Development of a new dynamometer for measuring grip strength applied on a cylindrical handle

[Impact of a simulated DIPJ Arthrodesis on Movement and Force Patterns]

PURPOSE

Distal interphalangeal joint (DIPJ) arthrodesis is a well-proven therapy for osteoarthritis in the DIPJ. Since the upper limb is effectively a linked chain which is moved by interlinked, joint-overlapping muscle-tendon units, impacts on movement and force patterns are expected to occur in response to arthrodesis. In this context, a real-time quantitative analysis has not been performed to date.

MATERIAL AND METHODS

Finger motion and force development during grasping were dynamically measured and quantitatively analyzed in 19 healthy volunteers with a simulated DIPJ arthrodesis using a TUB (Technische Universität Berlin) sensor glove during fist closure and evaluating two types of force grips compared with the physiological grip.

RESULTS

Typical motion patterns were found. During physiological fist closure, the average flexion angle was 71.5° in the metacarpophalangeal joint (MPJ), 76.8° in the proximal interphalangeal joint (PIPJ) and 37.3° in the distal interphalangeal joint (DIPJ). With DIPJ arthrodesis, the flexion angle decreased to 49.6° in the PIPJ, whereas it increased slightly to 77.3° in the MPJ. During force grip I, the average physiological flexion angles were 18.3° in the MPJ, 39.6° in the PIPJ and 42.6° in the DIPJ. With simulated DIPJ arthrodesis, the flexion angle in the MPJ increased to 28.4°, whereas it decreased to 25.2° in the PIPJ. Force grip II yielded physiological flexion angles of 30.9° in the MPJ, 36.6° in the PIPJ and 29.0° in the DIPJ. In response to simulated DIPJ arthrodesis, the angle in the MPJ increased to 34.4° while it decreased to 23.3° in the PIPJ. The forces measured with force grips were almost equally distributed under physiological conditions. In response to simulated DIPJ arthrodesis, the average decrease in the measured force of a finger was no more than 1.4%.

CONCLUSION

This study was the first to introduce a quantitative analysis of grasping with simulated DIPJ arthrodesis. Based on this analysis, the study demonstrates the dynamic interaction of the finger joints as well as force patterns on the individual finger rays of the hand in real-time.

Assessing Increased Activities of the Forearm Muscles Due to Anti-Vibration Gloves: Construct Validity of a Refined Methodology

OBJECTIVE

The primary aim was to test the construct validity of a surface electromyography (EMG) measurement protocol, indirectly assessing the effects of anti-vibration (AV) gloves on activities of the forearm muscles.

BACKGROUND

AV gloves impose a relatively higher grip demand and thus a higher risk for musculoskeletal disorders. Consequently, activities of the forearm muscles should be considered when assessing AV glove performance.

METHOD

Effects of AV gloves on activities of the forearm muscles (ECR: extensor carpi radialis longus; ED: extensor digitorum; FCR: flexor carpi radialis; FDS: flexor digitorum superficialis) were measured via EMG, while gripping a handle with two grip force levels. Fifteen subjects participated with 11 glove conditions, including one with bare hand.

RESULTS

Activities of ECR, FCR, mean of ECR and FCR (ECR_FCR), and mean of all four muscles were sensitive to wearing gloves. Compared with bare hand, combined ECR_FCR activities increased by 22%-78% (mean = 48%, SD = 28%) with gloves. The correlation coefficient (r) of ECR_FCR activities with glove thickness and manual dexterity scores were 0.74 (p < .05) and 0.90 (p < .001), respectively.

CONCLUSIONS

A refined EMG methodology was the most sensitive to AV gloves with specific forearm muscles (ECR and FCR) and the 50-N handgrip force. Its construct validity was further substantiated by correlations with glove thickness and manual dexterity.

APPLICATION

Assessment of the effect of AV gloves on activities of the forearm muscles can yield design guidance for AV gloves to reduce grip exertion by the gloved hand.

