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A PILOT STUDY OF AUTO-MYO-ELECTRIC CONTROL OF
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R. Thorsen1,2,4,5, J. Burridge3, |
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1Centro di Bioingegneria, Fond. Don
Gnocchi-Politecnico di Milano, Italy. 3University of 5University of Twente,
Enschede, The |
SUMMARY |
Myo-electric signals (MES) from the paretic anterior tibial muscle (TA) have been used to control functional
electrical stimulation ( the 3 SCI subjects showed increased torque
amplitude with the system. An immediate carryover effect was seen in one
subject. It is concluded that, for selected subjects, AutoMeCFES
can increase the muscle force of TA.
STATE OF THE ART
In some cases, a dropped foot may be corrected by use of Functional Electrical Stimulation (FES) /1/. Existing dropped-foot stimulators are most commonly controlled by a switch /1, 2, 3/. Although this method is satisfactory for many subjects it tends to over-ride the remaining muscle activity whereas myo-electric control may enhance it, which may therefore facilitate motor re-learning /4,5/. The feasibility of proportional myo-electrically controlled stimulation (MCS) for the upper extremity has been described /6/ and demonstrated /7/ although the system is still confined to laboratory tests.
In a previous paper /8/ we described the use of a
tracking test for evaluation of the torque enhancement by comparing the root
mean square (RMS) values of the subject generated tracks with and without MCS.
In this paper we have modified this method and included the results from 3 SCI
subjects with paretic TA.
MATERIAL AND METHODS
Subjects
Subjects were selected from the database
and medical records of >600 dropped-foot stimulator users at the Medical
Physics Department, Salisbury District Hospital (SDH) using the following
criteria: ³1 year after stroke; medically stable;
living within 60km from SDH; 18-60 years of age; near normal passive range of
movement of the ankle joint; some residual voluntary TA activity, but unable to
dorsiflex the ankle in standing; able to give
informed consent and comply with the test protocol; good response to FES and no
history of adverse effects due to stimulation; no serious medical or
heart/respiratory problems or using electrical life support devices .
All subjects were users of dropped foot stimulators. Approval from the ethics
committee and the subjects’ informed consent were obtained. The subjects are
listed in Table 1.
System
The AutoMeCFES system
was built in Centro di Bioingegneria
and programmed at University Twente, where
preliminary tests were performed. It amplifies and filters the MES (20Hz-500Hz)
using a dedicated amplifier /9/ and converts it to a 2.5kHz digital signal.
Signal processing is as follows: reduction of stimulation artefacts by
comb-filtering and elimination of initial 10ms post stimulation signal; root
mean square (RMS) calculation; smoothing by low-pass filtering; a noise offset
is subtracted and a gain determines the stimulation amplitude, which is limited
between zero and an upper limit individually defined for each subject. The
low-pass filter has a cut-off frequency of 1Hz.
The stimulation current
is amplitude modulated with rectangular biphasic balanced waveform. Further
information can be found in /7, 8/
Setup
Stimulation electrodes were placed, one just under the head of fibula and the other 2/3 down on the belly of TA, to induce dorsiflexion of the paretic foot. In all but one case (Subj: B) this placement coincided with the one the subjects used for their usual dropped foot stimulator. To minimise stimulation artefacts the recording electrodes were located between and aligned perpendicular to the stimulation electrodes.
A light-weight footrig
was designed and built to hold the ankle in 10° plantarflexion (from normal posture) and to
measure isometric dorsiflexion torque. Subjects were
placed comfortably on a couch with a free swinging shank and the foot clear of
the ground, thus avoiding accessory forces.
The Sessions
Each session was limited to two hours to avoid
fatigue. Maximal stimulation amplitude (IMax) was established as the upper comfortable limit (all
subjects had skin sensation) and the recruitment curve was measured. The
subject was then asked to produce a strong volitional contraction during
maximum stimulation. The torque produced during this pre-test was used as
the target amplitude in all the tracking tests (with and without stimulation).
