International Journal ofEnvironmental Researchand Public Health

ArticleContribution of Plantar Fascia and Intrinsic Foot Muscles in aSingle-Leg Drop Landing and Repetitive Rebound Jumps:
An Ultrasound-Based Study
Masanori Morikawa 1 , Noriaki Maeda 1 , Makoto Komiya 1 , Arisu Hirota 1 , Rami Mizuta 1 ,Toshiki Kobayashi 2 , Kazuki Kaneda 1 , Yuichi Nishikawa 3 and Yukio Urabe 1, *
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Citation: Morikawa, M.; Maeda, N.;
Komiya, M.; Hirota, A.; Mizuta, R.;
Kobayashi, T.; Kaneda, K.; Nishikawa,
Y.; Urabe, Y. Contribution of Plantar
Fascia and Intrinsic Foot Muscles in aSingle-Leg Drop Landing and
Repetitive Rebound Jumps: An
Ultrasound-Based Study. Int. J.
Environ. Res. Public Health 2021, 18,4511. https://doi.org/10.3390/
ijerph18094511
Academic Editors: Elena Cavarretta,
Fabrizio Perroni and Erica Gobbi

Division of Sport Rehabilitation, Graduate School of Biomedical and Health Sciences, Hiroshima University,
1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan; m-masanori@hiroshima-u.ac.jp (M.M.);
norimmi@hiroshima-u.ac.jp (N.M.); makoto-komiya@hiroshima-u.ac.jp (M.K.);
arisu-hirota@hiroshima-u.ac.jp (A.H.); m202263@hiroshima-u.ac.jp (R.M.);
kazuki-kaneda@hiroshima-u.ac.jp (K.K.)
Department of Biomedical Engineering, Faculty of Engineering, The Hong Kong Polytechnic University,
11 Yuk Choi Road, Hung Hom, Hong Kong, China; toshiki.kobayashi@polyu.edu.hk
Faculty of Frontier Engineering, Institute of Science & Engineering, Kanazawa University,
Kanazawa 076-264-5111, Japan; yuichi@se.kanazawa-u.ac.jp
Correspondence: yurabe@hiroshima-u.ac.jp

Abstract: The plantar fascia and intrinsic foot muscles (IFM) modulate foot stiffness. However, it isunclear whether the corresponding ultrasonography findings reflect it. This study aimed to examine
the effect of the plantar fascia and IFM morphologies on force attenuation during landing andreactivity when jumping in healthy adults (n = 21; age, 21–27 years). Thickness, cross-sectional area
(CSA), and hardness of the plantar fascia, abductor hallucis (AbH), and flexor hallucis brevis (FHB)
muscles were measured using ultrasonography. Single-leg drop landing and repetitive reboundjumping tests assessed the ground reaction force (GRF) and reactive jump index (RJI), respectively.
The CSA of FHB was negatively correlated with maximum vertical GRF (r = −0.472, p = 0.031) in the
single-leg drop landing test. The CSA of AbH was negatively correlated with contact time (r = −0.478,
p = 0.028), and the plantar fascia thickness was positively correlated with jump height (r = 0.615,
p = 0.003) and RJI (r = 0.645, p = 0.002) in the repetitive bound jump test. In multivariate regression
analysis, only the plantar fascia thickness was associated with RJI (β = 0.152, 95% confidence interval:
7.219–38.743, p = 0.007). The CSA of FHB may contribute to force attenuation during landing.
The thickness of the plantar fascia and CSA of AbH may facilitate jumping high with minimal
contact time.

Received: 25 March 2021
Accepted: 19 April 2021
Published: 23 April 2021

Keywords: muscle thickness; cross-sectional area; muscle hardness; drop landing; reactive jump index

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1. Introduction
The human foot comprises 28 bones that form three flexible arch structures. The
stiffness of these foot arches is passively and actively modulated by the plantar fascia andthe intrinsic foot muscles (IFM), respectively [1]. The plantar fascia and IFM decreaseor increase foot stiffness to attenuate external forces or transmit the internal force of theextrinsic foot muscle to the foot, respectively [2]. The IFM have been shown to be activatedaccording to load-induced arch deformations [3]. Moreover, IFM can protect the plantar
fascia from excessive strain or modulate foot stiffness in response to increased postural andloading demands [4].
In sports activities, force attenuation while landing is necessary to prevent injury [5];meanwhile, lower-limb stiffness at the time of jumping contributes to the ability to jumphigher with minimal ground contact time [6]. Indeed, athletes land and jump during

