Ready steady push--a study of the role of arm posture in manual exertions

This study investigated arm posture and hand forces during bi-manual pushing. Nine male and eight female participants performed isometric exertions at two reach distances (0 and elbow-grip) and six different positions of the hand interface (handle), defined by the plane (longitudinal, lateral, horizontal) and orientation (0 degrees and 45 degrees). Electrogoniometer instruments were used to measure the displacements/postures of the wrist and elbow joints and the forearm, and force measuring strain gauges were used to measure the exerted hand forces (x-, y- and z-components). The results showed that ability to vary arm posture, particularly the forearm, is important during build up of force and that people tend to seek for a balance in the forces applied at the hands by exerting more in the vertical direction. Also, lateral plane handle positions permitted exertion of greater forces than longitudinal and horizontal plane positions.     

Robotic sanding system for new designed furniture with free-formed surface

In this paper, a sanding system based on an industrial robot with a surface following controller is proposed for the sanding process of wooden materials constructing furniture. Handy air-driven tools can be easily attached to the tip of the robot arm via a compact force sensor. The robotic sanding system is called the 3D robot sander. The robot sander has two novel features. One is that the polishing force acting between the tool and wooden workpiece is delicately controlled to track a desired value, e.g., 2 kgf. The polishing force is defined as the resultant force of the contact force and kinetic friction force. The other is that no complicated teaching operation is required to obtain a desired trajectory of the tool. Cutter location (CL) data, which are tool paths generated by a CAD/CAM system, are directly used for the basic trajectory of the handy tool attached to the robot arm. The robot sander can be applied to the sanding task of free-formed curved surface with which conventional sanding machines have not been able to cope. The effectiveness and promise are shown and discussed through a few experiments.     

Perception of Hand Force in Power Grip for Females

Borg's rating of perceived exertion (RPE) and category ratio (CR‐10) scales are commonly used to quantify perceived muscular exertion for body segments. Twenty females participated in an experiment to study the power grip force at four perceived exertion levels using either dominant or nondominant hand under two posture conditions. It was found that the subjects tended to apply a higher power grip force (100% of perceived maximum voluntary contraction) than the levels they were requested to apply. The power grip forces between dominant and nondominant hands at low hand exertion levels were negligible. The grip forces between the two hands were significantly different when the exertion level was nearly maximal. Linear regression models were established for the subjects to link the relationship between the perceived hand exertion and measured grip force, hand used, and hand/arm posture. All the models were statistically significant (p < 0.0001) with R² values 0.97 or higher. These models provided better estimates in perceived hand exertion for dominant hand than for nondominant hand. A follow‐up experiment was conducted to measure the subjective rating of both the CR‐10 and RPE when a 98 N grip force was applied. It was found that the subjects reported higher exertion levels when they were using the CR‐10 scale than when they were using the RPE scale. © 2011 Wiley Periodicals, Inc.     

Shoulder strength of females while sitting and standing as a function of hand location and force direction

This study evaluated single-handed isometric push strength capabilities of females working at or above-shoulder level. We examined the influence of force exertion direction (vertical, horizontal and lateral), angle of shoulder flexion from horizontal (0°, 30°, 60° and 90°) and gross body posture (standing and sitting), on maximal volitional shoulder strength. Force exertion direction had the greatest affect on shoulder strength (p < 0.0001). Strength was greatest in the vertical axis pushing downwards and weakest in the horizontal plane pushing forwards. Angle influenced shoulder strength when considered together with direction (p < 0.0001). However, these effects were dominated by direction results. Marginal differences in strength existed between sitting and standing (p > 0.05). These results can be used to design workspaces that consider individual strength limitations and their dependence on force direction, work orientation, and gross body posture.     

