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Annals of Otology, Rhinology & Laryngology 123(4)
Figure 1.
Interval plot with the mean of each data set
presented with 95% confidence bars. The x-axis is scaled with
regard to intensity and testing session. The y-axis for root mean
square (RMS) is scaled in microvolts.
R² = 0.047
0
20
40
60
80
100
120
140
0
50
100
150
200
Session 2
(µV)
Session 1
(µV)
RelaƟonship between Session 1 & 2
Figure 2.
Scatter plot for voice task at the 65 dB target
between sessions 1 and 2. These sessions represent the most
consistent data from this study. Data points are mean root mean
square (RMS) values in µV for each participant. The
R
2
value
does not indicate a strong association between the data for the
2 sessions. Approximately 5% of the data from Session 1 can be
explained by Session 2.
been suggested that additional evidence-based research
concerning LEMG methodology and validity be con-
ducted.
8
Data from this study suggest that variables such as
data collection time (multiple sessions) and possibly vocal
intensity may play a role in the outcomes of LEMG assess-
ment, suggesting methodological limitations of LEMG in
terms of its clinical accuracy.
Our results indicated that to be 95% confident that a true
detectable change could be observed between testing ses-
sions, a change of 51 µV RMS was necessary with intensity
level held constant. It is likely, then, that uncontrolled vocal
intensity during LEMG procedures may operate as a con-
founding variable. Careful regulation of vocal intensity dur-
ing LEMG may be necessary if the clinical utility of LEMG
is to be determined with any degree of accuracy. In fact,
intensity would be expected to contribute to changes in the
RMS value of the LEMG signal. A near linear relationship
between muscle force and EMG activity has been found in
classic EMG studies.
22-24
Although EMG does not measure
muscle force directly, vocal intensity can be viewed as a
global indicator of performance effort and muscle loading
on the vocal apparatus. It was not surprising, then, that con-
trolling for intensity revealed changes in our calculated
RMS values.
It has been previously demonstrated that both intensity
and vocal frequency contribute to variability in quantita-
tive LEMG output with frequency being the greater factor
in TA recruitment variability, suggesting the need for con-
trol of both parameters for improved clinical assess-
ment.
25,26
In this study, participants were generally able to
maintain and regulate the intensity of their vocalization
constant at 65 dB across all trials. Although participants
were trained to reach the 75 dB target, many could not
produce this intensity level consistently for 1 second with
the LEMG needle in place. In the 75 dB condition, inten-
sity levels actually ranged from 66.70 dB to 75.22 dB with
a mean value of 70.42 dB. Thus, it can be inferred that not
only does intensity play a role in LEMG signal stability
but relatively small changes in intensity level (approxi-
mately 5 dB) can strongly affect RMS values, further
arguing for the need and importance of regulating vocal
intensity during LEMG diagnostics.
Limitations
The small sample size of 7 participants in this repeated mea-
sures study limits the ability to generalize our results to a
larger clinical population. Changes in vocal intensity were
limited to a 10 dB interval. Larger intensity intervals and addi-
tional participant data may better demonstrate differences in
mean RMS values across testing conditions. The standard
deviations of the RMS values in this study were large. This is
an inherent problem with attempting to quantify LEMG
because it is difficult to determine which variable(s), such as
ambient noise, movement artifact, interpersonal differences in
phonation, and so on, may be causing deviations in the sig-
nal.
11,27
Needle electrodes, as used in this study, have been
shown to demonstrate greater artifact at greater intensities.
28
Movement/vibration artifact cannot be alleviated but is a con-
cern because of the unsteadiness of the needle electrode and
the vibration of the vocal fold mucosa. Asolution to this prob-
lem may be to consider the use of hooked wire electrodes in
clinical LEMG studies to ameliorate these concerns.
Clinical Relevance and Future Directions
The results of this study demonstrate that even during con-
trolled laboratory conditions, the LEMG signal appears
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