Comparison of Longitudinal Sciatic Nerve Movement

[
research report
]
RICHARD F. ELLIS, PT, PhD1 • WAYNE A. HING, PT, PhD2 • PETER J. MCNAIR, PT, PhD3
Comparison of Longitudinal Sciatic
Nerve Movement With Different
Mobilization Exercises: An In Vivo
Study Utilizing Ultrasound Imaging
TTSTUDY DESIGN: Controlled laboratory study
using a single-group, within-subjects comparison.
TTOBJECTIVES: To determine whether different
types of neural mobilization exercises are associated with differing amounts of longitudinal sciatic
nerve excursion measured in vivo at the posterior
midthigh region.
TTBACKGROUND: Recent research focusing on
the upper limb of healthy subjects has shown
that nerve excursion differs significantly between
different types of neural mobilization exercises.
This has not been examined in the lower limb. It
is important to initially examine the influence of
neural mobilization on peripheral nerve excursion
in healthy people to identify peripheral nerve
excursion impairments under conditions in which
nerve excursion may be compromised.
TTMETHODS: High-resolution ultrasound imaging
was used to assess sciatic nerve excursion at the
posterior midthigh region. Four different neural mobilization exercises were performed in 31 healthy
participants. These neural mobilization exercises
used combinations of knee extension and cervical
spine flexion and extension. Frame-by-frame
cross-correlation analysis of the ultrasound images
was used to calculate nerve excursion. A repeatedmeasures analysis of variance and isolated means
comparisons were used for data analysis.
TTRESULTS: Different neural mobilization
exercises induced significantly different amounts
of sciatic nerve excursion at the posterior midthigh
region (P<.001). The slider exercise, consisting of
the participant performing simultaneous cervical
spine and knee extension, resulted in the largest
amount of sciatic nerve excursion (mean  SD,
3.2  2.0 mm). The amount of excursion during
the slider exercise was slightly greater (mean 
SD, 2.6  1.5 mm; P = .002) than it was during the
tensioner exercise (simultaneous cervical spine
flexion and knee extension). The single-joint neck
flexion exercise resulted in the least amount of
sciatic nerve excursion at the posterior midthigh
(mean  SD, –0.1  0.1 mm), which was significantly smaller than the other 3 exercises (P<.001).
TTCONCLUSION: These findings are consistent
with the results of previous research that has
examined median nerve excursion associated
with different neural mobilization exercises. Such
nerve excursion supports theories of nerve motion
associated with cervical spine and extremity movement, as generalizable to the lower limb. J Orthop
Sports Phys Ther 2012;42(8):667-675, Epub 18
June 2012. doi:10.2519/jospt.2012.3854
TTKEY WORDS: diagnostic ultrasound, nerve
biomechanics, nerve sliding, neurodynamics
A
recent systematic literature review highlighted
the heterogeneity of neural mobilization exercises
that have been assessed via
randomized controlled trials.28
Until recently, the proposed effects of
neural mobilization were based on theory
rather than research evidence. Accordingly, it is possible that many authors of
randomized controlled trials28 chose neural mobilization exercises on a theoretical
and ad hoc basis, rather than specifically
designing exercises to match clinical
conditions. In fact, several authors have
stated that the exercises utilized were
chosen solely on those used in previous
studies.1,3,4,32,41
The use and description of neural mo-
Senior Lecturer, School of Rehabilitation and Occupation Studies, Faculty of Health and Environmental Sciences, Auckland University of Technology, Auckland, New Zealand.
Associate Professor, Health and Rehabilitation Research Institute, School of Rehabilitation and Occupation Studies, Faculty of Health and Environmental Sciences, Auckland
University of Technology, Auckland, New Zealand; Professor, Faculty of Health Sciences and Medicine, Bond University, Gold Coast, Australia. 3Professor, Health and Rehabilitation
Research Institute, School of Rehabilitation and Occupation Studies, Faculty of Health and Environmental Sciences, Auckland University of Technology, Auckland, New Zealand.
