A comprehensive review of wearable assistive robotic devices used for head and neck rehabilitation

1. Introduction

The musculoskeletal system comprises of bones, muscles, tendons, ligaments and soft tissues. These work collectively and support the weight of the body and aid in movements. There are many factors involved in musculoskeletal dysfunction such as strenuous activity (high-intensity exercise), trauma (auto accidents), and aging, damaged due to wear and tear of daily activities. Owing to musculoskeletal pain, one might feel stiffness, fatigue in muscle; dis-oriented movements and disturbed sleep [1,2].

Each year it is estimated about 30% of the population of different ages are prone to the neck pain. In general people neglect this pain and opt for alternate pain reducing methods, but neglecting the neck pain for a long period of time without proper care could result in cervical spine dysfunction as shown in Fig. 1[3]. The major reasons for neck pain are discussed below:

Among the musculoskeletal disorders, Cervical/Neck pain is considered to the major reason to affect the performance of the person in performing daily-activities. An unattended, cervical spine pain could lead to head/neck posture imbalance and damage sensory inputs from neck to brain, and also causes death [[4], [5], [6]].

Whiplash injuries (accidents) are caused by the motor vehicle accidents, these patients often complain to have dizziness and show signs of reduced eye co-ordination as shown in Fig. 2 [7,8]. Wearing a helmet is recommended due to safety reasons while riding a bike, wearing an overweight helmet might produce compression load on the cervical spine, and in long run it might lead to neck pain [9].

Dizziness is an uncontrolled sudden fall of the head/neck motion, which can be commonly seen in elderly people due to the age, is also another cause for neck pain in elderly people [11]. Motor neuron diseases (MND), for example, amyotrophic lateral sclerosis (ALS), are usually considered to be the cause for neck pain. This leads to lack of stability in neck postures, dysfunction and unsynchronized head and eye movement [12]. Amyotrophic lateral sclerosis (ALS) has no cure yet, and the treatment significantly focused on improving the quality of life by providing aid in performing their daily activities [13]. A patient with this neurological disorder eventually ends in a wheelchair and exerts most of the energy in performing daily-activities [14,15]. As the neck muscles grow weak progressively, considering the average weight of a human head to be 5–6 kg patient faces difficulty in maintaining the head balanced against the gravity. If not taken care of this, the communication between the central nervous system and the body is interrupted and leads to abnormal or impaired functioning of the human and prolonged version could lead to patient's death [16].

Rehabilitation/Physical Therapy is a process designed to enhance the performance of an individual suffering with disability. This process aids in reducing the physical challenges in performing daily life activities and also helps in restoring the mobility of an individual. Rehabilitation is prescribed for an individual involved in accidents, surgery, musculoskeletal disorders (MSDs) and aged people to enhance the mobility of the individual. Benefits of physical therapy are, it can enhance physical capacity, reduce pain, strengthen muscles, improve gait, posture, coordination, flexibility and joint mobility. Disabled individuals put a lot of physical effort in the performance of the daily activities, for example eating, drinking and so forth, on their own. Amyotrophic lateral sclerosis (ALS), Dropped Head Syndrome (DHS) has no cure as yet, and the focus of the treatment is on improvement of the quality of life. An individual suffering from this neurological disorder eventually ends in a wheelchair and exerts most of the energy in the performance of daily-activities. As the neck muscles grow weak progressively, considering the average weight of a human head to be 5–6 kgs, the individual faces difficulty in maintaining the head balanced against gravity. Communication between the central nervous system and the body is interrupted if this is not taken care of and may lead to abnormal or impaired functioning of the human and prolonged version could lead to patient death. The use of cervical collars/braces is recommended for individuals suffering from neck pain for assistance in maintaining a good posture, aids the patient to stabilize the uncontrolled rotation of head, and enables to maintenance of social interaction [17,18].

Physical therapy is recommended to an individual suffering with musculoskeletal disorders. MSDs are occurred because of sudden or overexertion, straining the muscles by lifting or carrying heavy loads for a prolonged period time. Due to this, the muscle grow weak and lead to pain [[19], [20], [21], [22], [23]].

Physical therapy manipulation is performed in rehabilitation centers for the treatment of individuals suffering from musculoskeletal disorders. This clinical setting requires prolonged intense therapy sessions for the patients. These physical therapies involve special exercises and stretches designed to relieve pain, regain strength and controlled movements as shown in Fig. 3 [24,25].