A Review of Hand-Arm Vibration Studies Conducted by US NIOSH since 2000

Studies on hand-transmitted vibration exposure, biodynamic responses, and biological effects were conducted by researchers at the Health Effects Laboratory Division (HELD) of the National Institute for Occupational Safety and Health (NIOSH) during the last 20 years. These studies are systematically reviewed in this report, along with the identification of areas where additional research is needed. The majority of the studies cover the following aspects: (i) the methods and techniques for measuring hand-transmitted vibration exposure; (ii) vibration biodynamics of the hand-arm system and the quantification of vibration exposure; (iii) biological effects of hand-transmitted vibration exposure; (iv) measurements of vibration-induced health effects; (iv) quantification of influencing biomechanical effects; and (v) intervention methods and technologies for controlling hand-transmitted vibration exposure. The major findings of the studies are summarized and discussed.

Load distribution of the hand during cylinder grip analyzed by Manugraphy

STUDY DESIGN

Clinical measurement and basic research.

INTRODUCTION

Manugraphy allows assessing dynamically all forces applied perpendicular to a cylinder surface by the whole contact area of the hand with a high spatial resolution.

PURPOSE OF THE STUDY

To identify the physiological load distribution of the whole contact area of the hand during cylinder grip.

METHODS

A sample of 152 healthy volunteers performed grip force tests with 3 cylinder sizes of the Manugraphy system (novel, Munich, Germany) on 3 different days. The whole contact area of the hand was sectioned into 7 anatomic areas, and the percent contribution of each area in relation to the total load applied was calculated. The load distribution of the dominant and nondominant hands and with different cylinder sizes was compared. Furthermore, the load distribution between the finger phalanges of each finger was analyzed.

RESULTS

The results for the dominant and nondominant hands were in all 7 areas of the hand similar with the percent contribution differing within a range of 1%-4% (P > .138). Load distribution changed significantly with different cylinder sizes: all 7 areas differed between 1% and 7% with P < .001, most pronounced for the thumb. The load distribution of the phalanges showed that the contribution of the distal phalanges increased with ascending cylinder size, whereas the contribution of the proximal phalanges decreased. The interindividual variability of the load distribution pattern was noticeable.

DISCUSSION

For the clinical practice, Manugraphy might be a useful supplement to traditional grip force measurement for identifying the individual characteristics of a patient's dysfunction and monitoring the progress of hand rehabilitation.

CONCLUSIONS

There is no universal or typical load distribution pattern of the hand but only an individual pattern. To evaluate a compromised hand, it is permissible to compare it with the healthy opposite hand as a reference. Several cylinder sizes should be used for load distribution testing. Using smaller handles in the daily life can help to compensate impairment of the thumb and fingertips.

LEVEL OF EVIDENCE

2.

A Brief Review of Handgrip Strength and Sport Performance

Cronin, J, Lawton, T, Harris, N, Kilding, A, and McMaster, DT. A brief review of handgrip strength and sport performance. J Strength Cond Res 31(11): 3187-3217, 2017-Tests of handgrip strength (HGS) and handgrip force (HGF) are commonly used across a number of sporting populations. Measures of HGS and HGF have also been used by practitioners and researchers to evaluate links with sports performance. This article first evaluates the validity and reliability of various handgrip dynamometers (HGD) and HGF sensors, providing recommendations for procedures to ensure that precise and reliable data are collected as part of an athlete's testing battery. Second, the differences in HGS between elite and subelite athletes and the relationships between HGS, HGF, and sports performance are discussed.

Parameters influencing hand grip strength measured with the manugraphy system

BACKGROUND

This study aimed to determine whether sex, hand length and the individual training status affect hand strength and whether these measurements differ if they are recorded using the Jamar dynamometer or a new cylindrical measuring system.

METHODS

For this purpose, 152 healthy adults were examined using a new manugraphy measuring system (novel, Munich, Germany) comprising two measuring cylinders of different sizes and a Jamar electronic dynamometer with two grip positions corresponding approximately to the sizes of the cylinders. A descriptive analysis was performed as well as a correlation analysis using the Pearson correlation coefficient. To prepare predictive models, multiple linear regression analyses were carried out to determine factors that influence the force and p ≤ 0.05 was considered statistically significant.

RESULTS

A significant difference in the maximum and mean strength was observed that is dependent on sex, with men stronger than women, in line with expectations, and hand length, with small hands able to exert less force than large hands. No consistent increase in strength could be attributed to repetitive manual loads applied either at work or in leisure activities.

CONCLUSIONS

Both measurement techniques yielded similar results, suggesting that manugraphy is well suited for clinical research purposes because it not only takes measurements that are just as reproducible and valid as the conventional measurement technique but in doing so measures not just the total strength of a hand but also enables more precise comparisons of isolated hand regions applying dynamic measurements.

