Development and Evaluation of a Hybrid Measurement System to Determine the Kinematics of the Wrist.

Optical Motion Capture Systems (OMCSs) are considered the gold standard for kinematic measurement of human movements. However, in situations such as measuring wrist kinematics during a hairdressing activity, markers can be obscured, resulting in a loss of data. Other measurement methods based on non-optical data can be considered, such as magneto-inertial measurement units (MIMUs). Their accuracy is generally lower than that of an OMCS. In this context, it may be worth considering a hybrid system [MIMU + OMCS] to take advantage of OMCS accuracy while limiting occultation problems. The aim of this work was (1) to propose a methodology for coupling a low-cost MIMU (BNO055) to an OMCS in order to evaluate wrist kinematics, and then (2) to evaluate the accuracy of this hybrid system [MIMU + OMCS] during a simple hairdressing gesture. During hair cutting gestures, the root mean square error compared with the OMCS was 4.53° (1.45°) for flexion/extension, 5.07° (1.30°) for adduction/abduction, and 3.65° (1.19°) for pronation/supination. During combing gestures, they were significantly higher, but remained below 10°. In conclusion, this system allows for maintaining wrist kinematics in case of the loss of hand markers while preserving an acceptable level of precision (<10°) for ergonomic measurement or entertainment purposes.

Predictive factors of return-to-work trajectory after work-related rotator cuff syndrome: A prospective study of 96 workers.

OBJECTIVE: Sustained return to work after surgery for work-related rotator cuff syndrome (WRRCS) remains quite difficult. The main purpose of the present study was to identify predictive factors of a return-to-work (RTW) trajectory. METHODS: A total of 96 workers with WRRCS were identified by 4 surgeons. They were followed prospectively before and after the surgery, until 1 year after RTW, or for 20 months after surgery when they did not. Participants completed a series of standardized questionnaires related to working conditions, health, and beliefs, and performed functional tests at the inclusion time. During the follow-up period, they were regularly asked about their working conditions (present or not at work), activity (normal or lightened physical duties) and schedules (full- or part-time job). Statistical analysis was based on single- and multiple-factor models of prediction of the workers' trajectory. RESULTS: Three trajectories of RTW were distinguished, considering RTW and absenteeism that occurred during the follow-up: stable, unstable, and non-RTW. The median age of the sample was 49.5 [45.0-54.0], with 67.7% of workers employed in highly physically demanding jobs. In the multiple factor model, three factors were highly predictive of the trajectory: perceived health before surgery, having had a repaired ruptured-rotator-cuff tendinopathy, and the level of physical demand of the job. CONCLUSION: Three easy-to-collect predictive factors of RTW trajectory have been identified. They may be useful for healthcare professionals and care givers to identify vulnerable workers' risk of occupational dropout after arthroscopic surgery for rotator cuff tendinopathy.

Predictive factors for the duration until return to work after surgery for work-related rotator cuff syndrome: A prospective study of 92 workers.

BACKGROUND: Return to work (RTW) after surgery for a work-related rotator cuff disorder (WRRCD) is often difficult. The study's purpose was to identify preoperative factors predicting RTW. METHODS: Ninety-two workers with a WRRCD were identified by four surgeons. Before surgery, the workers completed a series of standardized questionnaires related to working conditions, health, and health beliefs. They were followed up prospectively for 20 months. Statistical analysis was based on single and multiple-factor Cox models of the duration of absence from the time of surgery until RTW. RESULTS: The median age at inclusion was 49 years (27-62), with 52 women included (57%). Sixty-one subjects (66%) were employed in highly physically demanding jobs. Forty-two (46%) stayed at work until their surgery, whereas preoperative sick leave exceeded 100 days in 20 subjects (21%). Twenty months after surgery, 14 were still not back at work. For the other participants, the mean duration until RTW was 225 days (SD 156). In the fully adjusted model, variables that were significantly predictive of the duration until RTW were: work physical demand levels, preoperative sick leave, the number of body parts causing pain or discomfort in the last 12 months, self-assessed 2-year workability, and the Readiness for RTW (RRTW) Scale. CONCLUSIONS: Several physical, psychological, and work-related factors, easily recorded, can be identified preoperatively. They may be predictive of delayed return, loss of employment, or employability as a result of shoulder surgery.

Study of between-subject and within-subject variability of electromyography data and its intrinsic determinants for clip fitting tasks.