In general, therapies are also recommended for individuals suffering from musculoskeletal injuries/disorders such as strokes, sprains, injuries related to spinal cord and post-surgical rehabilitation. Physical therapy, in most of the cases, requires one or more physiotherapists for treatment of the patient. Physiotherapist s require years of practice to master the technique to treat patients, which is an expensive process. This process involves prolonged hours of therapy sessions, leading to fatigue for the physiotherapist, and compromising the reliability of therapy Due to increase in demand for physical therapy, the training sessions is generally shorter, than the needed duration, which might not meet individual's requirement [[26], [27], [28], [29]]. As an alternative for the physical therapy cervical collars (static) are recommended for the individuals to maintain good posture balance and assist in holding the head for dropping movement.

Assistive Technology in general terms, comprises of assistive, flexible, and rehabilitative devices that support greater independence for individuals with disabilities. Unfortunately, the standard way of assistive devices for neck pain is cervical collars, these collars hold chin and limit actions like eating, drinking and rotating head. Besides, extensive usage of static collars could also produce strain on neck surface muscles because most of the time collars are wrapped around the user neck and might cause friction between neck skin and the collar while the user is performing ADL's, in addition to this due to heat and sweat generation while using the collars the user might feel discomfort and tries to move the head. As the existing cervical collars are static, research has been focused on the dynamic collars or wearable assistive devices, which could provide head/neck motions according to the user comfort. Therefore, mechanism based robotic wearable assistive devices (WADs) have been proposed by researchers to provide regular exercise, safety and comfort to individuals suffering with neck pain. The head/neck WADs can aid the individual to mimic, and assist in restoring the head/neck motions.

The objective of this review article is to evaluate the progress made in the area of wearable assistive head/neck rehabilitation devices, and discuss different mechanisms available for mimicking human head/neck motions and aid in recovery of neck pain. According to the existing literature, it is found that the assistive devices/collars are having the following research issues for developing a WAD: designing a light weight support collars which fits for various anatomical features, selection and incorporation of suitable mechanism for actuating the WAD. Based on the above research problems identified from the literature, the main goal of this review paper is to clarify and answer the following queries in detail: 1) Reasons for neck pain, necessity of therapy, and requirements of WADs for restoring and mimicking head/neck motions. 2)Existing robotic head/neck mechanisms and how the mechanisms are incorporated in the head/neck WADs. 3) Numerous design challenges faced by researchers for effectively developing the head/neck WADs. 4) Various methods of sensor interfacing with WADs for obtaining the head/neck motion data. 5)Current advanced machine learning techniques for analyzing the enormous sensor data produced by the head/neck motions.

Overall, this review article is divided into five sections. Introduction details about the reasons for the musculoskeletal disorders, necessity of manual physical therapy, and requirement of robotic assisted wearable devices for mimicking human motions. Furthermore, the importance of cervical spine in the head/neck motions, factors creating neck pain are discussed in detail in Section II. Importance of cervical immobilization for cervical spine injured individuals, existing static collars used for restricting the head/neck motions and their merits and demerits are described in Section III. The recent developments in head/neck mechanisms in humanoid robots, how the humanoid head/neck mechanisms influenced in the evolution of head/neck WADs, requirement of kinematics analysis for the head/neck motions, and importance of sensor interface for the WADs are presented in detail in Section IV. Finally, this article discusses on possible areas of the WADs can be further developed and applied, as shown in Section V.

2. Head/neck biomechanics

A detail knowledge of head/neck and cervical spine anatomy is required to comprehend the neck pain. Neck is an intersection which connects the head and torso, and cervical spine in the neck is responsible for head/neck movements. The neck comprises of nerves, ligaments, glands, digestive tracts and veins. The nerves and ligaments around neck supports the weight of head and stabilizes the head..

The arrangement of nerves around the neck are linearly parallel to one another, the load distribution on nerves is done equally with the help of cervical spine (C1 to C7) as shown in Fig. 4 [31].