Funding for this study was granted by the New Zealand Manipulative Physiotherapists Association (NZMPA) Scholarship Trust fund and Physiotherapy New Zealand (PNZ)
Scholarship Trust fund. This project was approved by the Auckland University of Technology Ethics Committee. Address correspondence to Dr Richard Ellis, Senior Lecturer,
School of Rehabilitation and Occupation Studies, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand. E-mail: richard.ellis@aut.ac.nz t Copyright
©2012 Journal of Orthopaedic & Sports Physical Therapy
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bilization to influence the mechanical
properties of peripheral nerves gained
popularity from the late 1970s through
the mid-1980s. However, the underlying mechanisms associated with clinical
improvements following neural mobilization remain unclear.11 There are many
theories that have been postulated, including physiological effects (removal
of intraneural edema11,15,20,28,44), central
effects (reduction of dorsal horn and
supraspinal sensitization13,15,21), and
mechanical effects (enhanced nerve
excursion15,18,21,22,28,38,44).
Future research needs to focus on
uncovering the underlying mechanisms
of neural mobilization. Such research
would, in part, increase understanding
of the mechanical effects of neural mobilization exercises on peripheral nerves.
Certainly, in the initial stages of such
research, more attention must be given
to understanding the influence of neural
mobilization exercises on normal nerve
excursion.
Previous studies have examined the
influence of neural mobilization exercises on nerve mechanics in cadavers18,21
and subsequently in vivo.22,26,27 Coppieters et al22 examined the difference in median nerve excursion between different
types of nerve-gliding exercises (including sliders and tensioners). Sliders utilize combinations of joint movements to
encourage peripheral nerve excursion by
increasing elongation at one end of the
nerve bed, thereby creating tension in
the nerve from that end, while simultaneously releasing tension from the other
end of the nerve.6,15,18,21,22,34,44 In doing so,
excursion is promoted without increased
nerve tension.18,21 In contrast, tensioners
utilize combinations of joint movements
that elongate the nerve bed from both
ends, in an attempt to stretch the neural
connective tissues.15,21,22,34,44
The conclusion has been that, of the
nerve-gliding exercises assessed, sliders produce greater amounts of median
nerve excursion compared to tensioners.18,21,22 It has also been shown that significantly less nerve excursion occurred
during nerve-gliding exercises initiated
from one end of the nerve bed, using a
single-joint movement, compared to
sliders.22
Neural mobilization exercises, derived
from neurodynamic tests, such as the
slump test or straight leg raise test, have
been advocated in clinical texts14,15,37,44
and as a result of published clinical trials.7,17,34,36,42,48 However, to the authors’
knowledge, no previous research has
been published that has examined in vivo
measurement of sciatic nerve excursion
in normal, healthy participants during
different types of neural mobilization
exercises. Such work would complement
previous work performed on the upper
limbs. The generalizability of neurodynamics and neural mobilization exercise
theories would be enhanced if it could
be shown that similar trends exist in the
lower limb, where anatomical structures
and motion are different.
The aim of this study was to examine
longitudinal sciatic nerve excursion at the
posterior midthigh region, using highresolution ultrasound imaging, during
slump-sitting neural mobilization exercises. This research was conducted with
healthy participants to assess normal
nerve excursion. Consistent with previous studies,18,21,22 it was hypothesized that
sliders would induce greater longitudinal
sciatic nerve excursion compared to both
tensioners and nerve-gliding exercises,
which utilize single-joint movements.
METHODS
Participants
T
hirty-one healthy participants
(22 women, 9 men; range, 21-61
years; mean  SD age, 29  9 years;
height, 170.5  7.5 cm; weight, 68.6 
13 kg; body mass index, 23.4  3 kg/m2)
were included in this study. Participants
met the inclusion criteria if they were
healthy, over the age of 18 years, and did
not have symptoms suggestive of sciatic
nerve dysfunction.
