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FUNCTIONAL ELECTRICAL STIMULATION ASSISTED SWIMMING
FOR PARAPLEGICS
22. Annual IFESS Conference
Constantin Wiesener
1
, Jens Axelgaard
2
, Rachel Horton
2
, Andreas Niedeggen
3
, and Thomas Schauer
1
1Control Systems Group, Technische Universität Berlin, Germany
2Axelgaard Manufacturing Co., Ltd., USA
3Treatment Centre for Spinal Cord Injuries, ukb Unfallkrankenhaus Berlin, Germany
wiesener@control.tu-berlin.de
Abstract:
A new method for functional electrical stimu-
lation (FES) assisted swimming has been developed. This
includes the development of waterproof electrodes, cables,
and a stimulator. In preliminary experiments, an extension
and flexion movement of the knee could be induced for a
complete paralyzed subject. Furthermore, the developed
setting stayed safe and dry during several sessions. To
investigate the overall benefit of FES assisted swimming
a pilot study was designed which started in 2018. Dur-
ing this study, up to ten complete paraplegic subjects shall
be trained to swim with FES assisted leg movement while
swimming speed and heart rate are compared to swimming
without FES-support. Additionally, the effects of the train-
ing on spasticity, the well-being, and the usability of the
technology shall be assessed.
Keywords: FES, Swimming, paraplegic
Introduction
A spinal cord injury (SCI) is often associated with paralysis
of the lower extremities which means a severe restriction
of physical activity and health for the affected subjects. De-
pending on the level and severity of the injury, this involves
a functional limitation of various body sensory and motor
functions below the level of lesion. In case of a traumatic
SCI, the physical inactivity is in stark contrast to the condi-
tion prior to the injury, especially for young patients.
Participation in physical and therapeutic activities following
a paraplegia is often limited due to the loss of voluntary
motor function and inefficient temperature regulation of
the affected extremities, autonomic dysfunction, and early
muscle fatigue. In addition, often specially adapted equip-
ment and assistants are needed. Despite all these obsta-
cles, sportive and therapeutic activity after paraplegia can
contribute to a reduction in concomitant diseases and to
an increase in the emotional well-being of those affected
[1, 2].
In the majority of cases, paraplegia results in complete
or incomplete paralysis of the lower extremities. There-
fore, effective and safe lower extremity training is limited
and training exercises of the upper extremities are recom-
mended such as arm-crank ergometer, wheelchair ergometer
or swimming. All these exercises can improve physical fit-
ness by up to 25 % with regular exercises [
3
]. Mobility
in the water is often the only experience of unaided body
movement (except for the transfer in and from the pool)
within the environment that most paralyzed patients enjoy.
In addition, there is a plurality of therapeutic effects de-
scribed in the literature as an increase of muscle strength,
improved coordination, reduction of spasticity and a reduc-
tion of contractures [4].
Functional electrical stimulation (FES) is used successfully
in FES cycling or rowing [
5
,
6
]. The corresponding muscles
for knee extension and flexion or hip extension and flexion
are stimulated depending on the crank or joint angle during
cycling or triggered by a pull switch while rowing. Due
to the combination of arm and leg training, a significantly
higher training effect can be achieved. In addition, an im-
provement in perfusion and lower limb bone density has
been observed in some studies [
5
]. In this paper, we want to
present our recent work on the combination of FES assisted
swimming in paraplegic patients.
Methods
During the relearn phase of swimming for paraplegics, the
independence in the water is first achieved on the back since
the prone position is more difficult to maintain without
muscle power at the hip joints. Furthermore, symmetri-
cal strokes are taught initially by the swimming instructor
during the relearn phase, because asymmetrical swimming
techniques cause the paralyzed limbs to roll and the patient
finds it difficult in maintaining a straight course [4].
Afterward, there are several possible swimming techniques
for paraplegics with slight modifications compared to non-
paralyzed subjects. For patients with thoracic or lumbar le-
sion height [
4
] recommends backstroke, breaststroke, crawl
stroke and butterfly stroke in which the butterfly stroke is
the most difficult to learn.
For normal breaststroke in unimpaired subjects, the so-
called frog kick is used as leg technique, which includes
knee flexors and extensors, thigh adductors and abductors,
gluteus and the plantar-flexors. In preliminary tests, we
found out that a complex movement like the frog kick is
1
Figure 1: Experimental FES swimming setting including the waterproof stimulator (Hasomed RehaMove3, CHIP embedded PC, scuba diving housing)
the user interface (Pebble I smartwatch, Laptop), waterproof IMU sensors and waterproof electrodes.
not realizable with FES. For backstroke and crawl, the so-
called flutter kick can be used for the lower legs with FES
support. For this technique, the knee extensors and flexors
and the gluteus are mostly involved. In [
7
,
8
] the knee
angle course for healthy non-expert swimmers for crawl is
analyzed. Here, after a short and strong extension phase,
a plateau phase can be observed where the knee joint is
fully extended. During the plateau phase, the other knee
is flexed until 40-50 degrees and then directly extended
until the plateau phase. To reproduce this movement for
paraplegic patients a periodic stimulation pattern is used
which is shown in Fig 3. The pulse width is kept constant
at a level of 300-400
µ
s while the current amplitude of the
biphasic pulses is ramped on and off.