Int. J. Environ. Res. Public Health 2021, 18, 4511. https://doi.org/10.3390/ijerph18094511

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Int. J. Environ. Res. Public Health 2021, 18, 4511

2 of 10training and competition [7], acting on the plantar fascia and IFM to attenuate and activate
different types of force, as required.
In recent years, ultrasonography has been used to evaluate the morphologies of theplantar fascia and IFM; their relationship with performance has been investigated in gait [8],running [9], and static [10] and dynamic [11] balance tests. However, it remains unclear
whether the morphologies of the plantar fascia and IFM are related to force attenuation
during landing or reactivity during jumping. This study aimed to examine the relationshipbetween the plantar fascia and IFM morphologies, and force attenuation during landing
and reactivity when jumping among healthy adults.
2. Materials and Methods
Twenty-three healthy recreationally active collegiate students were recruited for participation in this study from Hiroshima University. “Recreationally active” was defined as
participation in at least 150 min of moderate activity per week for at least 6 months [12].
Individuals were ineligible for this study if they met any of the following criteria: (1) historyof lower limb trauma; (2) neurological conditions; and (3) plantar fasciitis, ligamentous
injuries, bursitis, or orthopedic injuries to the lower limb.
The study was conducted in accordance with the principles of the Declaration of
Helsinki and was approved by the Ethics Committee for Epidemiology of Hiroshima
University (approval number: E-2090). All participants provided informed consent tostudy participation.
2.1. B-Mode Ultrasonography and Real-Time Tissue Elastography
Morphologies of IFM and plantar fascia (thickness, cross-sectional area (CSA), andhardness) were imaged by B-mode ultrasonography (HI VISION Avius; Hitachi Aloka
Medical, Tokyo, Japan) and real-time tissue elastography (RTE). An 8-MHz linear arrayprobe was used to obtain a continuous ultrasound image. An acoustic coupler (EZUTECPL1; Hitachi Aloka Medical, Tokyo, Japan) was put on the transducer with a plastic
attachment (EZU-TEATC1; Hitachi Aloka Medical, Tokyo, Japan) and used as referencematerial. The hardness value of reference material was 22.6 ± 2.2 kPa, which was a relativevalue based on the acoustic coupler in the material tests conducted by the manufacturer.
2.2. Evaluation of Thickness and CSA
Thickness and CSA values of selected tissue were measured with B-mode ultrasonography, as reported previously [10,13]; the aim was to measure the selected tissue in thecontraction-free position using a linear array probe. The following measurements weretaken in the prone position, with the subjects’ ankles in a neutral position and knees flexed
at 90◦ to minimize pressure on the tissues while ensuring that the probe made the best
contact with the skin:
1.
2.
3.

Thickness of the abductor hallucis (AbH), flexor hallucis brevis (FHB), and flexor
digitorum brevis (FDB) muscles;
CSA of AbH, FHB, and FDB, and;Thickness of the plantar fascia in the calcaneus.

The position of the probe was marked on the skin surface with semipermanent ink.
The probe was placed along a line perpendicular to the long axis of the foot on the anterior
aspect of the medial malleolus to record the CSA image of AbH; it was subsequently placed
approximately perpendicular to the same line to record the thickness image. Furthermore,
the probe was placed perpendicular to a line parallel to FHB to record its CSA image andwas subsequently placed along the same line to record the thickness image. Moreover, theprobe was placed along a line between the third toe and the medial calcaneal tubercle torecord the CSA image of the FDB muscle and was subsequently placed along the same line
to record the thickness image. Moreover, the probe was placed along a line between thesecond toe and the medial calcaneal tubercle to record the thickness image (Figure 1). TheseInt. J. Environ. Res. Public Health 2021, 18, 4511

3 of 10measurements are reliable in assessing the plantar muscle and plantar fascia thickness andCSA [13,14].

Figure 1. The probe’s position in the assessment of selected tissue thickness and CSA. Abbreviation:
AbH, abductor hallucis; FHB, flexor hallucis brevis; FDB, flexor digitorum brevis.

