This document describes a study that evaluates Laser Doppler Myography (LDM) as a non-contact method for measuring muscle contraction by comparing LDM signals to standard surface electromyography (sEMG) signals. LDM uses laser Doppler vibrometry to detect vibrations from contracting muscles without touching the skin. The study found that LDM and sEMG signals were similar in timing of muscle activation and that LDM signal amplitude increased with higher force production levels, suggesting LDM can reliably measure muscle contraction characteristics without contact. LDM may have advantages over sEMG such as being less prone to electrical artifacts and more directly related to muscle force.

1. Non-contact Assessment of Muscle Contraction:
Laser Doppler Myography
Sara Casaccia, Lorenzo Scalise, Luigi Casacanditella,
Enrico P. Tomasini
Dipartimento di Ingegneria Industriale e Scienze
Matematiche (DIISM)
Universitá Politecnica delle Marche
Ancona, Italy
s.casaccia@univpm.it
John W. Rohrbaugh
Department of Psychiatry
Washington University School of Medicine
Saint Louis, Missouri, USA
Abstract—Electromyography (EMG) is the gold-standard
technique used for the evaluation of muscle activity and
contraction. The EMG signal supports analysis of a number of
important parameters including amplitude and duration,
engagement of motor units, and functional characteristics
associated with factors such a force production and fatigue.
Recently, a novel measurement method (Laser Doppler
Myography, LDM) for the non-contact assessment of muscle
activity has been proposed to measure the vibro-mechanical
behavior of the muscles that conventionally is referred to as the
mechanomyogram (MMG). The fact that contracting skeletal
muscles produce vibrations and sounds has been known for more
than three centuries. The aim of this study is to report on the
LDM technique and to evaluate its capacity to measure without
contact some characteristics properties of skeletal muscle
contractions. This is accomplished with the very high vibration
sensitivity inherent in the Laser Doppler Vibrometry method (in
comparison to commonly used devices such as microphones,
piezo electric pressure sensors, and accelerometers). Data
measured by LDM are compared with signals measured using
standard surface EMG (sEMG) which requires the use of skin
electrodes. sEMG and LDM signals are simultaneously acquired
and processed. The LDM and sEMG signals are compared with
respect to the critical features of muscle activation timing, signal
amplitude and force production. LDM appears to be a reliable
and promising technique that allows measurement without the
need for contact with the patient skin. LDM has additional
potential advantages in terms of sensor properties, insofar as
there are no significant issues relating to bandwidth or sensor
resonance, and no mass loading is applied to the skin.
Keywords—electromyography; laser doppler vibrometry;
muscle contraction.
I. INTRODUCTION
Electromyography (EMG) is the most common biomedical
instrumentation method used for the evaluation of muscle
activity [1]. EMG provides measures of the electrical activity
of skeletal muscles, conveying information about the
composition and function of muscles. The contraction of a
muscle fiber is initiated when the neuronal action potentials
spread along the excitable membranes of the muscle fiber. A
motor unit action potential (MUAP) results from spatial and
temporal summation of individual action potentials as they
spread through the different muscle fibers of a single motor
unit. The EMG signal is generated, in turn, from summation of
the different MUAPs which are sufficiently near the recording
electrode.
Through the study of EMG signals it is possible to identify
many pathological process and the associated abnormalities in
the neuromuscular system. The EMG examination is used for
traditional diagnostic applications and also for ergonomics,
exercise physiology, rehabilitation, movement analysis,
biofeedback, and myoelectric control of prosthesis [2]. The
EMG signal is acquired using electrodes which can be needle
electrodes or skin electrodes which are adhered in contact with
the skin; the latter is commonly known as surface
electromyography (sEMG) [1, 3]. sEMG is not always easy to
perform because it requires patient compliance with the
application procedures, which involve skin preparation
(removing hair, skin detersion and abrasion) and close attention
to the placement of the electrodes and cable positioning. The
size of the muscle under study can pose problems, particularly
if the muscle is small in size in which case the EMG signal
would be affected by the contributions of adjacent muscles.
During contraction it is also possible to assess the
accompanying mechanical activity of muscles. This recording
method is generally called mechanomyography (MMG) and is
commonly defined as the recording of the low-frequency
lateral oscillation of active skeletal muscle fibers. Since muscle
contraction involves the repetitive activity of multiple,
simultaneously active individual motor units, the mechanical
signs are inherently vibrational in nature. The lateral
oscillations recorded by MMG are a function of:
• Gross lateral (expansion) movements of the
muscle at the onset of contraction due to non-
simultaneous activation of muscle fibers,
• Smaller subsequent lateral oscillation occurring at
the resonant frequency of the muscle,
This full text paper was peer-reviewed at the direction of IEEE Instrumentation and Measurement Society prior to the acceptance and publication.
