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Int J Sports Med. 6(2001) Datei: js801 Seite: 414 25.7.2001 ± 18:05 blackcyanmagentayellow
Orthopedics and Clinical Science
Ogon M, Aleksiev AR, Spratt KF, Pope MH, Saltzman CL. Foot-
wear Affects the Behavior of Low Back Muscles When Jogging.
Int J Sports Med 2001;
22: 414± 419
Accepted
after revision: October 30, 2000
nnnn
Use of modified shoes and insole materials has been
widely advocated to treat low back symptoms from running
impacts, although considerable uncertainty remains regarding
the effects of these devices on
the rate of shock transmission
to the spine. This study investigated the effects of shoes and in-
sole materials on a) the rate of shock transmission to the spine,
b) the
temporal response of spinal musculature to impact load-
ing, and c) the time interval between peak lumbar acceleration
and peak lumbar
muscle response. It was hypothesised that
shoes and inserts a) decrease the rate of shock transmission,
b) decrease the low back muscle response time, and c) shorten
the time interval
between peak lumbar acceleration and peak
lumbar muscle response. Twelve healthy subjects were tested
while jogging barefoot (unshod) or wearing identical athletic
shoes (shod). Either no material, semi-rigid (34 Shore A), or
soft (9.5 Shore A) insole material covered
the force plate in
the barefoot conditions and was placed as insole when running
shod. Ground reaction forces, acceleration at the third lumbar
level, and erector spinae myoelectric activity were recorded
si-
multaneously. The rate of shock transmission to the spine was
greater (p < 0.0003) unshod (acceleration rate: Means SD
127.35 87.23 g/s) than shod (49.84 33.98 g/s). The temporal
response of spinal musculature following heel strike was
significantly shorter (p < 0.023) unshod
(0.038 0.021 s) than
shod (0.047 0.036 s). The latency between acceleration peak
(maximal external force) and muscle response peak (maximal
internal force) was significantly (p < 0.02
1) longer unshod
(0.0137 0.022 s) than shod (0.004 0.040 s). These results
suggest that one of the benefits of running shoes and insoles
is improved
temporal synchronization between potentially de-
stabilizing external forces and stabilizing internal forces around
the lumbar spine.
n Key words: Ru
nning injuries, shoes, low backpain, insoles.
Introduction
The loading rate (the load amplitude divided by the time to
reach the maximal amplitude) has been shown to be an impor-
tant physical factor influencing injury to the musculoskeletal
system. Repetitive, rapidly applied impulsiv
e loading has been
shown to produce joint degeneration, whereas slowly applied
loads of equal or even greater magnitude often have no dele-
terious
effects [15 ±17]. With every step, an impulsive shock
wave is generated at heel strike that is transmitted from the
lower extremities through
the spine [7]. To avoid the jarring
and potentially damaging effects of this shock wave, the hu-
man body has evolved complex mechanisms for dampening
the shock wave.
The effectiveness of shock absorbing shoes on dampening this
shock wave during gait
has been discussed controversially [3 ±
5, 7,9,11,12]. Although there is a general agreement that in-
soles or shoe modifications may lower impact loading at heel
strike [3,4, 9,12], there
is some concern that insoles may lower
the shock absorbing behavior of the body at the same time
[8,18,20].
Despite this controversy, the use of shock absorbing insoles
has been successfully used to treat
low back patients [26],
and it has been shown that shock absorbers lower the shock
wave at low back level [25].
On the other hand, an active shock absorbing behavior of the
human body has been described that might be diminished by
soft inlays [8,18, 20]. In fact, it has been shown that the devel-
opment of external forces [11], as well as the transmissibility
of impact forces through the human body [7], are increased
by wearing soft soles. So far, the mechanism of active shock ab-
sorption is unclear.
