Inter-rater reliability of an inertial measurement unit sensor-based posture-matching method: A pilot study

Posture quantification is important for analyzing trunk and upper extremity loading and estimating the risk of work-related musculoskeletal disorders. This study introduced an inertial measurement unit sensor-based posture-matching (ISPM) method. The ISPM method is the concept that involves observers wearing sensors and simulating postures in photos or videos obtained at job sites to assess the postural risks for the trunk and upper extremity. We tested the inter-rater reliability of the method in 4 novice observers. For the trunk, shoulder/elbow, and wrist joint angle category, weighted kappa scores were 0.89, 0.52, and 0.26, respectively. The results of the ISPM method indicated moderate or high inter-rater reliability for trunk flexion, shoulder flexion/extension, shoulder abduction/adduction, and elbow flexion. The ISPM method was more reliable than conventional observation-based posture assessment tools used for trunk posture analyses. However, the assessment of upper extremity angles indicated that the reliability of ISPM was lower than that of conventional observation-based posture assessment tools. This was a proof-of-concept study conducted using a few samples. Therefore, further testing is necessary to support the findings.Relevance to industryThe ISPM method requires a minimal level of training. It uses conventional video recording and imposes almost no interruptions to the workers’ performance. The ISPM method showed moderate inter-rater reliability for the trunk, shoulder flexion/extension, shoulder abduction/adduction, and elbow flexion during the joint angle analysis.     

Anthropometric, muscle strength, and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting.

The rating of acceptable load (RAL) attained with a standard test (RALSt) and a wrk-simulating test (RALW) for postal parcel sorting was related to anthropometric, muscle strength, and spinal mobility characteristics of 18 male sorters. The subjects comprised a subsample of 103 experienced male sorters who carried out the RAL tests at postal sorting centres. The dynamic hand-grip endurance correlated significantly (p = 0.036) to the RALSt results. Correspondingly, there was a significant correlation (p = 0.044) between the ratio of maximal isometric strength of trunk extension to body weight and the RALW. The dynamic hand-grip endurance predicted 26% of the variation in the RALSt; in the RALW the maximal isometric strength of trunk flexion to body weight ratio predicted 24%. The subjects who rated heavier weights for RALSt tended to have a better trunk mobility. The dynamic endurance of hand-grip muscles, trunk strength, and spinal flexibility seemed to be the most powerful predictors for the psychophysically assessed 'acceptable loads' in experienced workers performing manual materials handling tasks.     

Test-retest reliability of isometric and isoinertial testing in symmetric and asymmetric lifting

The aim of this study was to evaluate test-retest reliability of a dynamometer in measuring lifting strength or force parameters under several combinations of ergonomic factors. Thirteen healthy participants were tested on peak force (PF), related variables and isometric strength (IS) twice, at intervals of 3 months. Correlation coefficients for all parameters in the sagittal plane were 0.60–0.85. Coefficients of variations (CVs) of methodology error for PF in the sagittal plane were 6.2–6.9%. Correlation coefficients and CVs for IS at 90° to the lateral plane were 0.51– 0.54 and 16.6–17.9%, respectively. In paired t-tests of the parameters under all conditions, there was no significant difference between test and retest. In the test and retest, ratings of perceived exertions for the low back and the right arm in isometric lifting were significantly higher than those in dynamic lifting. It was concluded that the test-retest reliability of dynamic forces in the dynamometer was high. The peak force in the sagittal plane was considered reliable. In isometric lifting, isometric strength in the sagittal plane seemed reliable, while that at right angles to the lateral plane was considered to be less reliable.     

Muscular strength profiles and anthropometry in random samples of men aged 31–35, 51–55 and 71–75 years

Maximal isometric muscular strength and anthropometric characteristics were studied among three random samples each containing about 180 Finnish men belonging to three generation cohorts, and having a mean age of 32·9 ± 1·4, 53·1 ± 1·5 and 72·7± 1·4 years. Strength was measured on special dynamometers for grip, trunk and knee extension and trunk and elbow flexion. The anthropometric characteristics included body height, weight, body-mass index, skeletal weight, fat weight and a weight factor.

