Development of wearable rehabilitation device using parallel link mechanism: rehabilitation of compound motion combining palmar/dorsi flexion and radial/ulnar deviation
© The Author(s) 2018
Received: 31 January 2018
Accepted: 16 May 2018
Published: 29 May 2018
In recent years, the total number of physical therapists is increasing in Japan. However, it is not sufficient to nurse of patients requiring long-term care. In order to cope with the shortage of manpower, it is desirable to develop the rehabilitation equipment. This paper describes the development of a wearable wrist rehabilitation training device using the parallel link mechanism. It is possible to train the translational and rotational motion of the wrist joint by the adoption of parallel links. Training of the translational motion of the wrist joint has not been discussed in existing methods. Therefore, compared to existing methods, this method can be expected to reduce the burden on the wrist joint. And it is possible to move about 60% of the wrist joint movable range of motion. This device performs repetitive training to prevent contracture of the wearer’s joints. In experiments, assumed wrist circumduction motion was trained to the six subjects. The correlation coefficient between the target trajectory and the training result was obtained and evaluated whether correct operation was trained. The validity of the proposed method was demonstrated.
A questionnaire survey performed in 2014 by Japan’s Care Work Foundation concluded that not enough people are working in the field of physical therapy and similar occupations [1, 2]. To address this issue, efforts are underway to develop training equipment to support and assist therapists. The existing rehabilitation equipment  is often operated using a serial link mechanism. It tends to accumulate errors as the number of joints increases; hence, the serial link makes it difficult to provide multiple degrees of freedom (DOF) of movement to a joint, which limits its use in joints, such as wrist joints that require a high DOF of movement. To address these issues, efforts are underway to develop rehabilitation training devices that use parallel link mechanisms. The existing training devices that use parallel links [4, 5] utilize a pneumatic cylinder as an actuator, which can be difficult to position and is noisy because of the use of a pump. This study hopes to improve the ease of operation of the device by adopting a linear servomotor. Accordingly, we develop a wearable rehabilitation device using parallel link mechanism that uses a servomotor . We target the wrist joint because it is a major focus of rehabilitation, and the complexity of its range of motion is clear. In previous studies , the mounting position tended to be slightly misaligned because the measuring and training devices were separate, it causes a decrease in training effect. In this study, we improve the equipment by integrating the measuring and training devices, reducing the weight of the mounting part, and expanding the range of motion. We first describe a motion experiment targeted at assumed rehabilitation training for wrist joint circumduction movements using the developed device, and then address the effectiveness of the device based on a discussion of the experiment.
Required specifications of the wrist training device
The circumduction motion of the wrist joint targeted for rehabilitation training herein is the action of moving the palm in a circular motion starting from the distal end of the ulna, as shown in Fig. 1 . This “circumduction motion”, which is a combination of the palmar/dorsi flexion action and the radial/ulnar deviation action at the wrist joint, is a complex operation that simultaneously performs all the movements of the wrist joint, and has been adopted as a functional evaluation exercise for rehabilitation training. The device must satisfy all DOF of motion of the wrist joint to train the circumduction motion in the wrist joint. Furthermore, passive rehabilitation training requires the ability to move the palm, which typically weighs approximately 500 g (the body weight-corrected value for the palm assuming a body weight of 60 kg). The training device used in our proposed method satisfies the abovementioned conditions.
Maximum range of motion of the device and the human wrist joint
Palmar/dorsi flexion (°)
− 70 to 90
− 50 to 60
Radial/ulnar deviation action (°)
− 50 to 25
− 45 to 45
Pronation and supination movements (°)
− 90 to 90
− 45 to 45
This section provides an overview of the wearable parallel link-type rehabilitation device that developed.
Overview of the parallel link-type training device
Structure of the developed integrated training system
Specification of the training device
Maximum stroke speed
Actuator maximum acceleration
Rated thrust (total value)
Standard deviation at training (angle)
Standard deviation at training (position)
Calculation of the position and posture of the wrist
The x-axis rotation angle of the coordinate system Σ0 is θ roll , the y-axis rotation angle is θ pitch , and the z-axis rotation angle is θ yaw . The position and posture of the palm equipment are denoted by Q = [0Opx 0Opy 0Opz θ roll θ pitch θ yaw ]T, using 0Op = [0Opx 0Opy 0Opz]T, θ roll , θ pitch , and θ yaw , where 0Op is the coordinate of Op in the coordinate system Σ0, and the order of rotation is with respect to the z-axis(θ yaw ), the y-axis(θ pitch ), and the x-axis(θ roll ). The points C1–C6 of the palm equipment are defined as pC1 = [pCxi pCyi pCzi]T in Σp. The points C1–C6 of the palm equipment are defined as 0C1 = [0Cxi 0Cyi 0Czi]T in Σ0. The points A1–A6 of the forearm equipment are defined as 0A1 = [0Axi 0Ayi 0Azi]T in Σ0. 0A1 and pC1 are constants determined during the design process. The slider lengths |L1|–|L6| are variables that depend on Q.
