Lower limb strength measurements by Hand Held Dynamometer assisted by optoelectronic system. A. Ancillao 1 , F. Patanè 1 , S. Rossi 2 , A. Pacilli 1 & P. Cappa 1 1 Dep. Of Mechanical and Aerospace Engineering, Sapienza University of Rome, Roma, IT 2 Dep. of Economics and Management – Industrial Engineering, University of Tuscia, Viterbo, IT Abstract Strength measurements on the lower limb muscles are popular in the medical practice to evaluate the health status and effectiveness of training programs. Quality of strength measures obtained by Hand Held Dynamometer (HDD) is anyway under discussion, as reliability depends on operator as well as testing conditions. In this work the authors used an optoelectronic system to assist HHD measurements in order to check quality and to propose a method to reduce interval of uncertainties due to HHD method. 1. Introduction Very often, in the medical practice and in rehabilitation programs, strength measurements are performed to evaluate the health status of patients and effectiveness of training programs. The force measurements by Hand Held Dynamometer (HHD) are also used to assess muscle strength in children with developmental disabilities, such as cerebral palsy (CP) [1]. The use of HHD is very popular because the device is inexpensive, easy to use and does not require specific patient preparation; in fact, the patient has just to assume the position defined by the protocol, e.g. sitting on a bench for knee flexion/extension trials, while the therapist applies the HHD on the leg and asks the patient to push against the HHD. At the end the therapist simply reads the force value on the display of the device. On the other side, HHD measures are affected by errors due to the operator and patient positioning. Studies were already conducted on the inter- tester reliability of the method and concluded that the method is questionable since due to the low reproducibility among trial repetitions [1], [2]. The reproducibility increases when the test is conducted by a trained clinician, but quality of measurements depends on application and positioning of the HHD [2]. The aim of this work is to conduct strength measurements while tracking position of the HHD with respect to the limb. Position and 3D orientation are recorded by an optoelectronic system. This will allow to quantify the error due to positioning, orientation and rotation of the HHD and to develop a method to correct the measurement. 2. Methods 2.1. Equipment Measures took place in the Motion Analysis and Robotics Laboratory at Ospedale Pediatrico Bambino Gesù, where a Vicon MX (Oxford, UK) motion capture system (8-camera-workstation, Nexus 1.7 software, 200 Hz, PlugInGait marker set based on the Davis protocol) is installed. A MicroFet™ dynamometer (Hoggan Scientific, Salt Lake City, UT) was equipped with four passive markers as shown in Fig. 1. Markers were placed on sticks fixed to the HHD in order to improve the visibility by the Vicon system. The central marker was used only for the static trial, to find the sensing HDD axis, and it was removed for the dynamic trials. Fig. 1: HHD Dynamometer equipped with passive markers. The Vicon system allowed to reconstruct the position of the subject and the HDD (Fig. 2) with an overall inaccuracy of 0.5 mm. Thus, the method proposed here allowed to compute the direction, orientation and application axis of the force measured by the HDD.