A new method of ergonomic testing of gloves protecting against cuts and stabs during knife use

The paper presents a new method of ergonomic evaluation of gloves protecting against cuts and stabs during knife use, consisting of five manual dexterity tests. Two of them were selected based on the available literature and relevant safety standards, and three were developed by the authors. All of the tests were designed to simulate occupational tasks associated with meat processing as performed by the gloved hand in actual workplaces. The tests involved the three most common types of protective gloves (knitted gloves made of a coverspun yarn, metal mesh gloves, and metal mesh gloves with an ergonomic polyurethane tightener) and were conducted on a group of 20 males. The loading on the muscles of the upper limb (adductor pollicis, flexor carpi ulnaris, flexor carpi radialis, and biceps brachii) was measured using surface electromyography. For the obtained muscle activity values, correlations were found between the glove type and loading of the upper limb. ANOVA showed that the activity of all muscles differed significantly between the five tests. A relationship between glove types and electromyographic results was confirmed at a significance level of α = 0.05.

Reduction in Cylindrical Grasp Strength Is Associated With Early Thumb Carpometacarpal Osteoarthritis

BACKGROUND

Advanced thumb carpometacarpal (CMC) osteoarthritis (OA) can cause substantial impairment in hand function, from grasping heavy objects to fine manipulation of implements and tools. In the clinical setting, we commonly measure the grip strength of gross grasp with a hand dynamometer in patients with CMC OA. Cylindrical grasp, which requires more thumb contribution than gross grasp, is an alternative method of measuring grip strength and one that may provide insight into thumb-related conditions. Because gross grasp and cylindrical grasp use the thumb in different planes, measurement of gross grasp alone might underestimate impairment. Therefore, it is important to evaluate cylindrical grasp as well. To our knowledge this tool has yet to be examined in a population with early thumb CMC OA.

QUESTIONS/PURPOSES

(1) Is cylindrical grasp and gross grasp strength reduced in subjects with early thumb CMC OA compared with asymptomatic control subjects? (2) What is the association of cylindrical and gross grasp to thumb CMC OA after adjusting for age, sex, and hand dominance?

METHODS

We recruited 90 subjects with early symptomatic and radiographic thumb CMC OA and 38 asymptomatic healthy control subjects for this multisite controlled study. Demographic information, hand examination, comprehensive histories, plain film radiographs, and cylindrical and gross grasp strength data were collected on all 128 subjects. Mean grasp strength was calculated for cylindrical and gross grasp in the population with early CMC OA and the control population. A t-test was performed on cylindrical and gross grasp to evaluate the difference between the mean in the control and early CMC OA populations. We used separate linear regression models for the two types of grasp to further quantify the association of grasp with a diagnosis of early thumb CMC OA controlling for age, sex, and whether the subject used their dominant or nondominant hand in the study.

RESULTS

Cylindrical grasp was weaker in the population with thumb CMC OA compared with healthy control subjects (6.3 ± 2.7 kg versus 8.4 ± 2.5 kg; mean difference, 2.1; 95% CI, 1.1-3.1; p < 0.001), but there was no difference in gross grasp force (29.6 ± 11.6 kg versus 31.4 ± 10.1 kg; mean difference, 1.7; 95% CI, -2.5 to 6.0; p = 0.425). When adjusting for age, sex, and handedness, cylindrical grasp reduction was related to CMC OA (β = -2.3; standard error [SE], 0.46; p < 0.001) (Y-intercept = 8.2; SE, 1.8; R² = 0.29), whereas gross grasp was not reduced in early thumb CMC OA (β = -2.8; SE, 1.6; p = 0.072) (Y-intercept = 34.3; SE, 6.3; R² = 0.48).

CONCLUSIONS

A reduction in cylindrical grasp is associated with early symptomatic and radiographic CMC OA, whereas gross grasp is not associated with early thumb CMC OA, suggesting that cylindrical grasp may be a better tool to detect changes in thumb and hand function seen during early disease stages.

CLINICAL RELEVANCE

Cylindrical grasp may serve as a more-sensitive measure for detecting early changes in early CMC OA. The associated decline in hand function also might provide an opportunity for measuring the effectiveness of treatment and intervention.