2.3. Evaluation of IFM and Plantar Fascia Hardness
The head of the probe was coated with a transmission gel to achieve acoustic coupling. The RTE was imaged by manually applying repetitive light compression (rhythmic
compression–relaxation cycle) with the transducer in the scan position. To obtain appropriate images for investigation, the examiner responsible for ultrasonography assessment
pressed the transducer against the selected tissue with constant repeated pressure (thestrain ratio in the range of −0.7 to 0.7) and monitored the pressure indicator incorporated
into the ultrasound scanner (Figure 2). For each task, three images were randomly selected,
and the average values were calculated after scanning. The hardness and thickness of theAbH, FHB, and FDB muscles and plantar fascia were measured in the supine position, withthe subjects’ ankles in neutral position and their knees flexed at 90◦ . The RTE assessment
took less than 10 min. The RTE images were displayed as translucent, color-coded, realtime images superimposed on the B-mode images, with the red signal indicating a large

Int. J. Environ. Res. Public Health 2021, 18, 4511

4 of 10

distortion in response to the compression (the softest component), blue indicating the least
distortion (the hardest component), and green representing the mean strain.

Figure 2. Assessment of muscle hardness using RTE and RTE images. We pressed the transducer against the selected soft
tissue with constant repeated pressure (compression–relaxation cycle) by checking the strain graph. Subsequently, we
selected the ROI of the coupler and soft tissue, and the strain ratio (muscle/coupler) was calculated. Abbreviation: RTE,
real-time tissue elastography; ROI, region of interest.

The mean strain in the region of interest (ROI) was monitored on a strain graph toadjust the force and frequency of compression. When the elastogram was successfully
superimposed on the ROI, the frequency was adjusted from 2 Hz to 4 Hz to maximize theimage quality. The strain rate within each ROI was measured automatically, using built-in
software, and the strain ratio (tissue/reference ratio; the value of the strain measured
in the reference ROI divided by the corresponding value obtained in the tissue ROI)
was calculated per image. The RTE in response to the compression force is smaller inharder than in softer tissue; hence, as the tissues become stiffer, the RTE value becomes
smaller [15]. RTE measurements were performed three times, and the mean value within
the ROI from these measurements was used in subsequent analyses. All measurements
and positioning of all ROIs were performed by the same examiner. High intra-observer
reproducibility was previously reported for the tissue/reference ratio of RTE measurements(tissue/coupler) [16,17].
2.4. Single-Leg Drop Landing Test
The single-leg drop landing test was conducted using the center of pressure (COP)
trajectory, and the vertical ground reaction force was measured using a force plate (Type9281 B; Kistler, Switzerland), 10 cm in height. The participants stood one-legged on theirright foot, static and barefoot on a box, 30 cm in height. They were asked to jump in a
forward direction from the box on a cue given by voice guidance from a personal computerwith relevant software (Dynamic balance evaluation applications ver. 1.2.5.1, Technology
Service Co., Ltd., Nagano, Japan). They were instructed to jump with their hands foldedin front of their chest, landing with their right foot on the center of the force plate placedin front of the box. The participants were asked to maintain the single-legged standing
posture for 5 s after landing, and to continue keeping both hands folded in the same
position. A 10-s interval was set between trials. Trials were excluded from the data analysis
when a participant (1) could not maintain a single-leg standing posture for 5 s after landing;(2) was disengaged from the force plate during or after landing; (3) moved the base of
support by shifting their foot after landing, trying to maintain standing posture; or (4)released their folded arms. The landing test was repeated until three successful trials were
performed and used for data analysis.