Cervical spine consists of various joints such as facet joints which are located between each vertebra, and these joints allow movement within the cervical spine, intervertebral discs are also located between C2 through C7 vertebras, these discs absorbs the dynamic shocks as well as distributes the pressure equally in all the directions. Among the seven vertebrae the first vertebrae, the atlas (C1) and axis vertebrae (C2) help head rotation. The overall neck Degree of Freedom (DoF) is considered as 6-DoF. Fig. 5 shows the head/neck Active Range of Motion (AROM), and also shows natural angular tilt/ROM for an adult head/neck motion. The human cervical spine undergoes a considerable amount of compressive preload in vivo. Cervical spine preload reaches to three times the weight of head owing to forces generated while trying to maintain the head in neutral position. The compressive loads on the cervical spine increases while performing the head/neck motions and other ADLs, and it is estimated 1200 N is the maximum load induced on the neck muscle and cervical spine. In healthy individuals these loads are easily sustained by the cervical spine without causing any damage to the other parts of the cervical spine, whereas in case of cervical spine injured individuals due to the discussed factors, it is difficult to stabilize the head while suffering with neck pain [32].

Hence, the concept of cervical orthoses/collars are introduced in 1950s by Blount [33] and Moe [34] as a non-operative orthotic device for spinal abnormalities. This orthosis was fabricated using stainless steel and leather, extended from pelvis to the back of neck. These collars are mainly recommended due to three reasons: provide support to the head, pain management, and head/neck position control. Based on these three parameters, numerous orthoses/collars were developed by the researchers such as hard collars, two and four-posture cervical orthosis, cervicothoracic orthosis and many others.

The cervical spine issues because of major accidents have received a lot of attention. Serious neck injuries may cause the harm of soft tissues surrounding the area of neck; if without appropriate care or treatment in time it might lead to death. In recent years, neck injury cases have been increased and fairly received a broad attention to thoroughly consider the issue and show effective solutions. The studies have illustrated that patients ignoring the pain or utilizing some relief techniques may help reduce the pain relatively; however, the pain often comes back more seriously. Rehabilitation considered as a non-invasive approach can resolve the pain and its key causes of the issue with a reasonable cost in the early treatment stage. In detail, non-invasive technique is usually a manual therapy which is utilized to possibly decrease the pain and boost the range of motion. With the support of physiotherapists, this technique of adjusting the cervical spine has been effectively implemented for numerous patients with neck and head disorders. The neck pain may also be together with unsteadiness and dizziness because of the age problem with the seniors, which may cause the posture instability as well as the head and eye movement control issue. In recent decade, the army helmets have elevated the weight and become the mounting platforms for the necessary combat equipment; in addition, for safety aim and related policies the motorcycle riders in most countries are obligatory to wear helmets when riding. Without correct and sufficient knowledge, the consumers have chosen helmets which have heavy weight utilizing the equipment for a long time, an individual may suffer from neck pain, and if this pain is not properly and timely treated it could boost the risk for serious neck injuries.

In general, field workers and office employees between 30 and 50 years old are the most impacted, especially when they become older. Many studies have analyzed that the major reasons for the work-related neck musculoskeletal disorders are thanks to the protracted static loads, very repetitive works, powerful efforts, dangerous postures, and not enough recovery, as well as the various combinations of these factors [35]. The neck disorders are a negative consequence of prolonged pain in the neck and shoulder muscles, once a field worker circumstance demands a significant force with two hands as holding and lifting the static loads and powerful manipulation that needs a large degree of stabilization at the shoulder and neck parts. In the case if this is repeated or performed for an extremely long time period, it usually causes serious shoulder-neck pain. Moreover, enforced postures during the work may be a high risk to the neck disorder. Some research has investigated the detailed relationship among the physical workloads on the arms and neck and the musculoskeletal disorders of the shoulders and neck in the workers. It can also be seen that the shoulder and neck disorders are related to the upper arm and head postures, their speed of motion and the muscular loads. Besides, some job types are closely related to the risk of shoulder and neck pain. Nowadays, neck pain can be considered to be one of the most significant forms of work incapacity. In fact, the neck musculoskeletal disorders related to work mostly occur in the individuals who have spent a lot of time to use computers frequently (for example, programmers, academic researchers, office employees, and software engineers). Nevertheless, this review article only focuses on the WADs, which could mimic head/neck motions and provide exercise for the individuals are explored. The upcoming section details about the traditional or prehospital setting recommended for the cervical spine injured patients during the rehabilitation process.

