Stroke is one of the most common causes of disability in adults, and its prevalence increases as the population ages, with about 40% of patients failing to recover after 5 years. Most patients have varying degrees of motor dysfunction, which seriously affects the quality of life of patients and also brings huge economic burden to the family and society [1-3]. Lower extremity dysfunction after stroke often affects the quality of life of patients, among which balance disorder is the most common lower extremity motor dysfunction after stroke, and poor balance function is the primary cause of increased risk of fall [4]. Gait improvement has also become the primary goal of post-stroke rehabilitation [5]. Traditional rehabilitation technology has limited effect on the rehabilitation of stroke patients' gait. Virtual reality technology, as a new rehabilitation technology, has begun to be applied in the rehabilitation of stroke patients. Abnormal gait is the focus and difficulty of post-stroke rehabilitation training. Virtual reality (VR), as a relatively new rehabilitation training method, plays its unique advantages in solving this problem.

1. Application of virtual reality technology in stroke rehabilitation

Visual virtual rehabilitation therapy was first proposed by Wann and Turnbull in 1993 [6]. Compared with traditional rehabilitation therapy, VR technology allows patients to visually see the operations they are performing, and through the immersive virtual environment experience, strengthen the awareness of training actions. The advantages of VR are mainly reflected in the three key links of repetition, feedback and motivation. Repetition is a necessary means of learning reinforcement process. Positive feedback, including the incentive conditions in VR technology, can bring positive driving force to the training experience of patients and give them a stronger sense of immersion. In addition, clear motivation allows patients to differentiate between goals to be achieved over a long period of time and to create a step-by-step training program. With the continuous development of virtual reality technology and the gradual improvement of software and hardware, it is more and more applied in medical undertakings. Virtual reality technology has been introduced into the evaluation and rehabilitation of stroke patients, creating a new era of rehabilitation medical treatment. Compared with traditional rehabilitation methods, virtual reality technology has the advantages of creating more realistic scenes, providing repetitive training for specific tasks, and having a stronger sense of participation [7-8]. The application of virtual reality in stroke rehabilitation mainly includes rehabilitation treatment of gait, upper limb motor function, balance function and cognitive function of stroke patients [9].

2. Application of virtual reality technology in gait rehabilitation

Gait abnormalities in stroke patients with hemiplegia are usually characterized by short, asymmetrical steps, slow gait speed, and reduced stride frequency. VR can simulate real-world environments, giving patients the feeling of walking in the real world. Patients can also simulate walking on city roads, parks, shops, etc. Current studies have shown that VR is effective in improving the gait of stroke patients [10-12]. Compared with non-VR walking intervention, VR-based training can improve walking speed with statistical significance, which confirms that VR-related walking training has a significant effect on increasing walking speed after stroke [13]. In this study, VR combined with rehabilitation robot was used to conduct gait function training for patients with cerebral infarction. After the training, Fugl-Meyer lower extremity motor function score showed that the scores of PATIENTS with VR were improved [14]. [15] Shema et al. VR training showed improved performance on the 2min walk Test and Four Square Step Test (FSST) after 5 weeks. Chen Peisun et al. [16] conducted a randomized controlled trial. In addition to routine rehabilitation training, the treatment group was also given VR training. After 4 weeks, it was found that the patients' motor ability, walking function and activities of daily living ability were significantly improved in combination with VR training compared with those who only received traditional training.

2.1 VR-based gait rehabilitation training

VR tasks: There are different types of virtual environments according to the degree of immersion [17]. The first, non-immersive VR, is a computer-generated environment projected on a screen or on the wall in front of the patient; The second type is semi-immersive VR or augmented reality, which superimposes virtual images on real images to increase the information content of real images. The third category is immersive VR, where the audience is part of the environment. For example, a head-mounted display (HMD), which is a device with a helmet, provides images inside the computer as a unique visual stimulus.

Training dose: Most studies used training duration of 40-60 minutes, while some studies used shorter (20 minutes) training duration [18-21]. The training frequency varies from two to five times per week, and the total training duration lasts from two to eight weeks. Thus, the entire VR intervention showed a wide range of variations between 2 and 22 hours. A typical training dose consists of training sessions lasting 40 to 60 minutes three to five times a week for three to six weeks.