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2. • Dimensional changes of the active muscle fibers
[4].
The muscle vibrations (or sounds) were recorded
electronically for the first time in 1923 and the development of
this technology has progressed steadily over the years [5].
Recording of the MMG vibrations can be made with a sound or
vibration detector (piezoelectric contact sensors, microphones,
accelerometers, and other transducers) on the skin over the
contracting muscle. The MMG signal can be processed and
analyzed in much the same way as the electrical (sEMG)
muscle signals. MMG can provide some notable advantages
over sEMG, including: 1) the MMG signal may have a more
focal spatial representation than the sEMG signal, 2) MMG is a
mechanical signal, so is less prone to electrical artifacts
associated with poor electrode contact, 3) the amplitude of the
MMG often appears to be more directly related to muscle force
production.
The aim of the present study is to demonstrate the
capability of Laser Doppler Vibrometry (LDV) technique to
measure the MMG signal without contact and in particular to
compare the MMG and sEMG signals under conditions
involving several levels of force production. The application of
LDV for recording MMG signals was designated Laser
Doppler Myography (LDM) when the first studies were
conducted to measure the timing of muscle contractions in
correlation with the standard technique (sEMG) [6-8] and in
particular a study was focused on the evaluation of an
algorithm to find the onset and offset of contraction for the
LDM and sEMG, showing effective capabilities of LDM in
respect to sEMG [7]. The LDM data presented below were
obtained under several movement conditions involving
controlled voluntary activation of arm muscles. It is shown that
the amplitude of the LDM signals are systematically related to
movement parameters. The LDM data are interpreted in
context of simultaneously obtained sEMG data, and are shown
to provide complementary information. The LDM method also
appears to have some technical advantages in comparison to
data obtained using conventional MMG sensors – which have
recognized limitations including low repeatability among
different sensors, insensitivity to low frequencies, mass loading
associated with direct contact with the skin (MMG), and (for
some methods) absence of meaningful calibration units. The
LDM findings converge with other findings, confirming the
general effectiveness of the LDV technique as a physiological
assessment method in a number of key physiological systems
[9-14].
II. MATERIALS AND METHODS
sEMG and LDM data were obtained from arm (bicep brachii)
muscles, on both left and right sides under three levels of
instructed isometric force production (as described in greater
detail below). In particular, the participant had to develop
three levels of force, with reference to maximum voluntary
contraction (MVC, described in %). MVC is the maximum
force which a participant could produce in a specific isometric
exercise.
A. Recording conditions
The study focused on the signals produced by the isometric
muscle contraction from the bicep brachii muscles, as
described in Table 1.
Table 1. Muscle group used for the tests and test
characteristics
Muscle
Right/Left
Electrode Placement Starting
posture
Clinical test
Bicep Brachii
Sitting on a
chair with
the elbow
resting on a
support and
an angle of
90°
between the
arm and the
forearm.
The
participant
lifts a barbell
with 3
different
weights and
maintains a
steady torque
angle during
sEMG and
MMG
measurement.
The tests involved synchronous acquisition of the sEMG
and LDM signals during isometric contraction of the target
muscle. The experimental setup is illustrated in Figure 1.
Figure 1. Measurement experimental setup for the tests.
sEMG electrodes and LDM laser position during the isometric
contraction of the bicep brachii.
To record the sEMG signal, two Ag-AgCl electrodes were
applied on the skin (with adhesive collars) overlying the target
muscle with an inter-electrode distance of 2 cm (center to
center). Placement of the electrodes was guided by the
instructions provided by the SENIAM (Surface
ElectroMyoGraphy for the Non-Invasive Assessment of
Muscles) Project. Prior to electrode application, the skin was
shaved and cleaned, and the cables were fixed in order to
minimize noise. Placement of electrodes (including ground
electrode on the wrist) and the starting postures are specified in
Table 1.

3. The LDM signal was measured with a single point LDV
system (Polytec PDV100; calibration accuracy ±0.05 mm/s,
bandwidth 0.05 Hz – 22 kHz, spot diameter < 1 mm, sensitivity
25 mm/s/V). The Polytec PDV100 utilizes a Class 2 (eye safe)
beam at 633 nm wavelength. The native output of the LDV
system was a velocity signal which was measured with a
nominal resolution of 5 nm/s (at 15 Hz). The laser beam was
oriented perpendicular to the skin surface at a distance about 30
cm and the laser spot was pointed midway between the sEMG
electrodes (as illustrated in Table 1).
sEMG and LDM signals were sampled at 1 kHz using a 12
bit acquisition board (ML865 PowerLab 4/25T) and were
filtered with different bandpass filters because these signals
have a different nature (electrical and mechanical) and
consequently they are characterized by different spectral
contents. The sEMG signal was filtered with a bandpass filter
(5-500 Hz) and the LDM signal was post-processing filtered
with a bandpass filter (5-100 Hz) to suppress noise.