However
, there is some evidence that the neurophysiologic
system is involved [8,18± 21], and it has been shown that glu-
teal muscle activation in walking is related to proprioception at
the level of the sole [2]. It has also been shown that a reduced
shock absorbing capacity of the human musculoskeletal sys-
tem from the femoral condyle to the forehead correlates with
the presence of low back pain
[25]. It is interesting to note that
Footwear Affects the Behavior of Low Back Muscles When Jogging
M. Ogon
1,2
, A. R Aleksiev
2
, K. F Spratt
2,6
, M. H Pope
2,5
, C. L Saltzman
3,4
1
Department of Orthopaedic Surgery, University of Innsbruck, Austria
2
Iowa Spine Research Center, University of Iowa, Iowa City, USA
3
Department of Orthopaedic Surgery, University of Iowa Hospitals & Clinics, Iowa City, IA, USA
4
Biomedical Engineering Department, University of Iowa, Iowa City, IA, USA
5
Department of Biomedical Physics and Bioengineering, University of Aberdeen, Scottland, UK
6
Iowa Testing Programs, University of Iowa, Iowa City, IA, USA
Int J Sports Med 2001; 22: 414± 419
Georg Thieme Verlag Stuttgart ´ New York
ISSN 0172-4622
414
Int J Sports Med. 6(2001) Datei: js801 Seite: 415 25.7.2001 ± 18:05 blackcyanmagentayellow
in this study the low back patients had no X-ray findings. Thus,
there is no evidence that a damaged disc or joint is the reason
for the decreased shock
absorbing capacity, but there might be
an altered behavior of the active part of the musculosceletal
system. In fact, it has been shown that the lumbar muscles of
low back sufferers react slower to sudden loading than the
spinal muscles of healthy individuals [1]. Nevertheless, the role
of the spinal muscles in energy dissipation is as yet unclear.
With each step internal muscle forces at the low back level are
generated to achieve
equilibrium and stabilize the lumbar
spine. Proper function of this system seems crucial, because
external forces which act earlier than the human control sys-
tem is able to
respond might injure the spine. Muscle activity
dissipates energy since the contracted muscle is a viscoelastic
element. The standing impact studies of Pope et al. [14] clearly
show the attenuation
in the bent knee stance. Sudden loads
have been noted in many epiodemiologic studies to be asso-
ciated with reports of acute low back pain. If the load is applied
to a spinal motion segment, then in the absence of muscular
stability the displacement will exceed the range of the neutral
zone and soft tissue structures (e. g. interspinous ligaments
and intervertebral disc) will be loaded.
The purpose
of this study was to explore the influence of shoes
and associated materials on shock wave transmission and, si-
multaneously, the motor response in the lower back to heel
strike impact.
Based on the positive experience in low back patients with
shock
absorbers [26] it was hypothesised that shoes would
demonstrate more protective effects than bare feet (a shoe
main effect), that soft materials would demonstrate more pro-
tective effects than hard materials or no materials (a material
main effect) and that shoes with a soft
insole would be more
protective than other combinations of shoes and materials (a
shoe by material interaction), where protective effects were
defined as:
1.
Decreases in the loading rate experienced at the spine.
2. Decrease of the response time of the spinal muscles to heel
strike (occurrence time relative to heel strike).
3. Reduction of the time interval between peak lumbar accel-
eration and peak lumbar muscle response.
Materials and Methods
Twelve generally healthy volunteers were recruited by local
advertising. Five were female and seven male. The mean age
was 32.9 years
with a standard deviation of 7.9 years. Ages
ranged from 21 to 48. All subjects signed an informed consent
statement that had been approved by the institutions review
board.
The experiment was a 2wShoe
3wMaterial 3wRepetition
factorial experiment where each subject ran at a self-paced
slow jogging speed on a laboratory runway of 8 m length. As a
completely
within subjects design, each subject was exposed
to all levels of each factor in the design (jogging both with
shoes and barefoot [2wShoe], with either no material, a hard
material or a soft material placed on the for
ce plate or in the
shoe as an insole [3wMaterial] and repeating each of these six
combinations three times [3wRepetitions]). Each repetition
was considered valid when the following
criteria were met: 1)
no change in the jogging style, 2) the right heel contacted the
force plate (Type 4060A, Bertec Corporation, Worthington, OH,
USA), and
3) the velocity was constant (+/- 15 percent).
Since a fundamental assumption in repeated measures designs
is
the independence of events across trials, a preliminary study
was done to evaluate the independence of responses across
trials. Once the three criteria for validity of repetitions were in-
cluded, trial to trial variations in
jogging speed and mechanics
(acceleration amplitude and acceleration rate at low back lev-
el) were minimal. Further,
the low demand and simple nature
of the task made learning or fatigue effects extremely unlikely.