This target amplitude was, for simplicity of the experiment, limited to 6, 13,
19 or 25Nm, whichever was just above the pre-test torque.
In the tracking test, a circle and a cross were
displayed to the subject on a computer screen. The circle (target signal)
oscillated vertically in a sinusoid (period = 5sec) for 30 seconds. The
subject’s task was to keep the cross, representing dorsiflexion
torque (tracking signal), within, or as close to the target as possible, by
contracting the dorsiflexors. The test was performed
first without stimulation (natural) and then with AutoMeCFES.
Evaluation
We define the peak torque (PT) as the difference between maximum and minimum
torque calculated as the mean value of, respectively, the 16 (1 sec) largest
and smallest sample values in the subject generated tracking. This gives us the
absolute range in which the subject can work. As a measure of the dynamic
range, we have chosen the standard deviation (STD) of the target as a score for
the variation around the mean value. This has the advantage over the root mean
square (as used in /7/) that it will not be influenced
by eventual offset that could be caused by increased muscle tone.
Thus is defined the torque enhancement in two ways.
PT Enhancement
= (PTMCS - PTNat.)
/ PTNat.
SD Enhancement = (SDMCS - SDNat.)
/ SDNat.
RESULTS&DISCUSSION
Subjects are denoted by ID number and the
result of the test is shown in Table 1.
|
ID |
A |
B |
C |
D |
E |
F |
G |
H |
I |
X |
Y |
Z |
|
Lesion type |
CVA |
CVA |
CVA |
CVA |
CVA |
CVA |
CVA |
CVA |
CVA |
SCI |
SCI |
SCI |
|
Age
[Years] |
62 |
50 |
38 |
58 |
27 |
48 |
53 |
65 |
63 |
47 |
25 |
30 |
|
Injury
[Years] |
6 |
7 |
2 |
4 |
13 |
7 |
12 |
9 |
8 |
5 |
8 |
2 |
|
IMax [mA] |
45 |
63 |
40 |
40 |
15 |
10 |
55 |
21 |
50 |
45 |
33 |
25 |
|
Target [Nm] |
25 |
19 |
12 |
19 |
11 |
6.2 |
6.2 |
12 |
25 |
37 |
50 |
37 |
|
PTNat. [Nm] |
13 |
11 |
11 |
10 |
5.6 |
3.5 |
0.62 |
2.6 |
8.4 |
24 |
21 |
22 |
|
PTMCS [Nm] |
23 |
17 |
13 |
9.5 |
8.7 |
3.5 |
4.5 |
9.3 |
12 |
26 |
23 |
25 |
|
STDNat. [Nm] |
4.8 |
2.5 |
3.7 |
3.8 |
1.6 |
0.94 |
0.21 |
0.96 |
3 |
7.8 |
6.5 |
7.1 |
|
STDMCS [Nm] |
8.7 |
5.6 |
4.5 |
3.5 |
2.9 |
0.88 |
1.4 |
2.6 |
3.9 |
9.3 |
6.1 |
8.2 |
Table 1. Shows, for each subject, the maximum values for stimulation (IMax), target torque, Peak torque (PT) and the standard deviation (STD) of the natural track and the MCS track.
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As it can be seen in Figure 1,
showing the torque enhancements, the trend of both PT and SD enhancement are
the same. Figure 2 shows selected tracking tests from the stroke subjects.
Subject B gained an offset during natural tracking immediately after the
first period in the test which reflects in a low PT enhancement but a high SD
enhancement. In 7 of the 9 stroke and 2 of the
3 SCI subjects the AutoMeCFES increased the torque
and thereby muscle force. In 3 subjects (D, F and Y) the torque was not augmented;
the enhancement scores are less than zero. The IMax was very low for F (see
Table 1) because this person found the sensation of FES very uncomfortable.