The youngest cohort had the highest and the oldest cohort the lowest values for strength as well as body height and fat-free body weight, while the middle-aged group had the highest and the youngest group the lowest values for body weight, fat weight, body-mass index and for the weight factor.

In correlation analysis grip strength was found to have the highest correlation with chronological age and to be least affected by the body anthropometric variables among the strength variables studied. On a percentage scale the decline of strength from the youngest to the oldest cohort was in the order: knee extension (47%), hand grip (42%), trunk extension (42%), trunk flexion (35%) and elbow flexion (35%). The body-mass index was found to be an important variable to be controlled in studying differences between age groups in muscle strength.     

Aerobic power and muscle strength among young and elderly workers with and without physically demanding work tasks

The aim of the study was to investigate the effect of waste collection on the physical capacity of the workers. A total of 19 young and 28 elderly male waste collectors and two age-matched control groups participated. The aerobic power was estimated. The maximal isometric muscle strength was measured for back extension and flexion, shoulder elevation and abduction, and handgrip. The aerobic power was lower among the elderly workers compared with the young workers of both groups. No differences were found between waste collectors and control groups. A general tendency to larger muscle strength was found for both young and elderly waste collectors compared with the control groups. Waste collectors generally have a higher physical capacity than the control groups which is an indication of an early selection of the young waste collectors. With respect to the elderly waste collectors, the job seems to have a training effect especially for the shoulder muscles. No training effect is found for the aerobic power, and a discrepancy between work demand and individual aerobic capacity may occur among elderly workers resulting in a negative health effect unless the work task is evaluated according to age dependent criteria.     

Shoulder load during machine milking An electromyographic and biomechanical study

Abstract

Ten professional milkers simulated 20 different machine-milking postures related to different vertical distances between milker and cow. The loading moment with respect to the shoulder joint bilateral axes was calculated, and EMGs from four shoulder muscles were recorded. The loading moments were related to isometric strength and EMG was standardized against an isometric maximum contraction. Muscle activity was low to moderate. Load moment increased with increasing level difference between milker and cow, but the simultaneous increase in strength utilization and muscular activation was lower than expected. About 25% of maximum shoulder flexor strength was required. The study illustrates some effects of the interaction between shoulder loading moment, trunk inclination and shoulder flexion. The method used gives load both in absolute form (loading moment) and in relation to individual capacity (strength utilization and normalized EMG).     

Anthropometric,muscle strength,and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting

The rating of acceptable load (RAL) attained with a standard test (RAL_st) and a work-simulating test (RAL_w) for postal parcel sorting was related to anthropometric, muscle strength, and spinal mobility characteristics of 18 male sorters. The subjects comprised a subsample of 103 experienced male sorters who carried out the RAL tests at postal sorting centres. The dynamic hand-grip endurance correlated significantly (p=0·036) to the RAL_st results. Correspondingly, there was a significant correlation ( p=0·044) between the ratio of maximal isometric strength of trunk extension to body weight and the RAL_w. The dynamic hand-grip endurance predicted 26% of the variation in the RAL_st; in the RAL_w the maximal isometric strength of trunk flexion to body weight ratio predicted 24%. The subjects who rated heavier weights for RAL_st, tended to have a better trunk mobility. The dynamic endurance of hand-grip muscles, trunk strength, and spinal flexibility seemed to be the most powerful predictors for the psychophysically assessed ‘acceptable loads’ in experienced workers performing manual materials handling tasks.     