Keyswitch orientation can reduce finger joint torques during tapping on a computer keyswitch

OBJECTIVE: To examine the effects of keyswitch orientation on joint torques. BACKGROUND: The fingertip produces primarily vertical forces during single-finger tapping on a computer keyswitch. However, horizontal force components within the sagittal plane of the finger could reduce net joint torques. METHOD: Eleven participants tapped on a keyswitch oriented in three directions: vertical, tilted 30 degrees such that when pressed it moved away from the user (similar to a positive-tilt keyboard), and tilted 30 degrees such that when pressed it moved toward the user (similar to a negative-tilt keyboard). Participants also tapped on a prototype cantilever keyswitch design in which the key cap moves along the arc of a bending beam gradually away from the user. Miniature electro-optical goniometers measured the finger posture, and a two-axis force sensor measured fingertip forces. RESULTS: Tapping on a keyswitch oriented such that it moves away from the user when pressed reduced net joint torques by 47% relative to tapping on a vertically orientated keyswitch and by 56% relative to tapping on a keyswitch oriented toward the user, whereas the cantilever design resulted in 14% decreases in net joint torque relative to the vertical orientation. CONCLUSION: Reductions of torques resulted from decreasing the moment arm of the fingertip force about the joints. APPLICATION: Keyboard design should incorporate keyswitch mechanism angles along with other postural and geometric constraints to reduce exposure of the finger joints and muscles to force during typing.     

Shoulder muscle activity in off-axis pushing and pulling tasks

Workspace design can often dictate the muscular efforts required to perform work, impacting injury risk. Within many environments, industrial workers often use sub-maximal forces in offset directions in to accomplish job tasks. The purpose of this research was to develop methods to estimate shoulder muscle activation during seated, static, sub-maximal exertions in off-axis (non-cardinal) directions. Surface EMG signals were recorded from 14 upper extremity muscles in 20 right-handed university aged, right-handed males (age: 22 ± 3 years, weight: 77.5 ± 11.1 kg, height 179.0 ± 7.0 cm) participated in this study. Each participant performed 60 submaximal exertions (40N) directed at 4 off-axis phase angles of 45° (45°, 135°, 225°, and 315°) in 3 planes (frontal, sagittal, and transverse) in 5 hand locations within a right handed reach envelope. The influence of hand location and force direction on muscle activity was evaluated with a forced-entry stepwise regression model. The ability of previously published on-axis prediction equations to predict muscle activity during these off-axis exertions was also evaluated. Within each muscle, activity levels were affected by both hand location and three-dimensional force direction and activation levels ranged from <1 to 37 %MVE. For each force direction there were 75 predictive equations selected and used, and the specific equation that best predicted activation depended on the muscle, exertion direction and hand location evaluated. This work assists ergonomic workplace design to minimize muscle demands during commonly performed off-axis exertions. These estimated demands can be employed to improve workplace design to reduce workplace injuries and enhance worker productivity.     

Postures adopted when using a two-wheeled cylinder trolley

Upper limb and trunk positions were investigated when subjects exerted force to start and move a two-wheeled cylinder trolley with four different handles. Three of the handles had orientations 35 degrees, 50 degrees and 70 degrees in the sagittal plane (relative to the back of the trolley). The fourth was the 50 degrees handles fitted with a link bar. Measurements were made of x (horizontal), y (lateral) and z (vertical) coordinates of the body joints in space throughout the motion, using a CODA MPX30 optical 3-D measurement system, and both linear and angular displacements were computed. The linear and angular positions of the body joints were found to change through the stages of the task. Orientation of the handle influenced the arm postures adopted in the task, and showed to produce different patterns of arm displacement in force exertion to tilt the trolley from standing position. For steady movement, the sagittal plane 50 degrees handle proved most desirable.     

Motor performance patterns between unilateral mechanical assistance and bilateral muscle contraction

Motor performance patterns for mechanical assistance on unilateral force control can be affected by simultaneous muscle contraction. This study investigated how muscle activity and motor performance during the cooperation between dominant-arm force control and assistive force are affected by simultaneous non-dominant arm muscle contraction with inertial loading. Eleven participants (age: 24.1 ± 1.7 years) performed trajectory-tracking task based on visual feedback of real-time isometric force control. Their force for dominant-arm elbow flexion was released from reference magnitude of 47 N to magnitude of 23.5 N by providing mechanical assistance of a linear actuator. A 47 N of inertial loading on non-dominant arm elbow flexion was conditionally provided. For four time epochs of the experimental task, we measured responses of the assisted arm in terms of: (1) surface electromyography (EMG) amplitudes of biceps brachii and triceps brachii muscles, (2) peak perturbation, and (3) motor performance of force variability and target overshoot during manual force output. Simultaneous loading on unassisted arm did not affect peak perturbation of assisted arm. However, it caused lower force variability and overshoot ratio during the time epoch of force release and higher EMG amplitudes of biceps brachii muscle during the time epochs after mechanical assistance is provided, compared to the non-loaded condition. Our results indicate that simultaneous muscle contraction affects unilateral force control with mechanical assistance aimed at enhancing motor performance by creating extra agonist muscle activity. These findings can be utilized for improving the performance of human-robot cooperation during manual material handling in many industrial sites.     