3. Cervical collars for cervical spine injury

Cervical Spine Immobilization is a pre-hospital setting commonly recommended in the case of Cervical Spinal Cord Injuries (SCI) during a traumatic incident. Immobilization of cervical spine is performed by cervical collars, which is a non-surgical process. These cervical collar/orthoses are external devices appropriately applied to the paralyzed segments or joints of the body to improve the function and stabilize the body part from dysfunction. Cervical collars are either used in the treatment for neck injuries or immobilization after the surgical procedures [36]. These collars are under continuous modified in aspects such as using light-weight materials and relatively comfortable while using the collars. Cervical Collars in particular are not flexible to fit the subjects head/neck dimensions, use of improper collar sizes might lead to over-extension and create separation among the cervical vertebrae. Cervical collars are widely used for treating trauma cases in emergency conditions. These collars stabilize and constraint head/neck motions to a single configuration, but users experience a great deal of discomfort as the head/neck is constrained to a single configuration for a longer period of time, and creates problems like immeasurable pain near the shoulders and also pressure on lower jaw [[37], [38], [39]].

Halo-vest a cranial-thoracic brace which is commonly used braces in treating patients with spine injuries. This brace consists of cranial fixation pins for reduction in the head/neck motion during daily activities. It has complications like loosening, pain, infection and skin breakdown at the pin contact region [40].

A comparative study was conducted between soft cervical collars and rigid cervical braces for validating of their effectiveness in restriction of neck motion. This study shows, that the soft cervical collars provide 82.7% of the uncontrolled neck movement and rigid cervical brace offer 37.1% of uncontrolled neck range of motion. The rigid cervical brace restricted its motion to 7.6° for flexion and the soft collar permitted 39.7° [41].

A study on healthy subjects, for validating the effects halo vest and cervical orthoses on the balance of the head. This study showed adverse effect caused by the halo-vest on the cervical muscle proprioception and lack of coordination in eye movement with respect to head movement. This effect was seen as minor in the case of healthy subjects but this impairment was greater in elderly patients. Post recovery of the patient was seen with problems in mobility, climbing stairs and reaching for the objects [42].

Sternal Occipital Mandibular Immobilizer (SOMI) collar, with rigid chest support with a manually adjustable chin platform. This brace provides moderate restriction on lateral bending and rotation. SOMI brace is preferred over Halo-vest is because it is less painful for the users [43].

Table 1 provides information about the existing static collars currently used for stabilizing the human head/neck, it is also observed that among the cervical collars soft collars are the most comfortable to wear and use, but does not have control over the involuntary motions. Hard collars are more durable and estimated to have 20%–25% of control over the head/neck motions than the soft collars. Cervical collars with chin, occiput or forehead straps/pads provided approximately 60% of head/neck motion control. It is estimated that these collars developed pressure ulcers in 38%–55% individuals, friction between facial hair, and ingrown facial hair.

Table 1. Cervical collars for stabilizing human head/neck.