Int. J. Environ. Res. Public Health 2021, 18, 4511

5 of 102.5. Repetitive Rebound Jump Test
Repetitive rebound jump (RJ) performance was assessed using an OptojumpTM system(Microgate, Bolzano, Italy). This system consists of two infrared photocell bars, with one
bar acting as a transmitter unit containing 96 light-emitting diodes positioned 0.003 m
above the ground, while the other bar acts as a receiver unit. Participants were instructed
to keep their hands on their hips to avoid upper-body interference, while jumping andlanding repetitively. They were asked to land on the same spot each time with legs initiallyextended and then flexed, while looking ahead at all times. Additionally, the participants
were asked to jump as high as possible and minimize the ground contact time, while
jumping again immediately. When a participant performed the RJ within the parallel bars
placed on the ground, the light was interrupted by the participant’s foot during the jump,which triggered the sensor in the unit to record a sampling frequency of 1000 Hz. Twosets of seven repetitive rebound jumps were performed with a 5-min rest between sets. Of
seven repetitive jumps in the second set, the second through fifth jumps were used in theanalysis, as this approach is associated with good inter-day reliability [18].
2.6. Data Collection
The voltage signal output from the force plate was amplified by an amplifier(Type9865E1Y28, Kistler, Switzerland) and digitized by an A/D converter (NI-USB6218BNC,
National Instruments, Austin, Texas, USA) at a sampling frequency of 1000 Hz. The force
plate data were filtered using a zero-lag, fourth-order, low-pass Butterworth filter with afrequency cut-off of 15 Hz. The vertical ground reaction force was analyzed for 5 s afterground contact and was defined as a vertical component of >5 N. The locus length ofthe center of pressure (COP) normalized by foot length (% foot length) was calculated
to quantify the magnitude of postural sway and the maximum vertical ground reaction
force normalized by body weight (VGRFmax) (% body weight, BW); time to peak (ms) andloading rate (% BW/ms) were similarly estimated to quantify force attenuation during
landing. The loading rate was defined as the peak of the first derivative of the force–timecurve [19]. The average of the values obtained in three successful trials was used for the
additional analysis.
OptojumpTM proprietary software (OptojumpTM Next software, version 1.9.9.0) wasused to automatically calculate the RJ performance variable, which included jump height(cm), contact time (s), and reactive jump index (RJI) (m/s). The jump height was estimated as:(1/2 × Tair × g)2 × (2 g)−1 ,(1)where Tair is the flight time (s) from the force record on the force plate, and g is theacceleration due to gravity (9.81 m/s2 ). The RJI was estimated as 1/8 × g × Tair2 /contact
time. The average of the values obtained from five repetitive jumps was used for the
additional analysis. Rebound jump index was used for assessing reactive strength, which
is an athlete’s ability to efficiently brake and absorb (eccentric) forces within specific timeframes, before subsequently generating a propulsive (concentric) force [18].
2.7. Statistical Analysis
Data were analyzed using SPSS (IBM Japan Co., Ltd., Tokyo, Japan). The Shapiro–
Wilk test was used to test the normality of data distribution. Normally and non-normally
distributed variables were presented as the mean ± standard deviation (SD) and the medianand interquartile range (IQR), respectively. Correlation coefficients were calculated with thePearson product-moment correlation test or Spearman rank correlation test, depending ondata distribution, and were used to estimate the possible relationships between thickness,
CSA, and RTE values of the assessed tissue (AbH, FDB, and FHB muscles and plantar
fascia), and rebound jump and drop landing assessment results. p-values of < 0.05 wereconsidered statistically significant.
Multiple linear regression with forced entry was performed to examine the association
between selected tissue morphology and rebound jump performance, which was usedInt. J. Environ. Res. Public Health 2021, 18, 4511

6 of 10as an RJI. To avoid overfitting due to the small sample size, we used variables withcorrelation coefficients of <0.10 between selected tissue morphology and RJ performanceas objective variables.
As performed in a previous study [20], a post hoc power analysis was conducted. The
procedure accounted for the population effect size (f2 ), the “α err prob,” sample size, andthe number of predictors included in the regression model to estimate the power of theomnibus F-test (“Power [1 err prob]”).
3. Results
Two of 23 prospective participants were excluded from this study due to hallux valgus.
The mean, median, IQR, and SD values of ultrasonographic evaluations and repetitive
rebound jump and drop landing assessments are presented in Table 1.
Table 1. Median (interquartile range) and Mean (standard deviation) Values of Assessed Parameters.
Parameters

Median(Interquartile Range)

Mean (Standard
Deviation)Age(years)23.0 (22.0–24.5)

23.3 ± 1.9

Body height(cm)

169.0 · (164.5–173.0)

169.8 ± 0.8

Body weight(kg)

60.0 · (54.0–65.5)

59.9 ± 9.0

BMI(kg/m2 )20.3 · (19.0–23.1)

20.7 ± 2.2

Foot length(cm)

24.7 · (24.1–25.7)

24.8 ± 1.4

Thickness of selected tissues (mm)11.0 · (9.5–12.4)
12.2 · (9.6–13.5)
9.6 · (6.3–11.3)
3.0 · (2.9–3.2)