Equations to predict female manual arm strength based on hand location relative to the shoulder

The purpose of this study was to develop regression equations to predict manual arm strength for a wide variety of hand locations within the reach envelope. Maximum voluntary manual arm strength was determined from 71 female participants in six exertion directions (superior, inferior, anterior, posterior, medial and lateral), in a total of 28 hand locations. Forces ranged from 51.3 to 164.4 N, and had a pooled coefficient of variation of 29.9%. Across all 168 combinations of hand locations and exertion directions, the multivariate regression equations explained 92.5% of the variance and had a root mean square error (RMSE) of only 6.4 N, using only the anterior, lateral and vertical location of the hand relative to the active shoulder joint as inputs. These equations provide a proof-of-principle for our novel regression approach, and represent a first step towards a more comprehensive equation to estimate maximum acceptable forces for occupational tasks.     

The principal features of maximal exertion in the sagittal plane

The postural stability diagram (PSD) is a plot of the vertical component of a manual force against the horizontal component. It may be used to assess the interactions of manual strength, frictional limitations at the feet and the forces demanded by a task. This paper presents data on human strength in the form of postural stability diagrams.

The observed features of maximal exertion, in all directions in the sagittal plane, are presented and analysed. Measurements were made on men and women using 12 different combinations of hand and foot placement.

Sex, the position of the hands and feet, and the direction of exertion were important determinants of strength. Evidence is presented to show that, in a minority of conditions, strength is limited by the distribution of body weight and the extent of the foot base. Centres of foot pressure were rarely located at the posterior limits of a subject's anatomical foot base. In certain combinations of hand and foot placement, pronounced peak forces were observed for a narrow range of directions of exertion in the LIFT/PUSH quadrant.     

Dynamic biomechanical model of the hand and arm in pistol grip power handtool usage

The study considers the dynamic nature of the human power handtool operator as a single degree-of-freedom mechanical torsional system. The hand and arm are, therefore, represented as a single mass, spring and damper. The values of these mechanical elements are dependent on the posture used and operator. The apparatus used to quantify these elements measured the free vibration frequency and amplitude decay of a known system due to the external loading of the hand and arm. Twenty-five subjects participated in the investigation. A full factorial experiment tested the effects on the three passive elements in the model when operators exerted maximum effort for gender, horizontal distance (30, 60, 90 cm), and vertical distance (55, 93, 142 190 cm) from the ankles to the handle. The results show that the spring element stiffness and mass moment of inertia changed by 20.6 and 44.5% respectively with vertical location (p<0.01), and 23.6 and 41.2% respectively with horizontal location (p<0.01). Mass moment of inertia and viscous damping for males were 31.1 and 38.5% respectively greater than for females (p<0.01). Tool handle displacement and hand force during torque build-up can, therefore, be predicted based on this model for different tool and workplace parameters. The biomechanical model was validated by recalling five subjects and having them operate a power handtool for varying horizontal distances (30, 60, 90 cm), vertical distances (55, 93, 142 cm), and two torque build-up times (70, 200 ms). Tool reaction displacement was measured using a 3D-motion analysis system. The predictions were closely correlated with these measurements (R = 0.88), although the model underpredicted the response by 27%. This was anticipated since it was unlikely that operators used maximal exertions for operating the tools.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a forcetransducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45° (SD 3.3°) downward to near horizontal, while the pulling force direction changed from pulling upward by 14° (SD 15.3°) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

The risk of distal upper limb disorder in cleaners: A modified application of the Strain Index method

