Evaluation of total grip strength and individual finger forces on opposing (A-type) handles among Koreans

The present study evaluated the effect of grip span on finger forces and defined the best grip span for maximising total grip strength based on the finger forces and subjective discomfort in a static exertion. Five grip spans (45, 50, 55, 60 and 65 mm) of the opposing (A-type) handle shape were tested in this study to measure total grip strength and individual finger force among Korean population. A total of 30 males who participated in this study were asked to exert a maximum grip force with two repetitions, and to report the subjective discomfort experienced between exertions using the Borg's CR-10 scale. The highest grip strength was obtained at 45 mm and 50 mm grip spans. Results also showed that forces of all fingers, except for the middle finger force, significantly differed over the grip spans. The lowest subjective discomfort was observed in the 50 mm grip span. The results might be used as development guidelines for ergonomic opposing (A-type) hand tools for Korean population.     

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.     

IEA Newsletter

Five grip spans (45 to 65 mm) were tested to evaluate the effects of handle grip span and user's hand size on maximum grip strength, individual finger force and subjective ratings of comfort using a computerised digital dynamometer with independent finger force sensors. Forty-six males participated and were assigned into three hand size groups (small, medium, large) according to their hands' length. In general, results showed the 55- and 50-mm grip spans were rated as the most comfortable sizes and showed the largest grip strength (433.6 N and 430.8 N, respectively), whereas the 65-mm grip span handle was rated as the least comfortable size and the least grip strength. With regard to the interaction effect of grip span and hand size, small and medium-hand participants rated the best preference for the 50- to 55-mm grip spans and the least for the 65-mm grip span, whereas large-hand participants rated the 55- to 60-mm grip spans as the most preferred and the 45-mm grip span as the least preferred. Normalised grip span (NGS) ratios (29% and 27%) are the ratios of user's hand length to handle grip span. The NGS ratios were obtained and applied for suggesting handle grip spans in order to maximise subjective comfort as well as gripping force according to the users' hand sizes. In the analysis of individual finger force, the middle finger force showed the highest contribution (37.5%) to the total finger force, followed by the ring (28.7%), index (20.2%) and little (13.6%) finger. In addition, each finger was observed to have a different optimal grip span for exerting the maximum force, resulting in a bow-contoured shaped handle (the grip span of the handle at the centre is larger than the handle at the end) for two-handle hand tools. Thus, the grip spans for two-handle hand tools may be designed according to the users' hand/finger anthropometrics to maximise subjective ratings and performance based on this study. Results obtained in this study will provide guidelines for hand tool designers and manufacturers for designing grip spans of two-handle tools, which can maximise handle comfort and performance.     

A new method for estimating hand internal loads from external force measurements

This study examines using force vectors measured using a directional strain gauge grip dynamometer for estimating finger flexor tendon tension. Fifty-three right-handed participants (25 males and 28 females) grasped varying-sized instrumented cylinders (2.54, 3.81, 5.08, 6.35 and 7.62 cm diameter) using a maximal voluntary power grip. The grip force vector magnitude and direction, referenced to the third metacarpal, was resolved by taking two orthogonal grip force measurements. A simple biomechanical model incorporating the flexor tendons was used to estimate long finger tendon tension during power grip. The flexor digitorum superficialis and the flexor digitorum profundus were assumed to create a moment about the metacarpal phalange (MCP) joint that equals and counteracts a moment around the MCP joint measured externally by the dynamometer. The model revealed that tendon tension increased by 130% from the smallest size handle to the largest, even though grip force magnitude decreased 36% for the same handles. The study demonstrates that grip force vectors may be useful for estimating internal hand forces.     