Participants were excluded if they
had a history of major trauma or surgery
to the lumbar, hip, buttock (gluteal), or
hamstring (posterior thigh) regions;
symptoms consistent with sciatic nerve
impairment (eg, paresthesias, weakness,
etc); or a positive slump test (a test that
determines mechanosensitivity of the sciatic nerve and its associated branches) as
described by Butler.15 Participants were
also excluded if they had a neurological
condition or other systemic disorders (eg,
diabetes) that might alter the function of
the nervous system.
Based on the data of the first 15 participants in the study, a power analysis and
sample-size calculation were performed
using G*Power 3.29,30 The dependent variable used for this analysis was the difference in sciatic nerve excursion among the
different neural mobilization exercises,
and calculations were based on a 30%
difference being observed. This value
was arbitrarily chosen, but thought to be
a notable change that would be relevant
to clinicians. It could also be considered
conservative, based on findings from previous research on the upper limb.22 With
power set at 0.8 and an alpha of .05, the
number of subjects required was 21. Following the power analysis, a review of the
procedures and methods was conducted,
with the conclusion that no changes were
necessary. Therefore, data from an additional 16 participants were added to
those of the first 15.
Participants were provided with both
written and verbal information concerning the testing procedures. Informed
consent was given by all participants. The
study was approved by the Auckland University of Technology Ethics Committee.
Participant Setup A Biodex System 3
Isokinetic Dynamometer (Biodex Medical Systems, Inc, Shirley, NY) was used to
provide a consistent, fixed, and supported
sitting position and to enable standardization of passive joint movement for
the neural mobilization exercises. Participants were positioned on the Biodex
seat and asked to adopt a slumped spinal posture, in which the thoracic and
lumbar spine would relax into a flexed
position. Once in the slump position, the
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trunk of each participant was brought
forward until the hips were at 90° of flexion, as measured by a universal goniometer. The slump position was maintained
with contact of the sternum against a
45-cm-diameter inflated ball, which was
placed on the participant’s lap. A belt was
then utilized to maintain this position
(FIGURE 1).
FASTRAK Electrogoniometer
Range of movement at the cervical spine
and knee was measured with a 3SPACE
FASTRAK (Polhemus, Inc, Colchester,
VT) electromagnetic motion tracking
system. The FASTRAK monitors the position of up to 4 separate sensors in real
time and with 6 degrees of freedom, in
respect to a source unit that emits a lowintensity electromagnetic field. It records
the position and orientation of each of
the sensors (angular and linear displacement) within the electromagnetic field at
a set frequency (30 samples per second
when using 4 sensors). The FASTRAK
system has been shown to be accurate
to within 0.2° when recording spinal
motion.40
For cervical spine motion, sensor 1 was
placed at the middle of the forehead, in
line with the bridge of the nose,2,23,35,46,47
using an adjustable elastic headband, and
sensor 2 was placed over the spinous process of C7 and secured with double-sided
tape and tape over the top of the sensor.
For knee-motion measurements, both
sensors were secured (using double-sided
tape) on the lateral aspect of the lower extremity, with sensor 1 positioned 100 mm
above the lateral joint line and sensor 2
located 100 mm below the joint line.12
To avoid potential interference from
metallic objects within the electromagnetic field, the electromagnetic source
unit was secured to a wooden pole and
elevated from the ground, as recommended by previous studies utilizing the
FASTRAK system.12,33 The supporting
pole was placed far enough in front of
the participant and other metal equipment to ensure that there was no interference. Preliminary calibration using
FIGURE 1. Experimental setup.
all 4 sensors, in pairs, against a universal
goniometer moved through a set angle,
demonstrated no interference affecting
the FASTRAK system.
The FASTRAK was linked to a computer that recorded the signals being
transmitted from the sensors. LabVIEW
2009 Version 9.0f2 (National Instruments Corporation, Austin, TX) computer software was utilized to allow real-time
visualization and recording of motion.
The joint-angle data for the cervical spine
and knee were then synchronized offline
to the recorded ultrasound sequences.
Imaging
Excursion of the sciatic nerve was assessed at the level of the posterior
midthigh (halfway between the gluteal
crease and popliteal crease). Initial transverse imaging at the posterior midthigh
allowed localization of the sciatic nerve.
