## 1 Introduction  
Spinal cord injury (SCI) causes quadriplegia or paraplegia depending on the height of the injured spinal cord, and ranges from complete paralysis to incomplete paralysis depending on the degree and severity of the injury. SCI restricts activities of daily living (ADL), and severe quadriplegia makes it impossible to perform almost all ADL, requiring a great deal of care and support. The incidence and causes of SCI vary according to geographical region, social background, and culture [1, 2]. Its incidence has been reported as 3.6 per million in Canada [3] and 195.4 per million in Ireland [4]. The most common causes of SCI have been reported as traffic accidents (67%) in West Africa, falls (60%) in South Asia, violence or self-harm (43%) in South Africa, and occupational accidents (23%) in Western Europe [1]. In recent years, the number of SCIs caused by falls of the elderly has increased in developed countries due to the aging of the population [5]. The latest national epidemiological survey of Japan, in 2018, which is the most aged country in the world, revealed that the proportion of elderly people is increasing, with an incidence of 49 per million and an average age of 66.5 years with a peak in those in their 70s [6]. In Japan, the most common severity of SCI in 2018 was Frankel grade D (46.3%), and fall was the most common cause (38.6%) [6]. In addition, the rate of cervical SCI increased from 75.0% in a national epidemiological survey conducted about 30 years ago to 88.1% in 2018 [6]. When elderly people are injured, the disability becomes severe due to complications and/or comorbidities, even in mild cases. It has been reported that the physical function and ADL of older patients with SCI do not improve significantly [7]. People with SCI often spend their life in a wheelchair as the paralysis becomes more severe, and suffer numerous problems such as decreased systemic tolerability and complications of metabolic syndrome. The high morbidity and mortality rates in persons with long-term SCI stem from cardiometabolic causes, which are likely to be associated with major changes in body composition [8]. In a recent report, one-month mortality risk was significantly higher at older than 75 years compared to younger than 55 years in the patients with traumatic SCI [9]. Therefore, it is very important to improve the function of people with SCI even in elderly people. Although rehabilitation treatment is performed, mainly by physical therapy and occupational therapy, it has been difficult to restore the function of an impaired spinal cord. These conventional rehabilitation treatments also limit the recovery of injured spinal cord function because of the previous thinking that nerves do not regenerate. However, since the discovery of neural plasticity [10], the frequency and task-specificity of rehabilitation treatment and neuropalasticity have been considered very important factors for effective rehabilitation treatment [11]. In recent years, neurorehabilitation treatment has been attracting attention as a means to overcome the limits of the past and as a more effective rehabilitation treatment. Neurorehabilitation is a new treatment method that attempts to reconstruct the motor circuit of the damaged spinal cord, and is expected to be more effective than the natural recovery of the cord. Neurorehabilitation for SCI employs various devices, including functional electrical stimulation (FES) [12,13], rehabilitation robots [14], and brain-computer interfaces (BCI) [15,16]. In addition, the realization of spinal cord regenerative medicine has been promoted worldwide [17,18]. In Japan, intravenous administration of human autologous bone marrow-derived mesenchymal stem cells was performed in 13 subacute SCI patients, which resulted in improved neurological function in 12 patients [19].