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Track distance runners exhibit 
bilateral differences in the plantar 
fascia stiffness
Hiroto Shiotani1,2, Ryo Yamashita3, Tomohiro Mizokuchi3, Natsuki Sado4, 
Munekazu Naito2,5 & Yasuo Kawakami1,2*
Human steady-state locomotion modes are symmetrical, leading to symmetric mechanical function 
of human feet in general; however, track distance running in a counterclockwise direction exposes 
the runner’s feet to asymmetrical stress. This may induce asymmetrical adaptation in the runners’ 
foot arch functions, but this has not been experimentally tested. Here, we show that the plantar 
fascia (PF), a primary structure of the foot arch elasticity, is stiffer for the left than the right foot 
as a characteristic of runners, via a cross-sectional study on 10 track distance runners and 10 
untrained individuals. Shear wave velocity (index of tissue stiffness: SWV) and thickness of PF and 
foot dimensions were compared between sides and groups. Runners showed higher PF SWV in their 
left (9.4 ± 1.0 m/s) than right (8.9 ± 0.9 m/s) feet, whereas untrained individuals showed no bilateral 
differences (8.5 ± 1.5 m/s and 8.6 ± 1.7 m/s, respectively). Additionally, runners showed higher left to 
right (L/R) ratio of PF SWV than untrained men (105.1% and 97.7%, respectively). PF thickness and 
foot dimensions were not significantly different between sides or groups. These results demonstrate 
stiffer PF in the left feet of runners, which may reflect adaptation to their running-specific training that 
involves asymmetrical mechanical loading.
During human locomotion, the medial longitudinal arch of the foot is lowered while being stretched out in 
response to weight-bearing, and then recoils as the load is removed. Such a spring-like property of the foot arch 
helps to attenuate impact forces and store/release elastic strain energy leading to energy saving in  running1,2. 
Previous studies indicate that the foot arch elasticity is attributed to the plantar fascia (PF)1,3,4. PF behaves vis-
coelastically under  load5,6, and its resistive tension helps to prevent the lengthening and lowering of the foot 
arch. During each foot contact of running, PF is repetitively loaded with the tension reaching as high as 0.6–3.7 
times bodyweight with its longitudinal strain up to 6%7–10. Such sizable stress concentrates around the proximal 
site of  PF11–13, which may be associated with the heterogeneity of mechanical and morphological properties (e.g., 
stiffness and thickness) of  PF14–16 as well as the occurrence of plantar  fasciitis17,18.
The localized stiffness of PF can be quantitatively assessed as the shear wave velocity (SWV) in vivo14–16. PF 
has higher SWV (i.e., stiffer) at the proximal site than middle and distal  sites14,16. Additionally, long-distance 
running induced a transient decrease of SWV at the proximal site of PF while long-distance runners showing 
smaller changes in SWV than untrained  individuals15, suggesting that runners had built up a more resilient 
PF. These findings are evidence of PF adaptability to site-specific and chronic mechanical stress, which can be 
reflected in its stiffness and morphology.
Human steady-state locomotion modes are symmetrical, leading to symmetric mechanical function of human 
feet in general; however, track distance running is performed always in a counterclockwise direction, i.e., the left 
leg being inside during curve running. In this phase, runners are required to generate greater forces with their 
left  legs19,20 to exert centripetal  force21. This is associated with the greater load on the left foot, resulting in the 
lowering of the left foot arch, and thus leading to an increase of mechanical stress to PF. Therefore, runners’ feet 
can be exposed to asymmetrical stress during running. This may induce asymmetrical adaptation in runners’ PF 
stiffness and morphology. Although PF SWV and thickness, and the foot dimensions were comparable between 
left and right sides in a healthy and untrained  population14, this may not be true for track distance runners. If 
OPEN
1Faculty of Sport Sciences, Waseda University, Saitama, Japan. 2Human Performance Laboratory, Comprehensive 
Research Organization, Waseda University, Tokyo, Japan. 3School of Sport Sciences, Waseda University, Saitama, 
Japan. 4Faculty of Health and Sport Sciences, University of Tsukuba, Ibaraki, Japan. 5Department of Anatomy, Aichi 
Medical University, Aichi, Japan. *email: ykawa@waseda.jp
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the bilateral differences in runner’s feet can be confirmed, this provides an indication of a threshold of mechani-
cal stress that causes adaptation of PF and foot arch functions. A profound understanding of PF adaptability is 
essential for improvements in their performance as well as prevention of plantar fasciitis.