The role of the abdominal musculature in the elevation of the intra-abdominal pressure during specified tasks

Abstract

A series of standardized tasks, isometric trunk flexion and extension and maximal Valsalva manoeuvres, were used to evaluate the role of the abdominal musculature in developing an increased intra-abdominal pressure (IAP). Seven male subjects were measured for IAP, myoelectric activity of rectus abdominis (RA), obliquus externus and intemus (OE and OI respectively), erector spinae (ES) and isometric trunk torque. IAPs in all experimental conditions were markedly greater than those that occurred while relaxed. In isometric trunk flexion, IAPs were increased with accompanying high levels of activity from the abdominal muscles. In contrast, little activity from the abdominal muscles occurred during isometric trunk extension, although levels of IAP were similar to those found in the isometric flexion condition. With maximal voluntary pressurization (Valsalva manoeuvre) slightly higher levels of IAP than those found in torque conditions were recorded, this pressure being produced with abdominal activities (OE and OI) less than one fourth their recorded maximum. When isometric torque tasks were added to the Valsalva manoeuvre, patterns of muscle activity (RA, OE, OI and ES) were significantly altered. For Valsalva with isometric trunk extension, activity from OE and OI was reduced while IAPs remained fairly constant. These findings indicate that in tasks where an IAP extension moment is warranted, abdominal pressure can be increased without the development of a large counter-moment produced by the dual action of the trunk flexors. Activation of other muscles such as the diaphragm and transversus abdominis is suggested as helping provide control over the level of IAP during controlled trunk tasks.     

Physical strain in the handling of gas cylinders

Four male employees were studied in the laboratory and during their daily work, distributing gas cylinders. Occupational activities consisted of truck driving with frequent stops for loading, unloading and dragging gas cylinders. Maximal voluntary contraction (MVC) force at handgrip, shoulder elevation, shoulder flexion and shoulder extension was measured and found to exceed the strength of control groups. Average heart rate during occupational work was 99beats/min (range 83-110), corresponding to 35% (range 33-37%) of their maximal oxygen uptake during bicycle ergometry. Heart rates varied within wide limits with peaks frequently approaching maximal values during the heaviest tasks. Photographs were taken to show the position of the limbs and trunk during traction of cylinders weighing 34-76?kg across the ground, and the traction force exerted by the arm was measured with a dynamometer. The results were used for a biomechanical analysis of the load on the upper extremity during traction of gas cylinders. 70% of handgrip MVC was used during traction of the heaviest cylinders. Through forward displacement of the body, a torque, exerted by the body weight and counteracting the drag of the gas cylinder was produced. Simultaneously a more favourable distribution of the traction force was obtained in the shoulder.     

The role of the abdominal musculature in the elevation of the intra-abdominal pressure during specified tasks

A series of standardized tasks, isometric trunk flexion and extension and maximal Valsalva manoeuvres, were used to evaluate the role of the abdominal musculature in developing an increased intra-abdominal pressure (IAP). Seven male subjects were measured for IAP, myoelectric activity of rectus abdominis (RA), obliquus externus and internus (OE and OI respectively), erector spinae (ES) and isometric trunk torque. IAPs in all experimental conditions were markedly greater than those that occurred while relaxed. In isometric trunk flexion, IAPs were increased with accompanying high levels of activity from the abdominal muscles. In contrast, little activity from the abdominal muscles occurred during isometric trunk extension, although levels of IAP were similar to those found in the isometric flexion condition. With maximal voluntary pressurization (Valsalva manoeuvre) slightly higher levels of IAP than those found in torque conditions were recorded, this pressure being produced with abdominal activities (OE and OI) less than one fourth their recorded maximum. When isometric torque tasks were added to the Valsalva manoeuvre, patterns of muscle activity (RA, OE, OI and ES) were significantly altered. For Valsalva with isometric trunk extension, activity from OE and OI was reduced while IAPs remained fairly constant. These findings indicate that in tasks where an IAP extension moment is warranted, abdominal pressure can be increased without the development of a large counter-moment produced by the dual action of the trunk flexors. Activation of other muscles such as the diaphragm and transversus abdominis is suggested as helping provide control over the level of IAP during controlled trunk tasks.     