This paper presents a modified application of the Strain Index (SI) method, in evaluation of effort-related variables in cleaning activities (n=40). EMG data were analyzed in the wrist flexor and extensor muscles. Effort-related variables were calculated to each record in four trigger levels (effort values were considered when the amplitude of the signal is above the trigger level for at least 0.5 s). Differences in effort time, intensity, frequency of efforts and in the resulting SI score were observed in the same activity when these variables are calculated with different trigger levels. Vacuuming, cleaning walls, floor scrapping, dusting offices and dusting horizontal surfaces were the most critical cleaning activities in terms of distal upper extremity (DUE) risk disorders; operating mono-disc and scrubber drier floor machines, dry and wet mopping and floor sweeping are among the cleaning activities with comparatively lower risk to DUE disorders. Global values of the cleaning activities (n=40) were calculated: Mean effort-intensity of 59.5% MVE; mean effort duration of 52.6% effort time; mean effort frequency of 7.8 peaks min⁻¹; mean SI score of 8.7 (for a task duration <1 h). The use of EMG data to evaluate effort related variables was found to be an useful alternative to observational methods, when efforts are not clearly associated to hand/wrist movements and when non-cyclical high-frequency activities makes virtually impossible the evaluation of effort variables (intensity, frequency and duration). However, the application of this method requires the definition of an appropriate trigger level and of an activation time.Relevance to industryJob Strain Index (SI) has been widely applied in industry, but not to cleaning activities. Difficulties may occur when observational or self-report methods are applied to activities where hand/wrist efforts are not associated to clear hand/wrist movements and when non-cyclical high-frequency activities make evaluation of effort variables difficult. This study presents a modified application of the original method to calculate wrist exertion variables and it is based on the analysis of EMG data.     

Fabrication and characterization of cantilevers with integrated sharp tips and piezoelectric elements for actuation and detection for parallel AFM applications

We have developed a new process to fabricate arrays of cantilevers with integrated tips for atomic force microscope (AFM) imaging and a piezoelectric layer for vertical actuation and detection. A good homogeneity of the tip shape is obtained thanks to a self-sharpening effect. The cantilevers have been characterized mechanically and electrically.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a force-transducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45 degrees (SD 3.3 degrees) downward to near horizontal, while the pulling force direction changed from pulling upward by 14 degrees (SD 15.3 degrees) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

Ergonomic evaluation of an alternative tool for cake decorating

AimCake decorating involves several hand intensive steps with high grip force during the application of icing. The purpose of this laboratory study was to evaluate forearm muscle activity, discomfort, productivity, and usability of an alternative tool for cake decorating compared to decorating with the traditional piping bag.MethodsParticipants (n = 17) performed 2 h of cake decorating tasks using the two tools. Subjective hand and arm fatigue, usability, upper extremity posture, and muscle activity from three forearm muscles were assessed for each tool. Outcome measures were evaluated using the Wilcoxon Signed Rank test and the paired t-test.ResultsLess fatigue was reported in the dominant hand (p = 0.001), forearm (p = 0.003) and shoulder (p = 0.02) for the alternative tool when compared to the piping bag. Average median (APDF 50%) and peak (APDF 90%) muscle activity was significantly less for the alternative tool across all three forearm muscles. The alternative tool significantly reduced grip force, an important risk factor for distal upper extremity pain and disorders. Participants rated usability of the alternative tool superior for refill and comfort but the traditional method was rated better for accuracy, stability, positioning and control.ConclusionsThe alternative tool was associated with less dominant arm fatigue, muscle activity, and grip force when compared with the piping bag. However, the alternative tool did not receive the best overall usability rating due to problems with accuracy and overflow, especially with smaller decorating tips. Recommendations were made for addressing these problems with the alternative tool.     