Therefore, the purpose of this study was to investigate the bilateral differences in mechanical and morpho-
logical properties of PF and foot dimensions in track distance runners, as contrasted to untrained individuals. 
We hypothesized that track distance runners have bilateral differences in PF SWV, thickness, and the foot arch 
height, and that runners show sizable differences in the left to right (L/R) ratios of measured variables as com-
pared to untrained individuals.
Results
In runners, SWV at the proximal site was significantly higher in left (9.4 ± 1.0 m/s) than right foot (8.9 ± 0.9 m/s) 
(p = 0.021, d = 0.813), but not at the middle (p = 0.782, d = 0.073) or distal sites (p = 0.554, d = 0.138) (Fig. 1). Even 
in a lefty runner (n = 1), PF SWV at the proximal site was higher for his left (10.0 m/s) than right foot (9.1 m/s). 
PF SWV at the proximal site was also higher for the left than the right feet both in rearfoot strike (n = 7, left: 
9.0 ± 1.0 m/s and right: 8.6 ± 0.8 m/s, respectively) and forefoot strike runners (n = 3, left: 10.2 ± 0.3 m/s and right: 
9.6 ± 0.5 m/s, respectively). In untrained men, SWV at each measurement site was not significantly different 
between left and right feet (p ≥ 0.222, d ≤ 0.264). PF thickness at each measurement site was not significantly dif-
ferent between left and right feet in runners (p ≥ 0.327, d ≤ 0.141) or untrained men (p ≥ 0.411, d ≤ 0.305) (Fig. 1). 
Foot dimensions were not significantly different between left and right feet in either of runners or untrained 
men (Table 1).
The L/R ratio of SWV at the proximal site was significantly higher in runners than untrained men (p = 0.027, 
d = 1.076), but not at the middle (p = 0.815, d = 0.107) or distal sites (p = 0.421, d = 0.369). The L/R ratios of thick-
ness at any of the measurement sites or foot dimensions were not significantly different between groups (Table 2).
Age, body height, body mass, BMI, and fractions of leg dominance and foot strike pattern were not signifi-
cantly different between runners and untrained men (Table 3). All participants were healthy and free from injury 
of the lower extremity in the past 12 months and had no present or past history of plantar fasciitis. The runners 
had kept habitual running of at least 10 km/week for the past year, mainly on a running track, and their running 
experiences ranged between 9 and 16 years. Their personal best time of 5000 m ranged from 14′ 15 to 15′ 30. 
The untrained participants were either sedentary or lightly active, and none of them had been involved in any 
structured training program or continuous sports participation at least 12 months before the measurements. All 
participants used conventional running shoes rather than minimalist, high cushion, or high motion control shoes.
Figure 1.  Bilateral differences in SWV and thickness of runners and untrained men. Data are shown as 
means ± SD.
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Table 1.  Bilateral differences in foot dimensions of runners and untrained men. Data are shown as 
means ± SD.
Runners (n = 10)
Untrained men (n = 10)
Left
Right
p value
Cohen’s d
Left
Right
p value
Cohen’s d
Foot length (mm)
245.3 ± 7.3
245.9 ± 8.6
p = 0.520
247.4 ± 8.7
248.2 ± 8.1
p = 0.445
d = 0.075
d = 0.107
Dorsal height (mm)
61.1 ± 4.4
60.8 ± 4.2
p = 0.760
60.5 ± 5.5
61.1 ± 4.0
p = 0.671
d = 0.070
d = 0.125
Navicular height (mm)
43.3 ± 6.2
41.9 ± 6.8
p = 0.080
41.3 ± 4.8
40.9 ± 5.4
p = 0.507
d = 0.215
d = 0.078
Arch height ratio (%)
17.7 ± 2.7
17.1 ± 3.0
p = 0.064
16.7 ± 1.8
16.4 ± 1.9
p = 0.400
d = 0.210
d = 0.162
Table 2.  Left/right ratios (%) of individual parameters in runners and untrained men. Data are shown as 
means ± SD. Bold fonts indicate significant difference between runners and untrained men (p < 0.05) with a 
“large” effect size (d ≥ 0.8).
