Keywords
Abductor hallucis - Foot contact area - Navicular height - Plantar fascia - Shear wave propagation velocity
Introduction
In basic human activities such as standing and locomotion, the foot plays a role in
sustaining balance, absorbing shock, and generating forceful propulsion against the
ground [1]. These foot functions are
closely linked to the arch structure. The abductor hallucis [2] and plantar fascia [3] are one of the important support
elements for the medial longitudinal arch. The volume of the abductor hallucis is
the largest among the intrinsic foot muscles [4], and fatigue of the muscle was accompanied by acute changes in the
medial longitudinal arch height [5].
Thus, investigating whether an intervention reduces the stiffness of the abductor
hallucis is useful, considering that muscle tissue can be stiffened due to fatigue
[6]. Regarding the plantar fascia,
it undergoes dynamic elongation and shortening, thereby modifying the windlass
effect during human locomotion [7]. The
plantar fascia that is excessively stiff might potentially impair this effect. In
addition, plantar fascia stiffness was higher in patients with type II diabetes than
in healthy individuals, which was associated with a slower gait speed and higher
fall risk [8]. A computational modeling
study [9] indicated that increased
stiffness of the plantar fascia was accompanied by corresponding changes in the
contact force through the metatarsophalangeal and tarsometatarsal joints, which
could result in metatarsalgia. Certainly, elucidating methods that decrease the
stiffness of the abductor hallucis and plantar fascia would be beneficial.
Instrument-assisted soft tissue mobilization (IASTM), which has been reported to have
a larger effect on joint range of motion compared to foam rolling [10] and has been advocated in sports
[11] and rehabilitation [12], would have the potential to change
the medial longitudinal arch height by altering the stiffness of the tissue
supporting the arch. The impact of IASTM on tissue stiffness is not well-documented,
and to the best of our knowledge, there are no studies available on its effect on
the tissue stiffness of the foot. Because a study conducted on the infraspinatus
[13] reported a decrease in muscle
stiffness, this method can be applied to the foot region. In contrast, a
cross-sectional study observed that the stiffness of the abductor hallucis and
plantar fascia did not differ between normal and flat feet in an unloaded position
[14]. Schuster et al. [15] investigated plantar fascia stiffness
and foot shape changes under low (sitting) and 100% (standing while supporting a
mass equal to body mass) body mass loading conditions. They showed that load-induced
change in plantar fascia stiffness was not a good predictor of corresponding changes
in foot structure. Based on these findings, despite potential alterations in the
stiffness of the abductor hallucis or plantar fascia, IASTM may not be sufficiently
effective to cause a change in the height of the medial longitudinal arch.
Therefore, the purpose of the present study was to examine the effects of IASTM on
tissue stiffness and structure of the foot. We hypothesized that IASTM on the
plantar surface would decrease resting abductor hallucis and plantar fascia
stiffness without changes in medial longitudinal arch height under low- and
high-loading conditions.
Materials and Methods
Participants
Twenty-eight healthy young men (age, 21±1 year; height, 173.5±5.4 cm; body mass,
69.9±8.7 kg; mean±standard deviation [SD]) participated in this study. The
inclusion criteria consisted of healthy young men, while the exclusion criterion
was defined as the presence of injuries or diseases affecting the lower limbs
and feet. Consequently, the level of physical activity of the participants
ranged from sedentary to active. The participants were requested to avoid
high-intensity exercise the day before and on the day of the experiment. It was
verified that there were no muscle pains in their lower legs and feet
immediately before the experiment. This study was approved by the Ethics
Committee of the last author’s institution and conducted in agreement with the
ethical guidelines of the International Journal of Sports Medicine [16]. Participants were informed of
the purpose and potential risks of the study and provided written informed
consent before participation.
The sample size estimation (G*Power 3.1.9.4, Kiel University, Germany) indicated
that 26 participants (52 feet) were necessary for a small-to-medium effect size
(f=0.20) with α=0.05 and statistical power=0.80 to detect the changes
in shear wave propagation velocity of the tissues over time between conditions.
Ikeda et al. [17] investigated the
effect of IASTM on musculoskeletal properties and reported a medium effect size
for the difference in the changes in joint stiffness between the IASTM and
control conditions. Because it was assumed to have larger variability in the
tissue stiffness than in the joint stiffness [17], the expected effect size of the
current study was set slightly lower than medium. Anticipating potential
dropouts, two more participants were recruited.
