Introduction

The human hand usually has five digits: four fingers plus one thumb, these are often referred to collectively as five fingers, however, whereby the thumb is included as one of Fingers. The hand has 27 bones, not including the Sesamoid bones, the number of which varies among people 14 of which are the phalanges (proximal, intermediate and distal) of the fingers and thumb. The metacarpal bones are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and connect the fingers and the carpal bones of the wrist which articulate with the forearm. Each human hand has at least five metacarpals and eight carpal bones.

Fingers contain some of the densest areas of nerve endings in the body and are the richest source of tactile feedback. They also have the greatest positioning capability of the body; thus, the sense of touch is intimately associated with hands. Like other paired organs (e.g. eyes, legs and feet), each hand is dominantly controlled by the opposing brain hemisphere, so that handedness (the preferred hand choice for single-handed activities such as writing with a pencil) reflects individual brain functioning.

Grip and pinch strength tests of the hand are fundamental assessments utilized across various health sciences and domains, including rehabilitation of injured members, ergonomics, sports medicine, and occupational health. These tests quantify the force exerted by the hand muscles during grasping and pinching activities, providing critical insights into hand function. Assessing of grip and pinch strength is vital for diagnosing neuromuscular conditions, evaluating hand function and developing effective solutions for individuals with hand impairments [1]. The particular objective of studying the grip and pinch produced by the human hand is to develop new rehabilitation activities and invent new orthopedic devices (these devices can use electrical stimulation or mechanical movement of the hand/finger muscles or a combination of the two methods) and allow people with limited hand use or non-whatsoever to provide improved hand dexterity or total use of their hands. This is especially important in the tetraplegic population (complete and incomplete) but can also include stroke victims, Parkinson’s disease sufferers, multiple sclerosis victims and individuals with other less common diseases. Indeed, amongst the spinal cord injury population, loss of the use of their hand function is considered the most debilitating feature that limits them from carrying out activities of daily life and improving their quality of life.

Hand strength is a crucial indicator of upper limb function and is closely linked to the performance of daily living activities. Diminished grip and pinch strength can significantly impact an individual's ability to carry out tasks requiring fine motor skills and manual dexterity [2]. Thus, reliable and valid assessment tools are essential for measuring hand strength accurately and monitoring changes over time. Ancient Egyptians used basic prosthetic devices, such as the wooden big toe found on a mummy dating back to around 950-710 BC. This prosthetic helped the individual walk and maintain balance, indirectly aiding hand functions by improving overall mobility. Another historical example is the writings of Hippocrates (c. 460 - c. 370 BC) describe splints and other devices used for rehabilitation after injuries. These early devices were designed to help patients regain functionality in their hands and other limbs.

Several factors influence grip and pinch strength, including age, gender, hand dominance, exercise, and the presence of musculoskeletal or neurological disorders [3]. Standardized testing protocols and normative data are necessary to account for these variables and ensure accurate comparisons across different populations. The development of portable and user-friendly dynamometers that use electrical, mechanical and hydraulic sensors, has enhanced the feasibility of conducting these tests in various settings, from clinical environments to field studies [4].

Characterizing hand grasp and pinch strength is essential in evaluating hand function, rehabilitation progress, and the effectiveness of therapeutic interventions. Standardized protocols and norms have been established to ensure consistent and reliable measurements. The American Society of Hand Therapists (ASHT) provides comprehensive guidelines for measuring grip and pinch strength using dynamometers and pinch gauges, recommending three types of pinch grips: lateral (key) pinch, tip-to-tip pinch, and three-jaw chuck pinch [2]. The Jamar dynamometer is widely recognized for assessing grip strength, with normative data stratified by age and gender [5,6]. In clinical practice, these measurements are typically conducted with the elbow positioned at 90 degrees, the wrist in neutral position, and the forearm in mid-pronation [7]. These protocols are crucial for diagnosing hand impairments, planning treatments, and assessing outcomes in both clinical and research settings.

Despite the widespread use of grip and pinch strength tests, there is a need for ongoing research to refine testing methodologies, establish comprehensive normative data, and explore the implications of hand strength measurements in different clinical and occupational contexts. For instance, Bohannon et al. highlighted the importance of grip strength as a predictor of overall health outcomes, emphasizing the need for standardized assessment procedures [8]. Moreover, current devices are unable to measure very low force or pressure values.

