects of tissue extracts. Indeed, serum of adult chicks appeared to have a trophic action similar to that of chick embryo extract.

Smooth muscle cells (SMC) form the major cell type in the arterial blood vessels. In the undamaged vessel wall they remain in a contractile state characterized by the absence of cell division, a low metabolic activity and a high actin-myosin content. As a reaction to injury of the vessel wall they can, however, change their phenotype and start to proliferate and migrate through the vessel wall from the media towards the intima. Extracellular proteolysis is increased during the migration of SMCs and local lysis of the surrounding extracellular matrix is observed. The SMC produces several proteases during migration, especially plasminogen activators and metalloproteinases. Many growth factors and cytokines can modulate the production and activation of extracellular proteinases and their inhibitors in the activated vascular SMC. The production and regulation of the extracellular proteolytic enzymes by SMCs in relation to cell migration are discussed in this review.

Optimization of the temporal aspects of task design requires a better understanding of the development of muscle fatigue in the neck and shoulder region over time. The objective of the study was to investigate this in two production companies and to determine the relationship between objective and subjective estimates of fatigue. Indicators of fatigue were obtained through electromyography (EMG) during test contractions and ratings of perceived discomfort. EMG amplitude increased during the day in both case studies while mean power frequency decreased only in one case. In both cases, a more detailed frequency analysis of the EMG signals showed an increase in lower frequency power accompanied by a decrease in higher frequency power. Local perceived discomfort in the neck and shoulder increased over the course of the day in both cases. However, no clear relationship between perceived discomfort and objective indicators of fatigue was found. Obtaining sufficient sensitivity to detect effects of temporal aspects of task design probably requires complementary or more refined methods (e.g. EMG arrays, mechanomyography).

In lifting, the abdominal muscles are thought to be activated to stabilize the spine. As a detrimental effect, they contribute to spinal compression. The existing literature is not conclusive about the biological relevance of this effect. From biological, mechanical and anatomical considerations it was hypothesised that the relative abdominal contribution to compression would be minor in the beginning of the lift, that the relative and absolute abdominal contribution to compression would rise throughout the lift, and that the obliques would contribute to a larger extent than the rectus abdominis. To investigate these hypotheses, 10 subjects lifted 0.5, 10.5 and 22.5 kg. EMG levels obtained from the rectus abdominis and the obliques were converted into force using normalized EMG, muscle potential and area values, and modulating factors for muscle length and contraction velocity. An anatomical model was applied to compute the abdominal effects on spinal compression in three consecutive phases within a lift. If expressed relative to the total spinal compression, the abdominal contribution for the three weight conditions was 7.1% (SD, 1.7), 10.4% (4.7) and 12.5% (4.4) in the begin and 21.0% (5.8), 19.0% (5.3) and 22.2% (6.6) in the end phase. Thus, the relative abdominal contribution to compression was minor in the beginning and increased towards the end. The absolute abdominal contribution was constant throughout the lift. The contributions could be retraced to the obliques rather than the rectus, while during the lift a shift in activation from the obliquus externus to internus was observed.

The aim of this study was to evaluate the effect of a passive upper body exoskeleton on muscle activity, perceived musculoskeletal effort, local perceived pressure and subjective usability for a static overhead task. Eight participants (4 male, 4 female) held a load (0 kg and 2 kg) three times overhead for a duration of 30 s each, both with and without the exoskeleton. Muscle activity was significantly reduced for the Biceps Brachii (49%) and Medial Deltoid (62%) by the device for the 2 kg load. Perceived effort of the arms was significantly lower with the device for the 2 kg load (41%). The device did not have a significant effect on trunk or leg muscle activity (for the 2 kg load) or perceived effort. Local perceived pressure was rated below 2 (low pressure levels) for all contact areas assessed. Half of the participants rated the device usability as acceptable. The exoskeleton reduced muscle activity and perceived effort by the arms, and had no significant negative effect on the trunk and lower body with regards to muscle activity, perceived effort and localised discomfort. © 2018 Elsevier Ltd