Once identified, the ultrasound transducer was rotated into the longitudinal
plane.22,26 A sonographer with 5 years
of experience performed all ultrasound
scans. The sonographer was blinded to
the analysis of all ultrasound imaging.
B-mode real-time ultrasound scanning was performed using an iU22 (Royal Philips Electronics, Amsterdam, the
Netherlands) ultrasound machine with
a 12- to 5-MHz, 50-mm linear-array
transducer. An ultrasound sequence of
the nerve in a longitudinal plane was recorded for each exercise trial. The video
sequence was captured over a 3-second
period at a capture rate of 30 frames per
second.
Ultrasound Image Analysis Each video
sequence was converted to a digital format (bitmaps). The image size for each
of the frames was 800 × 600 pixels. ImageJ Version 1.42 (National Institutes
of Health, Bethesda, MD) digital image
analysis software was used to calculate
the image resolution and to convert the
image scale from pixels to millimeters.
Image resolution varied between 7.3 and
10.4 pixels per mm, depending on the
depth of ultrasound penetration required
to capture the sciatic nerve.
Each video sequence was then analyzed offline, using a method of frameby-frame cross-correlation analysis that
was developed in MATLAB (The MathWorks, Inc, Natick, MA) by Dilley et al.24
This method employs a cross-correlation
algorithm to determine relative movement between successive frames in a sequence of ultrasound images.24 During
the analysis, the program compares the
grayscale values of speckle features from
the regions of interest within the nerve
between adjacent frames of the image sequence. In the compared frame, the coordinates of the regions of interest are offset
along the horizontal and vertical image
planes 1 pixel at a time within a predetermined range. A correlation coefficient
is calculated for each individual pixel
shift. The peak of a quadratic equation
fitted to the maximum 3 correlation coefficients is equivalent to the pixel shift/
movement between adjacent frames.24
Pixel-shift measurements for the nerve
were offset against (subtracted from)
pixel-shift measurements from stationary structures (ie, subcutaneous layers,
bone, etc) within the same ultrasound
field. This method allows for any slight
movement of the ultrasound transducer
to be eliminated from the analysis. This
method has proven to be highly reliable
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for the assessment of nerve motion.22,24,27
Video Selection Criteria
Each ultrasound video sequence was reviewed several times. To be selected for
analysis, the video sequence must have
had clear pixelation and clear identification of the sciatic nerve throughout
the 3-second duration. The sciatic nerve
must have stayed within the longitudinal
plane of the ultrasound transducer. Of
the video sequences that met these criteria, 2 were randomly chosen for each
of the 4 neural mobilization exercises per
participant.
During the ultrasound sequence selection and cross-correlation analysis, the
researcher was blinded to the participant
(including relevant demographic data),
the recording session, and the neural mobilization exercise performed (FIGURE 2).
Neural Mobilization Exercises
Slider Mobilization The Biodex System 3
Isokinetic Dynamometer performed passive knee extension (from 80° of flexion
to 20° of flexion, loading the sciatic nerve
caudally via the tibial nerve) while the
participant simultaneously performed
active cervical spine extension (from full
comfortable cervical flexion to full comfortable cervical extension, unloading the
nervous system cranially).
Single-Joint Mobilization (Knee) The
Biodex System 3 Isokinetic Dynamometer performed passive knee extension
(from 80° of flexion to 20° of flexion,
loading the sciatic nerve caudally via the
tibial nerve) while the participant looked
straight ahead to maintain the cervical
spine in a neutral position during the
movement of passive knee extension.
Single-Joint Mobilization (Cervical
Spine) The participant performed active
cervical flexion (from full comfortable
cervical extension to full comfortable cervical flexion, loading the nervous system
cranially). The knee was held stationary
by the Biodex System 3 Isokinetic Dynamometer at 80° of knee flexion.
