Prolonged Static Loading Research Articles

The ergonomics of laparoscopic surgery: a quantitative study of the time and motion of laparoscopic surgeons in live surgical environments.

Laparoscopic surgery presents multiple ergonomic difficulties for the surgeon, requiring awkward body postures and prolonged static muscle loading that increases risk of musculoskeletal strain and injury. This prospective study quantitatively measures the biomechanical movements of surgeons during laparoscopic procedures to determine at-risk movements from prolonged static muscle loading and repetitive motions that may lead to injury. A total of 150 video recordings of 18 surgeons, standing at the patient's left, were captured from three fixed camera positions during live gynecological laparoscopic surgery. Postoperative processing quantified surgeon movements at the neck, shoulders and elbows using computer software to measure extreme joint angles and time spent within defined joint angle ranges. Surgeons spent a median of 98% (range 77-100%) of surgical time with their neck rotated at 21° (range 0°-52°). The non-dominant arm was subjected to more extreme positions for significantly longer periods of time compared to the dominant, with shoulder flexion at 45°-90° for 35 vs. 0% (p<0.001) and elbow flexion at >120° for 31 vs. 0% (p<0.001) of total surgical time. Procedures involving power morcellation required significantly greater number of instrument insertion/removals-119 (range 56-182) compared with 12 (range 2-122) when morcellation was not used (p<0.001). Shorter surgeons maintained significantly greater degrees of neck rotation when viewing the monitor (p<0.003) and surgeons with shorter arm lengths spent longer in extreme positions with their non-dominant shoulder at >90° (p=0.04) and elbow at >120° (p<0.001) compared with taller surgeons. No significant correlations were found between BMI or surgical experience and more extreme joint positions. Four primary areas have been identified where surgeons are consistently demonstrating movements that increase their risk of harm: (1) extended periods of neck rotation; (2) asymmetrical loading between the dominant and non-dominant shoulders; (3) power morcellation and frequent insertions/removals of laparoscopic instruments resulting in repetitions of the most extreme shoulder positions and (4) a negative correlation between height and percentage time spent in more extreme positions.

Multimodal exercise program for promoting physical and cognitive health in people with mild to moderate dementia: a feasibility study

(mean = 24.5◦ ± 13.4◦, range = 9.3◦–66.1◦) and active device (mean = 8.0◦ ± 2.8◦, range = 2.7◦–15.4◦). 17 out of 36 subjects (44.7%) reported neck discomfort and 14 subjects (36.8%) reported ULD while playing with handheld device. With active device, 3 out of 36 subjects (7.9%) reported neck discomfort and 11 subjects (28.9%) reported ULD. During handheld device play, positive correlations were found between left CES activity and ULD (ρ = 0.362, n= 36, p= 0.30), and left UT activity and ULD (ρ = 0.362, n= 36, p= 0.30). Conclusion(s): The present results suggest that adolescents are susceptible to musculoskeletal discomforts when they play with EG devices which can lead to prolonged neck flexion posture and static muscle loading. This problem is especially accentuated in using the small-screen handheld game devices, and this may have an adverse effect on musculoskeletal health. Implications: Physical therapists play a leading role in public health education involving musculoskeletal health of the young population, who are becoming intensively relying on handheld electronic device. Acquiring knowledge from more experimental studies on how the musculoskeletal response with prolonged use of these devices, physical therapists can design effective programs to promote healthy use of electronic devices.

Comparative Analysis of Methods for Testing Materials for Fatigue Cracking Resistance

The conventional methods of testing for fatigue cracking resistance (compact samples) are compared with testing with a constant amplitude of the stress intensity factor (DCB samples). Examples are presented of the effect of the ambient environment on the growth rate of a crack in a DCB sample and constant growth rate of a crack in a double beam sample with constant amplitude of the stress intensity factor. The advantages and limitations of the testing methods studied are discussed. It is noted that DCB samples and double-beam samples with crack length independent stress intensity factor hold promise for studying the regularities of crack growth not only under cyclic but also under prolonged static loading.