Study design
The experimental procedure is illustrated in [Fig. 1]. Dependent variables were
measured before and after the IASTM intervention or while maintaining a relaxed
position with the same time interval as in IASTM. Among the participants (n=28),
24 participants were right-dominant, and others were left-dominant, which was
defined as that used for kicking a ball. One of the feet was assigned to the
IASTM condition and the other to the control condition, which was
counterbalanced between the dominant feet among the participants. Because the
effect of IASTM on the contralateral foot was unclear, we selected a
between-participant design in statistical analyses to examine the effect of
condition on dependent variables. The two conditions were conducted in a random
order. As for dependent variables that represent tissue stiffness, the resting
shear wave propagation velocity of the abductor hallucis and plantar fascia was
measured. Changes in navicular height during sitting and foot contact area while
standing on both feet were evaluated as indices of the corresponding change in
medial longitudinal arch height under low- and high-loading conditions,
respectively. We did not obtain navicular height during standing because of poor
repeatability in a pilot study. Before and after the intervention, measurements
were conducted in the following order: navicular height, shear wave propagation
velocity (in a random order between the two tissues), and foot contact area. All
measurements after IASTM were completed as expeditiously as possible. Data
collection and image analyses were conducted by the same investigator, who was
familiar with the measurements and blinded to the hypothesis outlined in the
introduction.
Fig. 1 Time course of the experimental procedure, typical examples
of ultrasound images, and schematic illustrations of ultrasound probe
placements. Each image shows the experimental setting for the
measurements or intervention. The area surrounded by the white line on
each ultrasound image represents the region of interest for shear wave
velocity analysis.
IASTM
The participant lay prone on a treatment table, and a single session of IASTM was
performed on their feet ([Fig. 1]).
The target for mobilization was the plantar surface within 20%–80% of the foot
length, focusing on the plantar fascia. A specialized apparatus with an upper
arc was used for mobilization. The procedure was performed by a single,
well-practiced examiner. To directly apply the specialized tool to the skin, a
water-soluble gel was used to enhance its smoothness. One repetition was defined
as a single round-trip movement within the range of 20% to 80% of the foot
length, and four sets of 30 repetitions were performed. Each set lasted for 20 s
with a 15-second interval between sets. To maintain a consistent speed, a
metronome was set at 90 beats/min. During the control condition, a rest period
of 125 s, which is the same time as the IASTM, was implemented.
Shear wave propagation velocity
The shear wave propagation velocity of the abductor hallucis and plantar fascia
was determined using a B-mode ultrasound apparatus (ACUSON S2000, Siemens
Medical Solutions, USA) with a 45-mm linear probe ([Fig. 1]). Participants sat on a
chair with ankle and knee joint angles of 0° and 90° (anatomical position=0°),
respectively [14]. An ultrasound
image of the abductor hallucis was acquired at a point equidistant from the apex
of the medial malleolus, where a line perpendicular to the ground was divided
into two equal parts. To measure the plantar fascia, an image was acquired at a
position 40% proximal to the foot length. Measurements were performed randomly
across tissues.
The images were analyzed in the same manner as previously described [18]
[19]. Briefly, the region of interest
(ROI) on the tissue was delineated to maximize its size within the color-coded
area of the elastography image, with the explicit exclusion of non-target
tissues (e. g. subcutaneous adipose tissues, aponeuroses, and non-target
muscles) using image processing software (Image J, NIH, USA). The average shear
wave propagation velocity value across the ROI was subsequently computed for
each image using proprietary analysis software developed in MATLAB (MATLAB
R2018a, Math Works, USA) [18]. This
software can transform the red-green-blue color model values of individual
pixels within the ROI into corresponding shear wave propagation velocity values,
aligned with the color scale of the elastography images. The mean of the three
images was used for further analysis. The velocity was used as an indicator of
tissue stiffness.