When subjects arrived for their scheduled participation, they
were fitted with the correct size of New Balance 600 running
shoe (New Balance Athletic Shoe, Inc. Boston, MA, USA), and
had the EMG electrodes and accelerometer devices fitted and
tested. Subjects were then informed about what was expected
of them in terms of trying to run in a natural and consistent
form across all repetitions
of the short jogging distance. Be-
cause the pilot efforts suggested little learning or fatigue ef-
fects associated with the protocol, the ordering of the trials
was fix
ed to minimize subject efforts in putting on and taking
off shoes. Subjects first nine trials were barefoot (unshod) and
their last nine trials were with shoes (shod). The nine unshod
trials varied the material on the force plate from none to hard
to soft with the material placed on the surface of the force
plate. The nine shod trials
varied the material insoles from
none to hard to soft, with the materials used as shoe insoles.
Greater detail concerning the experimental design is provided
in Table 1
.
Ground reaction forces, acceleration at L3 level and erector spi-
nae muscle activity were simultaneously monitored during
jogging. Ground reaction forces were measured by
a force
plate. Acceleration was recorded by a single-axis, lightweight
(0.4 g) accelerometer (Isoton, PE Accelerometer Model
2250A-10, Endevco, San Juan Capistrano, CA,
USA) attached to
the skin at the L3 spinal process with double-sided adhesive
tape [22, 27]. Low back muscle
activity was recorded 3 cm lat-
eral to the midline at L3 level by surface bipolar EMG elec-
Table 1 Experimental design
Shoe
Condition
Trials Material Combination of materials with shoe
or force plate
Unshod 1± 3 None Uncovered force plate.
Unshod 4± 6 Hard Force plate covered
with 10 mm
thick, semi-rigid (35 Shore A) shock
absorbing material
1
.
Unshod 7± 9 Soft Force plate covered with 10 mm
thick, soft (9.5 Shore A) shock ab-
sorbing material.
Shod 10± 12 None Shoe (80 Shore A) without insole.
Shod 13± 15 Hard A customized, 10 mm thick insole
from semi-rigid shock absorbing ma-
terial (PE Lite
).
Shod 16± 18 Soft A customized,
10 mm thick insole
from soft shock absorbing material
(PPT)
2
.
1
PE Lite
, Medium Density, Knite-Ride Inc., Kansas City, MO, USA
2
PPT, Blue, Langer Biomechanics Group Inc., Deer Park, NY, USA
Footwear Affects Low Back Muscles when Jogging Int J Sports Med 2001; 22 415
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trodes with buildup preamplifiers to reduce the artifacts fixed
at the right side. The same location of the bipolar surface elec-
trodes had already been proven
to be the best [6,13].
Data collection for
all three systems (EMG, force plate, and ac-
celerometer
readings) was triggered synchronously by a
switch, built into a ground based platform, located in the run-
way ahead of the force plate. To assess the rate of shock trans-
mission to the spine, acceleration ra
te was calculated as accel-
eration amplitude divided by the time to acceleration peak. The
time to acceler
ation peak was defined as acceleration duration
(latency between acceleration onset and acceleration peak)
(see Fig.1). To assess the low back muscle response time to heel
strike impact, the latency between heel strike and muscle re-
sponse onset (L1) was determined (see Fig. 1). To assess the la-
tency between maximal external and maximal internal force at
low back level, the
latency between acceleration peak and mus-
cle response peak (L2) was calculated (see Fig.1). The erector
spinae muscle response following heel strike was analyzed by
using inspection of digitally-magnified raw EMG signals with
resolution of 1 ms (Origin 3.5, Microcall Software Inc., North-
ampton, MA, USA), because all available time domain proces-
sing methods (average, integration, RMS, etc.) lead to an error
equal to at least the length of its time constant. Onset of muscle
response was
defined as the first increase in the EMG activity
following touchdown. Since there is also muscle activity due
to gait, only an increase of at least twice the magnitude of back
ground activity after touch down was consider
ed as muscle re-
sponse. To avoid a bias, the data were mixed in a random order
by one of the investigators and analyzed by another one.
A graphic representation of these measurements is summa-
rized in Fig.1.