This paper aims to provide a current overview of the literature on grip and pinch strength testing and introduce a novel device we have developed that makes it possible to evaluate extremely low values of grip and pinch strength (~ 100 Pa). This device will be highly useful as there are currently no grip or pinch evaluation tools that allow for such small values. This will give researchers and clinical practitioners the chance to investigate individuals with severe strength deficiencies and evaluate new tools to assess the strengthening of their muscles and nervous system. This paper seeks to contribute to the understanding of the measurement of hand function evaluation and support the development of evidence-based practices for improving hand strength and overall quality of life for individuals with hand-related impairments.

2. Materials and Methods

2.1. Summary of available hand grasp and pinch strength measuring devices

Assessing hand grasp and pinch strength is a critical component of evaluating hand function, with numerous commercial devices available for these measurements. The devices vary in design, functionality and accuracy, catering to different clinical and research needs. This summary highlights the key commercial devices used to measure hand grasp and pinch strength, based on current scientific literature.

Currently, devices used to evaluate grip and pinch strength are based on force or pressure measurements. Unfortunately, most currently available devices do not measure extremely low values (< .05kg) which are required for individuals who have a motor handicap due to peripheral or central nervous system damage which severely affects their hand strength. The minimum force and pressure measurements for evaluating grip and pinch strength in a human hand are typically determined by the sensitivity and resolution of the measuring instruments used which are in the range rather limited to relatively high values of the force and pressure (0.1 kg).

2.1.1. Jamar Hydraulic Hand Dynamometer

The Jamar hydraulic hand dynamometer is considered the gold standard for measuring hand grip strength due to its reliability and validity [1,18]. It features a hydraulic system that measures the force exerted and displays the results on an analog dial (newer systems use a digital readout). Due to its ease of use, and the fact that numerous studies have shown its high inter- and intra-rater reliability, making it a preferred choice in clinical and research settings [7].

2.1.2. Martin Vigorimeter

The Martin Vigorimeter uses air pressure to measure grip strength and is particularly useful for older patients [19,20] and patients with arthritis [21,22], as it requires less force to compress. It consists of a rubber bulb connected to a pressure gauge, providing an alternative for measuring grip strength in populations with reduced hand function. It has been shown that grip strength measurements are less dependent on the hand anthropometry when using the bulb design compared to the Jamar dynamometer design [23,24]. However, the realistic lower limit measurement for this device is above 3 kPa (0.5 PSI) [25].

2.1.3. Smedley dynamometer

Smedley dynamometers have a different design from hydraulic and pneumatic hand dynamometers. A Smedley dynamometer makes use of a calibrated spring to provide resistance during a grip test. The force gauge is oriented in front of the hand as opposed to above the wrist [26,27]. Similarly to the Jamar hydraulic hand dynamometer, in newer versions, results are displayed on a digital screen and can be saved for a limited number of users.

2.1.4. DynX training dynamometer

DynX training consists of a series of timed isometric Efforts performed by the muscle groups of the hand that can produce grip, at levels proportionate to Maximum Voluntary Contraction (MVC) measured for the muscles to be trained. These Efforts are interrupted with timed Rest Periods between each effort. The sustained isometric efforts of the DynX programs require only muscle tension. They are purely isometric regimens since there is no movement, tendons are only stressed and do not move in their sheath. As a result of the lack of motion, no outward work is being done. Compliance scores are calculated during the therapy and are totaled during Rest Periods. Scores indicate the percent compliance with the prescribed therapy, thereby providing feedback to both the patient and the therapist on the quality of exercise accomplished. Two training regimens, called Fixed Therapy and Stepped Therapy, are provided by DynX. Training compliance data is retained in non-volatile memory along with the parameters of the training modality. These data may be downloaded to a PC for permanent storage prior to initiating another training session. Real-time output of training may be monitored at any time [28].

2.5. Mechanical pinch gauge

Mechanical pinch gauge uses a mechanical measurement to asses pinch strength. Force exerted on the groove placed between the thumb and the fingers is measured and displayed on an analog gauge. The gauge is usually equipped with a maximum value indicator that remains until reset. Mechanical pinch gauge is used in clinical practice for the assessment of key, three-finger and two-finger pinch [29].

