Abstract— The hydraulically actuated Exo-Musculature, consisting of a network of artificial hydro-muscles, can be rapidly assembled and utilized as either perform-alone or wearable, human body-symbiotic robotic system. Its fundamental unit is a novel hydraulically actuated muscle (HAM), which is inspired by the capabilities of biological muscles. The HAM design has certain advantages over natural muscles, such as being able to maintain a position without expending energy. This design uses an elastic element to apply tensile force, which is released when hydraulic pressure is applied. This gives the muscle the unique characteristic of storing elastic energy when pressurized and releasing it to contract. Other artificial muscles, such as the McKibben, are similar to HAM in the respect that they are fluid-actuated and can be locked in place, but the McKibben is contractile in operation. Additionally, HAM utilizes incompressible fluid (water) and it is limited to expansion in only one dimension, which offers a higher energy density. It is much more compliant than traditional hydraulic cylinders, making it better suited for use in human rehabilitation and augmentation. Finally, HAM is constructed using common materials, making it an extremely low cost solution for both medical and robotic applications. I. I NTRODUCTION The term Exo-Musculature [1] refers to a soft, thin, light- weight, and compliant self-actuated garment without rigid links or singular joints. Traditionally, assistive or augmentative systems like orthotic or exoskeleton systems consist of rigid links and joints for the lower [2] and upper-body [3] extremities. Similarly, most previously created actuated systems for upper-body rehabilitation use rigid exoskeletons or rigid-link manipulators [4, 5]. However, this traditional approach limits natural degrees of freedom (DoFs) and reduces comfort for the user. Further, misalignments between biological and artificial joints are inevitable [6]. Misalignments occur due to: (1) substantial skin-bone relative motion, (2) changes in volume of the limb, (3) initial imprecision when putting on the exoskeleton. Clearly, misalignments make exoskeletons uncomfortable, and in regards to the lower extremities can even lead to skin lesions and bone fractures due to the large forces that they are subjected to [6]. M. Popovic is with the Worcester Polytechnic Institute, Worcester, MA 01609 USA (corresponding author, phone: 617-470-8198; fax: 508-831- 5886; e-mail: mpopovic@wpi.edu). D. Effraimidis (e-mail: danefre@wpi.edu), B. Jennings (e-mail: bjennings@wpi.edu), G. McCarthy (e-mail: gmccarthy@wpi.edu), N. Corso (e-mail: ndscorso@wpi.edu), C. D. Onal (e-mail: cdonal@wpi.edu), are with the Worcester Polytechnic Institute, Worcester, MA 01609 USA. Figure 1: Photograph of Exo-muscle with Parts Labeled Exo-Musculatures utilize natural anatomical structures (skeletal joints and bones) to provide support for the device and as a result, maintain the natural kinematic degrees of freedom (DoF) [7]. As there are no pre-specified synthetic rigid joints, an Exo-Musculature avoids misalignment problems at the joint level. Any misalignments that are still present in the system are less critical and can be addressed with more advanced sensory-motor control system [8]. The fundamental problem for mechanically actuated wearable Exo-Musculatures is the need for a large number of independently actuated and controllable DoF. For the human body, there are approximately 800 skeletal muscles, each of which is composed of one hundred or more individual motor units [1]. Consequently, if an artificial assistive Exo- Musculature is to mimic even a small percentage of the functionality of human musculature, the number of actuated DoF needs to be large. For mechanically actuated devices, conventional approaches involving one dedicated electric motor per actuated DoF results in systems that are large, heavy, and expensive. The Hydraulically Actuated Muscle (HAM) Exo- Musculature [9] solves this problem as it allows for a single actuator, pump, to drive multiple independently actuated and controlled DoFs. In difference to standard hydraulic and pneumatic systems and similar to the One-To-Many (OTM) systems [1,10,11] the HAM Exo-Musculature has one controlled energy reservoir per each DoF. In difference to the General OTM system [12] the current HAM Exo- Musculature is always coupled to end-effector/load while reservoirs are only actively coupled to pump, the prime mover. A hydraulically actuated Exo-muscle may have several advantages over a pneumatically actuated Exo-muscle. The system response times are typically much faster (sound propagates faster in water than in air), energy losses are much smaller, and for an incompressible fluid, forces per unit area may be much larger allowing for more compact design of system with the same peak dynamical output force Hydraulically Actuated Muscle (HAM) Exo-Musculature Gregory McCarthy, Daniil Effraimidis, Brian Jennings, Nicholas Corso, Cagdas D. Onal, and Marko Popovic