DEVELOPMENT OF ARDUINO-BASED HAND DYNAMOMETER ASSISTIVE DEVICE

The aim of this study is to develop a hand dynamometer that can act as a rehabilitation device by acquiring quantified data to enhance the power of hand grip strength. On the whole, patient that suffers from the hand injury is directly exposed to intricacy when performing daily task. Therefore, many hand aids have been developed to overcome the problem. A typical hand assistive device is able to measure the hand grip strength, which eventually increases its functionality. Like hand dynamometer, it is used for regular screening of hand grip strength and also for the preliminary and ongoing assessment of patients with hand dysfunction or trauma. Strain gauge-based system transducer acts as the measurement system together with Arduino microcontroller for the instrumentation, communication and controlling applications. The integration of strain gauges with a transducer is called a load cell which also made up the overall of force sensor to obtain readings from the hand grip movement. Microcontroller will further use this information to store and analyze data in the SD card. The percentage difference observed between hands across the sample of 25 subjects support the 10% rule. An overall 10.74% difference was found when combined dominant and non-dominant hand strength scores for all subjects were observed.

Review and Evaluation of Hand-Arm Coordinate Systems for Measuring Vibration Exposure, Biodynamic Responses, and Hand Forces

The hand coordinate systems for measuring vibration exposures and biodynamic responses have been standardized, but they are not actually used in many studies. This contradicts the purpose of the standardization. The objectives of this study were to identify the major sources of this problem, and to help define or identify better coordinate systems for the standardization. This study systematically reviewed the principles and definition methods, and evaluated typical hand coordinate systems. This study confirms that, as accelerometers remain the major technology for vibration measurement, it is reasonable to standardize two types of coordinate systems: a tool-based basicentric (BC) system and an anatomically based biodynamic (BD) system. However, these coordinate systems are not well defined in the current standard. Definition of the standard BC system is confusing, and it can be interpreted differently; as a result, it has been inconsistently applied in various standards and studies. The standard hand BD system is defined using the orientation of the third metacarpal bone. It is neither convenient nor defined based on important biological or biodynamic features. This explains why it is rarely used in practice. To resolve these inconsistencies and deficiencies, we proposed a revised method for defining the realistic handle BC system and an alternative method for defining the hand BD system. A fingertip-based BD system for measuring the principal grip force is also proposed based on an important feature of the grip force confirmed in this study.

The Effect of a Mechanical Arm System on Portable Grinder Vibration Emissions

Mechanical arm systems are commonly used to support powered hand tools to alleviate ergonomic stressors related to the development of workplace musculoskeletal disorders. However, the use of these systems can increase exposure times to other potentially harmful agents such as hand-transmitted vibration. To examine how these tool support systems affect tool vibration, the primary objectives of this study were to characterize the vibration emissions of typical portable pneumatic grinders used for surface grinding with and without a mechanical arm support system at a workplace and to estimate the potential risk of the increased vibration exposure time afforded by the use of these mechanical arm systems. This study also developed a laboratory-based simulated grinding task based on the ISO 28927-1 (2009) standard for assessing grinder vibrations; the simulated grinding vibrations were compared with those measured during actual workplace grinder operations. The results of this study demonstrate that use of the mechanical arm may provide a health benefit by reducing the forces required to lift and maneuver the tools and by decreasing hand-transmitted vibration exposure. However, the arm does not substantially change the basic characteristics of grinder vibration spectra. The mechanical arm reduced the average frequency-weighted acceleration by about 24% in the workplace and by about 7% in the laboratory. Because use of the mechanical arm system can increase daily time-on-task by 50% or more, the use of such systems may actually increase daily time-weighted hand-transmitted vibration exposures in some cases. The laboratory acceleration measurements were substantially lower than the workplace measurements, and the laboratory tool rankings based on acceleration were considerably different than those from the workplace. Thus, it is doubtful that ISO 28927-1 is useful for estimating workplace grinder vibration exposures or for predicting workplace grinder acceleration rank orders.

The Effect of Midcarpal Versus Total Wrist Fusion on the Hand's Load Distribution During Gripping

PURPOSE

To analyze the total grip force and load distribution of the hand with midcarpal fusion (MCF) and total wrist fusion (TWF).