A new method for estimating hand internal loads from external force measurements

This study examines using force vectors measured using a directional strain gauge grip dynamometer for estimating finger flexor tendon tension. Fifty-three right-handed participants (25 males and 28 females) grasped varying-sized instrumented cylinders (2.54, 3.81, 5.08, 6.35 and 7.62 cm diameter) using a maximal voluntary power grip. The grip force vector magnitude and direction, referenced to the third metacarpal, was resolved by taking two orthogonal grip force measurements. A simple biomechanical model incorporating the flexor tendons was used to estimate long finger tendon tension during power grip. The flexor digitorum superficialis and the flexor digitorum profundus were assumed to create a moment about the metacarpal phalange (MCP) joint that equals and counteracts a moment around the MCP joint measured externally by the dynamometer. The model revealed that tendon tension increased by 130% from the smallest size handle to the largest, even though grip force magnitude decreased 36% for the same handles. The study demonstrates that grip force vectors may be useful for estimating internal hand forces.     

A study of hand grip pressure distribution and EMG of finger flexor muscles under dynamic loads

A matrix of miniature and flexible pressure sensors is proposed to measure the grip pressure distribution (GPD) at the hand-handle interface of a vibrating handle. The GPD was acquired under static and dynamic loads for various levels of grip forces and magnitudes of vibration at different discrete frequencies in the 20–1000 Hz range. The EMG of finger flexor muscles was acquired using the silver-silver chloride surface electrodes under different static and dynamic loads. The measured data was analysed to study the influence of grip force, and magnitude and frequency characteristics of handle vibration on: (i) the local concentration of forces at the hand-handle interface; and (ii) the electrical activity of the finger flexor muscles. The results of the study revealed high interface pressure near the tips of index and middle fingers, and base of the thumb under static grip conditions. This concentration of high pressure shifted towards the middle of the fingers under dynamic loads, irrespective of the grip force, excitation frequency, and acceleration levels. The electrical activity of the finger flexor muscles increased considerably with the grip force under static as well as dynamic loads. The electrical activity under dynamic loads was observed to be 1·5–6·0 times higher than that under the static loads.     

Grip posture and forces during holding cylindrical objects with circular grips

Individual finger position and external grip forces were investigated while subjects held cylindrical objects from above using circular precision grips. Healthy females (n = 11) and males (n = 15) lifted cylindrical objects of various weights (05, 10 and 20kg), and varied diameters (50, 7-5 and 100cm) using the 5-finger grip mode. The effects of 4-, 3- and 2-finger grip modes in the circular grip were also investigated.

Individual finger position was nearly constant for all weights and for diameters of 5-0 and 7-5 cm. The mean angular positions for the index, middle, ring and little fingers relative to the thumb were 98°, 145°, 181°, and 236°, respectively. At the 10-cm diameter, the index and middle finger positions increased, while the ring and little finger positions decreased. There were no differences in individual finger position with regard to gender, hand dimension, or hand strength.

Total grip force increased with weight, and at diameters greater or lesser than 7-5 cm. Total grip force also increased as the number of fingers used for grasping decreased. Although the contribution of the individual fingers to the total grip force changed with weight and diameter, the thumb contribution always exceeded 38% followed by the ring and little fingers, which contributed approximately 18-23% for all weights and diameters. The contribution of the index finger was always smallest (>11%). There was no gender difference for any of the grip force variables. The effects of hand dimension and hand strength on the individual finger grip forces were subtle.     

Design of optimum grip and control area for one-handed manual control devices

The objective of this paper was to make a design specification of the control area in dual tasks of the hand grip and manual control. The grip postures were analyzed for three types of hand tools. An experiment was performed to measure the position of the fingers and the maximum finger forces at 4 different postures for nine subjects. It was found out that the finger forces were significantly affected by the subjects, the fingers, and the grip postures. The maximum force of women was 62% of men's. From the experiment, the primary control area was defined as 10–13cm and the secondary control area as 8–12cm from the wrist origin. The preferred hand posture of the index and the middle fingers was found to be 3045 degrees at metacarpophalangeal joint and 4050 degrees at proximal interphalangeal joint. It was also found out that the design of one-handed manual control devices should include the characteristics of the user, grip posture, finger force, and the control arrangement.     