A preliminary 3-way ANOVA was performed to evaluate the ef-
fects of the experimental conditions on jogging speed as an in-
itial screen for order effects. To control overall experiment-
wise error rate and to ev
aluate the overall effects of the shoe,
materials and repeated assessments on spine related out-
comes two 3-way
MANOVAs were done to simultaneously
evaluate the acceleration (amplitude and duration) and (mus-
cle onset latency [L1] and latency from acceleration peak to
muscle maximum res
ponse [L2]) criteria. A critical p-value of
0.10 was used to identify significance for the overall MANOVA
procedures. Follow-up univariate 3-way ANOVAs for the indi-
vidual outcomes, and
for the acceleration rate criteria were
planned for any of the significant MANOVA results, and post
hoc multiple comparisons were performed using Tukeys high-
est significant difference (HSD) follow-up procedures. Signifi-
cant interactions among the experimental factors in the ANO-
VA procedures were followed up using tests of simple effects
under the assumption that all of the factors in the
models re-
presented fixed effects. Critical p-values of 0.05 were used to
identify significant results from follow-up
univariate ANOVAs
and follow-up tests of main and simple effects.
Results
Regarding the shock transmission to the spine, the overall 3-
way MANOVA demonstrated
significant shoe and material ef-
fects by Wilks Lambda, F
2,10
= 57.03, p < 0.0001 and F
4,42
= 6.69,
p < 0.0003, respectively. Based on the 3-way ANOVA follow-
ups, acceleration amplitude demonstrated no significant shoe
(p = 0.18), material (p = 0.35), or repetition (p = 0.42) main ef-
fects, nor any interaction effects involving these factors. How-
ever, acceleration duration demonstrat
ed a significant shoe
main effect (p < 0.0001), with shorter durations in the unshod
(0.021 0.0
09 s) compared to the shod conditions (0.039
0.011 s). A significant material main effect (p < 0.0006), with
shorter durations in the no material condition (0.027
0.014 s) compared to
the hard and soft material conditions
(0.030 0.013 s and 0.033 0.014 s, respectively) was found.
The materials themselves were not significantly different from
each other based on Tuke
ys HSD follow-ups of the significant
materials main effect. The pattern of results for acceleration
rate were consistent with those observed for acceleration
duration. There was a significant shoe main effect (p < 0.0003),
where acceleration
rate was significantly higher unshod
(127.35 87.23 g/s) compared to shod (49.84 33.98 g/s). The
significant materials effect for acceleration rate (p < 0.02)
again demonstrated a similar
grouping pattern. Significantly
higher deceleration rates in the no material conditions
(106.79 91.42 g/s) compared to
the hard and soft material
conditions (78.07 61.56 g/s and 80.27 71.09 g/s, respective-
ly) were found, which were not themselves significantly differ-
ent from each other based on Tukeys HSD
follow-ups of the
significant materials main effect. Acceleration rate measured
in the different material conditions, barefoot and shod, are
presented in Fig. 2.
Fig.1 Accelero-
meter and EMG sig-
nal from one
trial,
recorded simulta-
neously with the
force plate meas-
urement. Heel strike
was determined by
the force plate.
Acc = acceleration,
Resp = erector spine
muscle response,
AD
= acceleration
duration, L1 = laten-
cy between heel
strike and muscle
response onset,
L2 = latency be-
tween acceleration
peak and muscle re-
sponse peak.
Fig. 2 Acceleration
rate at low back lev-
el following heel
strike in shod and
unshod conditions.
No material (no),
hard shock absorb-
ing material (hard),
and soft
shock ab-
sorbing material
(soft) was placed
on the ground and
in shoes, respective-
ly. Standard error
bars are indicated.
Int J Sports Med 2001; 22 Ogon M et al416
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The muscle onset latency (L1) and the latency between accel-
eration peak and muscle peak response (L2) were considered
simultaneously within a 3-way MANOVA, which
simulta-
neously considered the effects of shoe, materials, repetitions
and their interactions. Results revealed a significant shoe main
effect by Wilks Lambda, F
5,7
= 10.81, p < 0.0034. The materials
and repetition main effects were non-significant, nor were any
of the interaction effects significant.
Delay in the temporal response of the spine musculature to
heel
strike (L1) showed a significant shoe main effect
(p < 0.023.) The low back muscle response onset following heel
strike occurred significantly earlier in the unshod (0.038
0.021 s)
than shod condition (0.047 0.036 s). There was a
trend toward a material main effect (p < 0.064) with the pat-
tern of means in the expected order with fastest onset of mus-
cle response with no material (0.036 0.021 s), a slower re-
sponse with hard material (0.041 0.020 s), and the slowest
response with soft material (0.050 0.042 s) as shown in Fig. 3.