B. Participants and data acquisition protocol
Data were obtained from 5 male participants (3 right-
handed and 2 left-handed, mean weight = 72 kg, mean height =
183 cm, and mean age = 23 years).
Participants were tested under conditions involving
isometric contraction of bicep brachii (left and right) under
load conditions described below. Participants were instructed
and trained with the experimental protocol before the tests. The
bicep brachii contraction was achieved by requiring
participants to lift a barbell, with an angle of 90° between the
arm and the forearm, and with the elbow supported on a soft
surface. The first test was with the barbell without weights,
which is labeled here 0% MVC (although there was a small
nominal load of about 1 kg); for the second and third tests the
participants were asked to identify a probable weight
corresponding to their MVC, from which weights
corresponding to 30% and 90% MVC were identified. The
90% MVC is the almost maximum weight that the participant
can lift while the 30% has been find as about 1/3 of the 90%
MVC. The choice to use 30% and 90% MVC is based on
capacitive of the subjects to recognize a low load and an high
load. The 90% MVC was thus self-identified. For each weight
(0%, 30%, and 90% MVC), the participant was required to
repeat the test 2 times, each for a period of 15 s during which
they held a steady contraction. To minimize the likelihood of a
significant contribution from muscle fatigue, brief rest periods
of 2 min were given between subsequent contractions. The test
sequence started with the contraction of the right bicep brachii,
followed by a replicate sequence involving the left bicep
brachii.
III. RESULTS
A. General form of signals
sEMG and LDM signals were all observed to depend on the
force that the participant produced when he lifted the barbell.
The sEMG and LDM also showed clear changes during the
time of contraction, which varied according to the required
contraction strength. sEMG and LDM signals for a
representative participant are illustrated in Figure 2, where the
force-dependent changes are clearly evident. The signal
amplitudes show orderly increased over the three force levels
(0%, 30% and 90% MVC).
Figure 2. LDM signals and sEMG signals for one participant.
In red is the signal for a 0% MVC (barbell with a weight of ~1
kg), in green for a 30% MVC (barbell with a weight of ~6 kg)
and in blue for a 90% MVC (barbell with a weight of ~16 kg).
B. Force level characterization
Assessment of the bicep brachii force level of contraction for
each participant was based on calculation of the Root Mean
Square (RMS) of the sEMG and LDM signals. RMS was
computed according to the following equation:
(1)
For each sEMG and LDM signal the RMS was computed,
based on 1000 samples with an overlap of 500 samples for all
15 s of acquisition. Means RMS values were found for the
normalized RMS signals in order to support comparisons of
the sEMG and LDM techniques. To normalize signals, it has
been necessary to divide the signal values by the higher value
of the same signal. In Table 2 are shown the means between
the 2 acquisitions for each load (0%, 30%, 90% MVC force)
of sEMG and LDM normalized RMS signals (RMS norm).
The normalized RMS of the right and left biceps brachii
signals uniformly showed progressive increase in signal
amplitude when the weight of the barbell increased [15, 16].
That progressive increase of the RMS normalized parameter
was visible for each participant, for both left and right
movements, for both the LDM and sEMG signals, and would
provide an inerrant basis for classifying the level of
contraction force developed during each test (0%, 30% and
90% MVC).

