## 1 Introduction  
Measuring the applied force by fingers during task performance in real time can be challenging due to various factors. The low loads handled at high frequencies make it difficult to obtain accurate measurements. Additionally, rapid movements and sweat can cause the sensors placed on the skin to become loose or fall off. To overcome these issues, this paper proposes a prototype of a triaxial load cell. This device will provide more reliable measurements of the applied thumb force, ensuring better overall accuracy. The cell has been tested in a case study to validate its performance in monitoring the force exerted by the right thumb during a quality control task that involves testing disposable classic cigarette lighters to modulate the size of the flame. To determine if work tasks are causing work-related illnesses, ergonomic assessments must be conducted to monitor the interaction between humans and tools or products. When assessing tasks performed by hand, measuring the force exerted (applied force AF) is essential to ensure that the force falls within established safety standards [1]. If the AF magnitude is unsafe, it will be necessary to implement preventive actions [2]. Fine motor movements of the hand, particularly in the wrists or fingers, can be evaluated through AF measurements [3]. The muscle strength during isometric contraction remains relatively constant, with minimal variation in muscle length [4]. However, small loads generate AF during repetitive movements at a high frequency [2]. This means prolonged exposure to AF can result in high muscular loads, which is considered an ergonomic hazard (EH) [5; 6; 7; 8]. Work-related musculoskeletal disorders (WMSDs) are injuries in muscles, tendons, joints, and nerves due to EHs. Workers' most prevalent hand diseases are carpal tunnel syndrome, tendinitis, inflammation and repetitive motion injuries. These conditions cause pain and reduced wrist or finger mobility, negatively impacting employees' well-being [9]. [PERSON] et al. proposed a standardised technique for measuring grip strength in this context [10]. The method is squeezing a handgrip dynamometer for 3 s with maximum voluntary effort (MVE). It is a common technique used to evaluate the effects of AF on injuries and to monitor the progress of rehabilitating injured hands. It is important to note that this technique is used to follow up on the recovery of an injury and to monitor rehabilitation progress rather than as a tool to prevent ergonomic risks. Thus, the MVE method is not applicable for real-time ergonomic risk assessment in production areas. The mechanical problem of measuring AF developed by fingers requires knowledge of geometry, the properties of materials, and the conditions for their definition (forces, displacements, speeds, accelerations, temperatures, etc.) [11]. When forces exerted by hands and fingers are measured in real time, estimating the magnitude of triaxial forces is a significant challenge, i.e., force components should be monitored in three directions (x-, y-, and z-axis) during work movements, such as flexion, extension, radial deviation, ulnar deviation and torsion. The measurement technique must not interfere with work development and should not be invasive. A load cell transducer converts force into a measurable electrical output [12]. A load cell is a mechanical component that is linear elastic. When a force is applied, the load cell undergoes deformation, generating internal stresses and strains. These are detected by electrical extensometers, which convert them into an electrical signal. By using a calibration factor to this signal, the value of the applied force can be determined accurately [11; 13; 14]. An extensometer, also known as a strain gauge, is a type of sensor that can measure strain by detecting a change in electrical resistance resulting from tension or compression applied to the load cell. The strain parameter can either be positive, indicating tension, or negative, indicating compression.