Details of the Cervical Collars	Material	Advantages	Disadvantages
Soft Cervical Collar [44] (Weight: 181 g)	• High-density PU (polyurethane) foam, with very thick LDPE (Low-density polyethylene) sheet for uniform support and cushioning.	• It supports patient's cervical spine and head. • It can help realign cervical spine and relieve pain strains or sprains.	• Soft collars are soft and flexible but produce more heat, sweat, produces irritation and skin breakdown.
Rigid Collars [45]	• Plexiglass material.	• Advantages of these rigid collars are they confine motion in flexion and extension.	• Major drawbacks of these collars are that they could possibly increase/decrease mean interface pressure ranges from 0.3 to 9.1 mm Hg directly underneath the collar, ICP (Intra-Cranial Pressure) blood flow to brain, this could be fatal to the patient.
Philadelphia Cervical Collar [37] (Weight: 272 g)	• EVA-material is an elastomeric polymer that produces materials, which are “rubber-like”.	• This collar is used for preventing head movement post Cervical spine surgery.	• Restricts cervical spine flexion, extension and rotation. • Sensitive to heat in handling for a longer duration.
Aspen Cervical Collar [46] (Weight: 350 g)	• A plastic support is provided between the chin and chest to support the weight of the head, cushions are provided to comfort the user.	• This collar supports head/neck weight and aids in recovery of cervical spine and ligaments time to heal.	• This collar is a static collar which might cause muscular atrophy, loss of proprioception if worn for a longer duration.
Halo Vest [47] (Weight: 3.2 kg)	• Metal rods (the struts) are used for holding the patient's head.	• This brace is used for immobilizing the cervical spine post-surgery.	• Halo rings are placed around the forehead. A metallic ring is attached to the head with small pins that go into the bone of head. • Pin penetration into skull due to falls, and facial scarring at the pin site.
i2i head and neck positioning and support system [48] (Weight: about 1.5 kg)	• Durable wads are often made of closed-cell foam on a coated metal chassis, as well as tightly covered with a soft and durable stretch material.	• This support system helps for the user to hold the head and prevents from falling forwards or back.	• Limitation of this system is lower flexibility and also gets damaged if any external weight is added on the neck positioning system other than the user head weight.
The HeadMaster Collar [49] (Weight: 300 g)	• Padded tubular supports with PU (polyurethane) foam.	• Used for maintaining the up-right position of head in patients.	• This collar allows the user to restrict flexion motion because of the supports in the frontal region, but does not restrict extension motion because no additional supports were provided. • This could lead to over-extension of cervical spine for the ALS.
Cervigard [50] (Weight: about 1 kg)	• Nylon, polyester, EVA foam, steel hardware.	• Aids in treating Forward Head Posture (FHP) patients.	• This collar does not allow the user to have a comfortable jaw or chin motion, as the jaws attachments are used a hold the head from dropping. • This collar generates huge strain on the jaws, owing to arrangement as shown in the figure.
Non-Invasive Halo (NIH) [51]	• Padded carbon composite anterior-only chest plate. • A silicone pad 5 mm.	• This NIH is used to restrict the head/neck motion and protect the cervical spine and neck after surgery.	• This proposed NIH does have same disadvantage of conventional halos being loosened by the user, regularly requiring adjustments. • Male patients also have major problems during shaving which could result in ingrown facial hairs in the places concealed by the gel.

Based on the disadvantages of the static cervical collars/braces, researchers conducted study on developing mechanized collars which can address the aforementioned problems. The idea of incorporating mechanisms for mimicking head/neck motions also led to develop head/neck mechanisms for humanoid robots. Initially, humanoid robots neck has no DoF, it just has a rigid link to support the head of the humanoid. If the robot needs to look in different direction, the whole body of the robot requires to turn, which is similar to the person wearing a static or rigid collar. Hence, various mechanisms are developed and proposed for mimicking the head/neck motions. The importance of developing head/neck mechanisms for humanoid robots and also for the WADs are discussed in the following section.

4. Head/neck mechanisms

4.1. Robotic head/neck mechanisms

Humanoid robots or service robots are being researched in the area of medical and biomechanics. These service robots are built to mimic human movements and interact with the humans. In healthcare, the humanoid robots require human like motion (DoF) to aid the patients. Based on a study, it is found that the hospitalized children, Autism children responded positive to the treatment, as the robot mimic/copies the child head motions and also facial expressions [52]. Various facial robots are developed for interacting with children such as SAYA [53], KASPAR [54], ROMAN [55], HYDROïD [56], FLOBI [57], ATLAS [58], MARKO [59] and many others. Prior to this, the humanoid robot head is confined to look in the direction of motion and does not have any kind of head/neck motion, this sort of behavior in the humanoid robot would not be beneficial for Human Robot Interaction (HRI). Hence, researchers have not only concentrated on the facial robots, but also on the head/neck mechanisms which could provide mimic head/neck motions of the person. Numerous robotic head/neck mechanisms are proposed by the researchers, and a few of the head/neck mechanisms are listed below:

A 3-DoF serial mechanism with DC micro-motors was used as a part of the project Robot-Cub (iCub) [60]. A 4-DoF serial neck mechanism along with differential drive system, and allowed the robot to achieved an angular tilt of Pitch (±36°) Roll (±102°) Yaw (±49°) [61]. A 3-DoF cable-driven flexible parallel robot was designed to mimic human neck movements. This design consists of a moving platform connected to the fixed base using cables each separated 120° for each other, compression spring serves as cervical spine between the two platforms, and driven by motors located at the fixed platform [62,63]. A 2-DoF humanoid neck mechanism using a 3-legged Gough-Stewart platform, and mimicked head/neck motions such as tilt (35°), swing (14°) and pan (30°) [64]. A humanoid robot with 3-DoF neck mechanism was developed using RSSR (Revolute-Spherical-Spherical-Revolute) parallel configuration, driven by 2 motors [65]. A 3-DoF cable-driven neck mechanism was developed, where the length of each tendon is adjusted using three motors positioned at base, and motors are actuated using the PID (Proportional-Integral-Derivative) control to mimic head/neck motions [66].