Variable
Runners
Untrained men
p value
Cohen’s d
Shear wave velocity
Proximal
105.1 ± 6.2
97.7 ± 7.6
p = 0.027
d = 1.076
Middle
101.8 ± 12.9
100.2 ± 17.2
p = 0.815
d = 0.107
Distal
104.6 ± 13.7
99.7 ± 12.8
p = 0.421
d = 0.360
Thickness
Proximal
101.3 ± 6.4
99.6 ± 8.0
p = 0.610
d = 0.232
Middle
97.8 ± 5.9
98.8 ± 8.7
p = 0.756
d = 0.141
Distal
102.3 ± 14.1
97.6 ± 11.7
p = 0.429
d = 0.362
Foot length
99.8 ± 1.1
99.7 ± 1.3
p = 0.843
d = 0.083
Dorsal height
100.5 ± 3.9
99.1 ± 6.3
p = 0.583
d = 0.267
Navicular height
103.9 ± 5.8
101.2 ± 4.4
p = 0.270
d = 0.525
Arch height ratio
104.1 ± 6.0
101.6 ± 5.1
p = 0.329
d = 0.449
Table 3.  Physical characteristics of participants. Data are shown as means ± SD. BMI body mass index, RFS 
rear foot strikers, FFS forefoot strikers. Age, body height, body mass, BMI, and fractions of leg dominance and 
foot strike pattern were not significantly different between runners and untrained men.
Variable
Runners
Untrained men
p value
n
10
10
–
Age (years)
22.0 ± 0.7
22.5 ± 1.4
0.309
Body height (m)
1.68 ± 0.04
1.70 ± 0.05
0.392
Body mass (kg)
55.5 ± 4.2
58.4 ± 5.6
0.062
BMI (kg/m2)
19.6 ± 1.2
20.3 ± 1.7
0.113
Dominant leg (Lefty:Righty)
1:9
1:9
1.000
Foot strike pattern (RFS:FFS)
7:3
10:0
0.060
Running experience (years)
11.0 ± 2.2
–
–
Running distance (km/week)
43.7 ± 35.4
–
–
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Discussion
To the best of our knowledge, this is the first study to investigate bilateral differences in the mechanical and 
morphological properties of PF in track distance runners. The most striking finding of the present study was 
that track distance runners showed stiffer PF at the proximal site in their left than the right feet, unlike the 
untrained participants. A number of previous studies addressing running  mechanics22,23 and mechanical and 
morphological properties of the musculotendinous and fascial  tissues15,24 in runners have focused on unilateral 
leg by assuming bilateral symmetry. Our findings however suggest that asymmetry in runners is the major issue 
that needs to be carefully considered.
The greater load on the left foot during curve  running19,20 can induce an increase of mechanical stress on 
PF. We previously revealed that running causes a decrease in PF SWV at its proximal  site15, and this coincides 
with the simulation of stress distribution along  PF11–13. The bilateral differences in PF stiffness at the proximal 
site in runners may reflect the adaptation to such stress accumulation in this region of the left foot during track 
running, regardless of the lateral dominance or foot strike patterns. The fact that PF stiffness in the right feet of 
runners was comparable to that of both feet in untrained individuals suggests a threshold of mechanical stress 
that causes adaptation of PF stiffness, which is side-specific for track runners. The finding that PF can be stiffened 
in response to a sufficient load is valuable for the general population toward injury prevention and rehabilitation 
as well as improvements in human locomotor performance.
Since the runners in this study had no history of plantar fasciitis, they might have been successful examples 
who had been optimally adapted to their running-specific training. However, plantar fasciitis is one of the most 
common injuries in long-distance runners, regardless of their performance  levels25,26. This injury frequently 
occurs around the proximal site of  PF17,18 where the mechanical stress is  concentrated11–13. Thus, there is a clinical 
implication that the left foot of track runners can suffer from a higher incidence of this injury if their training 
adaptation does not work well. The bilateral imbalances in strength, morphology, and running mechanics are 
considered to be risk factors for injury of  runners27,28. Interactions between these factors for the occurrence of 
plantar fasciitis are worth examining in future studies.
No bilateral differences in PF thickness of runners and untrained individuals are consistent with previous 
findings that PF thickness was not different between recreational runners and untrained  individuals15, and that 
PF thickness was not influenced by physical  activity29. These results, together with our findings, discard the pos-
sibility of PF adaptability in terms of its thickness for reducing mechanical stress induced by distance running. 
