## 5 Discussion
### Comparison between the Modular Actuator and the Three-Chamber Actuator
#### 5.2.2 Multi-DoF Support Performance  
When using the identical single-chamber actuator to assist the four-finger flexion, the 3C-ACT achieved all the Kapandji scores (0-8, Figure 7a); however, the M-ACT was unable to make the thumb come in contact with the small finger (a Kapandji score of 0-5 and 8, Figure 7b). The result of the Kapandji test is affected by two factors: first, during the measurement of the two actuators, the four-finger flexion actuator may flex the four-finger to different angles, and second, the M-ACT may be inferior in thumb opposition support. Table 1 shows that when using the M-ACT and 3C-ACT to perform thumb-finger opposition motions, the four-finger flexion actuator was inflated to similar air pressure ranges. Accordingly, the two actuators had to flex and abduct the thumb to similar angles during the Kapandji test. Therefore, the latter would be the reason for the lower Kapandji score of the M-ACT. Unlike the air chambers in the 3C-ACT, which are in charge of the actuator bending in identical directions (the middle chamber for flexion, the left side-chamber for abduction, and the right side-chamber for adduction), each chamber of the M-ACT's CMC unit is responsible for the bending in two directions (the left module is responsible for flexion and abduction, and the right module for flexion and adduction). There would be two strategies for manually inflating the M-ACT for realizing thumb opposition movements. One strategy is to inflate the left module first to abduct the thumb fingertip to the same height as the four-fingertip, then inflate the right module to flex the CMC joint (Figure 1A). The pressurization of the right module did not flex the CMCjoint on the basis of a certain degree of abduction; instead, it sacrificed the abduction for flexion. The air pressure in the two modules must be adjusted repeatedly until the thumb's fingertip reaches the objective position, which is a difficult procedure to complete manually, as even a minor alteration at the CMC joint would deviate the fingertip (distal part) from the objective position. The other strategy is to first inflate the two modules for CMC flexion and then increase the air pressure in the left module for thumb abduction (Figure A1B). Either strategy is difficult to achieve a steady and fluent thumb-small opposition support, especially for postures that require both large thumb flexion and abduction because the deformation of the CMC unit in different DoF is not independent. The 3C-ACT, however, is simpler to manipulate. We could adopt either of the above-mentioned strategies to achieve thumb opposition with a less air tuning procedure because its deformation in different DoF is independent (additive), but not mutually depleting. Accordingly, the three-chamber structure of the 3C-ACT may be more beneficial to thumb opposition (multi-DoF) support. For the same reason, the 3C-ACT can realize all three types of thumb-small positions (Figure 8a), whereas the M-ACT can only realize the first type of thumb-small opposition, which only requires thumb flexion (Figure 8b). Notably, both side chambers of the 3C-ACT were inflated to facilitate CMC joint flexion; otherwise, type 1 and type 2 thumb-small oppositions are impossible to achieve. Though the required angle for CMC joint flexion was 13\({}^{\circ}\) (Table 1), a rather minor value, it is critical for the thumb to make contact with the finger or position that is away from it. Since the 3C-ACT outperformed the M-ACT in thumb opposition support and the M-ACT was proven to be proficient at single-DoF support in Section 5.2.1, we kept the 3C-ACT as the thumb actuator and utilized the M-ACT as the four-finger flexion actuator. This combination of 3C-ACT and M-ACT achieved all the Kapandji scores (Figure 7c) and a stronger pinch force than the other two combinations, especially for thumb-index pinch and thumb-middle pinch (Table 2).