566 IEEE TRANSACTIONS ON HUMAN-MACHINE SYSTEMS, VOL. 47, NO. 4, AUGUST 2017
TABLE II
D
EMOGRAPHIC CHARACTERISTICS OF THE THREE AMPUTEE SUBJECTS
Subject ID Gender Age (years) Affected side Residual stump length Cause of amputation Time since amputation Prosthesis usage
A1 Male 37 Right 23 cm Tumor 9 years All day, cosmetic
A2 Male 38 Right 16 cm Traumatic 10 years Half day, myoelectric
A3 Male 62 Left 16 cm Traumatic 9 y ears Half day, cosmetic
Fig. 2. Placement of hybrid EMG/NIRS sensors. (a) Targeted muscles for
deploying sensors; channels 1 and 2 are attached to FCU and FCR, respectively;
channels 3 and 4 are placed on ECRL and ED, respectively. (b) Sensor placement
of amputee subject A1. Anterior and posterior views of the stump for (c) A1
and (d) A2.
informed consents before the experiment and the procedures
accorded with the declaration of Helsinki.
The experiment included two separate parts. The first part
was for the offline evaluation of the sensor fusion approach that
combined the EMG and NIRS signals. In the second part, the
subjects performed the online Motion Test [11] for assessing the
real-time performance of controlling a virtual prosthetic hand.
Ten of the 13 able-bodied subjects and the three amputee sub-
jects completed the second part, and the time interval between
these two parts ranged from several days to three months.
B. Offline Data Acquisition and Pattern Classification
1) Sensor Placement: For every subject, four hybrid
EMG/NIRS sensors [39] were attached above flexor carpi ul-
naris (FCU), flexor carpi radialis (FCR), extensor carpi ra-
dialis longus (ECRL), and extensor digitorum (ED), respec-
tively, by using double-sided adhesive tapes, as shown in Fig. 2
(skin treatment with alcohol before the attachment). The hybrid
EMG/NIRS sensor integrated a bandpass filter (20–450 Hz) for
an EMG signal and a l ow-pass filter (0–300 Hz) for an NIRS
signal to attenuate the effects of unwanted noises. The EMG
and NIRS signals were amplified with gains of 500 and 1.3,
respectively, and were sampled at 1000 Hz. The near-infrared
light sources contained three wavelengths (730, 805, and 850
nm), and the near-infrared LEDs were switched ON and OFF
sequentially driven by a pulse train so that the detected signals
at each wavelength could be separated. The pulse frequency was
10 Hz, and the duty ratio was 50% [39].
2) Data Acquisition: During the experiments, the subjects
were required to naturally drop down their arms toward the
ground, and the following 13 types of contractions were per-
formed for ten steady-state repetitions: wrist flexion (WF), wrist
extension (WE), radial deviation (RD), ulnar deviation (UD),
pronation (PN), supination (SN), fist (FS), hand open (HO), in-
dex point (IP), fine pinch (FP), tripod grasp (TG), ball grasp
(BG), and rest. These motions were selected as they were fre-
quently encountered in ADL [9]. Each contraction was held for
5 s during a repetition to generate sufficient data for further of-
fline analysis including feature extraction and pattern classifica-
tion, which was widely adopted in similar studies [8], [13], [14].
Self-evaluated moderate contraction was used for all subjects,
and there was a 2-min break between two adjacent repetitions
to avoid muscle fatigue.
3) Feature Extraction: The raw EMG and NIRS data were
segmented into a series of 300-ms windows with an overlap of
200 ms, and features were extracted from these sliding win-
dows, as shown in Fig. 3. The 300-ms analysis window was
adopted for computing the feature vector to reduce the non-
stationary of EMG signals and was exactly matched with the
whole period of NIRS signals. A segment of EMG/NIRS sig-
nals with 300 ms contained enough information to predict a
motion intention. Moreover, the 200-ms overlapping (100 ms of
increment) of the sliding windows was selected to maximally
utilize the computing capacity and produce decision stream as
dense as possible to meet the real-time control requirement [7],
[13]. Four-dimensional EMG time-domain (EMGTD) features
[7] were extracted, namely, mean absolute value (MAV), zero
crossings, slope sign changes, and waveform length (WL). Three
NIRS features were extracted as NIRTD feature set, namely,
MAV, WL, and NIRV (the variance of the NIRS signal), which
were calculated as (1)–(3), respectively. The NIRTD feature set
represents the hemodynamics and blood flow properties during
muscle contractions. A concatenation of EMGTD and NIRTD,
which integrated the information of EMG and NIRS, was de-
noted as combined feature set (EMGTD-NIRTD)
x
mav
=
1
N
N
n=1
|x
n
| (1)
where N is the window size, and x
n
is the NIRS signal
x
wl
=
N
n=2
|x
n
| (2)