METHODS

Twelve patients with unilateral TWF and 12 patients with unilateral MCF were assessed at an average 64 months (range, 19-100 months) postoperatively. The total grip force and load distribution of both hands were measured by the Manugraphy system using 3 cylinder sizes. The load applied to 7 anatomical areas of the hand during cylinder grip was analyzed, comparing the operated and the nonsurgical hands.

RESULTS

For the 100 mm and 150 mm cylinders, a significantly lower total grip force was found in hands operated with either TWF or MCF. For the 200 mm cylinder, there was a significant difference between nonsurgical hands and those with MCF but not between nonsurgical hands and those with TWF. For the 100 mm cylinder, the difference between nonsurgical and operated hands was greater in hands with TWF than those with MCF. For the load distribution of the hand, no differences between the operated and the nonsurgical hand were found for either MCF or TWF.

CONCLUSIONS

MFC and TWF resulted in a reduced cylinder grip force. With respect to the load distribution, neither procedure influenced the relative contribution that each area of the hand produced during cylinder grip.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic III.

Multiaxis grip characteristics for varying handle diameters and effort

OBJECTIVE

A multiaxis dynamometer was used to quantify grip force vector angles and longitudinal centers of pressure (COPs) while varying handle size and effort used.

BACKGROUND

Authors of many studies have examined maximum grip strength using scalar instruments; a few have measured two-axis forces limited to one or more finger contact. This novel dynamometer uses two instrumented beams that are grasped by the distal fingers and proximal palm to compute two orthogonal components of force and the longitudinal COP through which the force acts.

METHOD

Sixteen healthy, right-handed participants grasped the multiaxis dynamometer with plastic handles ranging in diameter from 3.81 to 7.62 cm. They were required to scale their effort to 25%, 50%, 75%, and 100% of maximum.

RESULTS

Grip force vector angles were affected by both handle diameter and effort level, with angles increasing an average of 8.1° from the least to greatest effort. Longitudinal COP, averaged among the two beams, shifted 1.75 cm radially as handle diameter increased from 3.81 cm to 7.62 cm. Average COP along the beam in contact with the distal finger segments shifted 0.75 cm ulnarly as effort level increased from 25% to 100% of maximum.

CONCLUSION

Grip force characteristics changed with handle diameter and effort level. Overall grip force magnitude comprised both force components measured.

APPLICATION

Understanding grip characteristics should be important for handle and grip design and for evaluating hand function.

Grip force monitoring on the hand: Manugraphy system versus Jamar dynamometer

INTRODUCTION

For clinical grip force assessment, the Jamar dynamometer is a wide accepted tool. Users have to be aware that this method does not represent all grip efforts applied. The Manugraphy system is a tool that measure total grip force as well as identify load distribution patterns of the hand while gripping cylinders wrapped with calibrated capacitive matrix sensor mats. The aim of this study was to validate an assessment setting of the Manugraphy system for clinical use. Further, the relationship and difference between the Manugraphy system and the Jamar dynamometer were investigated.

MATERIALS AND METHODS

At two study centers, 152 healthy volunteers performed grip force tests with a digital Jamar dynamometer using handle positions 3 and 4 and the novel(®) Manugraphy system using two cylinders with circumferences of 150 and 200 mm. The subjects performed grip force testing with both devices on three different days. The intra- and inter-day variability for both methods was evaluated. To compare the values of both systems, the Spearman correlation coefficient was calculated.

RESULTS

The force values, as measured by the sensor matrix, were higher than those of the Jamar dynamometer. Analyses showed significant positive correlations between values obtained by the two measurement methods (p < 0.001). There was no significant inter-day variation for the 200-mm cylinder of the Manugraphy system. For the 150-mm cylinder, a significant variation was observed at center B, but not at A. Nevertheless, the fluctuation of the grip force values obtained with the Manugraphy system was equal or better than those obtained with the Jamar dynamometer.

CONCLUSIONS

The force values, obtained using the two systems, have a high correlation but are not directly comparable. Both systems allow valid and constant grip force measurement. As the sensor mat detects all forces applied perpendicularly to the cylinder surface, it characterizes grip force better than the Jamar dynamometer. In addition, information about load distribution of the hand is gained.