To our best knowledge, there is no study or work on FES
in water to produce a functional movement. There are
several works on Iontophoresis for the transdermal delivery
of pharmaceuticals using low DC currents. Furthermore, in
[
9
] waterproof surface EMG electrodes are used to compare
EMG signals on land and in water. Here, standard EMG
electrodes are fixed and waterproofed using 3M Tegaderm
as a waterproof oversize backing. Due to the fact that
chlorinated water in swimming pools has a conductance of
2.5-3 mS/cm which results in resistance of 333-400 Ohm
a direct stimulation with non-waterproof electrodes would
produce a parasitic short circuit between electrodes during
stimulation.
Therefore, Axelgaard Manufacturing developed a water-
proof electrode with a snap connector which stays water-
proof over at least 30 minutes (see Fig. 4). To fix the snap
connector a 3M Tegaderm adhesive has been used. In tests
with healthy subjects, it has been shown that the connection
between cable and electrode must be waterproof as well.
Therefore, a tight silicone tube is used as a cover for the
connection between electrode and cable.
During preliminary tests for the STIMSWIM pilot study, an
experimental FES swimming setting was developed (Fig.1).
For the stimulator, the RehaMove3 (Hasomed GmbH, Ger-
many) is used in combination with a wireless embedded PC
which executes the stimulation pattern and collects sensor
measurements. The stimulation is controlled via a Kivy
App running on a standard laptop. In the water, the subject
can control the stimulation via a customized smartwatch
App.
Preliminary experimental results
To find the best swimming technique and stimulation set-
ting several preliminary tests have been executed with one
paraplegic subject under medical supervision of the Un-
fallkrankenhaus Berlin
1
. The participant gave written in-
formed consent.
The stimulation of the gluteus was excluded due to the fact
that it is not possible to place the electrodes on a paraplegic
without help. Furthermore, the hip position of a paraplegic
in backstroke swimming technique depends on the level
of control over the waist and hip. In preliminary tests, we
found out that the lower the hip the less propulsion which
can be achieved by stimulating the knee flexors and ex-
tensors. As a result crawl stroke was decided to be used
in combination with FES for the planned study. Further-
more, during tests with floats at the ankle a default upward
movement of the ankle could be observed for crawl stroke
which results in a more streamlined position in the water.
In Fig. 4 three photos in the sagittal plane are displayed
which show the different states of the stimulation. During
this test, the subject was asked not to swim with his arms to
get a stable position. During the trial underwater video data
has been captured using a waterproof GoPro action cam-
era. Using the video data the left knee angle was tracked
as shown in Fig. 3 using the video processing software
Kinovea. Compared to [
7
,
8
] the plateau phase of the knee
1Ethical approval of Berlin Chamber of Physicians Eth-28/17
2
Figure 2: Photo and construction plan of Axelgaard Ultrastim
R
snap elec-
trode with oversize water fast backing with an electrode area of 22.9 cm2
[10, 11]
angle after the full extension is reduced. Due to the missing
arm movement, the knee moves automatically to the rest
angle of 90 degree. During swimming with arms knee an-
gle tracking was not possible due to the swimming speed
and role movement of the trunk. Furthermore, the reached
minimum flexion angles are 20 degrees higher compared to
non-paraplyzed swimmers. The stimulation setting includ-
ing the stimulator, cables, and electrodes stayed dry over
the full trial duration of 30 minutes. We found out that the
synchronization of the knee extension with the contralateral
elbow extension in the case of crawl stroke is needed to in-
crease the swimming speed and effectiveness. Furthermore,
if the arm movement is synchronized to the leg movement
then the rolling of the body around the longitudinal axis is
minimized. Therefore, we plan for the next trials to sense
the arm motion by an inertial sensor to control the stimu-
lation of the leg, or to provide a sensory stimulation to the
swimmer to inform him/her about the movement of the stim-
ulated leg as biofeedback. In addition, the influence of the
Hamstring stimulation was rated quite low compared to the
propulsion produced by the Quadriceps only. Therefore, for
the planned study either quadriceps only or the combination
of quadriceps and hamstrings shall be stimulated.