Математическое моделирование деформирования армированной бедренной кости при длительных статических нагрузках

Предложена двухслойная математическая модель шейки бедренной кости человека, армированной имплантатами различной конструкции, для моделирования напряжeнно-деформированного состояния, возникающего при хирургической методике профилактики переломов шейки бедра путeм принудительного введения металлических имплантатов. Приведены спроектированные конструкции имплантатов. Разработаны методика и программное обеспечение для геометрического моделирования бедренной кости с внедрeнными имплантатами. Сформулированы постановки новых краевых задач для оценки кинетики напряжeнно-деформированного состояния в армированной и неармированной шейке бедра при длительных статических нагрузках, соответствующих хождению человека, в условиях ползучести. Приведены эффективные упругие характеристики для кортикальной и губчатой кости, силовые и кинематические граничные условия задач. Построена феноменологическая модель ползучести компактной костной ткани, разработана методика идентификации параметров и выполнена проверка еe адекватности экспериментальным данным. На основе метода конечных элементов разработан численный метод решения поставленных краевых задач на уровне макромеханики сплошных сред. Многочисленные вариативные расчeты позволили выработать рекомендации по рациональному позиционированию имплантатов для максимального снижения концентрации напряжений. Выполненный анализ показал, что происходит существенная релаксация напряжений в наиболее нагруженных областях вследствие ползучести, причeм в армированной шейке бедра она происходит более интенсивно, чем в неармированной. Так, для имплантата «спица + спица» при длительности нагружения в течение 1 года при естественных нагрузках, соответствующих хождению человека, напряжeнность в наиболее нагруженной области шейки бедра вследствие ползучести снижается на 49% по отношению к напряжeнности в начальный момент приложения нагрузки. Установлено, что временная составляющая (длительная стационарная нагрузка) не ухудшает положительный эффект от снижения концентрации напряжений за счeт армирования шейки бедра, что является позитивным моментом с точки зрения медицинской практики.

Impact of shoulder position and fatigue on the flexion–relaxation response in cervical spine

BackgroundNeck pain is common among general population with a high prevalence among the people who are routinely exposed to prolonged use of static head–neck postures. Prolonged static loading can cause localized muscle fatigue which may impact the stability of the cervical spine. In this study, flexion–relaxation phenomenon was used to study the post fatigue changes in the stability of cervical spine by evaluating the synergistic load sharing between muscles and viscoelastic elements. MethodsThirteen male participants were recruited for data collection. The variables that influence cervical flexion–relaxation were studied pre- and post-fatigue using neutral and shrugged shoulder postures. The Sorensen protocol was used to induce neck extensor fatigue. Surface electromyography and optical motion capture systems were used to record neck muscle activation and head posture, respectively.FindingsThe flexion–relaxation phenomenon was observed only in the neutral shoulder position pre- and post-fatigue. The flexion relaxation ratio decreased significantly post-fatigue in neutral shoulder position but remained unchanged in shrugged shoulder position. The onset and offset angles and the corresponding durations of the silence period were significantly affected by the fatigue causing a post-fatigue expansion of silence period.InterpretationThe muscular fatigue of neck extensors and shoulder position was found to modulate the cervical flexion–relaxation phenomenon. Early shifting of load sharing under fatigued condition indicates increased demands on the passive tissues to stabilize the cervical spine. Shrugging of shoulder seems to alter muscular demands of neck extensors and make cervical flexion–relaxation phenomenon disappear due to continuous activation of the neck extensors.

Neck and shoulder muscle activity of orthodontists in natural environments

Work related musculoskeletal disorders (WMSDs) are common among dentists and possibly caused by prolonged static load. The aim of this study was to assess the contraction pattern of neck and shoulder muscles of orthodontists in natural environments.Electromyographic (EMG) activity of right sternocleidomastoid and trapezius muscles were recorded by means of portable recorders in eight orthodontists during working conditions, and both active and resting non-working conditions. Recordings were analysed in terms of contraction episode (CE) count, amplitude, and duration.The sternocleidomastoid and trapezius muscles contracted about 40–70times per hour in the natural environment. Their EMG activity pattern mainly consisted of short-lasting, low-amplitude CEs. The counts and amplitude of sternocleidomastoid CEs did not differ across vocational and non-vocational conditions. The number and amplitude of trapezius CEs were slightly but significantly higher during the vocational condition. There were highly significant (p<0.001) differences in duration of CEs across conditions, with two to threefold increase in the average duration of trapezius muscle contractions found in the vocational setting.During orthodontic work, operators commonly hold muscular contractions for significantly longer periods than are encountered in non-vocational settings. This behaviour may be associated causally with the increases seen in WMSDs through proposed pathophysiological mechanisms occurring at the motor unit level. Our findings may also be valid for other occupations characterised by seated static postures with precision hand and wrist movements.

