## 1 Introduction
By their generated motion, pneumatic motors are linear or rotary, and, as to their construction, they are equipped with a piston, blades or membrane [8]. The literature reveals the predominant deployment of pneumatic cylinders and pneumatic muscles in the construction of powered exoskeletons. The main applications of such systems are the rehabilitation and exercising of the upper and lower limbs. [PERSON] et al. present in [9] the construction of an exoskeleton device for active wrist rehabilitation based on pneumatic cylinders. The flexion and extension of the wrist is conducted by means of a bar system actuated by a pneumatic cylinder. In [10], [PERSON] et al. put forward a simple and low-cost pneumatic robotic mechanism for lower limb rehabilitation. Operational safety and force control are achieved by adequately adjusting the pressures in the chambers of the pneumatic cylinders that drive the rehabilitation robot. Other exoskeleton devices actuated by pneumatic cylinders are described in papers [11; 12; 13; 14]. The utilization of pneumatic muscles is gaining terrain in the construction of wearable assistive devices. This is due to the advantages offered by pneumatic muscles, such as higher force-to-weight ratio and the capacity to generate larger forces than other pneumatic motors. The low weight and flexibility of pneumatic muscles renders them easily integrated into an exoskeleton device [15]. In [16], [PERSON] presents an overview of recent developments in exoskeletons and robotic prosthetics that highlights the growing presence of pneumatic muscles in this field. Portable medical systems actuated by pneumatic muscles devised for the rehabilitation of the upper and lower limb joints are presented in a number of publications. [PERSON] et al. propose in [17] a wearable upper extremity therapy robot, actuated by pneumatic muscles. [PERSON] et al. also use pneumatic muscles to build a soft power support suit for the upper limb [18]. Other examples of deploying pneumatic muscles for the actuation of exoskeletons designed for mobilizing upper and lower limb joints are discussed in papers [19; 20; 21; 22; 23; 24]. In [25; 26], the authors present two patented applications of pneumatic muscles in the field of medical rehabilitation equipment, designed for the wrist, and hip and knee and ankle, respectively. In addition to characteristics such as compactness, low mass and portability, the published studies revealed a further significant requirement to be met by a powered exoskeleton, namely the highest possible energy efficiency. While all analyzed papers available in the literature describe the construction and performance of powered exoskeletons, the criteria for selecting a certain type of pneumatic motor (pneumatic cylinder or pneumatic muscle) are not explained. As a novelty, this paper introduces an energy-related approach to selecting a certain pneumatic actuator. In consideration of the global concern related to energy consumption, the paper presents a method for determining the energy-to-mass ratio (gravimetric energy density) of pneumatic actuators and puts forward this indicator to be used for their selection. Gravimetric energy density is the amount of energy stored in a given system or region of space per unit mass. The higher the gravimetric energy density of the air, the more energy can be stored or transported for the same amount of mass [27]. The reviewed literature discusses this energy-related indicator as a criterion for selecting pneumatic motors only to a rather slight extent. Only in [28], [PERSON] proposes the energy-to-mass ratio as a criterion for selecting a certain type of pneumatic actuator, his calculations yielding the conclusion that the highly praised advantages of pneumatic muscles are exaggerated. Other papers refer to the concept of power-to-weight ratio, without, however, offering a corresponding calculation course [29; 30; 31]. Paper [32] presents a comparison of a hydraulic and an electric motor as to their energy efficiency, based on the power-to-mass ratio.