B. Visser

Effects of precision demands and mental pressure on muscle activation and hand forces in computer mouse tasks

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 physical demands upon (Dutch) fire-fighters in relation to the maximum acceptable energetic workload

The aim of this study was to assess the tasks and activities that make physical demands on Dutch fire-fighters and to compare them with a guideline related to the development of excessive fatigue. The occupational physical demands on Dutch fire-fighters were assessed by conducting a task analysis during 85 24-h shifts. While workplace observations on the duration and frequency of physical tasks and activities were recorded, the heart rate was measured. This was then used to calculate the heart rate reserve percentage (%HRR) for predefined working periods, tasks and activities during 24-h shifts. The findings indicate that actual fire-fighting during 24-h shifts is characterised by a low frequency of incidents, a short ‘turn-out’ time, short tasks, and activities with a moderate to occasionally high energetic workload. Two tasks which sometimes occur in actual fire-fighting exceeded the guideline on energetic workload. The conclusion was that, though the number of incidents and the occupational demands are low during 24-h shifts, the peak loads for these two tasks are energetically high and could lead to excessive fatigue. Consequently, attention may need to be paid to health surveillance for persons exposed to such energetic peak loads, the development of physical and medical selection procedures, training, and workplace adjustments.

Job rotation as a factor in reducing physical workload at a refuse collecting department

The effect of job rotation on the physical workload was investigated for male employees working at a refuse collecting department. Before the introduction of job rotation, an employee worked as a street sweeper, as a refuse collector or as a driver. After the introduction of job rotation, every employee was allowed to alternate between two of the three possible jobs during the day, i.e. refuse collecting/street sweeping, refuse collecting/driving or street sweeping/driving. Two non-rotation groups (i.e. refuse collectors and street sweepers) and two rotation groups (i.e. refuse collectors/street sweepers and street sweepers/drivers) were mutually compared. The physical workload was determined by measuring the perceived load, energetic load and postural load during a full working day. Job rotation resulted in a significant decrease of the perceived load and energetic load and a slight decrease of the postural load. The results indicate that the total amount of work performed by means of job rotation resulted in an overall reduced physical workload of the employees of the refuse collecting department.

The effect of work pace on workload, motor variability and fatigue during simulated light assembly work

This study investigated the effect of work pace on workload, motor variability and fatigue during light assembly work. Upper extremity kinematics and electromyography (EMG) were obtained on a cycle-to-cycle basis for eight participants during two conditions, corresponding to “normal” and “high” work pace according to a predetermined time system for engineering. Indicators of fatigue, pain sensitivity and performance were recorded before, during and after the task. The level and variability of muscle activity did not differ according to work pace, and manifestations of muscle fatigue or changed pain sensitivity were not observed. In the high work pace, however, participants moved more efficiently, they showed more variability in wrist speed and acceleration, but they also made more errors. These results suggest that an increased work pace, within the range addressed here, will not have any substantial adverse effects on acute motor performance and fatigue in light, cyclic assembly work. Statement of Relevance:In the manufacturing industry, work pace is a key issue in production system design and hence of interest to ergonomists as well as engineers. In this laboratory study, increasing the work pace did not show adverse effects in terms of biomechanical exposures and muscle fatigue, but it did lead to more errors. For the industrial engineer, this observation suggests that an increase in work pace might diminish production quality, even without any noticeable fatigue being experienced by the operators.

Effect of job rotation on need for recovery, musculoskeletal complaints, and sick leave due to musculoskeletal complaints: A prospective study among refuse collectors

BACKGROUND Job rotation might be an effective preventive measure to reduce the prevalence of musculoskeletal complaints, although its effect has not been yet established. The aim of the present study is to evaluate the effect of job rotation in refuse collecting on need for recovery, prevalence of musculoskeletal complaints, and sick leave due to musculoskeletal complaints. METHODS A 1-year prospective study among refuse collectors was performed, using standardized questionnaires. Job rotation was performed between collecting two-wheeled containers and driving a refuse truck. The experimental groups of rotating refuse collectors at t(0) and t(1) (group R-R) and non-rotating refuse collectors at t(0) and rotating refuse collectors at t(1) (group NR-R) were compared with a reference group of non-rotating refuse collectors at t(0) and t(1) (group NR-NR). RESULTS The adjusted need for recovery of group R-R was marginally significantly lower than need for recovery of the reference group. Groups R-R and NR-R had a more than two times higher risk for complaints of the low back than the reference group. No other significant results were found. CONCLUSIONS Job rotation seemed to coincide with a reduced need for recovery and was associated with an increased risk of low back complaints. No effects were found on sick leave due to musculoskeletal complaints. The results might be influenced by the healthy worker selection effect in the reference group and its inverse in the rotating groups.

