## 1 Introduction
The 6000 Hand CPM OrthoAgility@is another example of electrically actuated equipment that is used for post-fracture recovery, reconstructive surgery on bone, cartilage, tendons and ligaments, and allows patients to achieve a full composite fist of 270\({}^{\circ}\)[7]. The WaveFlex Hand system developed by Remington Medical is a light construction deployable both in hospital and in patients' homes. It supports the performing of rehabilitation exercises within the limits of hand joint biomechanics [8]. In addition to the enumerated characteristics, the electrically actuated devices exemplified above also have a compliant behavior, that is, they adapt to a possible resistance put up by the patient upon the onset of pain. It is known that during rehabilitation exercising, certain limits of the motions can be exceeded; beyond that the patient may feel pain. The response time of the rehabilitation equipment from the sensing of pain to the execution of corrections, that is, until compliance is activated, should be as short as possible, this being an important feature of a recovery device. Ensuring the compliance of electrically driven rehabilitation devices, that is, the swift adaptation of the system to the concrete situation determined by the patient state, entails, however, excessive sensorization and the implementation of complicated control diagrams, all of these leading eventually to cost-intensive equipment of the order of thousands of euros [9]. Under these circumstances, it becomes a requirement to identify new solutions for reasonably costed rehabilitation equipment adapted to the budgets of health services or those of patients. A solution is the elimination of most sensors, thus simplifying the control diagrams and implicitly reducing costs. This can be achieved by introducing adjustable compliance actuators, known as ACAs, such as pneumatic muscles that due to the compressibility of air have an inherently compliant behavior. Pneumatic muscles ensure safe interaction with human operators also by their ability of storing and releasing energy into passive elastic elements [10]. Deploying pneumatic actuation in medical rehabilitation equipment is not widespread. Hesitation in using pneumatic drives is due to the requirement of air preparation systems, noisy operation or the impossibility of high accuracy positioning of the mobile elements. Notwithstanding, pneumatic actuation offers advantages such as the possibility of de veloping \"pneumatic springs\" for end-of-stroke damping, all being determined by air compressibility. Another favorable feature is compliance, the inverse of rigidity, which allows deviations from a certain position of equilibrium in case external forces act upon the system. To date, only few medical rehabilitation devices actuated by pneumatic muscles are known, most being patents and/or prototypes. An example of pneumatic muscle actuated rehabilitation equipment already available on the market for an affordable couple of hundred euros is the Hand Mentor Pro, developed for exercising the hand in view of the recovery of its gripping ability [11]. EXOWRIST (a prototype for wrist rehabilitation) is another example of rehabilitation equipment deploying four pneumatic muscles as actuators and ensuring two degrees of mobility. The advantages of this device are its low price and high portability for autonomous and independent use [12]. Another wrist rehabilitation system with a single degree of mobility is in its prototype phase, proposed by [PERSON] et.al. [13]. The equipment uses two pneumatic muscles in antagonist operation mode, causing the hand support to conduct flexion and extension motions. The review and analysis of the state of art of upper limb medical rehabilitation equipment constructions yielded certain conclusions regarding the requirements that these systems have to meet. Concretely, both portability and the compliant behavior of the equipment must be ensured at the lowest possible cost. Each of the known electrical or pneumatic equipment meets part of these requirements.