User capacities and operation forces: Requirements for body-powered upper-limb prostheses

In the Netherlands approximately 3750 persons have an arm defect: they miss (part of) their hand, forearm or even their entire arm. The majority of these people are in the possession of a prosthesis. This prosthesis can be purely cosmetic, or offer the user some grasping function. The latter can either be a body-powered or a myo-electric prosthesis. A myo-electric prosthesis is controlled by electrical signals originating from the contraction of muscles of the user and is powered by electric motors. Body-powered prostheses are operated by body movements, which are captured by a harness and transmitted through a Bowden cable to the prehensor. Unfortunately, 23-45% of the users are so dissatisfied with their chosen prosthesis that they decide not to wear it. Thus, prostheses need to be improved.This thesis focuses on the improvement of body-powered prostheses, which offer several advantages compared to myo-electric prostheses: they are much lighter, cheaper and more reliable and – perhaps most importantly – offer the user extended proprioceptive feedback about the prehensor’s movements and exerted grip force. On the down side, body-powered prostheses currently require high operation forces, causing pain and fatigue during or after use, and potentially limiting the inherent advantages in perception and control. Additionally, users complain about the comfort and outer appearance of the harness, the design of which still looks like that of the Count of Beaufort in 1860.Lowering the operation forces will most likely increase the pinch force control accuracy and reduce fatigue and pain during or after operation and therefore improve the prosthesis’ functionality. To which level cable forces need to be lowered is up till now unknown; it is assumed that lowering operation forces is effective, but only up the point where the control forces are still clearly distinguishable from noise (like inefficiencies in prehensor or cable friction).The goal of this thesis is to quantify the perception and control capabilities of prosthesis users as a function of body-powered prosthesis design elements, such as mechanical properties of the prehensor, or an alternative harness. The obtained quantified understanding is intended to guide improvements in body-powered prosthesis design, to enhance the quality of life of upper-limb prosthesis users and to prevent (repetitive strain) injuries.First, a range of maximum cable operation forces between 87 N and 538 N was established for a representative group of prosthesis users (Chapter 2). When the corrected values for fatigue-free operation (20% of the individually measured maximum force) were compared to the required operation forces of ten commercially available body-powered prostheses, it was concluded that only one of these could be operated fatigue-free. Based on the available results, cable forces should not exceed 38 N for the average female, and 66 N for the average male for most activities in daily life, to enable users to operate their prosthesis fatigue-free.A second study investigated the effect of cable operation forces (15 N versus 51 N) on the ability to transport a test object (Chapter 3). The object was a mechanical egg: too high cable forces would ‘break’ the object; too low cable forces would cause the operator to drop it. The results indicated that the egg was transferred successfully more often at the low cable operation force settings than at the high force setting.A third study investigated users’ perception and control abilities by utilizing a force reproduction task (Chapter 4). For successful object manipulation we desire a small difference between the intended and actually applied force on an object, as well as only minor fluctuations in the applied force level. In a force reproduction task the force reproduction error resembles the difference between the intended and actually applied force, whereas the force variability indicates the force fluctuations. The results showed a decreasing force reproduction error with increasing cable excursions for force levels of 10 and 20 N, and a decreasing force variability for decreasing operation force levels varying between 10 and 40 N. Thus, low force levels and large cable excursions contribute to improved force perception and control.In the fourth and final study an alternative harness design, the Ipsilateral Scapular Cutaneous Anchor System, was compared with the traditional figure-of-nine harness, as comfort of the harness was identified as being an issue in body-powered prosthesis (Chapter 5). In terms of perception and control capacities of users no differences between the two systems were found for operation forces ranging from 10 to 40 N. It could thus be concluded that the Anchor system appears to be a valid alternative to the traditional harness at low operation force levels as performance is comparable while comfort is reportedly better.In conclusion, this thesis shows that the operation forces which prosthesis users are required to exert are an important factor in body-powered prosthesis design. For most commercially available body-powered prostheses, the control cable forces are too high to be used on a daily basis. To enable users to operate a body-powered prosthesis fatigue-free during the day ‒ every day – with the provision of high quality feedback and adequate prehensor control, operation forces should not exceed 38 N for the average female and 66 N for the average male user. A long operation movement stroke and thus a large cable excursion does contribute to increased prehensor control. For the suggested low operation force levels the Ipsilateral Scapular Cutaneous Anchor System provides a good alternative for the traditional harness.Biomechatronics & Human-Machine Contro

