# Jumping Locomotion Strategies: From Animals to Bioinspired Robots
## 1 Introduction  
Animals' locomotion is robust, complex, and adaptive. Based on the different environmental scenarios, locomotion can be classified into three categories: (i) terrestrial locomotion; (ii) natural flight; (iii) swimming [1; 2; 3; 4]. Compared to flight and swimming [5], terrestrial locomotion is much more complex and more adaptive as a consequence of the environments' complexity and higher competition among terrestrial animals. Terrestrial locomotion includes walking, running (bipeds running and quadrupeds running), jumping (leaping and hopping), climbing, balancing and crawling, etc. [6]. Compared to other terrestrial locomotion modes, jumping permits better adaption to unstructured environments, stronger ability to overcome obstacles, and faster threats avoidance [2; 7]. Jumping requires a very short-time energy density. In nature, jumping is often combined with other locomotion modes such as walking, gliding, and flapping. In some cases, jumping represents itself the main locomotion mode, like in kangaroos and galagoes, while in others it is used to assist the main locomotion mode. The latter is, for example, the case of locusts, which exploit jumping to initiate flight. Jumping can help vertebrate and invertebrate animals overcome obstacles that, in some cases, are several times their body size, enabling them to perform fast movements, precious for escaping from predators and for catching scattered food. Jumping animals use different jumping strategies based on their own body and locomotor structures: (i) Continuous jumping; it is, for instance, the case of kangaroos, that can achieveenergy accumulation and continuous posture adjustment during both take-off and landing phases [8]; (ii) Intermittent jumping, mainly adopted by small animals, such as locusts and frogs, that need to reaccumulate energy after landing. This energy, in insects ranging from a mass of 1 mg (like fleas) to a mass of 2.5 g (like locusts) is accumulated in highly specialized energy storage components and then suddenly released to perform powerful jumps [9]. The research in this area is mainly focused on the jumping mechanisms of animals and jumping robots' design. The advantages of jumping locomotion and the excellent jumping ability of animals inspired engineers to develop jumping robots mimicking those animals. Compared to robots using conventional locomotion, e.g., wheels, walking and crawling etc., jumping robots can better adapt to unstructured environments because they can reach goals even in presence of only sparse footholds, by jumping from one to another [7]. Jumping robots are particularly indicated for applications characterized by uneven, rough terrain [10], such as battlefield reconnaissance, archaeological exploration, antiterrorism operations, freight transportation, patient care, disaster relief [11]. They can also be employed in interstellar detection [12], since jumping owes greater advantage over other locomotion modes in low-gravity environments, such as Mars and the moon. Research on jumping robotic artefacts was initiated by NASA in 1969 [13]. In that occasion, a jumping robot was embarked in Apollo series to be used for moon's surface exploration. The earliest studies, \"3D hopper\" [14] and \"Acrobot\" [15], were developed in the 1980s. \"3D hopper\" presented a rotation joint between the foot and body and another translation joint along the leg, pointing to the body. It could realize jumping and landing in a vertical plane, with the possibility to add a hinge joint for motion in three-dimensional space. \"Acrobot\", instead, was modeled as double pendulum, with only one actuated joint to keep balance and make power synchronously. Several significant theories and designs had been developed based on these two jumping robots [16; 17; 18; 19; 20], but studies were hardly able to build practical and useful experimental platforms, because there were too many assumptions at the basis of their model that were not possible in the real world [21].