3.3 Digital Interface
For signal conditioning, signal dc offset was set to
2.5V, which was also chosen as the patient reference
voltage, given through the reference electrode.
Therefore, this voltage represents the signal ground
for the circuit.
Data were conditioned by an Arduino Micro
board, based on an ATmega32u4 (by Atmel
Corporation, San Josè, California, USA)
microcontroller device.
In order to maximize the communication speed,
data were stored in a 14 byte register (2 byte each
10-bit value), and send as binary data (Serial.write()
command) without ASCII conversion (Serial.print()
command) every one millisecond. Arduino Micro
sends its data to the computer at a speed of 400kbps.
A Matlab application was developed to record
and save the data in text files, setting the baud rate to
460800 bps, the buffer length, and specifying the
data length (two bytes always positive) according to
the script uint16. The code reads 100 records at the
same time, each corresponding to a 10-bit digitalized
voltage printed by Arduino on the serial port, and
plots them in real time. The effective voltage is
obtained from its digital value from the equation
V=5N/1023.
Further noise was detected on the resulting
signal, which was filtered through a digital band-
pass Butterworth filter, with a low cutoff frequency
of 20Hz, and a high cutoff frequency of 495Hz, to
attenuate the high frequency harmonics generated by
the sampling process. The root mean square (RMS)
value is then calculated on a window of 300
samples, shifting it by 75 samples each time.
4 sEMG MEASUREMENTS
In this section data registration and modeling of
sEMG static measurements of abduction/adduction
posture of the thumb, index and middle fingers are
presented. Data were acquired from six able-bodied
subjects, 3 male (M1,M2,M3) and 3 female
(F1,F2,F3), five right-hand and one left-hand, each
one using his/her dominant hand.
Measurement results from different subjects
show a remarkable spread. Then assessment of
sEMG activity needs to be each time calibrated on
the subject. A personal characterization session was
defined to this purpose, where the number of
measurements were reduced as much as a provided
tolerance is still guaranteed by the extracted model.
At this point, the measurement session related to a
particular task can be start.
4.1 Wise Test
Since the novelty of our approach, a new test to
evaluate the repeatability and reproducibility of
finger abduction/adduction movement assessment
was created. It was based on the Wise test provided
for flexion/extension measurement, used to evaluate
the performance of the electronic gloves (Gentner
and Classen, 2009; Dipietro et al., 2003).
It consisted in placing and re-placing the hand in
known postures always with the glove and sEMG
sensors donned, to evaluate measurement
repeatability, and placing and re-placing the hand in
known postures after donning and doffing the
sensors, to evaluate the measurement
reproducibility. In particular, the postures were a)
flat hand with closed fingers (starting posture), b)
flat hand with 20° thumb abduction, c) flat hand
with 10° index abduction, d) flat hand with 10°
middle abduction.
A further posture with the maximal voluntary
contraction (MVC) is also drawn for each finger.
The three abduction angles to be measured are far
from the MVC reported in Table 1, then easily
performed and repeated by each subject (Merletti et
al., 1990), which had to open the fingers up to the
chosen abduction angle and hold it for 2s, during
which the sEMG signals are registered, then back to
the starting posture, where the resting sEMG signals
are recorded. This task was repeated 10 times with a
rest interval of 10s between them. After this
sequence, the subject was asked to perform
abduction to the MVC for each finger, in order to
identify a regression of the sEMG signal intensity
against the abduction angle with three points, that is
0°, 10° and MVC. Finally a data block is created.
This procedure is repeated 10 times with a resting
period of 3min each time. In order to evaluate the
repeatability, the same sEMG sensors were used to
measure the two positions (0°-10°), performing task
A-C, whereas the sEMG sensor were changed after
each sequence to evaluate the reproducibility (task
B-D).
Table 1: MCV values for index and middle abduction.
subject M1 M2 M3 F1 F2 F3
index 25 25 30 30 15 20
middle 25 25 30 30 25 20
Each block is composed of 2000 elements, obtained
from the RMS value of the signal samples registered