Abstract
In cases of
muscle partial deficiency, force enhancement can be achieved by electrical
stimulation (ES). In
the present study the volitional and electrically-induced torque components are
resolved under closed-loop activation. Isometric contraction of the Tibialis
Anterior (TA) muscle was studied on 5 healthy subjects, using an activation
protocol combining ES alone, volitional activation alone and combined
activation of these two modes. Torque and EMG were measured. A computational
algorithm was developed to dissociate the volitional from the overall torque,
based on EMG filtering and pre-determined EMG-torque calibration curves. The
results reveal that for a given overall torque, there exists a linear
relationship between volitional and ES-induced torque. However, the combined
effect of the two activations was found smaller than their algebraic summation,
indicating that not all fibers take part in the two activation modes.
1.
INTRODUCTION
Muscle force
deficiency may be caused by a variety of reasons including, among others,
spinal cord injury, stroke, cerebral palsy, muscle atrophy and ageing.
Temporary deficiency may be the result of muscle fatigue [3].Electrical
Stimulation (ES) of muscles is a well-known technique for the management of
muscle force deficiency. While in complete paralysis muscle activation is the
result of ES only, in other pathologies muscle activation may generally result
from the combined volitional and ES-induced activations. In these latter cases
ES is being used for the enhancement of muscle force.
Depending on the
level of stimulation, the proportion between volitional and induced activations
will vary. This question has been addressed only partly in the literature.
Earlier works [5, 6] evaluated the enhancement effect without addressing the
combined operation mode of the muscle. Later work assumed that the total output
force of a muscle subjected to voluntary and induced activations is the simple
summation of the forces that are generated by each of these components [2].
The present study
mathematically resolves each of the volitional and electrically-induced torque
components under closed-loop activation. Improving our knowledge will help us
to achieve better controllability of muscle force enhancement by electrical
stimulation.
2. METHODS
2.1.
Procedure
Isometric
contraction of the Tibialis Anterior (TA) muscle was studied on 5 healthy
subjects.
Each test was started with measurement of: (a) Maximal Voluntary Contraction
(MVC), (b) Volitional torque-EMG relationship. Real-time ankle torque feedback
was provided by a monitor placed in front of the subject. The subject was
restricted to stay within the displayed stripe while fulfilling his task.
Figure 1: single trial protocol
Each trial included 3 modes of muscle activation (Fig. 1): (I) 0-5s: torque
induced by ES only, (II) 5-10s: addition of a volitional torque acting together
to reach an overall displayed target torque, and (III) 10-15s: Cessation of ES
while maintaining the overall target torque by volitional activation only.
Normally 3 trials
were conducted for repeatability and statistics. Different trials were
performed by varying the induced component (ES intensity) and the target torque
level. In phase II an off-line analysis was performed to evaluate the
volitional torque from the overall torque.
2.2. EMG & Mechanical
Measurements
The torques were measured
in the sitting position by an instrumented beam attached on one side to a
static structure and on the other to a plate which embraced the foot. This
provided compensation of the gravitational torques. During measurement, the
ankle, knee and hip angles were set at
90ș.
Parallel to the
torque, the subject's TA EMG was measured using three surface electrodes: 2
active electrodes were placed on the belly along the longitudinal axis of the
muscle, and 1 ground electrode placed on the bony area of the knee. The
impedance between each pair of electrodes was less than 5KΩ. The three
electrodes were connected to a specially designed 10kHz bandwidth DC amplifier
with stimulus artifact suppression. All signals were sampled at frequency of
1KHz.
Figure 2: Experimental setup
2.3. ES Apparatus &
Protocol
A programmable
electrical stimulator providing constant current, rectangular, 100μs
mono-phasic pulses, at frequency of 20Hz was used. The pulse intensity was
varied between trials to achieve induced torques levels of 0-0.4MVC.
Stimulation was delivered to the muscle using two surface electrodes: one was
placed over the TA motor point, and the second, 10cm distally. The stimulation
parameters were controlled by PC, and an external trigger. During each trial,
the stimulator was active for 20s. Resting time between successive activations
was 5min.
2.4. Signal processing
In phase II where
volitional and induced activation act together, noise was first filtrated. The
load cell signals were used to calculate the overall ankle torque. Using the
comb-filter the volitional EMG was extracted from the raw EMG signals. Using
the EMG values in the EMG-Torque calibration curve yielded the volitional part
of the torque in the overall torque (Figure 3).
Figure 3: Computation of the volitional and induced torque
components
2.5. Graphs and statistics
Linear regression
was performed to describe the relationship between the induced torque (x-axis),
voluntary torque (y-axis) and their combined overall torque. The
difference between the regression coefficients was used to indicate significant
changes between the regression lines with α < 0.05 for significance.
3. RESULTS
The results
(Figure 4) indicate existence of a systematic relationship between the three
magnitudes as follows: (a) for given target torque, the volitional torque
decreases as the ES-induced torque increases, (b) for a given induced torque,
the volitional torque increases as the overall torque increases. This pattern
is well expressed by the linear curves (0.6 < RČ < 0.95)(Table1).
Applying statistical tests to the curves indicates that: (a) in 4 (out of 5)
subjects for different target torques the individual curves are statistically
different in their zero intersection point (
) but not in their slope (
); in one subject, though, this difference was found in both, and.
Target
torque: ■
+ Solid: 0.3MVC ♦
+ Dashed:0.2MVC
Figure 4: Typical relationship between the FES-Voluntary-Overall torques.
4. DISCUSSION AND CONCLUSIONS
Our methodology is
based on deriving the volitional EMG from the overall EMG signal [2]. This EMG
is then related to torque using calibration curves. This is a novel approach,
which relies in part on previously published works [1], [2].
Table 1: Regression curves and statistical difference
of different subjects (
).
|
0.2MVC |
0.3MVC |
||||
|
|
|
|
|
|
|
|
0.168 |
-0.62 |
0.57 |
0.28 * |
-0.72 |
0.86 |
|
0.18 |
-0.88 |
0.85 |
0.29 * |
-0.98 |
0.96 |
|
0.20 |
-0.51 |
0.58 |
0.28 * |
-0.42 |
0.86 |
|
0.18 |
-0.62 |
0.44 |
0.28 * |
-0.84 |
0.92 |
|
0.19 |
-0.30 |
0.23 |
0.31 * |
-0.76 * |
0.80 |
* Significant difference (P<0.05)
For a given overall
torque, a linear relationship between volitional, and ES-induced torque was
established. The y-axis intersection (= voluntary torque) value
coincidences with the required target torque. The line's slope is negative,
with values of (-0.3) (-0.99), the physical interpretation of this is that
with no
References
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[3] Mizrahi J., Verbitsky O., Isakov E., et
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[6] Thorsen R.,
Ferrarin M., Veltink P. Enhancement
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Acknowledgements
This study was supported by the Isler Foundation.