In order to control the motion of the device, most research platforms have implemented the PID or proportional–derivative (PD) control strategy for better control over the device to minimize the positional error and achieve precise head/neck positions and ROM. Table 2 provides the various robotic active head/neck mechanisms and their ROM. From this table, it is understood that the DC motors, tendon/cable-driven and serial mechanisms are widely used in the humanoid robots in order to achieve maximum ROM. As recent development, complaint mechanisms and PAM actuators are incorporated in the head/neck mechanisms to mimic head/neck motions. But serial, cable-driven mechanisms achieved maximum ROM, when compared to the complaint and PAM-based head/neck mechanisms.

Table 2. Active head/Neck mechanism for robotic systems.

DoF	Control Mechanism	Range of Motions (ROM)
6-DoF	• Force based Spring Mechanism. • Tendon-driven mechanism.	• Maximum Pitch of 18°, and Roll angle of 10° [67]. • Flexion/Extension - ± 20°, Lateral Bending - ± 50°, and Rotation ±5° [68].
4-DoF	• DC motor based neck mechanism.	• Flexion/Extension - ± 30°, Lateral Bending - ± 40° [69].
3-DoF	• RXS, X-shaped compliant joint based parallel mechanism. • Cable driven neck mechanism. • McKibben pneumatic actuators muscles (PAMs).	• Flexion/Extension - ± 22°, Right/Left bending - ± 20°, Right/Left torsion - ± 28° [70]. • Pitch and Roll - ± 60° [71]. • Pitch - ± 30°, Roll - ± 25°, and Yaw - ± 27° [72].
2-DoF	• Serial chain 4-bar Mechanism. • Bevel gear based differential mechanism.	• Maximum nodding of 70°, and Roll angle of 10° [73]. • Flexion/Extension - ± 100°, Rotation - ± 90° [59].

Where, P- Prismatic, R- Revolute, S- Spherical.

4.2. Wearable assistive device (WAD) for head/neck rehabilitation

The robotic head/neck mechanisms are considered as a foundation for the development of head/neck rehabilitation WADs. Considering the developed mechanisms such as serial, chain, and differential gear mechanisms for designing WADs for rehabilitation purposes is not recommended, because as these mechanisms are heavy in structure, not portable, mechanisms like serial does not provide stability and comprises the repeatability and performance of the device [74]. As a result, a special kind of robotic manipulator/WAD are required for positioning with high precision. Therefore, devices with high precision and user-friendly mechanical systems are being developed to aid both doctor and patient.

Robotic-assisted rehabilitation is a recent approach to overcome the limitations seen in clinical rehabilitation. Implementing of robotics in rehabilitation field/assistive device will increase the number of training sessions with consistent repetition and reduce therapy cost. Recently, studies in closed-loop manipulators in the area of rehabilitation have been increased. The closed-loop/Stewart platform/Parallel manipulators provide better load-to-weight ratio, load distribution, high precise positioning, and better rigidity [[75], [76], [77]].

The use of parallel manipulators (PM) in the area of medical field increased owing to their precision and high stiffness. Parallel manipulators are also used in brain scanning operations. According to the previous research work on the parallel manipulators, PM's possess high load carrying capacity, effective dynamic performance, and precise positioning. Most of the parallel manipulators considered for the study are 6-DoF which are inherited from the Stewart-Gough platform [[78], [79], [80], [81]].

Studies on dynamic braces/rehabilitation/assistive devices are limited, owing to minimal space availability, and incorporating a mechanism between head and shoulder is difficult. However, there are a few WADs are developed for providing the head/neck movements are discussed below:

A 6-3 PM exoskeleton was proposed for human head motions and sEMG (Surface Electromyography) with signals offered as input to feedforward neural network to acquire necessary force and torque to actuate exoskeleton. This exoskeleton can be used for rehabilitation applications or a helmet mount display support for virtual reality applications as shown in Fig. 6 [82].

A passive neck orthosis was developed for patients suffering from with head/neck dysfunction is shown in Fig. 7. This orthosis allowed the patient to achieve 10° flexion, 40° extension and up to 40° of left and right rotation of the head movements. This brace couldn't provide lateral bending for the patient, and no study was not conducted for measuring the mechanical losses [83