2.1.6. Five-position hydraulic pinch gauge

Hydraulic 5-level pinch gauge has an adjustable handle allowing pinch strength to be assessed at the varied pinch spans (from 2 cm to 6 cm) for accurate and repeatable pinch measurements[30]. Five-position strength test protocols (maximum voluntary exertion, MVE and modified maximum voluntary exertion, MMVE) now can be used for pinch measurements. Easy-to-adjust paddle accommodates to any size hand (other pinch gauges only have one pinch width). The pinch width without the paddle is the same width as standard pinch gauges, both mechanical and hydraulic. Five pinch positions permit MVE and MMVE protocols to be used with tip, palmar, and key tests [31]. Measurement results are displayed on a gauge facing toward the practitioner concealing the reading from the subject.

2.1.7. Hydraulic pinch gauge

A hydraulic pinch gauge employs a simple hydraulic system for pinch strength measurement. It can be used for the assessment of key, three-finger or two-finger pinch. Hydraulic pinch gauge provides repeatability over a long period as it does not rapidly degrade. The main shortcomings of the device are the accuracy which is typically above 0.4 kg and the lack of adjustability for different hand shapes and sizes.

2.1.8. Digital pinch gauge

The pinch strength measurement using the digitalized pinch dynamometer is reliable within the rater and between raters. It can be used to assess key, three-finger or two-finger pinch strength where the results are displayed on a digital screen and saved in memory of the device. Digital pinch gauge can be used interchangeably with the hydraulic pinch gauge suffering from similar shortcomings with fixed pinch span and lower limit for measurements of around 0.1 kg [32].

In summary, various commercial devices are available for assessing hand grasp and pinch strength, each with its advantages and specific use cases. Hydraulic dynamometers like the Jamar and Baseline remain the gold standard due to their proven reliability, while digital dynamometers and specialized devices like the Martin Vigorimeter provide additional options for specific populations and settings. Continued research and technological advancements will likely further refine these tools, enhancing their accuracy and utility in clinical practice.

2.2. Force mapping systems

Grip and pinch strength evaluations using force-sensing devices that incorporate piezoelectric sensors are crucial for assessing biomechanical function and aiding in research settings (Figure 1). Piezoelectric sensors convert mechanical stress into electrical signals, allowing for precise measurement of force and pressure applied by the hand. These devices are instrumental in diagnosing conditions such as carpal tunnel syndrome, monitoring rehabilitation progress, and enhancing ergonomic designs to prevent workplace injuries. Notable devices in this domain include the Biometrics E-LINK Evaluation System [33], which offers comprehensive hand function analysis, and the Noraxon myoFORCE [34], which integrates piezoelectric technology for detailed force measurement. Additionally, the Pliance Hand Diagnostic System by Novel provides dynamic pressure distribution measurement, further extending the capabilities of piezoelectric sensors in hand evaluations [35]. These advanced systems highlight the pivotal role of piezoelectric sensors in delivering accurate and reliable biomechanical assessments.

Figure 1. (a) Image of typical flexible, piezoelectric elements [36]; (b) Schematic image of a glove with piezoelectric elements positioned over critical areas of a glove that fits over a hand to detect force in the corresponding hand positions. Images courtesy of Tekscan.

A commonly used system for measuring pressure distribution and intensity in the hand is the Tekscan Grip™ system [37]. This system uses thin, flexible sensor arrays that can capture detailed pressure data from the hand while performing grip or pinch (Figure 2).

Figure 2. Tekscan Grip™ system for evaluating pressure from grasping objects [37]. The picture is that of a hand holding a glass with piezoelectric electric sensors fixed onto various Marshall’s. The circular insert in the bottom right corner depicts color-coded pressure level readouts, blue being the lowest and red the highest pressure measured. Image courtesy of Tekscan.

The Tekscan Grip™ system is an advanced tool for evaluating hand grip and pinch strength, leveraging the capabilities of its core component: thin, flexible tactile sensors. These sensors are based on a pressure-sensitive matrix that can measure force distribution across their surfaces. The sensors are comprised of a grid of conductive rows and columns, creating an array of intersecting points or that register changes in electrical resistance when pressure is applied. As force is applied to the sensor, the contact resistance decreases proportional to the applied force, allowing the system to quantify the force at each point. The grid is made with a high spatial resolution providing detailed mapping of pressure distribution across the sensor surface. Multiple sensors can be placed on the hand and produce real-time feedback, which is useful for clinical assessment and research applications. The thin and flexible sensors conform to the shape of the hand providing accurate measurements without significantly altering the natural grip or pinch posture.

The approximate price of the Tekscan Grip™ system can vary based on the specific configuration, included features and any additional services such as software licenses, training and support. As of the most recent information: The base price for a Tekscan Grip™ system generally starts around 10,000 to 15,000 USD. Additional features or upgraded versions of the system can push the price higher, sometimes exceeding 20,000 USD.