Rehabilitation scenario experiment
This section describes the rehabilitation exercise experiment performed to evaluate the effectiveness of the developed device.
Purpose of the experiment
We conducted an experiment based on the rehabilitation training of the circumduction motion of the wrist joint using the developed parallel link-type training device to demonstrate the effectiveness of our method.
The experiment was conducted on the left hands of six male subjects aged 21–31 with no history of injury to the wrist joint (mean ± standard deviation: age 25.7 ± 4.7 years old, height 168.2 ± 10.2 cm). The purpose and content of the research were fully explained in accordance with the Declaration of Helsinki. Moreover, the subjects’ informed consent was obtained prior to the experiment.
Results and discussion
Results of the experiment
Discussion of the experiment
The measurement of the circumduction motion can be modeled as the combined motions of palmar/dorsi flexion (θ roll ) and radial/ulnar deviation (θ pitch ) based on the device structure. Focusing on temporal changes of θ roll and θ pitch of all subjects in Fig. 9, it is possible to confirm periodic increase and decrease. And it can be confirmed that the peaks of the two increase/decrease cycles are deviated. Circumduction is an operation to move the palm in a circular motion. By simultaneously and periodically carrying out the dorsiflexion flexion motion and the radicular flexion motion, the motion is realized. The measured motion is a reasonable motion. A similar change can be confirmed in the measurement result and the training result. For about 1–2 s from the start of motion, each subject exercises a movement different from the periodic motion. This is expected to be the radial flexion in the thumb direction which is done at the start of the experiment, it is considered to be a reasonable operation.
Correlation coefficient between measuring and training
Examining Fig. 9, θ yaw changed over time in a different manner for each subject. The circumduction motion, which rotates the palm, was achieved by a combination of actions of the forearm muscle group, including the palmaris longus in the forearm. These muscles also contribute to the realization of the pronation and supination movements; hence, it is natural to unconsciously perform both movements at the same time. The measurement results from the experiment also indicated that θ yaw varied across all subjects. A typical training device, with its low DOF of movement, does not support this motion, therefore, become a burden to the wearer. Hence, our method would seem to be more effective for training exercises, such as the circumduction motion that requires a high DOF of movement.
The experiment found some degree of correlation between θ roll and θ pitch across the subjects, but almost no correlation among θ yaw , θ roll , and θ pitch because of the variations in how the subjects used their muscles when performing the circumduction motion. Therefore, predicting θ yaw from the θ roll and θ pitch values to create the motion patterns in advance was difficult. This result suggests that methods, such as this device, which take measurements that include θ yaw (pronation and supination movements), are effective for conducting training exercises with a high DOF of movement of the wrist joint.
The operation width difference between measuring and training
θ roll (°)
θ pitch (°)
θ yaw (°)
This study described a developed parallel link-type wrist joint rehabilitation device. The developed device can train not only three degrees of freedom of rotation but also three degrees of freedom of translation in the wrist joint. Using the developed device, we carried out training assuming the circumduction motion to six subjects, and measured the movement of the palm when training. We calculate the correlation coefficient between the training target and the training result, and confirmed that this method is effective because high correlation is obtained. And, the performance of this method was evaluated by calculated the difference of the range of motion between training target and training result. Since the evaluation results, the necessity of force control is indicated, and the implementation of force control on this method becomes a future task. The necessity of temporal agreement between the training target and the training result has been indicated, implementation of speed control is also a future subject.
YK carried out the design of the wearable rehabilitation device, the systems integrating, testing. TT carried out the design of device control circuit and testing. KY provided advice on future works and proof-reading of the paper. All authors read and approved the final manuscript.
This research was supported by JSPS Grant-in-Aid for Scientific Research JP15K01552.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
The experiment was approved by the Ethics Committee of Utsunomiya University, and the committee’s directives, including the completion of consent forms, were complied with. Written informed consent was obtained from the patient for the publication of this report and any accompanying images.
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