Joel R. Martin

Interaction of finger enslaving and error compensation in multiple finger force production

Previous studies have documented two patterns of finger interaction during multi-finger pressing tasks, enslaving and error compensation, which do not agree with each other. Enslaving is characterized by positive correlation between instructed (master) and non-instructed (slave) finger(s) while error compensation can be described as a pattern of negative correlation between master and slave fingers. We hypothesize that pattern of finger interaction, enslaving or compensation depends on the initial force level and the magnitude of the targeted force change. Subjects were instructed to press with four fingers (I index, M middle, R ring, and L little) from a specified initial force to target forces following a ramp target line. Force–force relations between master and each of three slave fingers were analyzed during the ramp phase of trials by calculating correlation coefficients within each master–slave pair and then two-factor ANOVA was performed to determine effect of initial force and force increase on the correlation coefficients. It was found that, as initial force increased, the value of the correlation coefficient decreased and in some cases became negative, i.e. the enslaving transformed into error compensation. Force increase magnitude had a smaller effect on the correlation coefficients. The observations support the hypothesis that the pattern of inter-finger interaction—enslaving or compensation—depends on the initial force level and, to a smaller degree, on the targeted magnitude of the force increase. They suggest that the controller views tasks with higher steady-state forces and smaller force changes as implying a requirement to avoid large changes in the total force.

Optimization and Variability of Motor Behavior in Multifinger Tasks: What Variables Does the Brain Use?

ABSTRACT The neural control of movement has been described using different sets of elemental variables. Two possible sets of elemental variables have been suggested for finger pressing tasks: the forces of individual fingers and the finger commands (also called finger modes or central commands). The authors analyzed which of the 2 sets of the elemental variables is more likely used in the optimization of the finger force sharing and which set is used for the stabilization of performance. They used two recently developed techniques—the analytical inverse optimization (ANIO) and the uncontrolled manifold (UCM) analysis—to evaluate each set of elemental variables with respect to both aspects of performance. The results of the UCM analysis favored the finger commands as the elemental variables used for performance stabilization, while ANIO worked equally well on both sets of elemental variables. A simple scheme is suggested as to how the CNS could optimize a cost function dependent on the finger forces, but for the sake of facilitation of the feed forward control it substitutes the original cost function by a cost function, which is convenient to optimize in the space of finger commands.

Effects of the index finger position and force production on the flexor digitorum superficialis moment arms at the metacarpophalangeal joints - a magnetic resonance imaging study.

BACKGROUND The purpose of this study was to use magnetic resonance imaging to measure the moment arm of the flexor digitorum superficialis tendon about the metacarpophalangeal joint of the index, middle, ring, and little fingers when the position and force production level of the index finger was altered. A secondary goal was to create regression models using anthropometric data to predict moment arms of the flexor digitorum superficialis about the metacarpophalangeal joint of each finger. METHODS The hands of subjects were scanned using a 3.0 T magnetic resonance imaging scanner. The metacarpophalangeal joint of the index finger was placed in: flexion, neutral, and extension. For each joint configuration subjects produced no active force (passive condition) and exerted a flexion force to resist a load at the fingertip (active condition). RESULTS The following was found: (1) The moment arm of the flexor digitorum superficialis at the metacarpophalangeal joint of the index finger (a) increased with the joint flexion and stayed unchanged with finger extension; and (b) decreased with the increase of force at the neutral and extended finger postures and did not change at the flexed posture. (2) The moment arms of the flexor digitorum superficialis tendon of the middle, ring, and little fingers (a) did not change when the index metacarpophalangeal joint position changed (P>0.20); and (b) The moment arms of the middle and little fingers increased when the index finger actively produced force at the flexed metacarpophalangeal joint posture. (4) The moment arms showed a high correlation with anthropometric measurements. INTERPRETATION Moment arms of the flexor digitorum superficialis change due to both changes in joint angle and muscle activation; they scale with various anthropometric measures.

Changes in the flexor digitorum profundus tendon geometry in the carpal tunnel due to force production and posture of metacarpophalangeal joint of the index finger: An MRI study

BACKGROUND Carpal tunnel syndrome is a disorder caused by increased pressure in the carpal tunnel associated with repetitive, stereotypical finger actions. Little is known about in vivo geometrical changes in the carpal tunnel caused by motion at the finger joints and exerting a fingertip force. METHODS The hands and forearms of five subjects were scanned using a 3.0 T magnetic resonance imaging scanner. The metacarpophalangeal joint of the index finger was placed in: flexion, neutral and extension. For each joint posture subjects either produced no active force (passive condition) or exerted a flexion force to resist a load (~4.0 N) at the fingertip (active condition). Changes in the radii of curvature, position and transverse plane area of the flexor digitorum profundus tendons at the carpal tunnel level were measured. RESULTS The radius of curvature of the flexor digitorum profundus tendons, at the carpal tunnel level, was significantly affected by posture of the index finger metacarpophalangeal joint (P<0.05) and the radii was significantly different between fingers (P<0.05). Actively producing force caused a significant shift (P<0.05) in the flexor digitorum profundus tendons in the ventral (palmar) direction. No significant change in the area of an ellipse containing the flexor digitorum profundus tendons was observed between conditions. INTERPRETATION The results show that relatively small changes in the posture and force production of a single finger can lead to significant changes in the geometry of all the flexor digitorum profundus tendons in the carpal tunnel. Additionally, voluntary force production at the fingertip increases the moment arm of the FDP tendons about the wrist joint.

Inverse piano technique for studying finger interaction during pressing tasks

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 mechanical 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.