Tensioner Mobilization The Biodex
System 3 Isokinetic Dynamometer per-
FIGURE 2. Neural mobilization exercises. (A) Slider mobilization, (B) single-joint mobilization (knee), (C) singlejoint mobilization (cervical spine), (D) tensioner mobilization.
formed passive knee extension (from 80°
of flexion to 20° of flexion, loading the
sciatic nerve caudally via the tibial nerve)
while the participant simultaneously performed active cervical flexion (from full
comfortable cervical extension to full
comfortable cervical flexion, loading the
nervous system cranially).
To specifically limit movement to the
knee and cervical spine, a rigid thermoplastic ankle-foot orthosis, set at neutral
(0° of ankle dorsiflexion), was worn by
each participant to eliminate the potential influence of ankle movement.
Knee joint motion occurred at an angular velocity of 20°/s. The participant
was instructed to perform the cervical
spine movement over 3 seconds. The exercises were completed in randomly assigned order to avoid any possible order
effects. Each participant completed all 4
exercises after performing 2 repetitions
of each movement to become familiar
with the trials. Then, a further 3 repetitions of each movement were performed
for data collection. There was 1 minute of
rest between each mobilization exercise.
Statistical Analysis
A 1-way repeated-measures analysis of
variance was utilized to assess differences in longitudinal sciatic nerve excursion across the 4 mobilization exercises
performed. Alpha was set at .05. Post
hoc pairwise comparisons of means were
then performed with Bonferroni adjustment, with the alpha level at .008 (.05/6
potential pairwise comparisons).39
The intrarater reliability of measuring longitudinal sciatic nerve excursion
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Neural Mobilization
Slider
Single-joint, knee
Total
Start Position
125.3  17.6
81.4  21.7
–43.9  17.9
End Position
0.6  1.0
27.1  18.2
26.5  18.2
Single-joint, cervical spine
118.9  26.4
–31.4  25.1
87.5  22.7
Tensioner
126.3  23.3
–29.5  21.3
96.8  21.7
*Values are mean  SD degrees. Positive values indicate cervical spine flexion; negative values indicate cervical spine extension.
from ultrasound footage using crosscorrelation software was also examined.
A 2-way, mixed intraclass correlation
coefficient (ICC2,1), with 95% confidence
intervals and standard error of measurement, was calculated to determine the
reliability of measurement.
Descriptive statistics were used to
report the amount of cervical spine and
knee range of motion for each of the
neural mobilizations. Descriptive statistics were also calculated to document
the cervical spine start and end positions
for each of the neural mobilizations. Dependent t tests (α<.05) were used to determine whether there were differences
in total cervical spine and knee range of
motion between specific pairs of neural
mobilization exercises.
RESULTS
O
f the 31 participants enrolled
in the study, data from 1 participant
were excluded, because the sciatic
nerve could not be maintained within the
longitudinal scanning plane during data
recording.
The descriptive data for cervical range
of motion for each of the 4 neural mobilization exercises are presented in the
TABLE. There was no statistically significant difference (P>.05) in the overall
range of cervical spine movement among
the slider, tensioner, and movement of
the cervical spine from full flexion to full
extension exercises.
For the 2 exercises that started with
the cervical spine in extension and ended
Sciatic Nerve Excursion, mm
TABLE
Descriptive Statistics for
Cervical Spine Motion for the
Neural Mobilization Exercises*
with the cervical spine in flexion, there
was no statistically significant difference
in cervical extension at the start of the
movement (mean  SD, 31.4°  25.1°
versus 29.5°  21.3°; P = .54). However,
the amount of flexion at the end of the
movement was significantly different
between exercises (87.5°  22.7° versus
96.8°  21.7°; P = .002). Cervical spine
extension was greater for the slider exercise than for the other 2 exercises
(cervical spine extension only and tensioner exercises) that used cervical spine
motion (P<.01). It should be noted that
for the slider exercise, cervical spine extension is the end position, as opposed
to the start position for the other 2 exercises. The cervical spine start position in
flexion with the slider exercise was significantly less than the cervical flexion
end position for the tensioner exercise
(P<.001).