Auxetic springs for seating

It is commonly believed that furniture spring systems are far more durable and friendly to use than foam systems. The aim of this research project was to design and realise models of individual auxetic compression springs and determine their resistance to cyclic as well as prolonged static loads. The study was conducted using numerical calculations and verification of results by laboratory experiments. The results of the performed investigations made it possible to conclude that the designed spring had stiffness characteristics similar to the compression characteristics of polyurethane foams. Cyclical and prolonged static loads have no significant influence on changes in spring heights; their impact on the comfort of seating is insignificant. New construction of springs guaranteed the auxetic properties of systems made up of many parallel-joined spring systems. Due to such properties, seats designed using auxetic springs can exhibit high comfort levels in comparison with traditional systems made of wire springs or foams.

Static and dynamic postural loadings during computer work in females: Sitting on an office chair versus sitting on an exercise ball

Seated computer work results in prolonged static loading, which has been associated with the development of musculoskeletal disorders. A popular alternative to sitting on an office chair while performing computer work is to sit on an exercise ball. Sitting on an exercise ball might affect static and dynamic aspects of working posture. We monitored posture, muscle activation and spinal shrinkage in 10 females performing a 1-h typing task, while sitting on an office chair with armrests and while sitting on an exercise ball. Sitting on an exercise ball resulted in 33% more trunk motion and in 66% more variation in lumbar EMG. Both of these findings can be considered to be an advantage for the exercise ball. However, the fifth percentile and average lumbar EMG were also higher when sitting on an exercise ball, with 38% and 78%, respectively. In addition, more spinal shrinkage occurred when sitting on an exercise ball than when sitting on an office chair. Arm flexion was reduced, but trapezius activation was unaffected when sitting on an exercise ball. It is concluded that the advantages with respect to physical loading of sitting on an exercise ball may not outweigh the disadvantages.

The effect of finite compressive strain on chondrocyte viability in statically loaded bovine articular cartilage

Recent studies have reported that certain regimes of compressive loading of articular cartilage result in increased cell death in the superficial tangential zone (STZ). The objectives of this study were (1) to test the prevalent hypothesis that preferential cell death in the STZ results from excessive compressive strain in that zone, relative to the middle and deep zones, by determining whether cell death correlates with the magnitude of compressive strain and (2) to test the corollary hypothesis that the viability response of cells is uniform through the thickness of the articular layer when exposed to the same loading environment. Live cartilage explants were statically compressed by approximately 65% of their original thickness, either normal to the articular surface (axial loading) or parallel to it (transverse loading). Cell viability after 12 h was compared to the local strain distribution measured by digital image correlation. Results showed that the strain distribution in the axially loaded samples was highest in the STZ (77%) and lowest in the deep zone (55%), whereas the strain was uniformly distributed in the transversely loaded samples (64%). In contrast, axially and transversely loaded samples exhibited very similar profiles of cell death through the depth, with a preferential distribution in the STZ. Unloaded control samples showed negligible cell death. Thus, under prolonged static loading, depth-dependent variations in chondrocyte death did not correlate with the local depth-dependent compressive strain, and the prevalent hypothesis must be rejected. An alternative hypothesis, suggested by these results, is that superficial zone chondrocytes are more vulnerable to prolonged static loading than chondrocytes in the middle and deep zones.