It has been suggested that fatigue affects proprioception and consequently movement accuracy, the effects of which may be counteracted by increased muscle activity. To determine the effects of fatigue on tracking performance and muscle activity in the M. extensor carpi radialis (ECR), 11 female participants performed a 2-min tracking task with a computer mouse, before and immediately after a fatiguing wrist extension protocol. Tracking performance was significantly affected by fatigue. Percentage time on target was significantly lower in the first half of the task after the fatigue protocol, but was unaffected in the latter half of the task. Mean distance to target and the standard deviation of the distance to target were both increased after the fatigue protocol. The changed performance was accompanied by higher peak EMG amplitudes in the ECR, whereas the static and the median EMG levels were not affected. The results of this study showed that subjects changed tracking performance when fatigued in order to meet the task instruction to stay on target. Contrary to our expectations, this did not lead to an overall higher muscle activity, but to a selective increase in peak muscle activity levels of the ECR. © 2006 Elsevier Ltd. All rights reserved.

High precision demands in manual tasks can be expected to cause more selective use of a part of the muscular synergy involved. To test this expectation, load sharing of the index finger and middle finger was investigated during a pinching task. Myoelectric activation of lower arm and neck-shoulder muscles was measured to see if overall level of effort was affected by precision demands. Ten healthy female subjects performed pinching tasks with three levels of force and three levels of precision demands. The force level did not significantly affect the relative contribution of the index and middle finger to the force. Higher precision demands, however, led to higher contribution of the index finger to the pinch force. Consequently, a more selective load of the forearm and hand occurs during tasks with high precision demands. The variability of the force contribution of the fingers increased during the task. No effects of precision demand on the activation of forearm and neck-shoulder muscles were found. Force level did affect the EMG parameters of several muscles. The effects were most apparent in the muscles responsible for the pinch force, the forearm muscles. Activation of these muscles was higher at higher force levels. In the trapezius muscle at the dominant side EMG amplitudes were lower at the high pinch force compared to the low force and median force conditions. © 2002 Elsevier Science Ltd. All rights reserved.

The aim of this study was to evaluate the effect of an industrial exoskeleton on muscle activity, perceived musculoskeletal effort, measured and perceived contact pressure at the trunk, thighs and shoulders, and subjective usability for simple sagittal plane lifting and lowering conditions. Twelve male participants lifted and lowered a box of 7.5 kg and 15 kg, respectively, from mid-shin height to waist height, five times, both with and without the exoskeleton. The device significantly reduced muscle activity of the Erector Spinae (12%-15%) and Biceps Femoris (5%). Ratings of perceived musculoskeletal effort in the trunk region were significantly less with the device (9.5%-11.4%). The measured contact pressure was highest on the trunk (91.7 kPa-93.8 kPa) and least on shoulders (47.6 kPa-51.7 kPa), whereas pressure was perceived highest on the thighs (35-44% of Max LPP). Six of the users rated the device usability as acceptable. The exoskeleton reduced musculoskeletal loading on the lower back and assisted with hip extensor torque during lifting and lowering. Contact pressures fell below the Pain Pressure Threshold. Perceived pressure was not exceptionally high, but sufficiently high to cause discomfort if used for long durations.

Shoulder fatigue has been suggested to be a useful risk indicator for shoulder disorders in repetitive, low-force work tasks. In contrast to high-force or purely isometric tasks, it is unclear whether measurable fatigue develops in realistic low-force work. The question addressed in this review was: 'Is there evidence of objective signs of fatigue in the shoulder region in realistic, low-force work tasks?' Studies on objective measures of fatigue applied in realistic low-force work tasks were systematically reviewed, using a task duration of more than 1 h and an intensity level of less than 20% maximum voluntary contraction (MVC) for the median trapezius activation level as inclusion criteria. Thirteen studies were found to fulfil the criteria. All these studies addressed fatigue-related changes in the electromyographic signal in the descending part of the trapezius muscle. Seven did find a combination of frequency decrease and amplitude increase over time, which is generally considered as an objective manifestation of fatigue. Thus, there is evidence of objective signs of fatigue in some of the realistic, low-force tasks. The intensity level appeared to be a main determinant here. In the studies demonstrating signs of fatigue an intensity level of 15%MVC or more was used, while the intensity level in the studies with a negative result was generally lower.