For exercises involving knee extension, the mean  SD range of motion was
58°  1.3° (from 78.5°  1.8° to 20.5° 
0.1° of flexion). No statistically significant
differences were observed between the
start and end positions for knee range of
motion across all 3 neural mobilization
exercises that included knee movement
(P>.3).
The repeated-measures analysis of
variance indicated that there was a statistically significant difference in motion of
the sciatic nerve at the posterior midthigh
region across the neural mobilization exercises (P<.001). Following Bonferroni
adjustment, there were statistically significant differences (P<.008) in sciatic
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
A
B
C
D
Neural Mobilization
FIGURE 3. Comparison of mean sciatic nerve
excursion (mm) between all neural mobilization
exercises (error bars represent 95% confidence
interval). (A) Slider mobilization, (B) single-joint
mobilization (knee), (C) single-joint mobilization
(cervical spine), (D) tensioner mobilization.
nerve excursion between the slider and
tensioner mobilization exercises (mean
 SD, 3.2  2.0 versus 2.6  1.5 mm; P
= .002) and between the slider and both
single-joint mobilization exercises (3.2 
2.0 versus 2.6  1.4 mm for knee extension exercises, P = .002; 3.2  2.0 versus
–0.1  0.1 mm for neck flexion exercises,
P<.001). There was no significant difference between the tensioner and knee extension exercises (P = .62), but both were
significantly different from the neck flexion exercise (P<.001) (FIGURE 3).
The reliability of measuring sciatic
nerve excursion across all trials for all 4
neural mobilizations combined was excellent, with an ICC of 0.95 (95% confidence interval: 0.92, 0.96; standard error
of measurement, 0.2 mm).
DISCUSSION
I
n support of the study hypotheses,
differences in the amount of longitudinal sciatic nerve excursion at the
posterior midthigh region exist between
different types of neural mobilization
exercises. Statistically, the slider mobilization generated significantly more
sciatic nerve excursion compared to the
tensioner mobilization; however, the difference between the 2 exercises was only
0.6 mm, which may or may not be clinically meaningful. The slider mobilization also generated statistically greater
sciatic nerve excursion compared to the
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[
single-joint mobilizations of knee extension and cervical spine flexion. These
findings are generally in agreement with
other research18,21,22 that has examined
similar techniques in the upper limb and
thus provide evidence that the theoretical construct of nerve excursion during
joint motion is also occurring in the lower
limb.
More specifically, it has been suggested that a slider mobilization is designed to maximize nerve excursion by
simultaneously elongating the nerve bed
from one end of the nerve while releasing tension from the other end. This combined action should result in the greatest
amount of nerve excursion.22 A singlejoint mobilization deliberately utilizes
movement from one end of the nerve bed
predominantly and, therefore, may have
less potential to induce nerve excursion,
due to possible increases in nerve tension
and less capacity to exploit any potential
cumulative effect.
Sciatic nerve excursion was significantly greater during the sliding technique (technique A, 3.2 mm) compared to
either the single-joint mobilization generated at the knee (technique B, 2.6 mm)
or the tensioner mobilization (technique
D, 2.6 mm). It is expected, from cadaver
studies, that full extension of the cervical
spine would allow a release of neural tension via the spinal cord and spinal nerve
roots,8,9,43 which potentially allows more
excursion of the sciatic nerve.
It is notable that no difference in excursion was seen between the single-joint
mobilization (technique B, 2.6 mm) and
the tensioner mobilization (technique
D, 2.6 mm). The tensioner mobilization
involved simultaneous elongation of the
nerve bed from both ends. However, the
inclusion of cervical flexion did not decrease the amount of sciatic nerve excursion as hypothesized, having elongated
the proximal aspect of the nerve bed. In
a similar manner, the single-joint mobilization generated at the cervical spine
(technique C) produced minimal sciatic
nerve excursion (–0.1  0.1 mm), which
was in fact smaller than the measure-
research report
ment error.
