The analysis was performed by calculating the performance outcomes of each trial (IQR or RMSE) and then pooling respective trial outcomes together across all able-bodied subjects, separately for each of the experimental scenarios (routine grasping, force steering) and conditions (bioFB, forceFB). All the results were reported as normalized forces, in fractions or percent, i.e., 100% corresponded to the maximum force of the prosthesis (~100 N). For the routine grasping scenario, the Bartlett multiple-sample test for equal variances was applied to test for statistically significant differences in dispersion (IQR) within the conditions overall, followed by Ansari-Bradley two-sample
test with Bonferroni correction for pairwise comparisons of the force variability between the conditions. The statistical analysis for the RMSE outcome of the force tracking task was performed using Wilcoxon signed rank test, as the data did not pass the normality test (one sample Kolmogorov-Smirnov). The threshold for statistical significance was adopted at p < 0.05.
3 R ESULTS
In the following paragraphs the results obtained from the two studies presenting the novel CASP system (Studies A and B, Appendix 1-2, [71], [72]) and the one study presenting the biofeedback (Study C, Appendix 4, [74]) will be shortly disseminated.
The results are reported in the format: mean ± standard deviation and are presented separately for able-bodied (figures and text) and the amputee subjects (text only).
3.1 Prosthesis Control (Studies A and B)
In Studies A and B, the CASP system was evaluated on a variety of objects (30 in total) in different positions and orientations, which presents a good database for evaluating the performance of the employed object modeling algorithm. The object size estimation error (SEE, Study A) and the orientation estimation errors (OEE, Study B) were 0.75 ± 1.1 cm (mean ± standard deviation) and 9 ± 5°, respectively.
3.1.1.1 Study A
Figure 3.1 summarizes the performance of the first CASP prototype for able-bodied subjects (PSR, TFR, and SFR) in the two test scenarios (AUTO and SEMI) which included 520 (13 subjects × 2 series × 20 objects) and 260 trials (13 subjects × 1 series
× 20 objects), respectively.
When the manual control was inactive (AUTO operation mode), the preshape success rate (PSR) was 79%. As soon as the user was allowed to correct for eventual errors (SEMI operation mode), the PSR improved significantly by 16% and reached 95%
(Figure 3.1a).
The task accomplishment outcome measures (TAR, SFR) are shown in the pie charts in Figure 3.1b. During the AUTO condition, the task was successfully completed in 73%
of trials. Successful completion of the task increased significantly to 81% in the SEMI condition. This improvement was due to a significant reduction of the system-induced task failures rate (SFR), which dropped from 10% to only 3%, whereas the myoelectric control induced failures remained virtually unchanged (16 – 17%).
Figure 3.1: Study A: a) Summary results for the CASP preshape success rate (PSR). b) Task accomplishment/failure rate (TAR/SFR) for the two operation modes: without (AUTO) and with manual user control (SEMI). Statistically significant differences are denoted by a star (*, p < 0.05). Image adapted from [71].
3.1.1.2 Study B
In total, 1360 grasping trials (10 subjects x 2 days x 4 conditions x 17 trials) were performed by able-bodied subjects. One amputee subject performed an additional 136 grasping trials (1 subject x 2 days x 4 conditions x 17 trials). All grasping trials were allocated evenly between the four control conditions (MAN1-3, and CASP). Figure 3.2 summarizes the results for able-bodied subjects.
The average time to grasp (TTG) for each of the four test conditions and training and evaluation sessions is shown in Figure 3.2a. During the evaluation session, the TTG in the manual control scenarios (MAN1-3) increased consistently with the number of controllable DoFs, i.e., it was 3.7 ± 1s for MAN1, 4.3 ± 1.7s for MAN2, and then
increased substantially to 11.2 ± 4.1s in MAN3. The differences were statistically significant between all MAN conditions. The TTG with CASP was 5.9 ± 1.9s, which was slower than in MAN1 and MAN2 but substantially faster than in MAN3.
Additionally, there was virtually no improvement between the training and evaluation sessions with CASP, contrary to the MAN2 and MAN3 conditions, which improved with training.