Abductor hallucis
Flexor hallucis brevis
Flexor digitorum brevis
Plantar fascia

11.1 ± 1.9
11.4 ± 2.4
8.8 ± 2.7
3.0 ± 0.4

Cross-sectional area of selected tissues (mm2 )261.3 · (179.0–285.8)
236.2 · (220.8–273.7)
204.0 · (175.2–259.2)

Abductor hallucis
Flexor hallucis brevis
Flexor digitorum brevis

245.0 ± 66.2
244.8 ± 39.0
214.3 ± 50.4

Ultrasound elastography of selected tissues (muscle/coupler)
1.20 · (0.65–1.95)
1.60 · (1.30–2.21)
2.60 · (1.70–3.70)
0.30 · (0.20–0.55)

Abductor hallucis
Flexor hallucis brevis
Flexor digitorum brevis
Plantar fascia

1.28 ± 0.76
1.95 ± 1.03
2.70 ± 1.24
0.40 ± 0.29

Drop landing test
COP length
VGRFmax
Peak time
Loading rate(%foot length)
(%BW)
(ms)
(%BW/s)24.6 · (21.6–27.3)
439.2 · (398.0–540.8)
54.0 · (48.8–62.8)
7.7 · (6.5–10.1)

25.4 ± 4.8
458.4 ± 90.6
57.2 ± 10.4
8.3 ± 2.5

Repetitive rebound jump test
Contact time
Jump height
Reactive jump index

(s)
(cm)
(cm/s)

0.30 · (0.27–0.33)
12.9 · (10.8–16.3)
45.6 · (34.9–53.4)

0.30 ± 0.05
13.5 ± 3.5
46.0 ± 15.1

Abbreviation: BMI, body mass index; COP, center of pressure; VGRFmax, maximum vertical grandreaction force; BW, body weight.

The CSA of FHB was negatively correlated with VGRFmax in forward drop landing(r = −0.472, p = 0.031) (Table 2). The thickness of the plantar fascia was negatively correlatedwith the COP length (r = −0.513, p = 0.018). In addition, the AbH muscle size and plantar
fascia thickness were correlated with increased RJ performance.

Int. J. Environ. Res. Public Health 2021, 18, 4511

7 of 10

Table 2. Correlation Analysis between Ultrasonographic Measurements and Repetitive Rebound Jump and Single-Leg Drop
Landing Assessment.
Drop Landing Test

Variables
COP Length

Thickness of selected tissues

Loading Rate

Contact Time

Jump Height

Reactive Jump
Index

r

p

r

p

r

p

r

p

r

p

r

p

r

p

0.030
−0.229−0.311−0.513 *

0.897
0.319
0.169
0.018

−0.043−0.055−0.211
0.051

0.854
0.811
0.360
0.825

0.093
−0.166
0.205
0.028

0.687
0.472
0.373
0.905

−0.0590.137−0.216
0.083

0.800
0.555
0.346
0.721

−0.230−0.0500.104−0.425

0.316
0.830
0.654
0.055

0.215
0.253
0.360
0.615 **

0.349
0.269
0.109
0.003

0.212
0.194
0.311
0.645 **

0.357
0.399
0.169
0.002

−0.055−0.163−0.291

0.814
0.481
0.201

−0.051−0.472 *−0.279

0.825
0.031
0.221

0.358
0.200
0.278

0.111
0.384
0.222

−0.176−0.339−0.351

0.447
0.132
0.119

−0.478 *
0.117
0.088

0.028
0.613
0.706

0.258
−0.066
0.333

0.260
0.776
0.141

0.376
−0.059
0.240

0.093
0.801
0.294

0.012
−0.023
0.004
0.219

0.957
0.920
0.987
0.341

−0.054−0.0170.292−0.071

0.815
0.942
0.199
0.760

−0.159−0.147−0.220−0.150

0.491
0.526
0.338
0.515

−0.011
0.032
0.267−0.028

0.962
0.891
0.242
0.903

0.159
−0.259−0.194−0.092

0.491
0.257
0.400
0.692

0.063
0.190
0.073
−0.299

0.786
0.410
0.753
0.187

0.008
0.227
0.153
−0.211

0.971
0.322
0.507
0.358

(mm2 )

Abductor hallucis a
Flexor hallucis brevis a
Flexor digitorum brevis a
Ultrasound elastography of selected tissues

Time to Peak(mm)

Abductor hallucis a
Flexor hallucis brevis a
Flexor digitorum brevis b
Plantar fascia of calcaneal portion a
Cross-sectional area of selected tissues

VGRF Max

Repetitive Rebound Jump Test

(tissue/coupler)

Abductor hallucis a
Flexor hallucis brevis b
Flexor digitorum brevis a
Plantar fascia b* p < 0.05, ** p < 0.01. a Each variable was tested by the Pearson correlation coefficient. b Each variable was tested by the Spearman’srank correlation coefficient. Abbreviation: COP, center of pressure; VGRFmax, maximum vertical grand reaction force normalized bybody weight.

