When a person moves or presses with an individual finger other fingers also produce a force (Kilbreath and Gandevia 1994; Li et al. 2004; Zatsiorsky et al. 2000). Several factors are known to contribute to this response: (1) peripheral mechanical coupling, (2) multi-digit motor units, and (3) diverging central commands. This phenomenon, known as enslaving, has traditionally been studied in isometric pressing tasks. The purpose of this project was to build a device, an Inverse Piano (IP), to study finger interaction in non-isometric pressing tasks. The IP allows for fingers to be unexpectedly raised or lowered during pressing tasks. Fingers are perturbed by linear motors located directly under uni-dimensional force sensors, which serve as the “piano keys”. Motors are triggered using National Instruments LabVIEW. This allows key position and finger force data to be recorded simultaneously. The IP makes possible the studying of several factors on the finger force outcome and coordination. In particular, the following factors can be explored: (a) Finger combination. There are 15 combinations of the key manipulation: four 1-finger tasks (I, M, R, L, where the letters designate the index, middle, ring, and little finger respectively); six 2-finger tasks (IM, IR, IL, MR, ML, RL); four 3-finger tasks (IMR, IML, IRL, MRL) and one 4-finger task (IMRL). (b) Predictability of the key raising. The options are innumerable but can be roughly classified into three groups: (1) both the sequence and time intervals are unknown to the subjects; (2) the sequence is known but the time intervals are unknown; and (3) both the sequence and time intervals are known in advance. (c) Amplitude of key movement. The IP is capable of displacing fingers up to 2 cm, in increments less than 1 mm. (d) The speed of key movement. The IP can vary key movement rates of between 2 mm/s to 4,687 mm/s. (e) Resistance of the keys to the external force. The resistance can mimic different mechani--cal properties, e.g. elastic reistance which is proportional to the key displacement, damping resistance proportional to the speed, dry friction, etc. The magnitude of the resistance, e.g. ‘stiffness’, can also be varied. (f) Feedback with various options: (1) visual feedback on the computer screen, the subject can also see his/her hand; (2) no visual feedback on the screen, however the subject can see his/her hand; and (3) no feedback on the screen, the subject cannot see his/her hand. Thus far experimentation using IP has only investigated effects of varying magnitude of displacement.
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