An investigation on the relationship between grip, push and contact forces applied to a tool handle

Owing to the strong dependence of the health risks associated with vibration exposure of the human hand and arm on hand force, a laboratory study was conducted to develop a methodology for measurement of the contact force at the tool handle–hand interface, and to identify the relationship between the contact force and the hand grip and push forces. A simulated tool handle fixture was realized in the laboratory to measure the grip and push forces using compression/extension force sensors integrated within the handle and a force plate, respectively. The contact force was derived through integration of the interface pressure over the contact area. These were measured using a capacitive pressure-sensing grid. The measurements were performed with 10 male subjects and three circular cross-section handles of different sizes under different combinations of grip and push forces. The hand–handle interface pressure data were analyzed to derive the contact force, as functions of the constant magnitudes of the grip and push forces, and the handle size. The results suggest that the hand–handle contact force is strongly dependent upon not only the grip and push forces but also the handle diameter. The contact force for a given handle size can be expressed as a linear combination of grip and push forces, where the contribution of the grip force is considerably larger than that of the push force. The results further suggest that a linear relation can characterize the dependence of the contact force on the handle diameter. The validity of the proposed relationship is demonstrated by evaluating the magnitudes of errors between the estimated contact forces with the measured data for the range of handle diameters, and grip and push forces considered in the study.

Relevance to industry

The methodology proposed in this study can be applied to measure the effective hand–handle contact force at workplaces for assessing the health risks associated with exposure to hand-transmitted vibration exposure and hand–wrist cumulative trauma. The relationship proposed in the study could be effectively applied for estimating the hand–handle contact force from known grip and push forces that are conveniently and directly measurable in laboratory studies involving vibration analyses of the human hand, power tools and relevant vibration attenuation devices. It is expected to be most useful in field applications, where it could provide an estimate of the range of magnitudes of the hand-grip force applied to the handle of an actual tool, which is quite difficult and expensive to measure. The relationship is also expected to contribute to the on-going standardization efforts for defining a correction factor to account for the effects of hand force on the vibration transmission and hand injuries.     

Effect of object width on precision grip force and finger posture

This study aimed to define the effect of object width on spontaneous grasp. Participants held objects of various masses (0.75 to 2.25 kg) and widths (3.5 to 9.5 cm) between thumb and index finger. Grip force, maximal grip force and corresponding finger postures were recorded using an embedded force sensor and an optoelectronic system, respectively. Results showed that index finger joints varied to accommodate the object width, whereas thumb posture remained constant across conditions. For a given object mass, grip force increased as a function of object width, although this result is not dictated by the laws of mechanics. Because maximal grip force also increased with object width, we hypothesise that participants maintain a constant ratio between grip force and their maximal grip force at each given width. Altogether we conclude that when the task consists in manipulating objects/tools, the optimal width is different than when maximal force exertions are required.     

Effect of object width on precision grip force and finger posture

This study aimed to define the effect of object width on spontaneous grasp. Participants held objects of various masses (0.75 to 2.25 kg) and widths (3.5 to 9.5 cm) between thumb and index finger. Grip force, maximal grip force and corresponding finger postures were recorded using an embedded force sensor and an optoelectronic system, respectively. Results showed that index finger joints varied to accommodate the object width, whereas thumb posture remained constant across conditions. For a given object mass, grip force increased as a function of object width, although this result is not dictated by the laws of mechanics. Because maximal grip force also increased with object width, we hypothesise that participants maintain a constant ratio between grip force and their maximal grip force at each given width. Altogether we conclude that when the task consists in manipulating objects/tools, the optimal width is different than when maximal force exertions are required.     

Study on the grip spans of combination pliers in a maximum gripping task

A newly developed system was applied in this study to evaluate the effects of the grip spans (45–80 mm) of combination pliers on the total grip strength, individual finger force, resultant force, and subjective discomfort. A total of twenty-six males participated and were asked to exert their maximum grip strength with two repetitions. The highest and the lowest total grip strength and resultant force (311.8 N and 737.9 N vs. 210.1 N and 501.7 N) were obtained at a 60 mm and 80 mm grip spans, respectively. In general, the participants considered the 50 and 60 mm grip spans as being the least discomfort, whereas the 80 mm grip span was considered as the most discomfort grip span in a maximum grasping task. The results can be utilized as basic data for the manufacturing and design industries of two-handle hand tools, such as pliers and wrenches.Practitioners summaryCustom-made combination pliers were applied in this study to evaluate grip strength, resultant force, and subjective discomfort, relative to five grip spans. The authors expect that the results of the present study will provide valuable information for the designers and users of pliers.     