The effect of arm and wrist supports on the load of the upper extremity during VDU work

OBJECTIVE To evaluate the effectiveness of arm and wrist supports in reducing the workload during computer work. DESIGN Female subjects (n=10) performed computer work in conditions with arm or wrist supports and in a condition without supports. BACKGROUND Sustained muscle tension in the trapezius muscle is a risk factor for trapezius myalgia. Arm and wrist supports are used at the workplace with the intention to reduce the muscle tension. The effectiveness of these aids in reducing the load is not clear. METHODS A typing task and mouse task were performed, each with four types of supports and without support. Electromyography and subjective ratings were used to quantify the workload. RESULTS Lower levels of trapezius muscle activation were recorded with the use of arm supports. Wrist supports did not reduce activation. The rated perceived workload did not discriminate. CONCLUSIONS Reduction of muscle activation in the neck-shoulder region during standard visual display unit work can be achieved with arm supports. Wrist supports do not reduce the strain on the neck-shoulder region. Subjective ratings seem not of use in selecting ergonomic aids in low intensity tasks. RelevanceVisual display unit workers are at risk of developing complaints of the neck and upper extremity. Arm and wrist supports are introduced at the workplace to reduce the workload. If arm and wrist supports are effective in reducing the workload they might be of use as preventive measures to reduce the risk of neck-shoulder complaints.

Weight and frequency effect on spinal loading in a bricklaying task

In manual materials handling jobs a reduction in the weight of materials often concurs with an increase in handling frequency. The effect of weight and inversely related frequency on spinal load was studied in two bricklaying tasks: building the skin and the floor of a steel ladle. In both tasks five subjects laid bricks of varying weight and frequency (obtained from field observations). The load parameters investigated were peak values and time integrals of the compressive force on the L5-S1 motion segment and stature loss, which is assumed to reflect motion segment creep due to compression. Peak compression was found to increase at higher brick weights. No differences in integrated compression were observed among four out of five combinations of weight and frequency (both in skin and floor building). Laying bricks for a fixed period of 47 min yielded average stature losses of 2.0 3.6 mm. Differences in stature loss among weight-frequency conditions were not significant. In conclusion, at lower weights peak loads decrease, but the benefit of this should be doubted because the frequency of exposure to these peak loads was found to increase. Moreover, this increase was such that no effects were found on spinal load estimates that incorporate both magnitude and time aspects of the load, like time-integrated compression and stature loss.

Predicting mechanical load of the glenohumeral joint, using net joint moments

OBJECTIVE To evaluate the relationship between net moments for the glenohumeral joint as calculated with a model that is easy to apply (Static Strength Prediction Program (SSPP)) and the internal forces, calculated with a simulation model of the human shoulder (Delft Shoulder Model, DSM). DESIGN Static recordings of bony landmarks of the shoulder girdle during prescribed arm motions using a three-dimensional (3D) digitizer. BACKGROUND The SSPP is a biomechanical model for the evaluation of workload. Concerning the shoulder the output of the model consists of net moments in the glenohumeral joint. For the glenohumeral joint the relationship between net joint moments and internal load predictions is unknown, therefore it is useful to investigate whether the SSPP can be used to predict mechanical load in the glenohumeral joint. METHODS 3D co-ordinates of bony landmarks of the shoulder girdle are recorded, in variable arm positions, in four different planes. RESULTS A strong linear relationship between compression forces and net moments is found, which makes it reasonable to assume a direct relationship between net moments and joint compression forces. CONCLUSIONS (a) Net joint moments appeared to be a good indicator for mechanical load in the glenohumeral joint in static situations. (b) The SSPP can be used to predict these joint moments for static situations. RELEVANCE biomechanical models predicting mechanical load can be used in work situations in order to prevent overload and injuries, as well as in many other areas, for instance to evaluate the mechanical load during wheelchair propulsion.

Effect of job rotation on work demands, workload, and recovery of refuse truck drivers and collectors.

Job rotation is often advocated to reduce workload, but its efficacy has seldom been investigated. The aim of this study is to compare the work demands, workload, and recovery among truck driving, refuse collecting, and rotating between these two jobs, between days and during the day. Three teams of 3 employees each participated in this study. Work demands were assessed by systematic observation of tasks and activities. Workload was quantified by means of heart rate, oxygen uptake, subjective ratings, and urinary excretion rates of catecholamines. Recovery was quantified by excretion rates of catecholamines after work. Job rotation between driving and collecting is an effective measure to reduce physical workload as compared with collecting only and to decrease mental workload as compared with driving only. However, job rotation resulted in increased physical workload as compared with driving only. Job rotation did not increase mental workload as compared with collecting only. No effects were seen on recovery. No differences were found between rotating between days and during the day. Actual or potential applications of this research include the recommendation that before job rotation is introduced, its efficacy be determined in terms of well-chosen workload measures because a reduction in work demands does not directly imply a reduction in workload. Therefore, job rotation might be less effective than expected.

The prevalence and incidence of musculoskeletal symptoms among hospital physicians: a systematic review

ObjectivePhysicians are exposed to a range of work-related risk factors that may result in occupational diseases. This systematic review aims at shedding light on the prevalence and incidence of musculoskeletal complaints among hospital physicians.MethodsA systematic literature search was performed in Pubmed and EMBASE (1990–2010), and methodological quality criteria were applied. A search was done for musculoskeletal complaints.ResultsFive medium-quality studies and three high-quality studies were included in this review. The definitions and assessment used in the studies for musculoskeletal complaints were different. In short, the frequently reported prevalence for hand and wrist pain was 8–33 and 0%, 17% for shoulder pain, and 9–28% for neck pain. Moreover, the annual prevalence of low back pain was between 33 and 68%.InterpretationThe limited number of studies makes it difficult to draw conclusions, and the results should be intepreted with care. In conclusion, musculoskeletal complaints may be work-related complaints in hospital physicians, which need future attention.