Most countries are reported with rising life expectancy and therefore a rapid increase in ageing population worldwide. Elderly people normally have physical deterioration and frailty, which imposes a heavy burden on the social health care system. The decreased physical capabilities owing to deterioration of neuromusculoskeletal system makes elderly people walking with a changed gait pattern and more cautious . They generally have increased step variability and metabolic cost of walking, lower walking speed, shorter step-length, and reduced range of motion of the ankle, knee, and hip joints [2,3]. In addition, the elderly people have difficulties in maintaining trunk stability and have a risk of falls . The lower limbs dysfunction and gait impairments are also common in elderly people, which could cause unnatural gait patterns [5,6]. Nearly three-quarters of all strokes occur in people over the age of 65 years. All those could reduce the mobility of elderly people and lead them to fewer independent lives and poor quality of life.
In addition, the patients with neurological disorders caused by diseases or injuries such as a stroke and spinal cord injury generally have muscle weakness, which could lead to insufficient force/torque at the hip joints during human locomotion . These individuals often have decreased capacities of self-balancing and increased falling risk . Therefore, approaches that can help elderly people and these patients to maintain a good walking pattern are desirable . The past decade has witnessed a remarkable progress in research and development (R&D) of wearable medical devices for the patients with gait impairments . The use of wearable medical devices such as robotic exoskeletons  and active orthoses  have become one of the most promising approaches to assist the individuals with gait disorders. It is predicted by a researcher that robotic exoskeletons would be commonly used in the community by 2024 .
Robotic hip exoskeletons integrate the robot power and human intelligence, and they can provide controllable assistive force/torque at the wearers’ hip joints with an anthropomorphic conﬁguration. One application of robotic hip exoskeletons is on gait rehabilitation. They are able to train the wearers’ muscles and assist their movements for therapeutic exercise. The robot-assisted rehabilitation can release therapists from the heavy burden of rehabilitation training and provide long training sessions for the patients with good consistency. Human regular walking is able to reduce the risk of strokes and coronary heart disease, and hence to improve the physical and mental health . Thus, it is promising to make human walking more efficient. Human effort is related to metabolic expenditure, and the other application of robotic hip exoskeletons is to augment human performance such as increasing the human strength and endurance.
By comparing with the human ankle joint, the hip joint needs higher metabolic cost for the generation of similar mechanical joint power owing to the differences in muscle characteristics . Therefore, in addition to the robotic ankle exoskeletons developed for metabolic benefit , the hip joint actuating is also a promising strategy because large positive torque is provided by the human hip during the activities of daily living . Robotic hip exoskeletons also have the potential to integrate into the factories. In warehouses and manufacturing environments, the workers often have to handle heavy goods, which could load their lumbar spine and increase the risk of physical injury such as low back pain and other work-related musculoskeletal disorders [18,19]. The work-related injuries could have a serious impact on the quality of life of these individuals. Robotic hip exoskeletons are able to assist these workers during manual handling of heavy-duty tasks.
The aim of this article is to review the aspects of engineering design and control strategies of robotic hip exoskeletons for the two applications, i.e., gait rehabilitation and human performance augmentation, and to discuss some possible future directions to improve the currently available robotic hip exoskeletons. We hope this review would provide useful information for the engineers and researchers to design desirable robotic hip exoskeletons, especially for those new to this ﬁeld and would like to make contributions to this important multidisciplinary biomedical engineering and orthopaedic rehabilitation filed.
In this article, the biomechanics of the human hip joint and pathological gait of individuals with hip dysfunction are first presented before reviewing the mechanical structure, actuators, sensors, and control strategies of the existing robotic hip exoskeletons. Finally, this article discusses the limitations of the available robotic hip exoskeletons and their possible R&D directions with respect to clinical applications.
Biomechanics of human hip and pathological gait
To increase adaptability and achieve minimal interference, bioinspired design of robotic hip exoskeletons is required. This section presents a brief description of biomechanics of the human hip joint and the pathological gait pattern of individuals with hip dysfunction, which provides a basis for the design and control of robotic hip exoskeletons.
Biomechanics of normal human hip joint
The human hip joint is a ball-and-socket joint and joins the pelvis to the femur. It is composed of the cup-shaped acetabulum and femoral head, which are connected and supported by several tissues and muscles [20,21]. In human locomotion analysis, the human hip behaves as a spherical joint with three degrees of freedom (DOFs), i.e., flexion/extension, abduction/adduction, and internal/external rotation. A human gait cycle is defined as a sequence of movements during walking and is basically composed of the alternating stance phase and swing phase , as shown in Fig. 1. According to the gait analysis of people with a normal gait pattern , the human hip joint will generate positive work to bear the body weight, propel the body forward, and stabilize the trunk during the period of 0–35% of a gait cycle. After this phase, the hip joint angle will cross the zero degree and the leg will become vertical.