Feedback: In addition to the obvious internal visual feedback perceived from the virtual environment, additional internal auditory, somatosensory, or proprioceptive information has been manipulated in some studies. Feng et al. used a 6-DOF motion platform to simulate the slope in the virtual environment to transmit the proprioceptive information consistent with walking on an inclined surface [22]. Deutsch et al. used tactile inputs to simulate turbulent or collision sensations. This multi-sensory feedback can be used as an important facilitator of task intrinsic learning and enhance participation in the virtual environment [23].

2.2 Gait virtual reality rehabilitation system

Rutgers Ankle rehabilitation system is Stewart platform force feedback rehabilitation system with 6 degrees of freedom [24]. The system consists of a display, a sensor, a computer and a controller. Patients fly in a virtual environment to train their lower limbs to avoid obstacles. The device reduces the weight on the affected limb, making it easier for the patient to adapt to training. The position and orientation of the patient's ankle joint movement are sensed by the sensor on the ankle joint, which can transmit the signal of the ankle joint movement to the computer, and control the movement of objects in the virtual environment through the patient's ankle joint movement. The powerful feedback device in the system can provide the corresponding resistance or power according to the patient's strength during training, so as to help the patient complete the rehabilitation training better.

Active and passive virtual rehabilitation training system can realize active and passive cooperative stimulation of patients [25]. The virtual reality system provides patients with a virtual scene of urban life streets. Patients can walk and talk with others in the virtual environment, and the movement information of patients can be feedback in real time through sensors. Through the training of this system, a closed circuit of information transmission can be formed in the process of patients' rehabilitation, so as to complete the active and passive synergistic stimulation of the damaged nerves, promote nerve remodeling, and realize the rehabilitation of patients' gait.

Exercise tablet training system is a virtual reality weight loss and balance training system that combines virtual reality technology and weight-loss treadmill training [26]. The system consists of a weight-loss flat panel, a large-screen TV, a computer and sensors. The weight reduction plate provides gravity supplement to prevent patients from falling and can carry a body weight of 163 kg. A large screen TV is mounted in front of the tablet to display the virtual environment. The tracking device monitors whether the patient is maintaining the correct posture and provides real-time feedback. When the patient's posture is incorrect, there will be auditory feedback to prompt the patient to correct the posture. Therapists can also timely detect the abnormal gait of patients, and give corrections, to better promote the rehabilitation of patients' gait. Paolini et al. combined Microsoft's THREE-DIMENSIONAL motion sensor camera with the virtual reality motion tablet system, which can track the foot position and direction in real time during gait training without wearing sensors on the feet [27].

The posture control system is based on virtual reality posture control system to carry out rehabilitation training for patients. The system allows patients to exercise postural control by watching visual feedback of their movements in real time. It consists of a visual feedback program and a HMD output that allows patients to watch their movements in real time, allowing them to adjust their posture in time. At the same time, the computer will record the patient's posture data for the rehabilitation therapist to analyze. Studies have demonstrated that virtual reality-based posture control training can improve gait ability of patients with sequelae of stroke. Virtual reality posture control training with real-time information is an effective method to increase gait control of patients with sequelae of stroke [28].

3 outlook

The emergence and development of virtual reality technology provides a new treatment method for gait rehabilitation of stroke patients. It can increase the patient's enthusiasm for treatment, provide real-time feedback according to the patient's training situation, and formulate training tasks according to the patient's personal conditions. It has incomparable advantages over traditional rehabilitation methods. However, at present, there are still many problems to be solved in the study of VIRTUAL reality in gait rehabilitation, such as the inclusion conditions of VIRTUAL reality patients, the time and intensity of training, and the choice of virtual reality mode, which all need further research. The effect mechanism of virtual reality gait rehabilitation training on brain functional recombination of patients also needs to be further explored by using molecular biology, physiology and other methods. The long-term efficacy of patient recovery also needs further confirmation. The equipment of virtual reality gait rehabilitation technology is expensive and difficult to be widely used in clinic. Developing a virtual reality gait rehabilitation system with low cost and small volume, so that patients can carry out community and even family rehabilitation is also the direction of future research. It is believed that with the development of science and technology, the gait rehabilitation training system based on virtual reality will be further improved and widely applied in the rehabilitation treatment of stroke patients.

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