TITLE: The effects of plantar flexor static stretching and dynamic stretching using
an aero-step on foot pressure during gait in healthy adults: a preliminary
study

  ABSTRACT:
[Purpose] The aim of this study was to examine whether plantar flexor static stretching
and dynamic stretching using an Aero-Step results in changes in foot pressure during gait
in healthy adults. [Subjects] Eighteen normal adults were randomly allocated to either a
dynamic stretching using an Aero-Step group (DSUAS) group (n = 8) or a static stretching
(SS) group (n = 10). [Methods] The DSUAS and SS participants took part in an exercise
program for 15 minutes. Outcome measures were foot plantar pressure, which was measured
during the subject's gait stance phase; the asymmetric ratio of foot pressure for both
feet; and the visual analogue scale (VAS) measured during the interventions. [Results]
There were significant differences in the asymmetric ratio of foot pressure for both feet
and VAS between the two groups after intervention. However, there were no significant
differences in foot plantar pressure during the gait stance phase within both groups.
[Conclusion] DSUSAS is an effective stretching method, as pain during it is lower than
that with SS, which can minimize the asymmetric ratio of foot pressure for both feet
during gait due to asymmetric postural alignment.

BODY.INTRODUCTION:
Most people warm up before exercising. However, the proper amount of warm-up exercise
required has been a subject of debate. An active warm-up is known to improve an athlete's
performance ability. Stretching is one component of warming up that improves muscle
flexibility, prevents muscle damage, and improves physical performance1,2,3). Problems mainly occur in the lower limbs, and pain and limitations
in joint working force mostly occurs in the ankle joints.  Practitioners report that loss of ankle dorsiflexion range of motion (ADFROM) causes hind
foot pronation as compensation, followed by altered patellofemoral tracking and finally;
anterior knee joint pain4, 5). In addition, limitations in gastrocnemius extensibility
cause limitations in ankle dorsiflexion during stance phase of gait, as well as compensatory
movements6, 7). Therefore, it is important to maintain ADFROM, as a loss of it
causes stress and pain to concentrations in the knee, hips, and lower back4, 8).  Traditionally, therapists have been evaluated a patient's soft tissue flexibility to apply
the proper intervention before treatment. In addition, they have used static stretching (SS)
to maintain and extend the lengths of connective tissues4). In fact, SS causes pain during its application in the clinic.
Moreover, SS prevents contracture of the joints, increases mobility, and improves muscle
flexibility. However, recent research studies insist it is hard to reduce damage
occurrence9). On the other hand, dynamic
stretching allows for movement of the extremities to the end range of the joint in a neutral
position. It also allows for the maximum stretch of muscles at the end range of joints and
causes the limb to return to the original position. This dynamic action is repeated with a
slow and naturally controlled motion for a specific duration of time1).  There are two opinions in the existing studies. One is positive and states that SS helps
reduce the injury ratio and helps in recovery? The other is negative and states that SS has
little influence on injury prevention and negatively influences body performance after
stretching1, 10,11,12). This means dynamic stretching can be recommended, as it can be a
substitute for a warm-up after stretching. Moreover, dynamic stretching can have a more
positive influence on body performance compared with SS when applied immediately after
stretching. Therefore, stretching is necessary for damage prevention. However, there are
different strengths and weaknesses depending on the stretching methods, as well as various
opinions on supplementing them.  Therefore, this study aimed to apply a new method of dynamic stretching on an unstable
plane to improve foot proprioception, as well as to induce a potential change in the
neuromuscular recruitment pattern around the lower extremity13). In addition, the weakness of SS, including pain and decreased
motor performance ability, were avoided in the new method to confirm the effect on foot
pressure and gait and determine an efficient stretching method.

BODY.SUBJECTS AND METHODS:
This study utilized 18 adult subjects (11 males and 7 females) who could walk normally
without any abnormalities in the musculoskeletal system or neurology that could affect gait.
The study was approved by the Kangwon National University Institutional Review Board. Before
the experiment began, the subjects were informed in detail about the purpose and methods of
the experiment, and they agreed voluntarily to participate in the experiment. A homogeneity
analysis of the two groups showed no significant difference. The average age, height, and
weight of the dynamic stretching using Aero-Step (DSUAS) group (n=8) and the SS group (n=10)
were 29.50 ± 2.36 years, 169.8 ± 6.14 cm, and 63.60 ± 11.29 kg; and 30.28 ± 2.81 years,
171.00 ± 7.571 cm, and 66.14 ± 12.96 kg, respectively.  Before the interventions, the subjects were measured for foot pressure during gait using a
Gait AnalyzerTM (Tech Storm Inc., Boryeong, Republic of Korea). They were
randomly allotted to the SS group and DSUAS group.  For SS, this study used a Slant Board (Rivers Edge Products, USA), which can be adjusted to
different angles, set at 30°. The subjects stretched for 15 min continuously. The subjects
were asked to maintain their pelvis in a neutral position. This study allowed leaning
against a wall when fatigued by long the SS of standing. If a subject claimed to be
experiencing pain in the lower limbs or any discomfort, the intervention was stopped
immediately.  For DSUAS, an Aero-Step XL (Togu, Prien-Bachham, Germany) was used to provide an unstable
base during performance of the exercise program. The Aero-Step was 51 cm in length, 37 cm in
width, and 8 cm in height. It is composed of soft rubber and two compartments filled with
air. The exercise program included the following exercises. For 15 min, three movements were
performed for 5 min each13).  Foot plantar pressure was measured during the subject's gait stance phase. The foot area is
divided into 10 areas, including F1 (fourth and fifth toes), F2 (third and second toes), F3
(first toe), F4 (outer forefoot), F5 (middle forefoot), F6 (inner forefoot), R3 (outer
middle foot), R4 (inner middle foot), R1 (outer heel), and R2 (inner heel). The collected
pressure information was analyzed using the Gait Analyzer application software (ver.
3.1)14). After the interventions, the
asymmetric ratio of both foot pressures (ARFP) was calculated by putting the foot pressure
values of both feet in the following formula: ARFP = | 1 − (lesser foot pressure (right or
left) / greater foot pressure)|. A large absolute value indicates good asymmetry, and vice
versa15).  This study measured the level of pain using the Visual Analogue Scale (VAS) during the
interventions.  The collected data were analyzed using PASW for Windows (ver. 18.0). To compare the changes
generated by the two interventions (dynamic stretching and SS), this study used the Wilcoxon
signed-rank test. The Mann-Whitney U test was used to compare the two interventions at the
significance level of (α) = 0.05.