Quantification of finger joint loadings using musculoskeletal modelling clarifies mechanical risk factors of hand osteoarthritis

Owing to limited quantitative data related to the loadings (forces and pressures) acting upon finger joints, several clinical observations regarding mechanical risk factors of hand osteoarthritis remain misunderstood. To improve the knowledge of this pathology, the present study used musculoskeletal modelling to quantify the forces and pressures acting upon hand joints during two grasping tasks. Kinematic and grip force data were recorded during both a pinch and a power grip tasks. Three-dimensional magnetic resonance imaging measurements were conducted to quantify joint contact areas. Using these datasets as input, a musculoskeletal model of the hand and wrist, including twenty-three degrees of freedom and forty-two muscles, has been developed to estimate joint forces and joint pressures. When compared with the power grip task, the pinch grip task resulted in two to eight times higher joint loadings whereas the grip forces exerted on each finger were twice lower. For both tasks, joint forces and pressures increased along a disto-proximal direction for each finger. The quantitative dataset provided by the present hand model clarified two clinical observations about osteoarthritis development which were not fully understood, i.e., the strong risk associated to pinch grip tasks and the high frequency of thumb-base osteoarthritis.

Is power grasping contact continuous or discrete?

During power grasp, the number of local force maxima reflects either the central nervous system's preferential use of particular hand regions, or anatomical constraints, or both. Previously, both bimodal and trimodal force maxima have been hypothesized for power grasp of a cylindrical handle. Here we measure the number of local force maxima, with a resolution of 4.8°, when performing pushing and pulling efforts in the plane perpendicular to the cylinder's long axis. Twelve participants produced external forces to eight targets. The number of contacts was defined as the number of local maxima exceeding background variance. A minimum of four and a maximum of five discrete contacts were observed in all subjects at the distal phalanges and metacarpal heads. We thus reject previous hypotheses of bimodal or trimodal force control for cylindrical power grasping. Since we presently observed only 4-5 contacts, which is rather low considering the hand's kinematic flexibility in the flexion plane, we also reject hypotheses of continuous contact, which are inherent to current grasping taxonomy. A modification to current grasping taxonomy is proposed wherein power grasp contains separate branches for continuous and discrete contacts, and where power and precision grasps are distinguished only by grasp manipulability.

Grip and pinch capability assessment system for children

The grip movement is essential for performing daily activities. However, the assessment of this movement is currently made subjectively, due to the lack of appropriate quantification equipment. The objective of this study was to develop, validate and test a virtual environment controlled by five trigger devices used to analyse cylindrical, spherical and hook grips as well as tip-to-tip pinch and pulp-to-side movements. Sensors and electronic circuits that detect the correct grip, the threshold of grip strength and the range of motion were inserted into the devices. To validate this system, tests were conducted with 20 children while being evaluated by three physical therapists, all specialists in neurology. The results from the evaluators and the system agreed to an extent of 86.6%. Following validation, the system was used by 35 children with no motor impairment and by 10 children with mild motor abnormalities in an upper limb; these assessments provided efficient and reliable results. The developments presented in this study may help to assess grip and pinch movements and facilitate the choice of strategies in therapeutic processes.

The effect of handhold orientation, size, and wearing gloves on hand-handhold breakaway strength

OBJECTIVE

The aim of this study was to quantify the effect of handhold orientation, size (diameter), and wearing a glove on the maximum breakaway strength between a hand and handhold.

BACKGROUND

Manual breakaway strength is known to be greatly reduced for vertical compared with horizontal handholds, but oblique orientations have yet to be studied.

METHOD

For this study, 12 young adults (6 female) attempted to hold on to fixed overhead cylindrical handholds with one hand in low-speed simulated falls as forces on the handhold were recorded in two experimental designs. Breakaway strength was measured for (a) three different-sized cylinders in four orientations while the participants were using the dominant hand and (b) a single-sized cylinder in four orientations while the participants were bare-handed or wearing a glove on the nondominant hand.

RESULTS

Handhold orientation (p < .001), handhold diameter (p < .001), and wearing gloves (p < .001) significantly affected breakaway strength. Breakaway strength increased 75% to 94% as the orientation of the handhold was moved from vertical to horizontal. Breakaway strength decreased 8% to 13% for large-diameter (51-mm) handholds as compared with smaller diameters (22 mm to 32 mm), depending on orientation. Gloves may increase or decrease the ability to hang on depending on interface friction; greater friction increased breakaway force.

