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.

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.

Figure 1. Illustration of (a) hand grasp; (b) finger pinch.

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.