Knowledge of motor variability is an essential step towards understanding a subject's working activity. This study aims to assess within-subject and between-subject variability and the intrinsic factors that drive variability during a clip fitting task in the laboratory. Muscular activity is recorded on six muscles of each upper limb. Four metrics are used: two characterizing muscular load and two characterizing intra-clip fitting dispersion. Factor analyses and muscle-by-muscle analyses are conducted. Independent variables linked to subject characteristics and to task performance are considered. Factor analysis reveals the combined activity of the different muscles. The short and highly constrained task shows both within-subject variability and between-subject variability. The latter predominates. Each of the independent variables induces a variability which affects the muscular load metrics, although only a few affect the within-clip fitting dispersion. The muscle-by-muscle and factorial score analyses results are consistent.

Analysis method for revealing human and organisational factors of occupational accidents with movement disturbance (OAMDs).

Slips, trips and other movement disturbances account for 20 to 30% of recorded occupational accidents (OAs). The causal representations of these accidents hamper their prevention. An analysis method dedicated to occupational accidents with movement disturbance (OAMDs) has been developed to change these representations. In France, the causal tree method (CTM) is very commonly used for analysing OAs. An initial version of an OAMD analysis method, which overcomes the problems encountered when analysing these accidents using the CTM, has been developed. This OAMD analysis method was reviewed by six targeted prevention officers and as a result some proposals have been discarded and this initial version has been transformed into three additional CTM modules. The purpose of these modules is to identify human and organisational factors and provide a formal representation of damage caused, beyond bodily injuries. Practitioner summary: A method for analysing occupational accidents triggered by a slip, a trip or any other movement disturbance has been developed in consideration of the practices and constraints in companies. In particular, this method allows us to highlight the human and organisational factors involved in the accident situation. Abbreviations: OA: occupational accident; OAMD: occupational accident with movement disturbance; CTM: causal tree method.

Objective assessment of knife sharpness over a working day cutting meat.

Knife sharpness is one of multiple factors involved in musculoskeletal disorders in industrial meat cutting. The aim of this study was to objectively evaluate, in real working situations, how knife sharpness changed over a working day cutting meat, and to analyse the impact of sharpening, steeling and meat-cutting activities on these variations. Twenty-two meat-cutting workers from three different companies participated in the study. The methods included measurements of knife sharpness in relation to real work situations and consideration of the way meat-cutting and sharpening operations were organised. Results showed that the type of meat-cutting activities, the steeling strategy adopted by the worker, including the types of tool used, and the overall organisation of the sharpening task all had a significant influence on how knife sharpness evolved over a 2-h period and over an entire working day. To improve MSD prevention, sharpening and steeling operations should not be considered as independent activities, but taken into account as a continuity of working actions. Appropriate assessment of knife sharpness by meat cutters affects how they organise meat-cutting and sharpening tasks.

Upper Limb Kinematics Using Inertial and Magnetic Sensors: Comparison of Sensor-to-Segment Calibrations.

Magneto-Inertial Measurement Unit sensors (MIMU) display high potential for the quantitative evaluation of upper limb kinematics, as they allow monitoring ambulatory measurements. The sensor-to-segment calibration step, consisting of establishing the relation between MIMU sensors and human segments, plays an important role in the global accuracy of joint angles. The aim of this study was to compare sensor-to-segment calibrations for the MIMU-based estimation of wrist, elbow, and shoulder joint angles, by examining trueness ("close to the reference") and precision (reproducibility) validity criteria. Ten subjects performed five sessions with three different operators. Three classes of calibrations were studied: segment axes equal to technical MIMU axes (TECH), segment axes generated during a static pose (STATIC), and those generated during functional movements (FUNCT). The calibrations were compared during the maximal uniaxial movements of each joint, plus an extra multi-joint movement. Generally, joint angles presented good trueness and very good precision in the range 5°-10°. Only small discrepancy between calibrations was highlighted, with the exception of a few cases. The very good overall accuracy (trueness and precision) of MIMU-based joint angle data seems to be more dependent on the level of rigor of the experimental procedure (operator training) than on the choice of calibration itself.

Assessment of knife sharpness by means of a cutting force measuring system.