CONCLUSION

Handles oriented perpendicular to the pull direction and high-friction gloves provide the greatest breakaway strength. Smaller handhold diameters than predicted by grip strength afford greater capability in these orientations.

APPLICATION

These insights can be used to design handholds that increase the ability to support one's body weight and reduce the effort needed to pull or lift heavy items.

Characterisation of forces exerted by the entire hand during the power grip: effect of the handle diameter

The objective of this study was to analyse the effect of the handle diameter on the grip forces exerted by the hand during a maximal power grip task. A handle ergometer, combining six instrumented beams and a pressure map, was used to determine the forces exerted by the palm side of the hand regrouping data from 10 anatomical sites (fingertips, phalanges, thumb, palm…). This methodology provided results giving new insight into the effect of the handle diameter on the forces exerted by the hand. First, it appeared that the relationship between the hand length/handle diameter ratio and the maximal grip force fit a U-inverted curve with maximal values observed for a handle diameter measuring 17.9% of the hand length. Second, it was showed that the handle diameter influenced the forces exerted on the anatomical sites of the hand. Finally, it was showed that the handle diameter influenced the finger force sharing particularly for the index and the little fingers. Practitioner Summary: This study analysed the effect of the handle diameter on the grip forces exerted by the hand during a maximal power grip force. This study showed that measurement of the totality of the forces exerted at the hand/handle interface is needed to better understand the ergonomics of handle tools. Our results could be re-used by designers and clinicians in order to develop handle tools which prevent hand pathologies.

Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration

Radial force (F(r)) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit F(r) tasks, and have not normalized for anatomical variance when considering F(r) distributions. The goals of the present study were (i) to explore F(r) during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in F(r) data. Twelve subjects grasped a vertically oriented cylindrical handle (diameter=6 cm) and matched target upward tangential forces of 10, 20, and 30 N. F(r) data were measured using a flexible pressure mat with an angular resolution of 4.8°, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that F(r) was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers' metatacarpal heads, and that, while increases in tangential force caused significant increases in F(r) for these regions, they did not significantly affect the F(r) distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated F(r) trends, and amplification of the test statistic. These results imply that, while subjects focus F(r) primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one's ability to draw anatomically relevant conclusions from grasping force data.

Individual finger contribution in submaximal voluntary contraction of gripping

The objective of this study was to evaluate individual finger force and contribution to a gripping force, the difference between actual and expected finger forces and subjective discomfort rating at 10 different submaximal voluntary contraction (%MVC) levels (10-100% in 10 increments). Seventy-two participants randomly exerted gripping force with a multi-finger force measurement system. The individual finger force, gripping force and discomfort increased as %MVC levels increased. The middle and ring fingers exerted more force and contributed to a gripping force more than the index and little fingers due to their larger mass fractions of the digit flexor muscles. It was apparent at <50% MVC; however, the index finger increased its contribution and exerted even more force than expected at more than 50% MVC. Subjective discomfort supported the results of the objective measures. This could explain the conflicting findings between index and ring fingers in previous finger contribution studies. STATEMENT OF RELEVANCE: Hand tool design is of special interest in ergonomics due to its association with musculoskeletal disorders in the hand. This study reveals a different contribution pattern of the fingers in submaximal voluntary contraction of gripping exertion.

Frictional properties of different hand skin areas and grasping techniques

High friction is crucially important in manipulation activities for reducing the hand grip forces and improving control of manipulative tasks. The aim of this study was to assess the coefficient of friction (COF) of various areas of hand skin. Static COF of nine different grasping techniques applied against two object coatings was assessed by means of the 'slip point' method in 16 participants. COF measures proved to be both highly reliable and considerably variable across participants (coefficients of variation ranging from 25 to 75%, depending on the applied grasp). COF was also higher in 'specialised' than in 'non-specialised' skin areas for grasping, as well as in palms, than in the tips of the fingers. The observed findings are of importance for optimisation of object manipulations and also emphasise the importance of measuring individual COF in ergonomic, biomechanics and motor control studies. STATEMENT OF RELEVANCE: The results reveal prominent differences in skin friction not only across various areas of the hand, but also across participants. While the former finding is relevant for optimisation of manipulation activities, the latter emphasises the importance of assessment of individual COF in studies of hand function.