Endurance time of grip-force as a function of grip-span, posture and anthropometric variables

The time to volitional exhaustion (endurance time) for sustained contractions is considered as a valid parameter to quantify fatigue and to determine the required rest pauses between two successive contractions. In this study, the effects of grip-span, shoulder posture and anthropometric characteristics on endurance time of grip-force during sustained 30% of maximal voluntary grip-force were investigated. Both subjective and objective measures of fatigue were used in determining the endurance times. Twelve male subjects performed sustained isometric handgrip contractions using a handgrip dynamometer at the combinations of three different grip span settings and two shoulder postures. The investigated three grip spans were the optimal, 2 cm narrower than the optimal, and 2 cm wider than the optimal. The investigated two shoulder postures were neutral and 25° flexion. The outcome measures were: endurance time, surface electromyography of related forearm muscles, heart rate, blood pressure, and ratings of perceived discomfort/pain. The results indicate that the endurance time decreases significantly as the grip span deviates from the optimal in both directions. On the other hand, the considered shoulder postures did not have a significant effect on the endurance time. Further analysis indicated a significant negative correlation between endurance time and rest pause and a marginal positive correlation between maximum voluntary grip-force and rest pause. Body mass index, and volume of forearm and hand had also significant negative correlation with endurance time. The comparisons are made with a number of existing endurance models and the impact of findings are discussed.

Relevance to industry

In accurate establishment of the time standards, muscular fatigue allowances need to be taken into account. The endurance time for sustained isometric contractions is correlated with the required rest allowances (pauses) for intermittent static contractions; and therefore, required muscular fatigue allowances can be estimated from the endurance times.     

Trunk postures and peak and cumulative low back kinetics during upright posture sheep shearing

Sheep shearing is the most demanding occupation in the wool harvesting industry and is known to have a high prevalence of low back pain. While use of a commercially available trunk harness reduces load on the low back, the extreme trunk flexion associated with shearing still remains. A novel, upright posture shearing technique has been designed to allow a more neutral spine posture. This study assessed this upright technique and found significant reductions in both trunk flexion and cumulative low back loading when compared to either the traditional method or the use of the trunk harness. Moments about the shoulder tended to be higher while using the upright shearing technique and further investigation of shoulder kinetics will be required to assess whether this creates injury risk to the upper extremity. Despite increased shoulder moments, the reduction in flexion and cumulative loading with the use of the upright technique has the potential to reduce risk of low back pain among shearers.     

Prediction of development of fatigue during a simulated ambulance work task from physical performance tests

The aims of this study were (1) to identify which physical performance tests could best explain the development of fatigue during a simulated ambulance work task, (2) to investigate the effect of height and weight and (3) to investigate in what respects these findings differ between female and male ambulance personnel. Forty-eight male and 17 female ambulance personnel completed a test battery assessing cardio-respiratory capacity, muscular strength and endurance, and co-ordination. The subjects also completed a simulated ambulance work task -- carrying a loaded stretcher. The work task was evaluated by development of fatigue. Univariate and multiple regression analyses were used to investigate to what extent the tests in the test battery were able to explain the variance of developed fatigue. The explained variance was higher for female than for male ambulance personnel (time > 70% of HRpeak: R2 = 0.75 vs 0.10, accumulated lactate: R2 = 0.62 vs 0.42, perceived exertion: R2 = 0.75 vs 0.10). Significant predictors in the models were VO2max, isometric back endurance, one-leg rising, isokinetic knee flexion and shoulder extension strength. Height, but not weight, could further explain the variance. The high physical strain during carrying the loaded stretcher implies the importance of investigating whether improved performance, matching the occupational demands, could decrease the development of fatigue during strenuous tasks.     

Back muscle strength, lifting, and stooped working postures

When lifting loads and working in a forward stooped position, the muscles of the back rather than the ligaments and bony structures of the spine should overcome the gravitational forces. Formulae, based on measurements of back muscle strength, for prediction of maximal loads to be lifted, and for the ability to sustain work in a stooped position, have been worked out and tested in practical situations. From tests with 50 male and female subjects the simplest prediction formulae for maximum loads were: max. load = 1.10 x isometric back muscle strength for men; and max. load = 0.95 x isometric back muscle strength - 8 kg for women. Some standard values for maximum lifts and permissible single and repeated lifts have been calculated for men and women separately and are given in Table 1. From tests with 65 rehabilitees it was found that the maximum isometric strength of the back muscles measured at shoulder height should exceed 2/3 of the body weight, if fatigue and/or pain in the back muscles is to be avoided during work in a standing stooped position.     