Navicular height
In the same participant’s posture for the shear wave propagation velocity
measurement, navicular height was determined by ultrasonography [20]. The participants placed their
feet on the pedestal without intentional inward or outward movements. After
palpating and identifying the approximate location of the navicular tuberosity,
a 60-mm linear ultrasound probe was positioned perpendicular to the medial
aspect of the foot to obtain a B-mode image visualizing the navicular tuberosity
([Fig. 2]). The edge of the
probe-imaging range was aligned with the surface where the sole of the foot
touched the ground. Measurement was performed three times. Navicular height was
calculated on the ultrasound image as the distance between the navicular
tuberosity and surface. The mean of the three images was used for subsequent
analysis.
Fig. 2 Schematic illustration of the foot and an example of an
ultrasound image obtained for navicular height measurement. The
navicular bone was gray with a thick frame. The vertex of the white
triangle in the ultrasound image indicates the navicular tuberosity. The
navicular height (two-direction arrow) was calculated on the ultrasound
image as the distance between the navicular tuberosity and the plantar
surface.
Foot contact area
Before and after IASTM, the foot contact area while standing at rest was measured
using a foot pressure distribution scan system with a 578×418 mm plate (footscan
USB, RS scan international, Belgium) [21]. The data obtained from the foot subjected to IASTM were utilized
as the IASTM condition data, whereas the data from the opposite foot were
employed as the control condition. The participant stood barefoot on the
scanning mat and was asked to stand with equal weight distribution on both legs.
A linear mixed model analysis demonstrated that the mean peak foot pressure was
not significantly different between feet and before and after intervention in
each foot (P=0.145–0.948). The participants gazed at a landmark 2 m in
front of them to control their gaze and minimize body sway during measurement.
Foot pressure distribution data were automatically obtained, and the foot
contact area was determined using the software.
Repeatability of dependent variables
The day-to-day repeatability of measurements was tested on five participants (ten
feet). No significant differences were found between the days for all variables
using paired t-tests. The coefficient of variation (CV) and intraclass
correlation coefficient (ICC [1,1]) for each variable were as follows: shear
wave propagation velocity of the abductor hallucis, 3.5±2.6%, 0.780; shear wave
propagation velocity of the plantar fascia, 3.1±2.8%, 0.899; navicular height,
1.3±1.1%, 0.968; and foot contact area, 3.0±2.6%, 0.736. The CVs values were
lower than the usual repeatability in biological systems (10–15%, [22]). The ICCs ranged from good to
excellent [23].
Statistical analyses
Statistical analyses were performed using SPSS (version 28.0, IBM, USA).
Normality of the data distribution was investigated using the Shapiro-Wilk test.
If the distribution was skewed, data were log-transformed. To facilitate
interpretation, all data are presented as mean±SD of the raw data. A linear
mixed model analysis was conducted for dependent variables with condition
(IASTM, control) and time (before, after) as fixed factors and participants as
the random factor. When a significant interaction was evident, Bonferroni
multiple comparisons were performed. For variables that showed a significant
interaction of condition×time, relationships between variables at baseline and
changes in them were tested using Pearson product moment correlation coefficient
(r). The significance level was set at P<0.05. In addition,
Cohen’s d (d) in between-subject designs [24] was calculated using the mean and
SD of the change scores. The d was interpreted as 0.20–0.49, 0.50–0.79,
and≥0.80 representing small, moderate, and large, respectively [25]. We considered the effect of
IASTM to be substantial, if d showed moderate or large with a significant
interaction of condition×time.
Results
[Fig. 3] shows the changes in shear wave
propagation velocity in the tissues. A main effect of time (P=0.010) was
significant for the abductor hallucis (Before>After; Control: Before, 2.89±0.41
m/s; After, 2.85±0.35 m/s; IASTM: Before, 3.04±0.43 m/s; After, 2.87±0.36 m/s)
without an interaction of condition×time (P=0.130) or main effect of
condition (P=0.355), with a small effect size (d=0.38). For the
plantar fascia, there was a significant condition×time interaction
(P<0.001). The 10.8% decrease (P<0.001) was found in IASTM
condition (Before, 3.59±0.48 m/s; After, 3.16±0.34 m/s; Control: Before, 3.44±0.58
m/s; After, 3.55±0.74 m/s) with a large effect size (d=1.24).