Power grip strength as a function of tool handle orientation and location

Thirty male volunteers participated in a study evaluating the effect of workspace envelope (work height and reach distance) and handle orientation on grip force capacity. Maximum voluntary power grip exertions were recorded using instrumented tool handles under three conditions: a pistol grip tool handle oriented horizontally and vertically and a right angle tool handle oriented horizontally. Significant main effects of handle height and reach location on normalized grip force capacity were observed with the horizontally oriented pistol grip and right angle handles, whereas only an interaction effect was observed with the vertically oriented pistol grip handle. Comparison of results to scores produced with a job assessment tool (RULA) is included as an appendix. The proposed methodology can provide information useful to job, workstation or tool design directed toward best accommodating the physical capacities of workers performing hand tool tasks.     

Power grip strength as a function of tool handle orientation and location

Thirty male volunteers participated in a study evaluating the effect of workspace envelope (work height and reach distance) and handle orientation on grip force capacity. Maximum voluntary power grip exertions were recorded using instrumented tool handles under three conditions: a pistol grip tool handle oriented horizontally and vertically and a right angle tool handle oriented horizontally. Significant main effects of handle height and reach location on normalized grip force capacity were observed with the horizontally oriented pistol grip and right angle handles, whereas only an interaction effect was observed with the vertically oriented pistol grip handle. Comparison of results to scores produced with a job assessment tool (RULA) is included as an appendix. The proposed methodology can provide information useful to job, workstation or tool design directed toward best accommodating the physical capacities of workers performing hand tool tasks.     

Effect of grip span on lateral pinch grip strength

Repetitive, high-force pinch grip exertions are common in many occupational activities. The goal of the current study was to quantify the relationship between lateral pinch grip span (distance between thumb and index finger) and lateral pinch grip strength. An experiment was conducted in which 40 participants performed maximal lateral pinch grip exertions at 11 levels of grip span distances (0, 10%, ... 100% of maximum functional lateral pinch grip span distance). The results show a significant effect of lateral pinch grip span, with strength at the maximum functional lateral pinch grip span 40% higher than that found at the smallest lateral pinch grip span considered. Between these two endpoints, strength increased monotonically with increasing pinch grip span. The application of these results in pinch grip design criteria for both high-force and long-duration exertions is discussed. Potential applications of this research include the design of hand tools and controls for which significant force is applied by the user.     

Development and application of a multi-axis dynamometer for measuring grip force

Objective: This paper describes the development and application of a novel multi-axis hand dynamometer for quantifying 2D grip force magnitude and direction in the flexion-extension plane of the fingers. Methods: A three-beam reconfigurable form dynamometer, containing two active beams for measuring orthogonal forces and moments regardless of point of force application, was designed, fabricated and tested. Maximum grip exertions were evaluated for 16 subjects gripping cylindrical handles varying in diameter. Results: Mean grip force magnitudes were 231 N (SD = 67.7 N), 236 N (72.9 N), 208 N (72.5 N) and 158 N (45.7 N) for 3.81 cm, 5.08 cm, 6.35 cm and 7.62 cm diameter handles, respectively. Grip force direction rotated clockwise and the centre of pressure moved upward along the handle as handle diameter increased. Conclusions: Given that the multi-axis dynamometer simultaneously measures planar grip force magnitude and direction, and centre of pressure along the handle, this novel sensor design provides more grip force characteristics than current sensor designs that would improve evaluation of grip characteristics and model-driven calculations of musculoskeletal forces from dynamometer data.     

