Assessing the Feasibility of a Novel Dynamometry Protocol for Measuring Finger Extensor Strength

The purpose of this observational study is to measure the strength of independent finger extension for the four middle digits (II-V). Independent extension involves actively extending one finger while keeping the other fingers fully flexed, while dependent extension involves actively extending all the fingers, including the index finger. The study protocol is based on a previous study (Matter-Parrat et al., 2017), where researchers used a similar device to compare dependent and independent finger extension. However, in contrast to the previous study, this study aims to measure extension forces for all fingers, not just the index finger.

To achieve this, the tests will be performed three times, on both hands, by three examiners and compared to determine the intra-examiner reproducibility and the inter-examiner reproducibility. The investigators will measure all fingers on participants' dominant hand first, following by their non-dominant hand. This sequence will be repeated three times for each evaluator. A five-minute interval will be stablished between each evaluator, as well as 1-minute break between each attempt, to minimize the risk of muscle fatigue.

During the test procedures, the participants' upper extremity will be positioned over a height-adjustable table, which will be placed on the side of the participant. This will allow for optimal support and stability during the measurement of finger extension strength. The height of the table will be adjusted to ensure that the participant's shoulder is in abduction, and their elbows are at a 90-degree angle of flexion. The forearms will be placed in a pronated position, while the wrists and metacarpophalangeal joints will be maintained in a neutral position to prevent any unwanted movement during the test. The participants will be seated comfortably with their back supported by a chair with no armrests to avoid any interference with the test procedures. In case active extension of the wrist is observed by an elevation of the metacarpal heads from the stand, the test will be repeated to obtain isolated extension of the fingers.

To measure participants' finger extension strength, a 3D printed support was specifically designed and printed in PLA (Polylactic Acid) on an ENDER 3 printer. Additionally, PLA is a suitable material that can be easily disinfected with 70% ethanol, maintaining hygiene standards between participants. We will print several scaled models of the support, ensuring the support can be tailored to best fit participants' hands. We will select the support whose size comfortably accommodate the dorsum of the participant's proximal phalanx, providing a secure and ergonomic fit. Once the support is connected to the force gauge's M6 Metric attachments, participants will be asked to extend each finger independently as forcefully as possible while keeping the other fingers in full flexion.

Data will be obtained by recording the peak value using the Baoshishan 500N force gauge, a commercially available device used to measure the strength of extensor digitorum muscles. The device, manufactured by Baoshishan Instrument Co., Ltd, China, has a measuring range of 0-500N (0-112.4 lbf) and an accuracy of ±0.5% of full scale. It features a resolution of 0.1N (0.0225 lbf) and a sampling frequency of 2000Hz. The gauge has a large, easy-to-read LCD display that shows force values in Newtons (N) or pounds (lbf). The device is compact and lightweight, measuring 170 x 86 x 38 mm and weighing 0.5 kg. Safety precautions will be followed to ensure participant well-being, including proper handling of the device, avoiding damage to internal components, and keeping it away from water and other liquids. The rechargeable battery will be charged according to the manufacturer's instructions, and the device will be stored in a dry, cool place when not in use.

All data analysis procedures will be performed using SPSS version 27.0 (IBM Corp., Armonk, NY, USA). Continuous data will be presented as mean ± standard deviation, and categorical data will be presented as numbers and percentages. The normality of the distribution will be assessed using the Shapiro-Wilk test and graphically. For all statistical analyses, a significance level of 0.05 will be considered, and confidence intervals will be reported where appropriate.

To assess the reproducibility of the measurements, the intra-class correlation coefficient (ICC) will be calculated. The confidence interval for the ICC will be determined using the bootstrap method with 10,000 replicates. An F-test will be performed to determine if the ICC significantly differs from zero, indicating nil reproducibility. Good intra-examiner and inter-examiner reproducibility will be defined as ICC values above 0.90 and 0.75, respectively.

Mixed linear regression models will be used to compare extension strength between the dominant and non-dominant hands, incorporating a 'subject' random effect to account for within-subject correlation. This analysis will allow us to examine potential differences in finger extension strength between the hands while considering individual variation.