The latency between peak acceleration and peak muscle re-
sponse at the lower back showed a significant shoe main effect
(p < 0.021) with the pattern of
means demonstrating signifi-
cantly shorter latencies in the shod conditions (0.0040
0.040 s) compared with the unshod
conditions (0.0137
0.022 s) (Fig. 4). No significant material (p = 0.62) or repetition
(p = 0.39) main effects or interaction effects were observed.
The average jogging velocity was 1.49 m/s (range, 1.45 m/s to
1.51 m/s). Jogging velocity was not significantly related to
shoe, material, or
repetition.
Discussion
This study was conducted to address the hypotheses that
wearing shoes and insert materials 1) decreases the rate of
shock transmission to the lower back, 2) decreases the re-
sponse time of the spinal muscles to
heel strike, and 3) reduces
the time interval between acceleration peak and muscle re-
sponse peak at the
lower back in jogging. The results support
hypothesis 1 and 3. Hypothesis 2 was not supported. On the
contrary, the muscle response was significantly later in run-
ning shod than in running barefoot and, furthermore, in-
creased with increasing softness of the sole material.
The study hypotheses were based on the positive experience
with shock absorbing materials in low back patients
[15]. From
this report it was assumed that shock absorbers shorten the la-
tency between external (passive shock, potentially destruc-
tive)
and internal (active muscular, protective) forces experi-
enced at the low back. The observed neuromuscular delay
caused by shoes and insert materials suggests, on the first
view, that this assumption was wrong. However, at the same
time, the latency between heel strike and acceleration peak at
low back level increased by wearing shoes, due
to an increased
time interval between acceleration onset and acceleration
peak. Thus, with shoes, the lower back experienced the impact
force peak later.
In fact, this mechanical delay predominates
the muscle response delay so that, after all, wearing shoes de-
creases the time interval between maximum external and
maximum internal forces
experienced in the lower back dur-
ing running. Since the shock wave tends to destabilize equilib-
rium and the muscle response may help regain equilibrium, it
seems that shortening of this time interv
al may have clear
benefits.
The results raised the question whether the proprioception af-
ferent from the heel, or low back muscle stretch reflex triggers
the neuromuscular shock-absorbing behavior at lumbar level.
The muscle responses occurred with a mean latency between
36 and 50 ms after touchdown. This is long enough for the
monosynaptic
stretch reflex (M1), which takes 30 to 50 ms
[8, 23]. The onset of acceleration at the low back occurred
about 18 ms after touchdown, and the latency between accel-
eration onset and muscle response onset was under 20 ms in
the barefoot situation, which is too
short even for an M1 reflex.
Based on these temporal data it appears that the lumbar mus-
cle activity was not triggered by a local shock wave at the lum-
bar level, but by proprioception
at the heel level. The proprio-
ceptors appear sensitive enough to discriminate differences of
the magnitude of the standard
deviation of the impact dura-
tion (about 0.01 s). This indicates that heel afferent proprio-
ception could be sensitive to heel loading rate.
Perhaps the single biggest limitation of this study is the lack of
multiple shoe conditions, which kept this study from consider-
ing
the possible differential effects of various shoe designs rel-
ative to various introductions of insole materials. The inability
Fig. 3 Latency between heel strike and erector spinae muscle re-
sponse onset (L1). Resp Onset = muscle response onset, Resp End =
muscle response end. Standard error bars are indicated.
Fig. 4 Latency between acceleration peak (Acc Peak) and muscle re-
sponse peak (Resp Peak) following heel strike in shod and unshod
condition (L2). Standard error bars are indicated.
Footwear Affects Low Back Muscles when Jogging Int J Sports Med 2001; 22 417
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of the current study to demonstrate the anticipated interaction
between the shoe (shod, unshod) and material (none, hard,
soft) factors was, on first consideration, a surprise.
The most
obvious explanation, lack of power owing to the relatively
small sample size of twelve subjects, although plausable and
almost obligatory,
was unsatisfying as the pattern of results
across the material conditions in the shod and unshod condi-
tions did not hint toward any anticipated effects. Similarly,
confounding effects due to lack of randomization of the order-
ing of the shoe/material combinations was not judged a likely
explanation for either the observed shoe and mat
erial main ef-
fects or for the lack of the anticipated shoe x material interac-
tion effects. The jogging task was simply too similarly executed
across trials and
shoe/material combinations to suggest either
learning or fatigue effects.
