Stimulation points were affected by two factors and its corresponding finger angles.

<p>(A) The effect of stimulation point displacement relative to the skin on the finger activation threshold and the selective range of each individual finger were investigated. (B) The negative electrode was placed at distances of 0L, 1L, 1.414L, and 2L away from the center of the original electrode to study whether different electrode positions caused the changes in the activation threshold and selective activation range of individual fingers. (C) The IMU sensor module was used to record the 3D finger movement angle data.</p

Forearm length and actual size corresponding to 2L (1/12 of the forearm length) for each of the 8 participants.

<p>Forearm length and actual size corresponding to 2L (1/12 of the forearm length) for each of the 8 participants.</p

Activation thresholds of several adjacent stimulus points under different forearm positions (Participant D).

<p>Activation thresholds of several adjacent stimulus points under different forearm positions (Participant D).</p

Electrode placement on the forearm for selective stimulation of finger extension/flexion

<div><p>It is still challenging to achieve a complex grasp or fine finger control by using surface functional electrical stimulation (FES), which usually requires a precise electrode configuration under laboratory or clinical settings. The goals of this study are as follows: 1) to study the possibility of selectively activating individual fingers; 2) to investigate whether the current activation threshold and selective range of individual fingers are affected by two factors: changes in the electrode position and forearm rotation (pronation, neutral and supination); and 3) to explore a theoretical model for guidance of the electrode placement used for selective activation of individual fingers. A coordinate system with more than 400 grid points was established over the forearm skin surface. A searching procedure was used to traverse all grid points to identify the stimulation points for finger extension/flexion by applying monophasic stimulation pulses. Some of the stimulation points for finger extension and flexion were selected and tested in their respective two different forearm postures according to the number and the type of the activated fingers and the strength of finger action response to the electrical stimulation at the stimulation point. The activation thresholds and current ranges of the selectively activated finger at each stimulation point were determined by visual analysis. The stimulation points were divided into three groups (“Low”, “Medium” and “High”) according to the thresholds of the 1^st activated fingers. The angles produced by the selectively activated finger within selective current ranges were measured and analyzed. Selective stimulation of extension/flexion is possible for most fingers. Small changes in electrode position and forearm rotation have no significant effect on the threshold amplitude and the current range for the selective activation of most fingers (<i>p</i> > 0.05). The current range is the largest (more than 2 mA) for selective activation of the thumb, followed by those for the index, ring, middle and little fingers. The stimulation points in the “Low” group for all five fingers lead to noticeable finger angles at low current intensity, especially for the index, middle, and ring fingers. The slopes of the finger angle variation in the “Low” group for digits 2~4 are inversely proportional to the current intensity, whereas the slopes of the finger angle variation in other groups and in all groups for the thumb and little finger are proportional to the current intensity. It is possible to selectively activate the extension/flexion of most fingers by stimulating the forearm muscles. The physiological characteristics of each finger should be considered when placing the negative electrode for selective stimulation of individual fingers. The electrode placement used for the selective activation of individual fingers should not be confined to the location with the lowest activation threshold.</p></div

The angle variation trends of the stimulation points in the "Low", "Medium" and "High" groups.

<p>The results are expressed as the means (<i>n</i> = 8). *<i>p</i> < 0.05, as determined by one-way ANOVA.</p

Coordinate system established for the determination of stimulation points of the finger extension and flexion.

<p>Coordinate system established for the determination of stimulation points of the finger extension and flexion.</p

The angle curve of finger A in its selective current range at a stimulus point.

<p>Note: T_a, T_b: Activation thresholds; <i>I</i>₁ = <i>ceil</i> (Ta); <i>I</i>₂ = <i>I</i>₁+ 1; <i>I</i>₃ = <i>I</i>₁ + 2.</p

Forearm length and actual size corresponding to 2L (1/12 of the forearm length) for each of the 8 participants.

<p>Forearm length and actual size corresponding to 2L (1/12 of the forearm length) for each of the 8 participants.</p

Activation thresholds (left) and selective ranges (right) of individual fingers.

<p>(*) represents a difference in the activation thresholds at different electrode positions. (†) indicates a significant difference in the selective activation thresholds of fingers at the same grid point in different forearm positions. The results are shown as the means ± SD (<i>n</i> = 8). *^, † <i>p</i> < 0.05 as determined by two-way analysis of variance.</p

Trends of angle variations of the thumb MCP and IP joints.

<p>The angles were tested in supine and neutral forearm postures.</p