In the pre-test for D, it was revealed that stimulation did not increase
force above maximum voluntary contraction. None of the subjects had been
trained in the tracking test and they only tried 2-4 tracking tests before
the measurements were taken. This part of the study, therefore, does not
take into account the effect of an eventual learning, which might improve the
control. Case story: Subject G declared that the foot was ‘dead’; he
was not aware of any volitional contraction whatsoever. During pre-tests of
the session, the level of myo-electric activity was
displayed instead of the torque without application of stimulation and the
subject regained a volitional torque of 2Nm and 6Nm with MCS. Immediately
after the session, the subject was thus able to perform slight dorsiflexion against gravity. This can be interpreted as
a short-term carry-over effect from the biofeedback that the AutoMeCFES provided. The SCI subjects had curves
morphologically similar to subject A but with less torque enhancement. CONCLUSIONWe conclude that, in some cases of
paretic TA due to stroke or SCL, AutoMeCFES can be
used to increase isometric force and that the control can be continuous
rather than on/off. |
Figure 1. Torque enhancement calculated as both PT enhancement and SD enhancement. The data to the left of the vertical dividing line are the stroke subjects and to the right are the subjects with a spinal cord injury. |
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Figure 2. Three selected tracking tests. Top: An example of a good tracking
test. Middle: A subject that was unable to return to the baseline between the periods, due to increase muscle tension. Bottom: A poor tracking but high torque enhancement. |
REFERENCES
/1/
P. Taylor, J. Burridge, et. al., “Clinical audit of 5
years provision of the Odstock dropped foot
stimulator”, Artif Organs, vol. 23, pp. 440-2, 1999.
/2/
W. T. Liberson, H. J. Holmquest,
et. al., “Functional Electrotherapy: Stimulation of the Peroneal
Nerve Synchronized with the Swing Phase of the Gait of Hemiplegic
Patients”, Archives of Phys. Med. Rehab.,
vol. 42: 101-105, 1961.
/3/[#58] R. Dai, R. B. Stein, B. J. Andrews, K. B. James, and M. Wieler, “Application of tilt sensors in functional
electrical stimulation,” IEEE Trans Rehabil Eng, vol. 4, pp. 63-72, 1996.
/4/
J. Cauraurgh, K. Light, et.al.,
“Chronic Motor Dysfunction After Stroke, Recovering Wrist and Finger Extension
by Electromyography-Triggered Neuromuscular Stimulation”, Stroke, vol. 31, pp. 1360-64, 2000.
/5/
R. A. Schmidt and T. D. Lee, “Motor Control and Learning”, Human Kinetics ISBN
0-88011-484-3, pp. 24, 1999.
/6/ M. Popovic;,
T. Keller;, I. P. I. Papas;, V. Dietz;, and M. Morari;,
“Surface-stimulation technology for grasping and walking neuroprostheses,”
IEEE Engineering in Medicine and Biology
Magazine ,, vol. 20, pp. 82 -93, 2001.
/7/ R. Thorsen, R. Spadone, and M. Ferrarin, “A pilot study of myoelectrically controlled FES of upper extremity,” IEEE Trans. Neural
Syst. Rehab. Eng., vol. 9, pp. 161 -168, 2001.
/8/ R. Thorsen, J. Burridge, N. Donaldson, M. Ferrarin, J. Norton, and P.
Veltink, “Auto-Myo-Electric Control of FES on Tibialis Anterior. A Pilot Study With Stroke Patients.,” Proceedings of the 6 th
Annual Conference of the International Functional Electrical Stimulation
Society, pp. 147-149, 2001.
/9/
R. Thorsen, “An artefact suppressing fast-recovery myoelectric
amplifier”, IEEE Trans Biomed Eng., vol. 46, pp. 764-6, 1999.
ACKNOWLEDGEMENTS
This study was only possible due to the helpful co-operation from stroke patients and medical staff from Het Roessingh Rehabilitation Institute, The Netherlands, during the preliminary investigation and Salisbury District Hospital, England during these experiments. This work has been supported by the European Union TMR programme NEUROS
AUTHOR’S ADDRESS
NeuroPro
Post.Doc.