Objective: To implement a prototype specific for human ankle kinematics studies in limited spaces, immobile, or lying down patients. Based on anatomy and anthropometry, using a screw theory model, draw-wire and inertial sensors were employed Methods: We included ankle injury studies to highlight the importance of measuring the in vivo range of motion; we studied the ankle anatomy, biomechanics, and anthropometry to estimate the size and movements of the device. We simulated the biaxial representation of ankle motion through the product of exponential mapping. Finally, we designed a structure based on trilateration by projecting tetrahedrons, an acquisition circuit with firmware and calibration software. Results: The prototype has two main parts: support and adjustable platform. We proposed a method to find the position by projecting three apexes on the base using draw-wire sensors, an acquisition board, a single-board computer, a display, Bluetooth, Wi-Fi, and two inertial measurement units. The power source had battery backup with boost and buck converters. Conclusion: We proposed an ankle model in the screw theory framework, a method for localization, and a novel device for in vivo measurements specific for lying patients on a bed, the ground, outdoors, or remote locations without complex setups. The double-battery management is robust and long lasting. Significance: The device is an alternative for measuring the range of motion in laying down patients. We will use it in modeling, diagnosis, and rehabilitation.