Following a brief description of the problem of musculoskeletal disorders (MSDs) in the meat industry and use of knives, this paper presents a study of the influence of the main intrinsic technical characteristics of a knife on its cutting performance. This study prompted design of a specific system for measuring initial cutting capacity and cutting edge retention. This design process preceded research into test conditions (cutting speed, sample nature and thickness, knife inclination angle, etc.) offering optimum test bench operation. This equipment was then used to study the influence of blade inclination angle, steel grade and sharpening angle on cutting performance. It has been shown that cutting force varies with blade inclination. Use of knives with curved blades and/or a blade inclined with respect to the knife handle is therefore preferred in relation to our cutting force reduction objective. It has also been shown that choices are in fact governed by compromises in relation to other parameters (steel grade and edge angle). These observations confirm the need to set up suitable training of knife users to achieve best possible cutting performance.

Influence on grip of knife handle surface characteristics and wearing protective gloves.

Ten subjects were asked to apply maximum torques on knife handles with either their bare hand or their hand wearing a Kevlar fibre protective glove. Four knife handles (2 roughnesses, 2 hardnesses) were tested. Surface electromyograms of 6 upper limb and shoulder muscles were recorded and subject opinions on both knife handle hardness and friction in the hand were also assessed. The results revealed the significant influence of wearing gloves (p<0.0001), knife type (p<0.0005) and handle hardness (p<0.005) on the applied torque. Wearing Kevlar fibre gloves greatly increased the torque independently of the other two parameters. Under the bare hand condition, a 90 degrees ShA slightly rough handle provided the greatest torque. Subject opinion agreed with the observed effects on recorded torque values except for the hardness factor, for which a preference for the 70 degrees ShA value over the 90 degrees ShA value emerged.

Design and ergonomics. Methods for integrating ergonomics at hand tool design stage.

As a marked increase in the number of musculoskeletal disorders was noted in many industrialized countries and more specifically in companies that require the use of hand tools, the French National Research and Safety Institute (INRS) launched in 1999 a research project on the topic of integrating ergonomics into hand tool design, and more particularly to a design of a boning knife. After a brief recall of the difficulties of integrating ergonomics at the design stage, the present paper shows how 3 design methodological tools--Functional Analysis, Quality Function Deployment and TRIZ--have been applied to the design of a boning knife. Implementation of these tools enabled us to demonstrate the extent to which they are capable of responding to the difficulties of integrating ergonomics into product design.

Relevance of the EMG/grip relationship in isometric anisotonic conditions.

The aim of the present study was to develop a relationship to evaluate grip force using the electromyogram (EMG) in isometric anisotonic conditions. The EMGs of the flexor digitorum superficialis (FDS) and the extensor digitorum (ED) were recorded in 3 flexion-extension positions of the wrist (30 degrees flexion, 30 degrees extension, and 60 degrees extension) associated with 3 positions of the forearm (70 degrees pronation, prono-supination, and 70 degrees supination). For each position, the participants had to follow linear ramp targets (2 rates of increase and decrease) displayed on an oscilloscope. The results show the best fit is a quadratic type force-EMG relationship. Some aspects such as the rate of force variation and the forearm and wrist positions are then discussed along with the limitations of the relationship.

Integration of ergonomics into hand tool design: principle and presentation of an example.

The development of ergonomic tools responds to health protection needs on the part of workers, especially the work related musculoskeletal disorders of the upper limbs and to the development of ergonomic tools to take into account the needs of the factories. Only an ergonomic design process can enable tool manufacturers to meet these requirements. Three factors are involved: integration of ergonomics into the design process, definition of the different ergonomic stages involved, and finally knowledge of the different factors involved in hand tool design. This document examines these 3 elements in more detail and presents briefly a project of research whose main purpose is to integrate ergonomic criteria into a design process.

Evaluation of Grip Force Using Electromyograms in Isometric Isotonic Conditions.

The purpose of this study was to develop a relationship to evaluate the grip force (forcerel) using the electromyogram (EMGrel) of the flexor digitorum superficialis (FDS) and of the extensor digitorum (ED) according to the flexion-extension wrist angle (θf.e) and to the pronation-supination forearm angle (θp-s). Fifteen participants had to exert 3 levels of grip forces in 4 positions of the wrist combined with 3 positions of the forearm. The relationship is: forcerel = 0.0045· θf-e· EMGrel(FDS) + 0.48· EMGrel(FDS)-0.0014 · θf-e · EMGrel(ED) -0.0016 · θp.s · EMGrel(ED) + 0.4· EMGrel(ED) This relationship can be used to estimate grip force for levels of strength lower than 50% of the maximal voluntary contraction.