## 1 Introduction  
Injuries and pathologies affecting muscles and joints in the lower limb are commonly assessed by physiatrists and orthopedic surgeons via visual observation and manipulation of the relevant body segments. Several musculoskeletal and neurological conditions, such as damages and pathologies of the central and peripheral nervous system, along with traumatic events and complications following surgeries, can result in insufficiency of the ankle plantarflexor and/or dorsiflexor muscles. This may lead to altered gait patterns and significant kinematic compensations in the more proximal lower limb joints, such as the knee and the hip [1]. A rather common pathology which affects the muscles in the anterior compartment of the leg is the compression or injury of the peroneal nerve. Individuals with reduced ankle plantarflexion strength typically walk more slowly and compensate at the hip and knee joints, for example, by increasing the hip extension force [2, 3]. Several scoring systems for the clinical assessment of the ankle muscles' forces are available, such as the Medical Research Council (MRC) scale for muscle strength, which is based on a 0-5 scoring system [4]. This scoring system relies on the operator sensitivity in assessing the residual force of the muscles acting at the ankle either with or without the effect of gravity. In particular, the patient lies supine or sits with their ankle in neutral position and is requested to apply the maximum ankle plantar- and dorsi-flexion force against the manual resistance applied to the foot by the operator. Like other operator-based clinical scores, this scoring system is based on a limited number of discrete values, and the assessment of the degree of impairment is subjective and depends on the evaluator's experience. Accurate assessment of muscle strength is a critical element of any physical evaluation in standard clinical practice to score the degree of impairment and to identify the most suitable treatment. It has been shown that the maximum voluntary ankle force exerted by the plantar- and dorsi-flexion muscles is affected by the knee [5] and the ankle [6] flexion angles. However, comparing ankle force data across studies is difficult since different subjects' postures--prone, supine, and sitting--have been used in the literature [7]. Indeed, a variety of apparatuses have been proposed for the objective instrumental measurement of ankle forces. Some devices are manually held [8; 9; 10] or fixed [6; 9] force transducers. Rehabilitation devices, such as the AnkleBot [11] (developed at Massachusetts Institute of Technology, Cambridge, MA, USA), the Biodex System (Biodex Medical Systems, New York, NY, USA) [12], and the Prima Plus (Easytech, Florence, Italy) [5] have also been employed to assess the ankle strength properties. These devices have both advantages and disadvantages. While manually held devices are easy to use, the measurements are likely affected by the operator-dependent unstable and non-consistent positioning of the transducer with respect to the foot [8; 10]. Conversely, the force transducers fixated to a stable support do not always allow for the replication of clinical evaluation conditions [9]. In addition, most of the devices currently available on the market are rather bulky and require large, dedicated rooms and do not always represent a cost-effective solution for those centers with limited resources. Therefore, the aim of this study was to propose a simple, instrumental-based cost-effective solution to measure both ankle dorsi- and plantar-flexion forces in quasi-isometric contractions, replicating commonly used clinical evaluation methods based on the manual assessment of ankle strength. The repeatability of the measurement procedure has been assessed in a population of 30 healthy subjects with no history of foot/ankle pathologies or surgeries.