3.3 Optimized Design Structure
3.3.1 Hand flexion/extension experiment
The hand rehabilitation system in this research is exercising the patient’s impaired fingers with natural motions repetitively and intensively. Therefore, the research for the finger trajectory gen- erated by the rehabilitation system is important issue during the design process of the exoskeleton structure.
Previous studies for finger motions were reviewed to investigate a suitable trajectory of the fin- ger F/E motion. Kamper et al. studied the fingertip trajectory when the subjects grasped a variety of objects. [103] They found that the DIP joint angle shows linear relation to the PIP joint angle, however, there was no specific dependency between the MCP and PIP joints because the human can control two joints independently. Conti et al. captured the joint trajectory of a normal person and designed an exoskeleton for the hand following the normal person’s trajectory [104]. In addition, Yang et al. researched a finger exoskeleton using the tendon mechanism based on the relationship between finger joints [105]. When a normal person moved the index finger, the researchers mea- sured the angles of the three joints in F/E motion and investigated the their relationships during motions. Since the finger joints were adapted to the size and shape of the objects for stable grasp, investigating the specific relationship between the angles of MCP and PIP joint during grasping var- ious objects is difficult. Furthermore, since the most of people have their own habits according to the specific hand anatomy, it is hard to obtain the similar motions by many people. Therefore, many researchers were used one normal person’s motion to design the finger trajectory of the rehabilita- tion structure, but it is difficult to verify whether the motion by only one person with the individual habit is good for the rehabilitation of the patient’s finger. Thus, the hand F/E motion experiment by a variety of people was conducted to investigate the general relationship between the MCP and PIP joint angles.
First of all, five subjects with normal hands were participated in the hand flexion/extension experiment. They wore a finger motion measurement glove with inertial measurement unit (IMU) on right hand and flexed/extended the hand without specific motion instructions. After all subjects’
movements were measured, the MCP and PIP joint angles were calculated separately.
Figure 3-3 shows the MCP and PIP joint angles during the hand flexion/extension without mo- tion instructions. Although the curved fit was obtained using a first-order equation, the angle data
Figure 3-3: Hand flexion/extension experiment without motion instructions
were distributed throughout the ROM of the MCP and PIP joints and did not follow the curved fit well. Since the MCP and PIP joints can move independently, the instructions to minimize the individual habits should required. Therefore, a new experiment for measuring hand motions was conducted.
The subjects participated in this experiment should followed the instruction described in Fig. 3- 4 (a) to eliminate the effect of the individual habits when they grasping the fingers. The figure shows three postures of the hand: flexion, relaxed hand, and extension. The relaxed hand shows a partially flexed posture, induced by the passive recoil force generated by flexor digitorum profundus (FDP) [20]. To perform the grasping motion according to the instruction, the subject relaxed his/her hand from the flexion posture and made minimal force to move their fingers to the extension posture.
By contrast, the subject with an extended hand relaxed his/her hand, then moved the hand to a flexion posture. By using the minimum force and natural contraction of the fingers, the potential for idiosyncratic motions was minimized. Therefore, by this instructions for hand F/E motion, the general finger motions with similar tendency were obtained although many subjects participated the experiment.
In the hand F/E experiment, four subjects (aged: 25.3±1.71 years) participated while following the given instructions. The subjects proceeded to flex/relax/extend their hands for 1 min per trial while attaching the markers for capturing the finger motions at the joints of the index finger. There were three trials with 1 minute rest between trials. The positions of the markers were captured by the motion capture system (Prime 13, Optitrack, USA) during the experiment and the finger joint
Flexion Relaxed Extension hand
Moving the fingers using minimal force Relaxing the hand
Relaxing the hand Moving the fingers
using minimal force
(a) Instructions for hand flexion/extension
(b) Relationship between MCP and PIP joint angles
Figure 3-4: Hand flexion/extension experiments
angles of the subjects were analyzed. All experiments were conducted with the approval of the institutional internal review board (IRB) (IRB approval number: UNISTIRB-17-23-A).
Figure 3-4 (b) presents the experimental results for MCP and joint angles performed by the subjects. The black dots and the red line represent the obtained joint angles by the subjects and its polynomial curve fitted to the experimental data, respectively. The polynomial fit followed the curvilinear trend and the finger motions by the subjects were distributed around the polynomial curve. Therefore, the equation for the obtained polynomial in the experiment was as follows:
y =−0.0004239x3 + 0.0392x2 + 0.8507x (3.1)
wherexandyare the MCP and PIP joint angles, respectively. The ROM by the subjects is presented in Table 3.1.
Table 3.1: The obtained ROM from the experiment Joint Range of motion (ROM)
MCP (◦) 0 - 70
PIP (◦) 0 - 106