4. Table 2. Means RMS values of both sEMG and LDM
normalized signals for the right and left bicep brachii, for three
level of force (0%, 30% and 90 % MVC)
sEMG signals
of right bicep
brachii
RMS norm
(0% MVC)
RMS norm
(30% MVC)
RMS norm
(90% MVC)
Participant 1 0.04 0.14 0.62
Participant 2 0.07 0.22 0.47
Participant 3 0.03 0.12 0.56
Participant 4 0.03 0.11 0.59
Participant 5 0.22 0.44 0.46
Mean 0.08 0.21 0.54
sEMG signals
of left bicep
brachii
RMS norm
(0% MVC)
RMS norm
(30% MVC)
RMS norm
(90% MVC)
Participant 1 0.08 0.09 0.24
Participant 2 0.06 0.16 0.62
Participant 3 0.04 0.17 0.42
Participant 4 0.02 0.08 0.68
Participant 5 0.23 0.31 0.58
Mean 0.09 0.16 0.51
LDM signals of
right bicep
brachii
RMS norm
(0% MVC)
RMS norm
(30% MVC)
RMS norm
(90% MVC)
Participant 1 0.12 0.47 0.62
Participant 2 0.08 0.25 0.41
Participant 3 0.13 0.25 0.73
Participant 4 0.20 0.25 0.48
Participant 5 0.14 0.22 0.32
Mean 0.13 0.29 0.51
LDM signals of
left bicep
brachii
RMS norm
(0% MVC)
RMS norm
(30% MVC)
RMS norm
(90% MVC)
Participant 1 0.12 0.16 0.60
Participant 2 0.08 0.37 0.59
Participant 3 0.22 0.40 0.70
Participant 4 0.11 0.34 0.60
Participant 5 0.11 0.27 0.44
Mean 0.13 0.31 0.59
Thus, for each participant it was possible to discriminate the
load (0%, 30% or 90% MVC) for the right and left sEMG
signals (Figures 3-4) and for the right and left LDM signals
(Figures 5-6).
Figure 3. RMS normalized values for the sEMG signals from
right bicep brachii. In green are the means of the RMS values
at 0%, 30% and 90 % MVC for the 5 participants.
Figure 4. RMS normalized values for the sEMG signals from
left bicep brachii. In green are the means of the RMS values at
0%, 30% and 90 % MVC for the 5 participants.
Figure 5. RMS normalized values for the LDM signals from
right bicep brachii. In green are the means of the RMS values
at 0%, 30% and 90 % MVC for the 5 participants.

5. Figure 6. RMS normalized values for the LDM signals from
left bicep brachii. In green are the means of the RMS values at
0%, 30% and 90 % MVC for the 5 participants.
Figure 7 illustrates the increase of mean normalized RMS
values for each load, for right/left sEMG and LDM signals. It
is clear that an increase of force level produces an increase of
the RMS parameter.
Figure 7. Mean of the normalized RMS values for the
right/left sEMG and LDM acquisitions during the different
tests with 0%, 30% and 90% MVC.
IV. DISCUSSION
The purpose of this research was to investigate the
effectiveness of a novel non-contact method based on Laser
Doppler Vibrometry (LDV) for measuring muscle activity
during isometric contraction of bicep brachii, when the
participant lifted a barbell with different weights. For purposes
of validation and comparison, the conventional surface EMG
(sEMG) was simultaneously recorded. Robust LDM signals
were observed for arm muscles on both the left and right sides,
for each of 5 individuals tested. The findings confirm that the
MMG measured with the LDM method agrees in general form
with prior descriptions based on other recording modalities. In
brief, the LDM method was shown to be effective, insofar as
the signals were found to be systematically related in amplitude
characteristics to level of force production.
The analysis of the sEMG and LDM signals in terms of
Root Mean Square (RMS) amplitudes showed the same trend
for the right and left bicep brachii signals over the 0%, 30%
and 90% MVC levels. When the force level increased there
was a uniform and progressive increase of the RMS parameter.
The RMS signal amplitudes would support perfect
discrimination among the three different barbell loads that the
participant was required to lift. It should be noted that there
were appreciable differences across individuals in the LDM
and sEMG signal amplitudes. These differences may well have
been related to individual differences in such factors as muscle
strength and habitual exercise levels, subcutaneous fat, and
body dimensions. The present study did not provide a basis for
formally assessing such factors, which will remain as important
issues for future research.
In conclusion, this study, with the previous works, has the
capability to evaluate the efficiency of the Laser Doppler
Vibrometry to detect the muscle contraction. In this paper, it
has been shown the experimental set-up and the measurement
procedure used to analyze it without contact and with an high
sensitivity of acquisition. The LDM method also appears to
have some technical advantages in comparison to data obtained
using conventional MMG sensors – which have recognized
limitations including low repeatability among different sensors,
insensitivity to low frequencies, mass loading associated with
direct contact with the skin, and (for some methods) absence of
meaningful calibration units. In this context, the LDV method
would appear to offer significant technical advantages.
It has to be highlighted that, in parallel with the discussed
advantages, the proposed technique presents some limitations,
as the device cost (higher than a standard sEMG) and the
necessity to keep the measurement point (< 1 cm²) on the
desired area (which at the moment limits its use to isometric
tests). Up to now only single point tests have been carried out,
while multipoint sEMG is available. Moreover, test performed
for this work were laboratory tests, operated in controlled
conditions, it is necessary to explore the method capabilities
during clinical tests in order to evaluate the full operational
effectiveness capability of the LDV to measure the
characteristic of muscle contraction with the same results.
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