Development of residual strains in human vertebral trabecular bone after prolonged static and cyclic loading at low load levels

Development of irreversible residual strains in trabecular bone may be a mechanism by which age-related non-traumatic vertebral fractures occur. To investigate this concept, static and cyclic loading tests were conducted at low loading levels for cylindrical cores of cadaveric vertebral trabecular bone. Stresses were applied equivalent to elastic strains of either 750 or 1500 microstrain. Creep strains were measured during the tests, which lasted for 125,000 seconds (about 35 h), and for an additional 125,000 seconds after complete unloading. Emphasis was placed on the residual strains that developed, defined as the strain remaining at the end of the unloading phase. The results indicated that appreciable residual strains did develop, and were similar for static and cyclic loading. Irrespective of the applied load levels and loading modes, the residual strains that remained after the unloading phase were similar in magnitude to the originally applied elastic strain. Extrapolation of the observed residual strains to full recovery indicated that the time that would be required for full recovery was over 20 times longer than the duration of the applied loads. These results indicate that human vertebral trabecular bone does not creep in a linear viscoelastic fashion at low stress levels, and that creep mechanisms dominate the residual strains regardless of the loading mode. Taken together, these findings support the concept that non-traumatic vertebral fractures may be related to long-term creep effects because the trabecular bone does not have sufficient time to recover mechanically from creep deformations accumulated by prolonged static or cyclic loading.

Biomechanical responses of the intervertebral joints to static and vibrational loading: a finite element study

Objective. This study was performed to investigate the time-dependent responses of the intervertebral joint to static and vibrational loads. Design. A poroelastic finite element model was established to analyse the fluid flow, stress distribution and deformation of the intervertebral disc. Background. Long-term exposure to whole body vibration is highly associated with disc degeneration and low back pain. It is hypothesized that moderate vibrational loading may increase the efficiency of fluid and nutritional transport of the intervertebral disc while prolonged static and excessive vibrational loading may have deleterious effect. Methods. A three-dimensional finite element model was established using the actual geometry of the L4–L5 lumber motion segment. Nonlinear poroelastic properties were assigned to the intervertebral disc and cancellous bone. Static and vibrational loads were applied and the responses of fluid flow and stress distributions were analysed. Results. The finite element model showed that the loads carried by the annulus and the facets increased with time under static loading. The fluid flow and deformation of the intervertebral disc were dependent on the loading frequency. Conclusion. Vibration loading may be able to enhance disc fluid exchange via the fluid pumping mechanism. Relevance The predicted responses implied that vibrational motion may be important in facilitating fluid and metabolic transport of the intervertebral disc.

Neuromuscular disorder in response to anterior cruciate ligament creep

Objective. To determine the effect of creep developed in the anterior cruciate ligament and other viscoelastic knee structures on the function of the flexor and extensor muscles of males and females. Design. Static load applied to the proximal tibia of young healthy male and female subjects in a laboratory setting with maximal voluntary knee flexion and extension performed before and after the load application. Background. Static loads applied to various joints during occupational and sports activities are epidemiologically linked to higher than normal rates of disability reports. The physiological and biomechanical processes active in the development of such a neuromuscular disorder are not known. We hypothesize that creep developed in the anterior cruciate ligament due to prolonged static load will have pronounced impact on the reflexive activation of the associated musculature in a manner that may increase the risk of injury. Females are expected to be exposed to higher risk than males. Methods. Male and female groups performed maximal voluntary knee flexion and extension before and after applying 200 and 150 N, respectively, to the proximal tibia for a 10 min period. Flexion and extension forces as well as electromyograph from agonist and antagonist muscles were measured at 35° and 90° knee flexion. Data was analyzed through repeated measures of analysis of variance. Results. It was found that in extension, quadriceps electromyographic activity increased significantly after anterior cruciate ligament creep while hamstrings co-activation did not change. There was also a trend towards increased extension force after creep was developed, with significant effect of gender (larger increase in females). Similarly, significant increase in hamstrings electromyographic activity and a trend towards increased force during knee flexion was observed but with no effect of gender. Electromyographic spasms from the flexors and extensors were recorded in 30% of the subjects during the 10 min static loading period at 90° angle and from the flexors only at 35°. Creep in the ligament was marginally greater in females than in males with a significant effect of angle, being greater at 35° than in 90° knee flexion. Conclusions. The results suggest that ligament creep may develop a neuromuscular disorder consisting of spasms, increased electromyography and force in the agonist muscles without compensation from the antagonist. Static loading of a joint, therefore, may develop a neuromuscular disorder compounded with laxity of the ligaments and subject the individual to increased risk of injury. Relevance The data provides evidence that prolonged static loads applied to the anterior cruciate ligament and associated viscoelastic structures results in unbalanced muscular activation which puts individuals at increased exposure to injury. Work and sports activities should be scheduled while minimizing periods of static joint loading and emphasizing sufficient rest periods to allow recovery of creep and return to balanced muscular activation and co-activation.