The Tekscan Grip™ system is highly versatile, finding applications in various fields such as biomechanics, rehabilitation, ergonomics, and sports science. In clinical settings, it can be used to assess hand function in patients recovering from injury or surgery, providing detailed data on grip strength and distribution that can guide rehabilitation strategies. In ergonomics, it helps in designing tools and interfaces that optimize human performance and comfort by analyzing how forces are applied during use. Sports scientists use the system to study athletes' grip techniques, aiming to enhance performance and reduce injury risks.

Other instruments similar to the Tekscan Grip™ system include the F-Scan and the T-Scan systems, which also utilize thin-film pressure sensors. The F-Scan system, for instance, is primarily used for gait analysis, employing sensors that measure plantar pressure distribution to provide insights into foot biomechanics [38,39]. The T-Scan system is designed for dental occlusion analysis, using pressure sensors to map the distribution of bite forces [40].

The transducers used in these systems share a common principle: they are constructed from piezoresistive materials that change resistance under mechanical stress. This allows them to measure and map pressure distribution accurately and in real time. For example, in the F-Scan system, the sensors are embedded in insoles worn by the subject, capturing dynamic pressure patterns as they walk or run. Similarly, the T-Scan's dental sensors capture the bite force distribution, assisting in diagnosing and treating occlusal disorders.

Devices using similar principles include the XSENSOR X3 PRO V7, which is used for pressure mapping in seating and bedding applications, and the NOVEL Pedal system, which is another tool for dynamic plantar pressure measurement. These devices, like the Tekscan Grip™ system, rely on the piezoresistive properties of their sensors to provide detailed, real-time force and pressure data, proving indispensable in fields requiring precise pressure analysis.

Instruments that incorporate piezoelectric elements like the Tekscan Grip™ system and similar devices exemplify the sophisticated application of piezoresistive transducers in various domains, from medical rehabilitation to ergonomic design and sports science. The detailed, quantitative data they provide is crucial for enhancing performance, preventing injury, and improving overall human-machine interaction. However, it is difficult do use them to determine an overall grasp or pinch measurement for a patient, but they are fundamental for studying individual muscle groups. These devices are invaluable in clinical settings for evaluating hand function, diagnosing conditions, and planning rehabilitation programs. They provide precise and detailed information about the pressure exerted during grip and pinch activities, even at very low levels.

3. Results

Realizing that there is a need for extremely high sensitivity measurements, we have developed the device schematically portrayed in Figure 3. This device is a modified Martin Vigorimeter. It uses the same principle of squeezing a sealed flexible container (a latex bulb with 0.08 mm thickness and an 800% stretch factor) and measuring the corresponding pressure change due to volume change, but it is modified to measure extremely low pressures and can be retrofitted with an external pressure regime regulator to allow it to measure high levels of pressure measurement typical of commercially available devices. Its innovative nature is that it can measure the lowest available reported pinch and grasp pressure values and span the highest range obtainable compared to current devices. Furthermore, we achieved this without an increase in cost to the consumer.


Figure 3. Schematic image of a differential pressure measurement grasp and pinch Dynamometer. Pictured are a) ultra-soft water, balloon (made from latex with a thickness of approximately 0.08 mm and a stretchiness factor of over 800%), b) partially filled with water, c) water trap, d) external pressure regimen regulator, e ) Air tube, f) differential pressure measurement device, f) ultra-low read out, note: the current limit in the commercial device sold as the Martin Vigorimeter is 1 kPa which represents a two order magnitude improvement.

