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Myasthenia gravis and Lambert-Eaton syndrome are examples of neuromuscular junction disorders. Muscular dystrophies and inflammatory myopathies such as polymyositis are examples of primary muscular (myopathic) disorders.

Neurodegenarative classification encapsulates the progressive loss of structure or function of the neurone and a number of conditions exhibit this form of progression. Hence, amyotrophic lateral sclerosis (ALS), Parkinson's and Huntington's are classified as neurodegenerative diseases that affect movement. These conditions exhibit characteristically slower movement compared to healthy people – hypokinesis – or excessive and abnormal involuntary movements – hyperkinesis [156]. Some common examples of hypokinesis include bradykinesia, freezing, rigidity and stiff muscles, while those belonging to hyperkinesis are chorea, dyskinesia, myoclonus, tics and tremor [242].

The most common neurological [215] and adult movement disorder, essential tremor (ET), is about 20 times more prevalent than Parkinson's disease itself. Patients with ET are likely to have tremors with 4–12 Hz and the risk factors associated with this are age, ethnicity and family history [215]. The condition affects the performance of work-related tasks and activities of daily living (ADLs) and a number of medical and physical rehabilitation approaches are in use as treatments for the condition [39, 295].

An estimated 7 to 10 million people worldwide are living with the second‐most common neurodegenerative disorder, Parkinson's disease (PD) [90]. Approximately 60 000 Americans are diagnosed with PD each year while in western Europe this figure is 160 for every 100 000 over the age of 80 [89]. In China, approximately 1.7 million above the age of 55 [396] are suffering from the condition. The movement disorders experienced by a PD patient can be classified into three stages [244]. In the initial stage, the patient may exhibit a forward stooped posture, festinating gait, rigidity, etc. During the first 10 years of PD, characteristic movements such as resting tremor, hypokinesia and micrographic handwriting are common. During the later phase, patients may exhibit dyskinesia, akinesia, postural instability, etc. In terms of treatment, various kinds of medical therapies, such as levodopa, as well as surgical approaches and deep brain stimulation are utilised to control symptoms in addition to physical rehabilitation therapies [171].

Although these two conditions are common and have a significant impact on the quality of life, they are not fatal diseases. In contrast, stroke is one of the most fatal conditions in developed countries [374]. However, a majority of stroke suffers may be alive after the initial injury, albeit losing some motor functions lifelong or for a prolonged period of time [193]. In 2005, there were 5.7 million deaths in low‐ and middle‐income countries due to stroke, which has increased significantly to 6.5 million and 7.8 million, respectively, in 2015 [338]. Age, gender, race, ethnicity and heredity are considered as important markers of risk factors [273], while hypertension, cardiac disease, diabetes, glucose metabolism, lipids, cigarette smoking, alcohol, illicit drug use, lifestyle, etc., are considered to have an adverse influence on the likelihood of stroke [273]. Similar to other disorders, physiotherapy is widely used as a rehabilitation therapy to assist stroke patients to regain physical functionality [291].

Such a complex locomotor and neural system are frequently influenced by various injuries and disorders. Therefore, a number of approaches have been explored to treat people suffering from conditions leading to abnormal movements and even disabilities. Among these methods, physical rehabilitation is commonly utilised to assist patients in recovery and reacquire ADLs.

Defined as “the treatment of disease, injury, or deformity by physical methods such as massage, heat treatment, and exercise rather than by drugs or surgery” [279], physiotherapy (also known as physical therapy) has been applied in clinics for thousands of years. A number of therapies are included in physical therapy, such as mechanotherapy, hydrotherapy, balneotherapy and so on [349], among which mechanotherapy was documented as early as the 1840s. In recent decades, physical therapy has been applied extensively for various musculoskeletal injuries and neurological movement disorders [107, 142]. The detailed examples can be found in Section 1.2.

Although traditional physical therapy has shown its effectiveness for the rehabilitation of physical functions of patients with movement disorders [95], a series of drawbacks can also be observed [9, 57], which are summarised as follows.

 These rehabilitation programmes are “boring” and patients are demotivated by these repetitive exercises.

 Computerised sensing techniques are not involved in these programmes, which may lead to incorrect interpretation of observed data.

 A one‐to‐one form of delivering rehabilitation services makes conventional rehabilitation very inefficient and costly.

 Costly equipment is required by traditional rehabilitation therapies.

 Insufficient funding for rehabilitation services results in making access to these services unaffordable.

 The workforce in the rehabilitation field is inadequate in number.

 Rehabilitation centres are usually distributed in urban areas, while a large number of people needing rehabilitation services live in rural regions.

In light of the above, in 1998, the term “telerehabilitation” was “first raised” in a scientific article, attempting to overcome the shortage in conventional rehabilitation services [303]. Co‐existing with the opportunities for telerehabilitation, such as economy of scale, interactive and motivation, reduced healthcare costs, patients' privacy prevention and so on [57], a number of challenges can be seen from an engineering point of view as well. These challenges are also closely related to the aim of this book.

First of all, developing affordable, high‐quality and robust hardware [300] is critical to capture human movements for further analysis. High‐quality and effective hardware may provide more accurate monitoring of the movements, thereby offering better feedback to patients to correct their movements and more valuable information to therapists to make further treatment decisions. In addition, similar to the fourth issue in traditional rehabilitation mentioned above, costly equipment used in telerehabilitation services may prevent patients with low economic status from accessing them. Thus, how to develop affordable devices is critical for the development of telerehabilitation services.

Secondly, an advanced approach in representing human movement data [381] is important after capturing human motion. As pointed out in Theodoros and Russell [350], one difficulty in telerehabilitation is how to reduce information collected from sensors, thereby producing meaningful results to therapists. Therefore, an emerging challenge is to discover which features can be utilised to represent human motions so that their rehabilitation‐related details can be well preserved while other irrelevant information can be reduced as much as possible.

Thirdly, developing an outcome measurement scheme to quantitatively and objectively represent the performance of patients accessing telerehabilitation services in also a challenge [382]. As previously described [10], one of the goals of RERC is to develop assessment tools to monitor the progress of patients accessing telerehabilitation services. These tools can not only be a feedback to stimulate the patients to perform more exercises but can also provide therapists with general information about their patients in terms of functional rehabilitation.

Last but not the least, enabling patients to access physical telerehabilitation services regardless of their location and time is a challenge [382]. As one purpose of rehabilitation services is for patients to recover the ability to perform ADLs, enabling them to perform rehabilitation exercises in their most familiar and natural environment is very important. Due to the different preferences between patients, developing telerehabilitation services that can be run on mobile devices is extremely useful. By doing so, patients' kinematic performance in telerehabilitation exercises and daily living can be assessed pervasively.

Because of the importance of telerehabilitation, as well as the automated kinematic performance assessment tool, a significant amount of effort has been made to improve the physical telerehabilitation, which will be discussed in Sections 1.5 and 1.6.