The recorded shoulder joint angles in MAN1 and 2 differed significantly in abduction and external rotation compared to the conditions with manual (MAN3) or automatic (CASP) rotation control. Between MAN1 and 2, the angles were similar, which also held for MAN3 versus CASP (for exact values please refer to the published article in Appendix 2, [72]). Representative shoulder configurations for MAN1 and CASP are depicted in Figure 3.2b when a cup was positioned horizontally (left picture) and vertically (right picture). Since the wrist rotation was inactive, in MAN1 the user had to perform extensive compensatory movements consisting of either shoulder abduction and external rotation (left picture) or adduction and internal rotation (right picture) in order to orient the hand appropriately for grasping the object. On the other hand, there were no such over-extensive movements when using CASP and in that case the shoulder angles remained virtually unaffected by the object orientations, since the automatic control adjusted the hand orientation accordingly, using the active wrist joint.
Similar trends were also observed in the amputee subject where the average TTG was 2.5 ± 1s, 4.7 ± 1.8s, 10 ± 2.2s, and 5.5 ± 1.8s for MAN1-3 and the CASP conditions respectively. The subject successfully learned how to use the system and, although he was an experienced user of a classic 1-DoF myoelectric prosthesis, the results were similar to the ones obtained for able-bodied subjects. The CASP system was approximately twice as fast as manual control for the same number of DoFs (MAN3).
Figure 3.2: Study B: a) Summary results for the average time to grasp (TTG) an object across conditions (MAN1-3, CASP) and experimental sessions (training, evaluation).
The statistically significant differences are denoted by a star (*, p < 0.05); the symbol
‘C‘ indicates that the difference exists across all conditions that were performed within the same experimental session. b) 3D model showing the arm positions recorded shortly before the object was grasped. An object placed horizontally (left) and vertically (right) was grasped using MAN1 and CASP control schemes. Image adapted from [72].
3.2 Prosthesis Feedback (Study C)
Summary results for ten able-bodied subjects and all conditions (force- and bio-feedback) and experimental tasks (routine grasping and force steering) are presented in Figure 3.3.
Providing EMG biofeedback significantly improved the consistency in generating grasping forces at all three force levels (Figure 3.3a). Without EMG biofeedback, the IQRs were 10%, 14% and 16% for the target force of 30%, 50%, and 70%, respectively,
and they were approximately twofold lower when EMG biofeedback was transmitted (i.e., 6%, 6%, and 7%, respectively). Likewise, with the force feedback, the force variability increased significantly for the higher target forces (forceFB [30%] vs.
forceFB [50%] and forceFB [70%]), which was not the case for the EMG biofeedback condition.
Figure 3.3:Sudy C: a) Routine grasping task performance in two feedback conditions (forceFB and bioFB) and at three target force levels (30, 50, and 70%). Boxplots depict the median (red line), interquartile range (blue box), maximal/minimal values (whiskers) and outliers (red crosses). Dashed gray lines are the target force levels. b) Force-tracking task performance. The toot mean square tracking error is given for two conditions (forceFB, and bioFB). Statistically significant differences are denoted by a star (*, p < 0.05). Image adapted from [74].
During the force steering task, providing EMG biofeedback reduced tracking error (Figure 3.3b). The decrease was modest but statistically significant (15.5 ± 2% for forceFB vs. 13.5 ± 2% for bioFB, p < 0.001).
The results for the two amputee subjects demonstrated a similar trend as in able bodied
subjects. The provision of the EMG biofeedback reduced the IQR of the generated forces from 13%, 9% and 16% for forceFB to 9%, 8%, and 10% for bioFB for the target forces of 30%, 50%, and 70%, respectively. The relative improvement was, however, less than in able-bodied subjects. The same trend can be observed in force tracking task: The tracking errors decreased from 16.8% and 18.8% in forceFB to 13% and 17.8% in bioFB for amputee 1 and 2, respectively.
4 D ISCUSSION
In this dissertation the contributions from four peer-reviewed publications addressing the topics of prosthesis control and feedback were presented. Specifically, in the publications presented in Appendix 1-2 ([71], [72]), the novel context and user aware system for prosthesis control (CASP) was developed and evaluated in 23 able-bodied and 1 amputee subject (Studies A and B). Likewise, in the publications presented in Appendix 3-4 ([73], [74]), closing the loop with prosthesis was addressed by presenting a comprehensive closed-loop framework (CLF) in which the novel biofeedback (bioFB) concept for enhancing myoelectric prosthesis control was developed and evaluated in 10 able-bodied and 2 amputee subjects (Study C). Hereafter, the relevant results and the methods from the four aforementioned publications will be summarized and discussed.