To realize these features, first, the chamber is manufactured out of extremely thin, soft and easily compliant plastic (0.08 mm thick latex rubber with an 800% stretch compliance), (ex. much less compliant than a standard balloon). Second, the chamber is partially filled (as depicted in Figure 3) with liquid water, or a less viscous fluid, to expand its size so that a common hand can be maneuvered over the chamber (the chamber size can be changed to accommodate different hand sizes). In addition, it is important that the chamber is only partially filled with fluid so that we can take advantage of the compliant air-filled region during its stretching. Once the chamber is squeezed, the pressure difference caused by its volume reduction is measured using a highly accurate, differential pressure regulator. By using a differential pressure device, instead of an absolute pressure gauge, we can achieve the lowest most accurate pressure measure determined by the electronics and the pressure transducers used. If on the other hand, a high-pressure measurement is desirable the same device can be used by inserting a “regime pressure regulator” (i.e. a pressure resistor see Figure 3) in the airflow tube. By inserting, this regulator, squeezing upon the container, will not register a value until it has overcome the pressure resistor, and the final value registered on the differential pressure device will be that of the resistor, and the applied force over the measurement chamber. In other words, the device can be used to measure the lowest and highest pressure possible that can be applied to the chamber, unless under extremely high pressures for which the chamber ruptures. However, if this were to happen, then we could simply change the chamber material and make it more rigid to withstand higher pressures. In other words, the newly developed device can measure lower than the lowest device on the market by simply using the highest sensibility differential pressure sensor instead of previously used absolute pressure indicators, and the highest measured pressures by using downstream pressure regulators (see Figure 3 d)), That is, the device is easily transformed into a higher grip measurement pressure device by putting a high-pressure regulator in line with the device. Moreover, not only is it a simple device, it is very low cost, because each of the elements are standard, and can be acquired at high volumes, it does not exceed currently commercially available device prices. The device can be used for grip or pinch, depending upon how the individual applies pressure to the chamber and configures their experimental setup.

4. Discussion

The human hand, with its intricate anatomy and dense concentration of nerve endings, plays a crucial role in daily activities and overall quality of life. Evaluating hand strength, particularly grip and pinch strength, provides essential insights into hand function, aiding in the diagnosis and rehabilitation of various conditions. This study aimed to address the limitations of current devices in measuring extremely low grip and pinch strength values by introducing a novel, highly sensitive device.

Current commercial devices such as the Jamar dynamometer, Martin Vigorimeter, and Smedley dynamometer, while reliable, have limitations in detecting very low force values. These devices typically cater to higher strength ranges, making them inadequate for individuals with severe hand strength deficiencies, such as those resulting from spinal cord injuries, strokes, or neurological disorders. The newly developed device, a modified Martin Vigorimeter, addresses this gap by using an ultra-soft, compliant latex chamber and a differential pressure measurement system to detect minute pressure changes. This modification allows for the measurement of extremely low values, significantly enhancing the sensitivity range compared to existing devices.

The Tekscan Grip™ system and similar force-mapping systems exemplify the sophisticated application of piezoresistive transducers in hand strength evaluation. These systems provide detailed, real-time data on pressure distribution and force, essential for accurate diagnosis and rehabilitation planning. However, the high cost and complexity of these systems limit their accessibility, particularly in resource-limited settings. The modified Martin Vigorimeter offers a cost-effective alternative, providing high sensitivity without a significant increase in cost, thus potentially increasing accessibility for a broader range of clinical and research applications.

Force sensor devices incorporating various technologies, such as the Biometrics E-LINK Evaluation System and the Noraxon myoFORCE, offer precise force measurement capabilities. These devices are instrumental in diagnosing conditions like carpal tunnel syndrome and monitoring rehabilitation progress. The modified Martin Vigorimeter, while not incorporating piezoelectric technology, provides a comparable level of sensitivity and accuracy in measuring low-force values, demonstrating the potential for simpler, more cost-effective solutions in hand strength evaluation.

The development of this novel device highlights the importance of continued research and innovation in the field of hand strength evaluation. By addressing the limitations of existing devices and enhancing measurement sensitivity, this new tool offers significant potential for improving the assessment and rehabilitation of individuals with severe hand strength deficiencies. Future research should focus on validating the device's accuracy and reliability across diverse populations and clinical settings, as well as exploring its potential applications in various occupational and therapeutic contexts.

5. Conclusion

In conclusion, the human hand's complexity and its critical role in daily activities necessitate accurate and sensitive tools for evaluating hand strength. Existing devices, while reliable, fall short in measuring extremely low grip and pinch strength values, limiting their utility for individuals with severe hand impairments. The novel device developed in this study addresses this gap by significantly enhancing measurement sensitivity through the use of a highly compliant latex chamber and a differential pressure system.

This innovation provides a cost-effective alternative to more complex and expensive systems, potentially increasing accessibility and utility in various clinical and research settings. The modified Martin Vigorimeter demonstrates that it is possible to achieve high sensitivity and accuracy in hand strength measurement without a significant increase in cost or complexity.

Future research should focus on further validating this device's performance and exploring its applications across different populations and clinical contexts. By continuing to refine hand strength evaluation tools, we can better diagnose, treat, and rehabilitate individuals with hand impairments, ultimately improving their quality of life.

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