Fig. 3 Individual (circle plots) changes in the shear wave propagation
velocity of the abductor hallucis and plantar fascia under control and
instrument-assisted soft tissue mobilization (IASTM) conditions. The rhombus
indicates the mean value. *Significant change after intervention with a
significant main effect of time. † Significant change after intervention
with a significant interaction of time and condition.
The individual changes in navicular height and foot contact area are shown in [Fig. 4]. There was no significant
interaction (P=0.262) or main effect (time: P=0.292; condition:
P=0.645) for navicular height (Control: Before, 4.18±0.49 cm; After,
4.17±0.45 cm; IASTM: Before, 4.18±0.48 cm; After, 4.28±0.44 cm), with a small effect
size (d=0.30). Similarly, no significant interaction or main effects
(P=0.332–0.769) were found for the foot contact area (Control: Before,
163.6±15.0 cm2; After, 160.9±16.7 cm2; IASTM: Before,
163.6±10.9 cm2; After, 163.0±14.8 cm2, d=0.16).
Fig. 4 Individual (circle plots) changes in navicular height and foot
contact area under control and instrument-assisted soft tissue mobilization
(IASTM) conditions. The rhombus indicates the mean value.
[Fig. 5] shows the relationship between
the shear wave propagation velocity of the plantar fascia before IASTM and its
relative changes. There was a negative correlation between the variables
(r=− 0.660, P<0.001).
Fig. 5 Relationship between the shear wave propagation velocity of the
plantar fascia before instrument-assisted soft tissue mobilization (IASTM)
and change in shear wave velocity in IASTM condition.
Discussion
To the best of our knowledge, this is the first study that clarified the acute
effects of IASTM on the stiffness of the abductor hallucis and plantar fascia
comprising the medial longitudinal arch and foot structure. The present study
revealed that IASTM on the plantar surface substantially decreased the resting
plantar fascia stiffness. The decrease was greater in individuals with a higher
stiffness of the plantar fascia. In contrast, the effects of IASTM on resting
abductor hallucis stiffness, navicular height during sitting, and foot contact area
while standing on both feet were not substantial. The present results demonstrated
that IASTM on the plantar surface affected tissue stiffness but did not change the
structure of the foot.
IASTM significantly reduced plantar fascia stiffness ([Fig. 3]). While the mechanism by which
IASTM alters tissue stiffness remains unclear, a review [12] suggests potential influences such as
fascial release, increased blood flow, and desensitization of pain receptors, which
might be related to the current results. Brandl et al. [26] demonstrated an increase in
bioimpedance following IASTM, likely attributed to a decrease in water content
within the targeted tissues. This alteration could potentially affect the
viscoelastic properties of the treated tissues, as indicated by the reduced plantar
fascia stiffness observed in the current study. In contrast, the effect of IASTM on
abductor hallucis stiffness was unclear ([Fig. 3]). An increase in the sample size may result in a significant
interaction; however, the expected effect is anticipated to be small. Hence, the
likelihood of obtaining a practically meaningful effect is low. These results are
consistent with those of previous studies [13]
[17]. Bailey et al. [13] reported a decrease in stiffness of
the infraspinatus through IASTM. They specifically mentioned conducting IASTM
sessions to reduce muscle stiffness. Conversely, Ikeda et al. [17] did not observe a decrease in muscle
stiffness when using IASTM. They performed a wide range of IASTM not only on muscles
such as the gastrocnemius and soleus but also on surrounding tissues and
muscle-tendon junctions. The muscle lengths and sizes of the medial gastrocnemius
and soleus [27] are greater than those
of the infraspinatus [28] and foot
region, and the area beneath the skin is limited in the case of the soleus muscle.
Within limited instrument sizes or time frames, the effectiveness of mobilization
may vary depending on the length and size of the targeted muscle, which might have
posed challenges in observing the effects of IASTM in the previous study [17]. In the current study, IASTM was
applied to the plantar surface by a well-practiced practitioner, without direct
intervention on the skin above the abductor hallucis, which would be related to the
substantial effect of IASTM on plantar fascia stiffness but not on abductor hallucis
stiffness. Taken together, to reduce stiffness using IASTM, it may be necessary to
directly apply it to targeted tissues.