Features of the Deformation and Failure of Molybdenum-Tungsten Alloy with High-Temperature Repeated Static Loading

The low-cycle strength and cyclic creep, structural changes and the nature of failure for molybdenum-tungsten alloy of the system Mo – 30%W – NbC – C prepared by powder metallurgy at 1770, 2020, and 2270 K are studied. At these temperatures a single dependence for the steady-state cyclic creep rate on the ratio of the equivalent stress to the material yield point at the corresponding temperature and a generalized low-cycle strength curve for the alloy are obtained. It is shown that the cyclic nature of the applied load accelerates directional plastic deformation and failure of powder metallurgy alloy by a factor from 1.5 to 7 compared with prolonged static loading with retention of similarity for the cyclic and static creep curves. At these temperatures the alloy essentially fails as a result of the occurrence of a critical polygonization stage. The structural changes found are in complete conformity with previously predicted features of this mechanism. Under nonstationary loading conditions the incubation period of the critical polygonization stage is shorter, which results in acceleration of creep compared with testing under stationary conditions.

Occupational risk factors for radial tunnel syndrome in industrial workers.

The aim of the study was to evaluate both nonoccupational and occupational factors associated with radial tunnel syndrome (RTS) among industrial workers in 3 large plants. Twenty-one cases of RTS were compared with 21 referents matched for gender, age, and plant. RTS was associated with carpal tunnel syndrome (CTS) in 9 cases. Past medical history, household activities, and ergonomic and organizational characteristics of the job were analyzed. The study found 3 occupational risk factors for RTS. Exertion of force of over 1 kg [odds ratio (OR) 9.1, 95% confidence interval (95% CI) 1.4-56.9] more than 10 times per hour was the main biomechanical risk factor. Prolonged static load applied to the hand during work was strongly associated with RTS (OR 5.9, 95% CI 1.2-29.9). Work posture with the elbow fully extended (0-45 degrees) was associated with RTS (OR 4.9, 95% CI 1.0-25.0). Full extension of the elbow, associated with a twisted posture of the forearm, stressed the radial nerve at the elbow. However, personal activities, household chores, and sport and leisure activities were not associated with RTS. The study confirms that RTS occurs in workers performing hard manual labor that requires forceful and repetitive movements involving elbow extension and forearm prosupination.

Prediction of Low Back Pain Reporting in Industry from Estimates of Cumulative Loading on the Spine

Adverse effects of excessive accumulative loading on spinal structures, from either high repetition or prolonged static loading of even relatively low loads, have been known from in vitro studies for many years. However, cumulative compression and shear on low back tissues, although suspected to be risk factors for the reporting of low back pain at work, have been epidemiologically confirmed only relatively recently. Software has been developed which combines a quasi-dynamic biomechanical spine model and dose-response curves from epidemiological data to estimate a predictive index. In this paper, a method is described for estimating cumulative spinal loading and other risk factors and using the data to predict the probability of reporting low back pain in the workplace.

Upper limb disorders and work: The importance of physical and psychosocial factors

Upper limb, shoulder, and neck disorders cause both acute and chronic pain, as well as significant functional impairment. They impose a heavy financial burden on societies, industries, and individuals. Our understanding of the pathology of many of the disorders is poor. The epidemiological pursuit of causal relationships is hampered by the nature of the disorders and by the diverse and interactive exposures both at, and away from, the workplace. Current studies suggest that forceful, repetitive manual work, along with prolonged static loading and exposure to vibration are established areas of risk. Much less is known about the possible contribution of psychological factors. Perception of work characteristics, for example, low decision latitude and lack of social support, appear to show increased associations with a number of upper limb disorders, although mechanisms to explain these observations are still broadly theoretical. Research into individual factors is limited, but age and gender both appear to be important.

