IEEE Robotics & Automation Magazine - December 2017 - 50

fibers within a relatively short timespan. Secondary complications, e.g.,
Sz
in the cardiovascular system and the
Knee Joint
reduction of bone fitness, also develop more frequently [1], [2].
Currently, FES, a technique that
ϕHL
ϕIMU,SL
ϕKL
allows paraplegics to move again, is
Sx
the only option for an adequate nonϕHR
compensatory training of affected
εz
ϕKR
Hip Joint
extremities. By placing surface electrodes onto the skin, currents are
α
applied to a muscle's nerves to initiate a muscle contraction. The first
εy
intention for the application of electrical stimulation in paraplegics was
εx
to restore the ability to walk [3].
Ankle Joint
However, in most cases, this is difficult to achieve because of the high
t
t
t
Figure 1. The geometry of the lower limbs for calculating the joint angles { H , { K , { H ,
number of degrees of freedom for
and {t K based on the segment inclination angles (exemplary for { S ) derived from
inertial sensors (orange boxes) [7]. The crank-angle a can be determined by using the
gait. FES cycling, where the stimuinclination estimates (see [8]).
lation is triggered using, e.g., the
crank angle, was developed for peoGermany, together with Hasomed GmbH of Magdeburg, ple with SCI and has the advantage that cycling can be
Germany, and Hempel GesundheitsPartner of Berlin (an maintained for reasonably long periods of time, depending
orthopedic supply store) as well as the treatment center for on the training status, with a minimized risk of falls. Tricyspinal cord injuries at the Unfallkrankenhaus Berlin founded cles are used as ergometers that are operated while sitting
Team Hasomed. With our athlete, Hanno Voigt, who had in a wheelchair. The legs are fixed to the pedals using
been paralyzed for more than 35 years at the time of the Cyb- ankle-joint orthoses. Various studies have shown that regathlon, we joined the FES cycling race, where neither motor ular training with FES cycling can produce a significant
nor arm support was allowed.
enhancement to the cardiovascular system and raise lowerAn SCI leads to muscle atrophy, a reduction in muscle limb circulation in paraplegic patients (see, e.g., [3]-[6]).
mass, a decreased ability of the muscle to contract, and a
transformation of type I muscle fibers to type II muscle Methods and Apparatus
R

Table 1. A summary of the joint-angle estimation
algorithm.
Inputs

acceleration and angular rate of four inertial
sensors at both thighs and shanks
for every sample do:
for every sensor do:
estimate orientation
calculate azimuth angle
calculate azimuth-free orientation
calculating inclination angle (e.g.,
{ IMU S ) around main rotation axes

Algorithm

end for
if all inclination angles are available:
calculate joint angles {t H and {t K
for both legs
end if
end for
Outputs

estimated joint angles: {t HR, {t HL, {t KR, {t KL

This table shows the required inputs and the calculated outputs as
well as the sequence of instructions (see [7] for details).

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

Inertial-Sensor-Based Stimulation Pattern
The standard approaches for FES cycling (see, e.g., [3]) use
the crank angle to generate the stimulation pattern for inducing a pedaling motion. For the majority of available systems,
the SCI patient sits with his or her wheelchair in front of an
ergometer for the training period while the mobile cycling is
performed on recumbent tricycles. Therefore, a measurement system for the crank angle is needed, e.g., an encoder at
the crank. Furthermore, the muscle stimulation phases with
positive torque regarding the crank angle have to be determined according to the seating position and the seat-tocrank distance (e.g., [3]). This is relevant, e.g., when the
wheelchair is placed differently in front of an ergometer
every day or when a new subject uses a mobile cycling
device. To our knowledge, all competing teams used a crankangle-based stimulation pattern during the Cybathlon.
In [7], we proposed a method for estimating joint angles
based on the orientation estimates of the shanks and thighs
for stationary cycling with an ergometer. In Figure 1, the
fundamental geometric setting for FES cycling and the
inertial measurement unit (IMU) placement are illustrated.
A summary of this method can be found in Table 1. Using

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2017