It is generally accepted that, although
greatest closer to the axis of joint rotation, nerve excursion becomes less at
more distant regions along the same
nerve tract.19,26,27,31,44 Cervical flexion has
been suggested to increase tension within the lumbar nerve roots, resulting in
an unfolding of resting slack within the
nerve root.9,10,43 Cervical flexion has been
shown in research involving cadavers to
cause 3 to 4 mm of cranial movement
of spinal nerve roots within the thoracic
spine.9 Cervical flexion has been widely
advocated as an additional maneuver to
the slump test to further tension the neural system, leading to neural sensitization
and symptom reproduction.10,14,15,44 However, it is unclear whether the increased
neural sensitization stems from an increase in neural tension, neural excursion, or both. Hall et al31 reported that
the addition of cervical flexion during a
straight leg raise test did not show any
significant change in hip flexion, to first
onset of manually perceived resistance,
in either healthy individuals or a group
with lumbar radiculopathy. These authors concluded that cervical flexion did
not have a significant effect on neural
tissue compliance during the straight leg
raise.31 Our findings are consistent with
this theory, in that minimal sciatic nerve
excursion was evident during isolated
cervical flexion.
Previous studies that have assessed
nerve excursion during neural mobilization exercises in cadavers18 and in vivo22
have observed greater amounts of median nerve excursion, both in absolute
terms and between the different mobilizations, compared to the present study
of sciatic nerve excursion. It is difficult to
make direct comparisons here due to the
different kinematics between movement
of the upper and lower limbs (eg, in relation to joint ranges of motion). In respect
to certain techniques, similarities can be
seen between upper-limb and lower-limb
data. For example, sciatic nerve excursion during the tensioner technique (2.6
mm) is close to that seen for the median
]
nerve during a tensioner mobilization
(1.8 mm).22
In the current study, the maximum
mean excursion of the sciatic nerve was
3.2 mm. Direct comparisons of the present study to the available evidence are difficult, due to methodological variations.
It is not possible to directly compare cadaveric measures of nerve excursion to
those taken in vivo, as methods of tissue
dissection and preservation have the potential to alter nerve mechanics. Unfortunately, there is a paucity of research that
has examined the excursion of the sciatic
nerve in vivo. However, a study utilizing
cadavers by Beith et al5 examined the increase in length of the nerve bed of the
sciatic-tibial-medial plantar nerve tract
and found an increase (mean  SD, 42.2
 2.4 mm) in nerve bed length when
moving the knee from 90° of flexion to
terminal extension. The same nerve bed
increased by 6.8  0.69 mm from 20°
of ankle plantar flexion to a neutral (0°)
ankle position.5
Of note is the small magnitude of
sciatic nerve excursion. The only other
study that has examined sciatic nerve
excursion using in vivo ultrasound assessment found similar levels of sciatic
nerve excursion at the posterior midthigh
region.27 However, direct comparison to
this previous study is limited, as differing methods were utilized. An alternative
means of comparison for in vivo nerve
excursion during neural mobilization
is from studies performed in the upper
limb.22,26 From these studies, the mean
and median nerve excursion ranges recorded were 3.4 to 10.2 mm22 and 0.8
to 3.0 mm,26 respectively, utilizing different methods and neural mobilization
exercises.
At this time, the clinical implication of
the small magnitude of nerve excursion
during neural mobilization exercises is
unclear. A key factor that limits further
scrutiny of these values is the controversy
over whether reduced nerve excursion
may be a factor in peripheral nerve disorders and, if impaired nerve excursion
is evident, whether neural mobilization
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may influence a potential loss of nerve
excursion. Although the magnitude of excursion between the different techniques
appears small, given the accuracy of measurement (as seen from the ICC and standard error of measurement values), these
values reflect true differences. However,
the clinical relevance of this small change
is uncertain.
The neural mobilization techniques
used in this study utilized a combination
of passive knee movement and active
cervical spine movement. It would have
been ideal to use movements at the knee
and cervical spine that were either both
active or passive. In regard to using active knee movement, there are technical
difficulties with using ultrasound with an
actively moving limb, particularly at the
posterior midthigh, as activation of the
hamstrings makes it difficult to maintain
posterior midthigh transducer contact.