Another limitation is the fact that it is extremely difficult to
measure the force that acts in vivo on the lumbar spine direct-
ly. However, accelerometers have been proven useful to esti-
mate impact loading. T
o reduce the bias in the acceleration
outcomes, we followed the fundamental requirements to
properly measure bone vibration in vivo by skin attached ac-
celeromet
ers: a thin layer of soft tissue between accelerometer
and bone [22], use of a light weight accelerometer [10], and a
tight attachment to the skin [22,2
7].
Even if there is latency difference in the other
mutually ortho-
gonal directions at the lumbar level, it would
not significantly
influence the results and conclusions of the study, because of
the too brief latency of the axial component of the accelera-
tion. The few significant differences in external
forces between
the hard and the soft insole materials should also be interpret-
ed with some caution. In this study we investigated two of the
most commonly used inshoe orthotic materials - PeLite and
PPT. These materials are US orthotic industry
standards, and
represent the range of softness-hardness that can be used
comfortably without adversely affecting gait. It might be ex-
pected that testing harder and softer insole mate
rial than
these we used might provide a more robust test of the hypoth-
eses. In this initial study
, however, scientifically well charac-
terized materials were used for the sake of the clinical signifi-
cance, practical application and future reproduction of the
study design.
Robbins and Gouw [19], in a review of the litera-
ture and their own results on athletic footwear and chronic
overloading, summarized that soft shoes with thick yielding
midsoles are probably dangerous because they
attenuate tac-
tile plantar sensations required for protective impact moderat-
ing behavior. They developed the theory of a plantar surface,
sensory-mediated feedback system
for neuromuscular control
of the shock absorbing behavior [18,20, 21]. Their theory has
been supported by the experimental observation that stereo-
typic ipsilateral
hip flexion and contralateral hip extension fol-
lowing a rapid, heavy loading of the leg increases in amplitude
as the irregularity of the plantar surface support increases
[21].
The low back muscle response to running was dramatically af-
fected by use of shoes or soft insole shoeing materials. The la-
tency from heel
strike to muscle response onset was pro-
longed. This is consistent with the observation of Robbins and
Gouw who found that the active damping of the external force
energy started later and progressed less efficiently with
use of
shoes or soft insole materials [18,20]. Thus, too soft shoes
might be worse, especially for low back patients since their
ability to react to sudden load
is diminished anyway [1].
Our results help to explain other observations of shock absorb-
ing behavior [7, 24]. Forner et al. [7] recently examined the
properties of shoe insert materials as they
effect shock wave
transmission between tibia and forehead. They studied the dif-
ference between materials with lower rigidity and loss tangent
(low energy absorbing) and higher rigidity with
high loss tan-
gent. They found more transmission of acceleration from the
tibia to the forehead with the least rigid material. Our study
suggests that this decrease in shock absorbing behavior
is due
to an increased latency of spinal muscle response when wear-
ing very soft shoes.
In summary, we found that shoes and insert materials not only
reduce the loading rate, but affect the low
back muscles. They
can protect the lower spine from heel strike impact in two
ways: by reducing the impact loading rate and by minimizing
the
latency between maximum external and internal force.
Acknowledgment
This study was supported by an Erwin-Schroedinger Grand
(Austria), NIH (G 50111), and the University of Iowa (P10542).
The authors
thank Donald Shurr, PT, C.P.O., for his expert ortho-
tic advice. Presented at the 53. AAOS (American Academy of
Orthopaedic Surgeons) Meeting. Specialty Day of the
AOFAS
(American Orthopaedic Foot and Ankle Society). San Francisco,
CA, USA, February 16th, 1997.
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Corresponding Author:
Associate Professor Michael Ogon, M. D.
Department of Orthopaedic Surgery
University of Innsbruck
Anichstrasse 35
A-6020 Innsbruck
Austria/Europe
Phone: +43 (512) 504-2697
Fax: +43 (521) 50
4-2701
E-mail: Michael.Ogon@uibk.ac.at
Footwear Affects Low Back Muscles when Jogging Int J Sports Med 2001; 22 419