Design of an optical soft sensor for measuring fingertip force and contact recognition

Among the various ways to estimate user intention in hand exoskeletons, a contact force measurement is definitely the most straightforward and intuitive method. A force sensor, located at the center of a fingertip usually, hinders the tactile sensation of the user by blocking the contact between an object and the fingertip. To overcome this problem, a soft force sensor with horse shoe shape is utilized to measure the contact force and provide the tactile sensation to the user. This work presents the mechanical design, implementation and evaluation of a soft fingertip force sensor. To maximize tactile sensation of the user, we adopted a horse shoe shape structure to leave the finger pad exposed. An optical sensing mechanism was selected for its relatively fast response compared to other soft sensors. The whole sensor system has a soft exterior providing flexibility and a user-friendly interface. To evaluate the sensor’s performance, we carried out sensor optimization process and calibration experiment with a customized test bed. Then, we investigated both static and dynamic response and observed the mechanical behavior and light intensity changes caused by the cross sectional shape and base/agent ratio of PDMS. Lastly, we applied the proposed sensor to the glove type fingertip force monitoring system. The sensor estimates the index finger tip force with high accuracy (R ² = 0:96) within 5N range.     

A study of the difference between nominal and actual hand forces in two-handed sagittal plane whole-body exertions

Given a task posture, changes in hand force magnitude and direction with regard to joint locations result in variations in joint loads. Previous work has quantified considerable vertical force components during push/pull exertions. The objective of this work was to quantify and statistically model actual hand forces in two-hand, standing exertions relative to the required nominal horizontal and vertical hand forces for a population of widely varying stature and strength. A total of 19 participants exerted force on a fixed handle while receiving visual feedback on the magnitude of force exerted in the required horizontal or vertical direction. A set of regression equations with adjusted R(2) values ranging from 0.20 to 0.66 were developed to define actual hand force vectors by predicting off-axis forces from the required hand force magnitude. Off-axis forces significantly increase the overall magnitude of force exerted in two-hand push/pull and up/down standing force exertions. STATEMENT OF RELEVANCE: This study quantifies and statistically models actual hand forces in two-hand, standing exertions. Inaccuracies in hand force estimates affect the ability to accurately assess task-oriented strength capability. Knowledge of the relationship between nominal and actual hand forces can be used to improve existing ergonomic analysis tools, including biomechanical simulations of manual tasks.     

Development of a touch probe based on five-dimensional force/torque transducer for coordinate measuring machine (CMM)

Systematic errors due to the pre-travel variation and the probe tip shape are irreducible to the traditional touch trigger probing systems. This paper describes the development of a probing system based on five-dimensional force/torque transducer for coordinate measuring machines. The compensation for pre-travel variation is accomplished through the five-dimensional force/torque information acquired by the integrated transducer and the stiffness matrix of the stylus. From the relationship between the obtained force/torque information and the geometrical shape equation, coordinates of the exact contact point immune from the contact error are acquired. After calibration, the combined measurement uncertainty is estimated to be less than ±0.3 μm.     

Field measurement of hand forces of palm oil harvesters and evaluating the risk of work-related musculoskeletal disorders (WMSDs) through biomechanical analysis

Hand force data is critical in evaluating work-related musculoskeletal disorders (WMSDs). Nevertheless, earlier studies on oil palm workers relied on estimated or laboratory measurements, which may not accurately reflect the actual hand forces. This study is the first report on the field measurement of hand forces for palm oil harvesters using a chisel and sickle to harvest low and tall palm trees, respectively. The dynamic hand forces and ground reaction forces were measured using instrumented harvesting tools and force plates, while wearable motion (IMU) and electromyography (EMG) sensors were incorporated for quantifying postural angles and muscle activations, respectively. Additionally, the spinal loadings, continuous Rapid Entire Body Assessment (REBA) scores, and subjective pain scores were determined to evaluate the risk of WMSDs. A total of 10 harvesters were recruited to perform the palm pruning tasks using a chisel and sickle. Resultantly, the sickle and chisel recorded a maximum cutting force of 1601.23 ± 424.26 N and 420.80 ± 96.00 N, respectively. All pruning tasks were found to be highly risky to harvesters, with a peak REBA score of 12. Likewise, all investigated muscles were activated for over 40% MVC, thus inducing moderate pain in the muscles. The peak L5-S1 compression forces for all tasks exceeded the safety threshold (>3400 N), but the values were not significantly different. The shear force of the L5-S1 was extreme in pruning with a sickle (1446.10 ± 411.00 N) compared to using a chisel. In conclusion, palm harvesters were at a high risk of developing WMSDs following poor postures, high physical exertion and muscle activity, and excessive spinal loads.