Evaluation of upper body kinematics and muscle activity during milking attachment task

The objective of the study was to evaluate the effects of udder height on upper body kinematics and muscle activity during a simulated attachment task in a parallel parlor set up, and the effects of udder access method (back or side) on the task biomechanics. Twenty males performed the task under conditions that simulated three udder heights and two udder access methods. The muscular load and kinematics during the task confirmed that milking is a physically demanding task. Trunk flexion angle increased with decreasing udder height, and the erector spinae activation was higher when the udder was below shoulder height compared to at or above. Compared to accessing the udder from side of the cow (herringbone parlor style), accessing from behind (Parallel parlor style) was associated with lower trunk flexion, greater shoulder horizontal adduction, lower shoulder elevation, and greater anterior deltoid activation. Milking in herringbone parlor style and with the udder at or above shoulder level may help reduce strain on the trunk/neck.     

Trunk posture: reliability, accuracy, and risk estimates for low back pain from a video based assessment method

It has been recently reported that both dynamic movement characteristics, as well as the duration of postures adopted during work, are important in the development of low back pain (LBP). This paper presents a video-based posture assessment method capable of measuring trunk angles and angular velocities in industrial workplaces. The inter-observer reliability, system accuracy, and the relationship of the measured exposures to the reporting of low back pain are reported. The video analysis workstation consisted of a desktop computer equipped with digital video capture and playback technology, a VCR, and a computer game type joystick. The operator could then use a joystick to track trunk flexion and lateral bending during computer-controlled video playback. The joystick buttons were used for binary input of twisting. The inter-observer reliability for peak flexion and percentage of time spent in posture category variables were excellent (ICC>0.8). Lower reliability levels were observed for peak and average velocity and movement related variables. The video analysis system time series data showed very high correlation to the criterion optoelectronic imaging system (r=0.92). Root mean square errors averaged 5.8° for the amplitude probability distribution function data. Trunk flexion variables including peak level, peak velocity, average velocity indicators, and percent time in flexion category indicators all showed significant differences between cases and controls in the epidemiological study. A model consisting of the measures peak trunk flexion, percent time in lateral bend and average lateral bending velocity emerged after multivariable analysis for relationship to low back pain.

Relevance to industry

Risk of injury for the low back is multifactorial. The trunk position and movement velocity are emerging as important parameters. This analysis confirms the importance of these factors and demonstrates the utility of a video-based method to measure them in industrial settings.     

Endurance limits in different modes of load holding

This study examines the influence of different modes of load holding on the isometric strength exertion and endurance capacity. Six healthy males underwent longitudinal testing with the trunk, knee and elbow in 10 different (straight and bent) postures. The load heights (LH) of 320, 440, 850 and 1250 mm from the floor level with subject-load-distance (S-L-D) of 120 and 320 mm were used. It was noted that the level and duration of force exertion were higher when the load was placed closer to the body. The endurance times were consistently high with the increase in LH, highest being recorded in Position 9 - i e, with the load placed at the shoulder level and S-L-D of 120 mm. As evident from MVC and endurance times, the back straight/knee bent postures may not be preferred to back bent postures.     