Table 3 summarizes the results of multiple regression analysis of the association
between IFM and plantar fascia morphology and RJ performance. There was no association
between contact time, the CSA of AbH, and the thickness of the plantar fascia. Only the
thickness of the plantar fascia was associated with the RJI. In post hoc power analysis, themultivariate regression model for the association between ultrasonographic assessment
items and the rebound jump index demonstrated sufficient power (1 err prob = 0.836).
Table 3. Multiple Regression Analysis of the Association between Ultrasound Assessment and Repetitive Rebound Jump.
Contact Time
Variables

CSA of abductor hallucis
Thickness of plantar facia

β−0.369−0.284

95% CI interval
Lower

Upper

−0.001−0.086

0.000
0.019

Reactive Jump Index
p
0.101
0.200

β
0.152
0.587

95% CI interval
Lower

Upper

−0.0577.219

0.126
38.743

p
0.437
0.007

Explanatory variables were CSA of abductor hallucis and thickness of plantar fascia. β, standardized partial regression coefficient; CI,
confidential interval; COP, center of pressure; CSA, cross-sectional area.

4. Discussion
The main finding of this study indicated that the morphology of the FHB muscle andplantar fascia was related to force attenuation in single-leg drop landing; meanwhile, theAbH muscle and the plantar fascia were related to reactivity in the repetitive rebound jump
test. Moreover, the thickness of the plantar fascia is associated with the RJI. Since the RJI is
considered to be the indicator of an athlete’s ability to efficiently brake and absorb eccentric
forces within specific time frames, before subsequently generating a propulsive concentric
force, this present study suggests that the thicker plantar fascia may indicate an ability tojump higher with minimal contact time during jumping.
Olsen et al. [21] reported the kinematic characteristics of the metatarsophalangeal(MTP) joint during forward drop landing. At the moment of ground contact, the first MTP
joint was rapidly extended to 20◦ . This phenomenon may increase the absorptive functionof the foot by engaging the windlass mechanism and tightening the plantar fascia. In thisstudy, we showed that a larger FHB correlated with a lower VGRFmax during single-leg
drop landing. This finding suggests that the eccentric contraction of the FHB muscle
may attenuate the force in response to toe extension during ground contact. Anotherpossible explanation is that the FHB muscle is anchored to the calcaneus in the medial
tubercle region (i.e., the origin of the plantar fascia) through its attachment to the medial