Feedback in voluntary closing arm prostheses: Investigation of optimal force feedback in shoulder controlled arm prosthesis operation

High rejection rates indicate that prosthetic users are not satisfied with the performance of their arm prostheses. In theory, one of the advantages of shoulder controlled prosthesis, compared to myo-electric prostheses, is that the user gets direct proprioceptive feedback about opening width and pinch force of his prosthetic hand. However, commercially available voluntary closing prostheses require too high operating forces, which leads to discomfort and disturbs the direct proprioceptive feedback. The purpose of this study is to find an optimal operation force, at which the prosthetic user gets the best force feedback during comfortable prosthesis operation. During experimental research, subjects where asked to reproduce a certain reference force, with and without a visual representation of the force produced. The subject’s performances of blind generated forces regarding the reproducibility, stability and repeatability were evaluated to find an optimal cable force. The performances of male and female subjects, as well as the performances of subjects with and without arm defect were compared. As a result the optimal operation force level was found between 20 and 30 N for male and female subjects without arm defect. No differences in stability and repeatability performance were found between subjects with and without an arm defect. However, subjects with arm defect were found to have difficulties to reproduce high force levels. In line with this, the reproducibility optimum was found between 10 and 20 N for subjects with arm defect. It is concluded that of today’s commercially available arm prostheses only one is capable of creating pinch forces at the optimal cable activation force level of 20-30 N. The created pinch forces with this prosthesis are not sufficient to handle objects in daily life. Future prosthesis design should not exceed cable forces of 30 N when realizing the desired pinch forces for daily activities. Therefore transmission ratios or servo mechanisms might be needed to optimize prosthesis design.BMD/BCDBioMechanical EngineeringMechanical, Maritime and Materials Engineerin

Ipsilateral Scapular Cutaneous Anchor System: An alternative for the harness in body-powered upper-limb prostheses

Background: Body-powered prosthesis users frequently complain about the poor cosmesis and comfort of the traditional shoulder harness. The Ipsilateral Scapular Cutaneous Anchor System offers an alternative, but it remains unclear to what extent it affects the perception and control of cable operation forces compared to the traditional shoulder harness.Objective: To compare cable force perception and control with the figure-of-nine harness versus the Ipsilateral Scapular Cutaneous Anchor System and to investigate force perception and control at different force levels.Study design: Experimental trial.Methods: Ten male able-bodied subjects completed a cable force reproduction task at four force levels in the range of 10–40 N using the figure-of-nine harness and the Anchor System. Perception and control of cable operating forces were quantified by the force reproduction error and the force variability.Results: In terms of force reproduction error and force variability, the subjects did not behave differently when using the two systems. The smallest force reproduction error and force variability were found at the smallest target force level of 10 N.Conclusion: The Anchor System performs no differently than the traditional figure-of-nine harness in terms of force perception and control, making it a viable alternative. Furthermore, users perceive and control low operation forcesbetter than high forces.Biomechatronics & Human-Machine Contro

Fatigue-free operation of most body-powered prostheses not feasible for majority of users with trans-radial deficiency