Prediction of development of fatigue during a simulated ambulance work task from physical performance tests

The aims of this study were (1) to identify which physical performance tests could best explain the development of fatigue during a simulated ambulance work task, (2) to investigate the effect of height and weight and (3) to investigate in what respects these findings differ between female and male ambulance personnel. Forty-eight male and 17 female ambulance personnel completed a test battery assessing cardio-respiratory capacity, muscular strength and endurance, and co-ordination. The subjects also completed a simulated ambulance work task — carrying a loaded stretcher. The work task was evaluated by development of fatigue. Univariate and multiple regression analyses were used to investigate to what extent the tests in the test battery were able to explain the variance of developed fatigue. The explained variance was higher for female than for male ambulance personnel (time > 70% of HR_peak: R² = 0.75 vs 0.10, accumulated lactate: R² = 0.62 vs 0.42, perceived exertion: R² = 0.75 vs 0.10). Significant predictors in the models were VO₂max, isometric back endurance, one-leg rising, isokinetic knee flexion and shoulder extension strength. Height, but not weight, could further explain the variance. The high physical strain during carrying the loaded stretcher implies the importance of investigating whether improved performance, matching the occupational demands, could decrease the development of fatigue during strenuous tasks.     

Interfering effects of the task demands of grip force and mental processing on isometric shoulder strength and muscle activity

The purpose of this study was to examine the interfering effects of physical and mental tasks on shoulder isometric strength in different postures. Fifteen volunteers (seven women, eight men) performed a series of isometric shoulder exertions at 30°, 60° and 90° of both shoulder flexion and abduction alone and with the addition of a 30% grip force, a mental task (Stroop test) and both additional tasks simultaneously. The shoulder tasks were completed either at maximal intensity, or while maintaining a shoulder posture without any additional effort. Surface electromyography (EMG) from seven muscles of the shoulder girdle and shoulder moment were collected for each 6 s shoulder exertion. When normalized to maximum exertion, no differences were found between genders and no differences existed between conditions when subjects maintained each posture without exerted force. In the maximal shoulder exertion trials, an increase in shoulder angle (in either plane) resulted in an increase in EMG in most muscles, while shoulder moment decreased in flexion and remained constant in abduction. Shoulder moments and muscle activation were greatest in the shoulder exertion alone condition followed by adding a 30% grip and the Stroop test, with the addition of both tasks further reducing the exerted shoulder moment and EMG. However, muscle activity did not always decrease with shoulder strength and remained elevated, indicating a complex coactivation pattern produced by an interfering role of the tasks. Overall, it was found that a mental task can have the same or greater effect as a concurrent grip and should be considered when assessing muscular loading in the workplace, as typical biomechanical modelling may underestimate internal loads. The results not only provide valuable shoulder strength data but also practical strength values, depending on additional tasks.     

Active trunk stiffness during voluntary isometric flexion and extension exertions

OBJECTIVE: Compare muscle activity and trunk stiffness during isometric trunk flexion and extension exertions. BACKGROUND: Elastic stiffness of the torso musculature is considered the primary stabilizing mechanism of the spine. Therefore, stiffness of the trunk during voluntary exertions provides insight into the stabilizing control of pushing and pulling tasks. METHODS: Twelve participants maintained an upright posture against external flexion and extension loads applied to the trunk. Trunk stiffness, damping, and mass were determined from the dynamic relation between pseudorandom force disturbances and subsequent small-amplitude trunk movements recorded during the voluntary exertions. Muscle activity was recorded from rectus abdominus, external oblique, lumbar paraspinal, and internal oblique muscle groups. RESULTS: Normalized electromyographic activity indicated greater antagonistic muscle recruitment during flexion exertions than during extension. Trunk stiffness was significantly greater during flexion exertions than during extension exertions despite similar levels of applied force. Trunk stiffness increased with exertion effort. CONCLUSION: Theoretical and empirical analyses reveal that greater antagonistic cocontraction is required to maintain spinal stability during trunk flexion exertions than during extension exertions. Measured differences in active trunk stiffness were attributed to antagonistic activity during flexion exertions with possible contributions from spinal kinematics and muscle lines of action. APPLICATION: When compared with trunk extension exertions, trunk flexion exertions such as pushing tasks require unique neuromuscular control that is not simply explained by differences in exertion direction. Biomechanical analyses of flexion tasks must consider the stabilizing muscle recruitment patterns when evaluating spinal compression and shear loads.