No significant changes in navicular height during sitting and foot contact area
during standing were observed following IASTM ([Fig. 4]), despite the substantial
decrease (d=1.24) in plantar fascia stiffness. Theoretically, reduced
stiffness would result in a corresponding alternation of the medial longitudinal
arch height. The present results suggest that although sufficient reduction in
plantar fascia stiffness was achieved through IASTM, inducing changes in the medial
longitudinal arch height proved to be challenging. Consistent with the results of
the present study, load-induced changes in plantar fascia stiffness were poorly
associated with changes in the foot shape [15]. Previous cross-sectional studies showed that plantar fascia
stiffness was not different between individuals with normal foot and flat foot in
unloading [14] and standing positions on
both feet [29]. Furthermore, a modeling
study [30] demonstrated that a decrease
in the medial longitudinal arch height associated with a reduction in plantar fascia
stiffness was less likely in stiffer plantar fascia. In the current study, it was
found that individuals with a stiffer baseline demonstrated a greater reduction in
the stiffness of the plantar fascia ([Fig.
5]). Combining the findings of the present study with those of previous
research suggests that individuals with initially higher plantar fascia stiffness
are more prone to experiencing a reduction in stiffness; conversely, a decrease in
medial longitudinal arch height is less likely to occur in these individuals.
Collectively, the impact of stiffness changes in the plantar fascia on the medial
longitudinal arch height would be small.
For the lack of significant change in the foot contact area, another possible factor
is the potential improvement in neuromuscular activation of the intrinsic foot
muscles under loading conditions. Osailan et al. [31] reported an increase in muscular
power after a single session of IASTM. These findings imply that IASMT can enhance
neuromuscular activation of the mobilized muscle. Fiolkowski et al. [2] showed that the reduced neuromuscular
activation of the abductor hallucis due to the tibial nerve block was accompanied by
increased navicular drop, implying that the abductor hallucis activation prevents
the load-induced decrease in the medial longitudinal arch height. This suggests that
the facilitation of intrinsic foot muscle activities by IASTM may counteract a
decline in the medial longitudinal arch height induced by a decrease in plantar
fascia stiffness. As we did not obtain any neuromuscular activation data in the
current study, further research is necessary to clarify this matter.
The present study has some limitations. First, we evaluated the dependent variables
in resting and static conditions. Careful attention is required to associate the
role of the investigated tissues during dynamic movements with the results of the
current study. Second, the participants in this study were limited to young males;
thus, it is uncertain whether similar results would be obtained in females or older
individuals. A previous study reported that the stiffness of the plantar fascia was
similar between females and males [32],
whereas another study demonstrated lower stiffness in males [33]. Older individuals have been reported
to exhibit lower stiffness in muscle and fascia tissues than young individuals [18]. These may suggest the effectiveness
of IASTM may be smaller in males and older individuals. Finally, we measured only
the shear wave propagation velocity of the abductor hallucis and plantar fascia from
one region. It was reported the regional differences in the plantar fascia stiffness
along the foot [33]. Future research
should investigate whether IASTM affects other muscles or tissues constructing the
medial longitudinal arch, and whether such effects are similar among the different
regions of the tissues within the foot.
The current study provides some practical implications. The present results indicated
for the first time that IASTM can be applied to the foot region. Considering the
different magnitudes of the IASTM effect on the plantar fascia and abductor hallucis
stiffness, IASTM should be directly applied to tissues to achieve a substantial
effect on their stiffness. A key finding is that the greater the stiffness of the
plantar fascia, the more significant the reduction in stiffness by IASTM ([Fig. 5]). This suggests that stiffness
at baseline can be useful in predicting the IASTM effect, which would help the use
of IASTM in sports and rehabilitation situations (e. g. in type II diabetes patients
who have stiff plantar fascia [8]).
In conclusion, the present study revealed that instrument-assisted soft tissue
mobilization (IASTM) on the plantar surface decreased plantar fascia stiffness. The
effect was greater in individuals with higher stiffness of the plantar fascia,
suggesting the practical usefulness of IASTM in the foot region. In contrast, the
effects of IASTM on abductor hallucis stiffness, navicular height, and foot contact
area were not significant. These results might be related to the lack of direct
IASTM intervention on the skin above the abductor hallucis and potential improvement
in neuromuscular activation of the intrinsic foot muscles by IASTM. The findings
provide robust evidence that IASTM affects tissue stiffness without affecting foot
structure.