Int. J. Environ. Res. Public Health 2021, 18, 4511

8 of 10intermuscular septum [22]. In other words, as in the windlass mechanism of the plantar
fascia, the contraction of the FHB muscle may modulate the stiffness of the MTP joint
and foot.
The present study showed a significant negative correlation between plantar fascia
thickness and COP length in single-leg drop landing. Generally, COP is transferred bypostural sway. If its structure is too flexible, postural sway would increase because it needs
to be compensated by proximal joint. In other words, the ability to regulate the stiffnessof the foot structure is considered to be important for reducing COP displacement. The
plantar fascia is a fiber that attaches from the calcaneus to the proximal phalanx, and canchange the stiffness of the foot structure through the windlass mechanism [23]. Thus, inthe case of single-leg drop landing phase, the plantar fascia would be stretched by theextension of the toes and contraction of the triceps surae muscle, and can respond to thepostural demand for loading. A thick plantar fascia may have a high potential for thatdemand, thus the correlation between plantar fascia thickness and COP may have been
observed in the current study.
To the best of our knowledge, this is the first study to showing a relationship between
the CSA of AbH and RJI. A previous study showed that the RJI is positively correlatedwith the height of the medial longitudinal arch [24]. This finding indicates that the medial
longitudinal arch plays a role in storing and releasing elastic strain energy during repetitive
dynamic movement. AbH is the largest single muscle of the IFM [25] and arises from theprocess of calcaneal tuberosity at the base of the first phalanx of the hallux and supports themedial longitudinal arch. Indeed, the CSA of AbH contributes to the height of the medial
longitudinal arch [26]. The present findings expand on the relationship between the RJI
and the IFM, using ultrasonography assessment.
This study has demonstrated an association between the thicker plantar fascia andgreater RJI. A previous study explored the relationship between the thickness of the plantar
fascia and the RJI and reported likely correlation between them [27]. The RJI reflects theability to jump as high as possible with minimal contact time; the spring-like function of thefoot is important to RJ because it efficiently attenuates the impact in the landing phase andgenerates explosive power in the take-off phase. In particular, the plantar fascia contributes
to attenuating the impact and managing the stiffness of the foot through the bow-stringarrangement of the medial longitudinal arch [28]. Moreover, as the triceps fascia tension
increases, so does that of the plantar fascia [1]. This is called the Achilles–calcaneus–plantar
system, which is known to effectively transport the force generated by the lower limb to
the foot during a jump [29]. Overall, the thicker the plantar fascia, the higher the potential
for storing and releasing energy, which may improve RJ performance in healthy adults.
Previously, it was unclear if the morphology assessed by ultrasound of the plantar
fascia and IFM could reflect sports performance during the drop landing and repetitive
rebound jump. In the present study, these soft tissue morphologies and actual sports performance were correlated. Therefore, this indicated the possibility to assess the plantar fascia
and IFM by using ultrasound imaging devices in order to quantify sports performance inthe future. In addition, it was also unknown which of plantar fascia or IFM affect attenuating force during landing and repetitive rebound jumping. This study demonstrated therelationship between the plantar fascia, flexor hallucis brevis, and abductor pollicis brevis
and sports performances. These results may be important findings for specific training forthe foot and consideration of the mechanism of injury in the future.
This study has some limitations that need to be considered when interpreting its
findings. First, this study only included healthy adults without foot abnormalities. However, previous studies have shown that the plantar fascia is thicker in populations withplantar tendinitis [30]; thus, a relationship different from that presented in this study may
be observed in this population. Second, this study did not account for the impact of sex. A
previous study reported that the menstrual cycle may affect the thickness of the plantar
fascia. However, this association was not examined in this study; therefore, further researchis required to elucidate the differences between men and women in this context. Third,

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we assessed the foot morphology in static and non-weightbearing position, so we could
not investigate the dynamics of the intrinsic foot muscles. Fourth, other intrinsic footmuscles, such as quadratus plantae and extensor digitorum brevis, were not assessed byultrasonography. Lastly, this study recruited subjects engaged in not the same but a varietyof sports (such as baseball, basketball, classic ballet, football, orientating, swimming, tennis,
track and field, volleyball, and weightlifting). Therefore, further research including avariety of loading conditions and/or intrinsic foot muscles in the same sports player isneeded to understand foot functions during sports-related movement.
5. Conclusions
In conclusion, there were significant correlations between the thickness of the plantar
fascia and the CSA of FHB and VGRFmax during landing, and correlations between theplantar fascia and AbH and reactivity during repetitive jumping among healthy adults.
Thus, the present findings suggest that the CSA of FHB contributes to force attenuation
during landing, and that the thickness of the plantar fascia and the CSA of AbH facilitate
jumping high with minimal contact time during repetitive rebound movement performedby healthy adults.
Author Contributions: Conceptualization, Y.U. and N.M., M.K.; methodology, M.K. and A.H.;
software, A.H. and R.M.; formal analysis, M.M.; investigation, M.M., M.K., A.H. and R.M.; data
curation, M.K., A.H., R.M.; writing—original draft preparation, M.M.; writing—review and editing,Y.U., N.M., M.K., K.K., T.K. and Y.N.; visualization, A.H.; supervision, Y.U.; project administration,
Y.U., N.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research did not receive any specific grant from funding agencies in the public,commercial, or not-for-profit sectors.
Institutional Review Board Statement: The study protocols complied with the principles of the
Declaration of Helsinki and were approved by the Ethical Committee for Epidemiology of Hiroshima
University (approval number: E-2090).
Informed Consent Statement: Informed consent was obtained from all participants.
Data Availability Statement: The data presented in this study are available on request from thecorresponding author. The data are not publicly available due to ethical consideration.
Acknowledgments: We would like to thank all volunteers for their participation in this study.
Conflicts of Interest: The authors declare no conflict of interest.

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