Development and validation of a palpation device for arbitrary muscle stiffness measurements within subjects

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Objective: Neuromuscular disorders can often be quantified by the amount of muscle activity and is generally measured through electromyography. However, this method cannot provide arbitrary data as it only detects fluctuations in electrical potential. There is a need for arbitrary and standardised assessments of muscle tone to enable physiotherapists to make correct diagnoses. In this work, we present the rationale and results of a newly developed device to measure arbitrary muscle stiffness. Approach: Three prototypes were developed and iteratively improved. The first one relied on the theory of electrical impedance myography and the two other models managed a mechanical approach using a palpation probe and two reference areas. The final mechanical palpation principle was optimised and tested for two devices and two operators. The experiments were conducted in 39 subjects for left and right arm for relaxed and tensioned m. biceps brachii. Main results: The results show the reliability of the palpation device in terms of repeatability between identical measurements (ICC = 0.93), between different operators (ICC = 0.84) and between the two devices (ICC = 0.88). The device detects significant differences between relaxed and tensioned biceps brachii conditions (p < 0.001). Significance: The study indicates the feasibility of a mechanical muscle stiffness method to provide arbitrary data. The opportunities of the mechanical palpation device to identify variations in neuromuscular impairments in a reproducible procedure is confirmed. Further studies are required to evaluate the tonus of various muscles under diverse circumstances and to pinpoint further applications in clinical practice.