# Foot-Mounted Inertial Measurement Units-Based Device for Ankle Rehabilitation
## 1 Introduction  
Ankle strains are common injuries that can produce temporary or permanent impairments. An ankle strain occurs when the ligaments are torn because of being stretched beyond their limits. Ankle strain causes approximately 15 to 20% of sports injuries, when considering all the different grades of tears [1]. It can take several weeks or even months for a strained ankle to heal. When a strain is not properly treated, a more severe strain can occur and permanently weaken the ankle, making it more likely for new injuries to take place. Due to the lack of correct treatment, repeated ankle strains can lead to long-term problems, such as chronic ankle pain, ongoing instability, and arthritis [1]. In this case, rehabilitation is the part of the treatment that helps restore a patient to the regular use of their injured extremity through eliminating the impairments caused by the injury, when there has been no permanent damage. The problem is that unsupervised rehabilitation is prone to mistakes. Well-performed medical treatment can be undermined by poor rehabilitation [2]. Ankle movement can be described as a three-dimensional object with three degrees of freedom. The basic movements used in ankle rehabilitation are: abduction, adduction, plantarflexion, dorsiflexion, inversion, and version. The term sprain, also known as twisting or entorsis, indicates ligament injury by traction, which is executed by an indirect mechanism that extremely stretches the ligament structure beyond what the ligaments can tolerate. The mechanism of this injury is related to the inversion or version movements. The inversion movement is the most common acute injury that damages the lateral ligaments of theankle. Its inadequate treatment can lead to problems, such as a decreased range of motion, joint instability, and pain [1]. Depending on the degree of the injury, this traumatism can be catalogued into the following categories [1]:  
* Grade I: Involves the stretching of any ligament without tearing and slight signs of pain and/or inflammation. * Grade II: Involves the partial tearing of one or more ligaments and moderate pain and inflammatory signs. * Grade III: Involves full tear of ligaments and joint instability; pain and inflammatory signs are significant, and there is a loss of ankle function and mobility. The diagnosis of ankle sprain starts with the evaluation of the situation and mechanism of the injury. This is done to determine the current degrees of possible ankle movement, and the required treatment for this specific situation. This evaluation is based primarily on physical exploration using techniques such as inspection, palpation, mobilization, and radiology. Due to the frequency of this injury, it is necessary to develop a set of strengthening exercises for the rehabilitation of the ankle that allows recovering the amplitude of the movement of the joint and helps strengthen the muscles of this extremity. Telerehabilitation is the clinical application of Information and Communication Technologies (ICT) to provide rehabilitation services remotely and without the presence of a therapist all of the time. Telerehabilitation has been proven to be technically feasible; the case has been made as to how this technology can improve access to health services, as well as to extend the reach of clinicians beyond the physical walls of a traditional health care facility [1]. Devices that are used for ankle sprain rehabilitation can be grouped by complexity. Examples of low-complexity devices include elastic bands, roller foams, and wobble boards. Intermediate complexity devices provide more consistent results; in these devices, the patient plays a passive role in the rehabilitation process. Examples of intermediate devices include the DOF JACE Ankle A330 CPM system and the DOF Optiflex Ankle CPM system. High-complexity devices have high capabilities for data acquisition, storage, transmission, and providing written feedback. However, some disadvantages include being expensive and bulky, which restrict the use outside of a clinic.