Background: Body-powered prostheses require cable operation forces between 33 and 131 N. The accepted upper limit for fatigue-free long-duration operation is 20% of a users’ maximum cable operation force. However, no information is available on users’ maximum force. Objectives: To quantify users’ maximum cable operation force and to relate this to the fatigue-free force range for the use of body-powered prostheses. Study design: Experimental trial. Methods: In total, 23 subjects with trans-radial deficiencies used a bypass prosthesis to exert maximum cable force three times during 3 s and reported discomfort or pain on a body map. Additionally, subjects’ anthropometric measures were taken to relate to maximum force. Results: Subjects generated forces ranging from 87 to 538 N. Of the 23 subjects, 12 generated insufficient maximum cable force to operate 8 of the 10 body-powered prostheses fatigue free. Discomfort or pain did not correlate with the magnitude of maximum force achieved by the subjects. Nine subjects indicated discomfort or pain. No relationships between anthropometry and maximal forces were found except for maximum cable forces and the affected upper-arm circumference for females. Conclusion: For a majority of subjects, the maximal cable force was lower than acceptable for fatigue-free prosthesis use. Discomfort or pain occurred in ~40% of the subjects, suggesting a suboptimal force transmission mechanism. Clinical relevance: The physical strength of users determines whether a body-powered prosthesis is suitable for comfortable, fatigue-free long-duration use on a daily basis. High cable operation forces can provoke discomfort and pain for some users, mainly in the armpit. Prediction of the users’ strength by anthropometric measures might assist the choice of a suitable prosthesis.Biomechatronics & Human-Machine Contro

A need for a more user-centered design in body powered prostheses

Users of body powered prostheses (BPP) complain about too high operating forces, leading to pain and/or fatigue during or after prosthetic operation. In the worst case nerve and vessel damage can occur [1, 2], leading to nonuse of prostheses. Smit et al. investigated cable forces and displacements required to operate commercially available voluntary closing and voluntary opening hands and hooks [3, 4]. The capacities of prosthetic users to operate these terminal devices remain unknown. Taylor reported in 1954 forces and displacements measured with 50 ‘normal’ subjects for arm flexion (280±24 N; 5.3±1.0 cm), shrug (270±106 N; 5.7±1.5 cm) and arm extension (251±29 N; 5.8±1.7 cm) (mean±SD) [5]. Unfortunately, the measurement procedure is unclear. Moreover, the study reported forces and displacements from isolated movements instead of combinations of movements typically used for BPP operation. Our recent pilot experiments on 10 male subjects (28±2 years old) also without arm defects using a BPP harness revealed average values of 475 N and a peak value of 970 N for one subject. Although these values are higher, it remains unclear if these force levels are sufficient to comfortably operate a BPP, or too low leading to non-use. Importantly, knowing the capacities and limitations of prosthetic users will aid in choosing and redesigning future BPPs to prevent non-use.Biomechatronics & Human-Machine Contro

Perception and control of low cable operation forces in voluntary closing body-powered upper-limb prostheses

Operating a body-powered prosthesis can be painful and tiring due to high cable operation forces, illustrating that low cable operation forces are a desirable design property for body-powered prostheses. However, lower operation forces might negatively affect controllability and force perception, which is plausible but not known. This study aims to quantify the accuracy of cable force perception and control for body-powered prostheses in a low cable operation force range by utilizing isometric and dynamic force reproduction experiments. Twenty-five subjects with trans-radial absence conducted two force reproduction tasks; first an isometric task of reproducing 10, 15, 20, 25, 30 or 40 N and second a force reproduction task of 10 and 20 N, for cable excursions of 10, 20, 40, 60 and 80 mm. Task performance was quantified by the force reproduction error and the variability in the generated force. The results of the isometric experiment demonstrated that increasing force levels enlarge the force variability, but do not influence the force reproduction error for the tested force range. The second experiment showed that increased cable excursions resulted in a decreased force reproduction error, for both tested force levels, whereas the force variability remained unchanged. In conclusion, the design recommendations for voluntary closing body-powered prostheses suggested by this study are to minimize cable operation forces: this does not affect force reproduction error but does reduce force variability. Furthermore, increased cable excursions facilitate users with additional information to meet a target force more accurately.Biomechatronics & Human-Machine ControlHuman-Robot Interactio