The human physiological system is stressed to its limits during endurance sports competition events.We describe a whole body computational model for energy conversion during bicycle racing. About 23 per cent of the metabolic energy is used for muscle work, the rest is converted to heat. We calculated heat transfer by conduction and blood flow inside the body, and heat transfer from the skin by radiation, convection and sweat evaporation, resulting in temperature changes in 25 body compartments. We simulated a mountain time trial to Alpe d'Huez during the Tour de France. To approach the time realized by Lance Armstrong in 2004, very high oxygen uptake must be sustained by the simulated cyclist. Temperature was predicted to reach 39°C in the brain, and 39.7°C in leg muscle. In addition to the macroscopic simulation, we analysed the buffering of bursts of high adenosine triphosphate hydrolysis by creatine kinase during cyclical muscle activity at the biochemical pathway level. To investigate the low oxygen to carbohydrate ratio for the brain, which takes up lactate during exercise, we calculated the flux distribution in cerebral energy metabolism. Computational modelling of the human body, describing heat exchange and energy metabolism, makes simulation of endurance sports events feasible. This journal is © 2011 The Royal Society.

To study the effects of a tactile feedback signal in a computer mouse on reduction of hovering behaviour and consequently on changes in muscle load, productivity, comfort and user friendliness, a comparative, experimental study with repeated measures was conducted. Fifteen subjects performed five trials with different mouse actions and a standardised task, once with a mouse with the feedback signal and once with a mouse without the feedback signal. Holding the hand just above the mouse caused higher muscle loading than clicking and scrolling. Holding the hand on the mouse caused higher muscle loading than resting the hand on the desk. The feedback signal effectively decreased hovering behaviour. It also led to a more dynamic activation pattern of the extensor muscles of the forearm. The overall opinion of the feedback signal for future use was rated as somewhat variable. No effects on discomfort or productivity were found. The use of a mouse with a tactile vibrating feedback signal seems promising for preventing arm complaints, although more research is needed to establish the clinical relevance.

The objective of this study was to investigate the role of the plasminogen activation system in the migration of human vascular smooth muscle cells in vitro. After wounding of confluent human smooth muscle cell cultures by stripping cells from their extracellular matrix, cells start to migrate from the wounded edge into the denuded area. Addition of plasmin to the culture medium resulted in an approximately 50% increase of migrated cells after 24 hours. The plasmin inhibitor aprotinin was able to reduce this effect to control levels. Migration could also be stimulated by addition of urokinase-type plasminogen activator (HMW-u-PA) (30%) or tissue-type plasminogen activator (t-PA) (28%). Simultaneous addition of aprotinin reduced this increase below control levels, indicating that both HMW-u-PA and t-PA mediated plasminogen activation contributes to smooth muscle cell migration. Addition of low molecular weight u-PA (LMW-u-PA), a u-PA form lacking the receptor binding domain, or the aminoterminal fragment of u-PA (ATP), lacking the active site, had no effect on migration. These results suggest that both t-PA and u-PA can contribute to human smooth muscle cell migration in vitro, most likely via plasminogen activation. For the stimulation of migration by u-PA, activity as well as binding to its cell-surface receptor appears to be involved.

1. Two growth trials were performed to measure the effects of dietary methionine and cystine (SAA) on growth rate, food conversion efficiency and breast meat deposition in male broilers. 2. In experiment 1, broilers were grown on 6 experimental diets covering a range from 6.9 to 9.6 g SAA/kg. The diets were fed from 15 to 33 d of age. Similarly, in experiment 2, 6 diets containing 6.0 to 8.5 g SAA/kg were fed to finishing broilers 33 to 43 d of age. In each experiment 60 birds per treatment were processed, and carcase yield and breast meat percentage were determined. 3. Significant responses in weight gain, efficiency of food conversion and breast meat percentage were detected, which could be described well by exponential regression curves. Dietary SAA requirements to obtain maximum efficiency of food utilisation and maximum breast meat deposition were estimated to be about 9.0 g/kg from 15 to 33 d of age, and about 8.0 g/kg from 33 to 43 d of age. 4. Economic aspects were considered to calculate optimum SAA specifications from the results. In both trials, the dietary optimum of SAA was found to be higher for birds to be further processed than for birds to be marketed as whole carcases.