BODY.RESULTS:
The foot pressures of the subjects were measured, and then the results were compared. Both
DSUAS and SS showed no significant difference in each region (p>0.05).  Comparison of the VAS scores showed that there was a significant difference between the
DSUAS and SS groups. In addition, there was a significant difference in ARFP between the
groups after the interventions (p<0.05) (Table
1Table 1.Between-group comparisons of VAS scores and ARFPsDSUASStatic stretchingz aVAS0.62±0.916.00±1.150.00*ARFP0.10±0.040.16±0.03−2.43*aBetween-group comparison. Mean ± SD. VAS: visual analogue scale; ARFP:
asymmetric ratio of both foot pressure; DSUAS: dynamic stretching using Aero-Step.
*p<0.05).

BODY.DISCUSSION:
Acute stretching generates an analgesic effect that improves injured athletes' performances
by minimizing muscular inhibition caused by pain3). The previous studies, which analyzed the effect of stretching using
neurophysiological and mechanical factors, indicates that stretching techniques allow for
neural inhibition through muscle stretching. In other words, stretching reduces reflex
activity and muscle resistance, which causes increased joint range of motion16). Likewise, the effectiveness, efficiency,
and neurophysiological foundations of stretching have been proven through various preceding
studies. Moreover, many earlier studies dealt with SS17); however, recent studies focus more on dynamic stretching18).  In this study, there was no significant difference in foot pressure between the two
stretching groups. This result is consistent with the previous study that reported reduced
performance and disconnected force power immediately after stretching3). Due to the design of this study, significant changes in
foot pressure could not be observed, though acute changes were confirmed after the
interventions.  One interesting thing, however, is the decreased mean value of the R4 (inner middle foot)
region. An increase in the R4 region is a typical found in individuals with flatfoot. The
reason for the decreased mean value of the R4 region is improvement of the contraction and
tightness of the gastrocnemius-soleus muscle by the two types of stretching, which increased
ankle dorsiflexion. This result suggests that, the gastrocnemius-soleus muscle could easily
transit to a supple and rigid position and maintain the sole's concavity during the stance
phase and push-off phase of gait19).
Meanwhile, the DSUAS group showed a greater decrease in the mean foot pressure in the R4
region compared with the SS group. This is because DSUAS requires more muscle contraction
and induced central programming of muscle contraction/coordination, as well as warm-up
activity3). On the other hand, SS is
simply application of passive stretching of the muscles. Therefore, it could not generate
the organic transition of the gastrocnemius-soleus muscle between contraction and release
during gait. Therefore, it did not cause as great a reduction in the R4 region as DSUAS.  Meanwhile, this study showed a meaningful change in the ARFPs. The DSUAS group stretched
their muscles on an unstable support plane, which generated increased stimulation of the
proprioceptors and mechanoreceptors of the plantar skin and ankle joints13). As a result, muscle activation occurred
from the ankle to the knee and from the knee to the hip; and improved the postural alignment
during gait.  One more interesting thing in this study was that the two groups showed significant
differences in pain while stretching. This result suggests that the pain that occurs during
an intervention changes muscle recruitment, and disturbs maintenance of the ideal
(symmetric) posture. A previous study reported that the analgesic effect caused by
stretching influenced not only pain fibers but also other nerves (proprioceptive nerve)3).  Therefore, the DSUAS applied in this study can stretch the gastrocnemius-soleus muscle
properly, decreasing pressure on the R4 (inner middle foot) region and preventing flatfoot.
In addition, it is considered an effective stretching method, as less pain is experienced
during it than SS, which can minimize the ARFP during gait due to asymmetric postural
alignment. However, this study was a preliminary study conducted with a small number of
healthy subjects. Therefore, future studies should deal with how to relieve symptoms and
improve foot pressure symmetry during the gait of flatfoot patients.

Question: Compared to Static stretching (SS) group what was the result of Aero-Step group (DSUAS) group on Foot plantar pressure during the gait stance phase?
Evidence: there were no significant
differences in foot plantar pressure during the gait stance phase within both groups.
Answer: no significant difference