Influence of strain rate on the nature of failure under prolonged static and cyclical loads. 2. Examples of calculation

Based on a physicomechanical model of intercrystallite failure in materials, such as presented in the previous communication, we have proposed a computational algorithm for the endurance of polycrystalline material under cyclical load. We have provided examples for the calculation of the cyclical strength in austenitic steel under various strain rates. Comparison of the theoretical results against the experimental data demonstrated the possibility of using the model developed here to describe the quantitative relationships governing intercrystallite failure of the material over a broad range of load regimes.

Neck and shoulder complaints among sewing-machine operators: A study concerning frequency, symptomatology and dysfunction

The occupation of sewing-machine operators (SMO) involves monotonous and repetitive tasks, performed in a work position equivalent to a static component of muscular load on the neck and shoulder. The present study concerns the occurrence of neck-shoulder problems in a population of SMO. A total of 224 SMO from four textile factories in the western part of Sweden were subjected to a comprehensive questionnaire about demographic, vocational, medical and psychosocial data. The Nordic Ministry Questionnaire specifically directed towards neck-shoulder complaints showed a prevalence rate during the last 12 months of 75% and during the last seven days a rate of 51%. Daily problems were experienced by 26%. Some 27% had had problems leading to restraints in work time and 37% in leisure time. Those SMO screening positive were clinically examined in an attempt to describe the clinical picture behind the complaints. Diagnoses were made according to specific criteria. The tension neck syndrome (TNS) was most frequent, followed by the cervical syndrome. In half of those examined, symptoms and findings were too unspecific for diagnosis. A positive correlation between the TNS and working hours per week suggests a daily prolonged static load on the neck and shoulder to be of importance for neck-shoulder problems among the SMO.

Transient creep of metals at elevated temperatures and reduced stresses

1. Rheological relationships for high-temperature creep of metals in the region of reduced stresses should take account of the change in material volume from the instant of load application. In the case of a complex stressed state the relationships for describing the two primary creep stages are $$\begin{gathered} d\varepsilon _v^p = \psi (\sigma _0 ,\sigma _* ,\alpha _* ,\lambda )d\lambda ; \hfill \\ D_{d\varepsilon } = l_0 (\sigma _* )D_\sigma d\tau , \hfill \\ \end{gathered} $$ where σ0, σ*, α* are tensor invariants for true stresses. 2. In the case of a uniaxial stressed state (prolonged static loading) the linear approximation for devp/d with λ=const gives relationship (8) for creep strain ep, which may be written in the form of expressions (2) or (10) which are confirmed by numerous experimental data. 3. Relationships (9) and (10) are valid for a broad range of stresses and they are suitable for describing retarded creep with different strain portion for the grain boundary strain component. With relatively high stresses or moderate temperatures transient creep constants Δe1 and γ do not depend on external conditions.

Changes in the mechanical properties of intervertebral lumbar disks produced by prolonged static compression forces

These results indicate that the mechanical properties of intervertebral disks upon prolonged static compression undergo certain changes. The intradisk pressure in unaltered and moderately degenerated disks have a definite tendency to decrease and the dynamics of this process is related to the extent of degenerative changes in the disk. The changes in the intradisk pressure correlate with the changes in the tangential stretching of the annulus fibrosus. An overstretching effect was found for the annulus fibrosus in prolonged compression. The annulus fibrosus of the low lumbar disks which has a greater reserve of elasticity is more resistant to prolonged compression. In moderately degenerated disks, the turgor of the nucleus pulposus and elasticity of the annulus fibrosus are reduced and, thus, the reduction in the intradisk pressure and the extensibility of the annulus fibrosus upon the action of prolonged loads are not as pronounced. However, the changes found in these disks are very stable and are retained for a long period after the removal of compression. The intradisk pressure and tangential stretching of the annulus fibrosus at high loads remained virtually invariant during prolonged compression. This finding indicates that the structural elements of the disk under these conditions function at their limiting capacity. All the changes in the mechanical properties of the disks found are small, which indicates the high stability of human intervertebral lumbar disks relative to prolonged static compressive loads.