The STIMSWIM pilot study
The STIMSWIM pilot study shall investigate the technical
feasibility of FES to support swimming motions by the legs
9 10 11 12 13 14 15 16 17
Time in s
50
60
70
80
90
100
110
120
130
140
150
160
Left knee angle in degree
0
20
40
60
80
100
120
Current in m A
Left knee angle
Stimulation curr ent Quadriceps
Stimulation curr ent Hamstring
Figure 3: Left knee angle for three consecutive strokes during stimulation
of the Quadriceps and Hamstring muscle group.
Rest / No stimulation
Extension of right leg with FES
Extension of left leg with FES
Figure 4: Experiment with a paraplegic subject (Th5, ASIA scale A)
with floats at the feet with asynchronous stimulation of the left and right
quadriceps and hamstrings.
in up to 10 subjects with complete paralysis of the lower
extremities after spinal trauma. The subjects have to be over
18 years and must give written informed consent. There are
three main questions which shall be answered within this
trial. Does the swimming speed increase compared to non-
assisted swimming? 2. Does the general well-being of the
subject improve during the trial? 3. How is the acceptance
of the technology by the user? The trial comprises of a land
and a swimming phase. After the recruitment and initial
assessment, the subject is asked to carry out a four-week
FES cycling training. During the land training, he/she is
asked to train at least three times a week for 30 minutes
with a standard RehaMove FES cycling ergometer. At the
beginning and end of this training phase, the thigh diameter
and maximum cycling power are assessed. This preliminary
FES cycling training is needed to build up a defined baseline
for the swimming trial. During the swimming phase, the
3
with FES no FES no FES with FES
Pause Pause Pause
with FES with FES
Pause Pause Pause
with FES with FES
Assessment session:
Normal Training session:
25 m 25 m 25 m 25 m
Figure 5: Comparison of assessment and training session. For each as-
sessment session, the subject shall swim the same distance while the time
needed is measured. The distance of each swimming trial will be fixed to
25 m.
FES cycling shall be reduced to two times a week.
Table 1: Trial plan for the swim training during the STIMSWIM study. A
means training with FES and B without.
ID Time after assessment Assessment
T0
Four weeks after re-
cruitment/assessment
and FES cycling
training (3 times a
week)
Execution of Modified
Ashworth Scale Test,
Measurement of swim
distance and speed with
and without FES (ABBA)
T1
After 4-6 training ses-
sions (maximum 30
days after last assess-
ment)
Execution of Modified
Ashworth Scale Test,
Measurement of swim
distance and speed with
and without FES (ABBA)
T2
After 8-12 training ses-
sions (maximum 30
days after last assess-
ment)
Execution of Modified
Ashworth Scale Test,
Measurement of swim
distance and speed with
and without FES (ABBA)
T3
After 12-18 training
sessions (maximum
30 days after last
assessment)
Execution of Modified
Ashworth Scale Test,
Measurement of swim
distance and speed with
and without FES (ABBA)
Afterward, the swimming training starts which follows the
trial protocol shown in Tab.1. The swimming assessment
consist of a strict ABBA pattern to eliminate the effect of
fatigue. Furthermore, the swimming trials of an assess-
ment session either with FES or without are separated by a
five-minute pause. In between the assessment, the subjects
shall train with FES only in four separate training blocks as
shown in Fig. 5. The training is supervised by a pool atten-
dant to eliminate the risk of severe events. At the beginning
and end of each assessment, the spasticity shall be speci-
fied using the Modified Ashworth Scale test. Furthermore,
the subject is asked to fill in a questionnaire about his/her
well-being and the usability of the FES swimming training.
At the moment 3 subjects could be recruited and we started
the FES cycling training. The study shall be finalized by
the end of this year.
Discussion
A new concept for FES assisted swimming for paraplegics
was proposed which uses a waterproof stimulator and elec-
trodes to produce a swimming movement of the paralyzed
legs. During experiments, a periodic FES induced extension
and flexion of the knee could be shown which produced a
propulsion movement in the water. To assess the improve-
ment in speed, swimming distance and well-being a pilot
trial was set up. If it is possible to show that an effective
swimming training including the paralyzed legs can be re-
alized a completely new aqua therapy for paraplegics can
be built up. In addition, the stimulation could also be used
recreationally by paraplegics for diving. Furthermore, it
is conceivable that not only complete paraplegic patients
but also incomplete paraplegic patients or stroke patients
could benefit from an FES-assisted gait therapy in water.
An improvement in physical functions and walking ability
in manual underwater training has been shown in several
studies [12, 13].
Acknowledgment
We would like to acknowledge Axelgaard Manufacturing
Co., Ltd., USA for developing, producing and donating the
stimulation electrodes. Furthermore, we would like to thank
all participants and partners in the STIMSWIM clinical trial
for their support and commitment.
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