Evaluation of handles in a maximum gripping task

Various hook handles were tested to evaluate the effect of handle design characteristics on subjective discomfort ratings and phalange forces in a maximum gripping task. A force glove system with 12 thin force sensitive resistor (FSR) sensors was used to measure phalange forces on the hook handles. Thirty subjects (15 males and 15 females) were tested, and generally subjects preferred 30 or 37 mm (the latter for large handed males) double frustrum handles followed by 30 mm oval handles, whereas overall they showed less preference for 37 mm oval handles and 45 mm double frustrum handles. The phalange force was more related to handle shape than to handle size in this study, i.e. the individual phalange forces on oval handles were about 8% higher than those on double frustum handles. The force distributions in the maximum gripping task showed significant differences in finger and phalange forces, in the order of middle, index, ring, and little fingers and distal, middle, and proximal phalanges from the highest to the lowest forces. The findings of this study may provide guidelines for designing double frustum handles for satisfying user's preference and oval handles for obtaining high phalange forces in a maximum gripping task.     

Ergonomics evaluation of a foam rubber grip for tool handles

An important consideration in tool design is the avoidance of hand discomfort and a reduction of work efficiency by using proper grip designs. To measure grip force distribution on a tool handle, Force Sensing Resistors were calibrated, placed on two different types of grip (wood and foam) and interfaced to a personal computer. The results, using six common garden tools - loppers, hedge shears, shovels, leaf rakes, hoes, garden rakes - indicated a very uneven distribution in grip force, but with the foam grip providing a more uniform distribution. Unfortunately, in most cases, the tool grip force was greater for the foam grip due to the deformation of the foam and a 'loss of control' feeling in the subjects. However, most subjects strongly preferred the foam grips.     

Variability of grip strength during isometric contraction

The use of measures of strength variability as a means of determining sincerity of effort is becoming a more common practice, particularly in medico-legal and rehabilitation settings. The stability of such variability measures, however, has not been documented. This research investigated, in two studies, the trial-to-trial variability of grip strength under maximal and submaximal effort conditions. In the first study, 63 subjects were asked to give 100% grip effort, and in the second study 40 subjects from the original group were asked to give 50% grip effort. The Jamar hand dynamometer was used to measure grip strength in both experiments, and a coefficient of variation (CV) was calculated for every three repeat measures at each handle position. Testing was conducted on two separate occasions for both experiments. Although the interoccasion reliability of grip strength was very high, in comparison, the CV was not stable over test occasions, with interoccasion reliability indices close to zero. Factors significantly influencing CV were effort level, with submaximal effort producing larger CVs, and gender, with females having greater strength variability. If the rule is applied that one or more CVs above the 7·5% cut-off value could indicate submaximal effort, then for this sample of subjects giving maximal effort, 97% of females and 64% of males would be misclassified. Applying a single CV classification cut-off value to a mixed sample of subjects appears to unfairly discriminate against the females. Further research into the factors associated with high CV values is essential before the CV can be used with any confidence in a clinical setting as a method for determining sincerity of effort.     

Evaluation of handles in a maximum gripping task

Various hook handles were tested to evaluate the effect of handle design characteristics on subjective discomfort ratings and phalange forces in a maximum gripping task. A force glove system with 12 thin force sensitive resistor (FSR) sensors was used to measure phalange forces on the hook handles. Thirty subjects (15 males and 15 females) were tested, and generally subjects preferred 30 or 37?mm (the latter for large handed males) double frustrum handles followed by 30?mm oval handles, whereas overall they showed less preference for 37?mm oval handles and 45?mm double frustrum handles. The phalange force was more related to handle shape than to handle size in this study, i.e. the individual phalange forces on oval handles were about 8% higher than those on double frustum handles. The force distributions in the maximum gripping task showed significant differences in finger and phalange forces, in the order of middle, index, ring, and little fingers and distal, middle, and proximal phalanges from the highest to the lowest forces. The findings of this study may provide guidelines for designing double frustum handles for satisfying user's preference and oval handles for obtaining high phalange forces in a maximum gripping task.