Implementing passive cervical spine
movement, with the participant in an
upright, slump-sitting position, through
a standardized range would have been
logistically difficult. Therefore, active
cervical movement was utilized. It is acknowledged that this is a methodological
limitation, in that the active movement
presents less rigorous control than standardized passive motion does. However,
all cervical spine motion amounted to
each participant’s normal and full active range. This could be interpreted as
adding clinical relevance to the use of
the participant’s normal and full cervical
spine range of motion.
Furthermore, analysis of the cervical
spine position at the start and end of each
mobilization showed some variation between individual participants. It is likely
that this variance is related to the variance of cervical spine postures within
the participant sample and the effect of
the slump position on cervical spine posture and range of motion. This variance
was also highlighted in the differences in
cervical spine range of motion from the
various start and stop positions between
each exercise. As previously discussed, it
is acknowledged that this is a limitation
of the study.
Two points are noteworthy. The first
is that there was no significant difference
in nerve excursion seen between exercises
B and D. The difference in performance
between the exercises was the addition
of cervical spine flexion for the tensioner
mobilization (D). Evidently, the cervical
spine flexion used for D did not result in
significantly more sciatic nerve excursion.
Furthermore, the amount of sciatic nerve
excursion seen when flexing the cervical
spine in isolation, as seen with C, was not
larger than measurement error. Second,
the mean overall range of active cervical
spine movement, used between the different neural mobilization exercises, was
not statistically different. Although there
was variance seen in cervical spine start
and end positions for each of the mobilizations, the overall ranges of movement
were not significantly different, indicating a sufficient level of consistency of
cervical spine movement between participants across the different exercises.
It may be worthwhile for future studies
to utilize more standardized methods of
passive cervical spine motion to add further rigor.
In light of the small difference in sciatic nerve excursion between the different neural mobilizations, other potential
features, which were not examined in this
study, may influence nerve excursion. For
example, nerve strain has been shown to
influence nerve excursion.21 Likewise,
symptom reproduction may also limit
joint range of motion and, subsequently,
nerve excursion. Sciatic nerve excursion
at the posterior midthigh region does
not offer any direct reflection as to what
may occur more proximally along the
sciatic nerve tract. These features, along
with further analysis of nerve mechanics,
are worthy of consideration in regard to
future research of neural mobilization.
Future research investigating the therapeutic efficacy of neural mobilization
should select specific exercises based
on a better understanding of the neural
mechanical effects that are likely to be
exploited.
CONCLUSION
T
he findings of this study provide evidence that lower-limb
nerve excursion agrees with the
theory related to such movements. Different types of neural mobilization exercises induced different amounts of
longitudinal sciatic nerve movement. A
slider mobilization produced the most
nerve excursion compared to singlejoint mobilizations and a tensioner mobilization. It may be relevant to take this
information regarding nerve mechanics
in healthy participants, together with
similar findings from related research
in the upper limb, to enable more specific design and prescription of neural
mobilization. t
KEY POINTS
FINDINGS: Different types of neural mo-
bilization exercises produce different
amounts of longitudinal sciatic nerve
excursion. The use of cervical extension
during a slump slider resulted in the
largest amount of sciatic nerve excursion.
IMPLICATIONS: Knowledge regarding the
varied amounts of sciatic nerve excursion observed with different exercises
may be important to aid in specific design of neural mobilization exercises.
CAUTION: Details regarding the amount
of sciatic nerve excursion can only be
inferred from the slump-sitting exercises that were utilized. Data on sciatic
nerve excursion are only relevant to
recordings taken at the level of the posterior midthigh and may be different
at regions beyond that location. This
study did not comment on the clinical
efficacy of these techniques and did not
attempt to make any inferences regarding nerve mechanics in pathological
populations.
ACKNOWLEDGEMENTS: The authors would like
to thank Trish Knight (BHSc) and Sandee
Hui (BHSc) for their assistance in collecting
data, and David Mabbott for providing the
illustrations.
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