No bilateral difference in the foot arch dimensions suggests that the arches of both feet of runners can fulfill their 
imposed roles through different mechanical properties with comparable morphology.
We could not obtain the running mechanics and the foot arch deformation during running. This is one of the 
limitations of the present study. As our findings suggest a threshold of mechanical stress that causes adaptation of 
PF stiffness, quantifying the mechanical stress applied to bilateral feet during track running will lead to a better 
understanding of the nature of PF adaptability. Additionally, the runners who participated in the present study 
can be categorized as recreational  level30. Runners of different performance levels (e.g., competitive and elite 
runners) show different running  mechanics31,32 and fatigue  responses33,34. Thus, competitive and elite runners 
have the possibility to exhibit different signs of adaptation in PF and foot morphology. Comparisons between 
runners in different performance levels should be incorporated in these future studies. Moreover, there is a vari-
ation in the training volume of runners (Table 3). As we previously reported that long-distance running induced 
transient decreases of PF  SWV15, the training volume can be a potential factor that affects PF properties and their 
adaptation. In addition, the material and mechanical properties of the running surface (e.g., rubber, asphalt, 
or grass) as well as shoe sole have the possibility to affect the magnitude of stress to the  foot35–38. Future studies 
addressing the chronic effects of training volume and environment on PF adaptation as well as foot arch functions 
are needed. Lastly, it can be assumed that sprinters, participating in the event of 200 and 400 m in particular, and 
long/high jumpers may also apply asymmetrical stress to their feet with a greater magnitude of stress compared 
to distance runners. Further investigation of bilateral differences in PF characteristics and foot dimensions in 
other events and sports athletes can be an option of the future theme in understanding PF adaptability.
In conclusion, this study showed bilateral differences in the mechanical but not in the morphological proper-
ties of PF and foot arch dimensions in track distance runners as compared to untrained individuals. PF SWV at 
the proximal site was higher in the left feet of track distance runners while their right feet showing comparable 
values to that of untrained individuals. These results demonstrate stiffer proximal PF in the left feet of runners, 
which may reflect adaptation to their running-specific training that involves asymmetrical mechanical loading.
Methods
Study design and participants. 
A cross-sectional study was conducted at Waseda University (Tokoro-
zawa campus) in Japan from August to November 2017. This study was approved by the Human Research Ethics 
Committee of Waseda University (reference number: 2016-310) and was carried out in accordance with the 
Declaration of Helsinki. Written informed consent was obtained from all participants before data collection.
The necessary sample size was calculated from our preliminary results (n = 6 in each group; total = 12). A 
priori power analysis (G*Power v3.1, Heinrich Heine-Universität Dusseldorf, Germany) with an assumed type 
1 error of 0.05 and a statistical power of 0.80 was conducted to find significant differences in PF SWV between 
left and right feet of runners and between groups, respectively. The critical sample sizes were estimated to be 
at least 7 runners and 9 in each group (total = 18), respectively. Thus, 10 track distance male runners and 10 
untrained men were recruited in this study (Table 1). Twelve runners were eligible for participation in this study. 
Of these, 2 runners met the exclusion criteria of history of plantar fasciitis and operative treatment of the lower 
limb (Fig. 2). Finally, 10 runners and 10 untrained men who matched the baseline physical characteristics with 
those of runners were successfully recruited in this study.
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Before the main measurements, the profiles including age, body height, body mass, dominant leg, athletic 
experiences, exercise habits for the past year, foot strike pattern, history of injuries and operative treatment, and 
model of their running shoes were collected from all participants. The dominant leg was determined according 
to the participant’s favorite leg for kicking a ball. The foot strike pattern (rearfoot or forefoot strikers) of partici-
pants was visually confirmed on another  occasion15. Additionally, the training environment (e.g., affiliation and 
running surfaces), personal best time of 5000 m, and running volume/week for the year were asked for runners. 
To avoid any confounding factors, we recruited participants attending the same university, and attempted to 
match the baseline physical characteristics of untrained participants with those of runners. Participants were 
not allowed to perform any strenuous exercises for at least 24 h before the measurement.
Ultrasound measurements. 
The supersonic shear imaging (SSI) and B-mode ultrasonography techniques 
with an Aixplorer ultrasound scanner (version 6.4, Supersonic Imagine, Aix-en-Provence, France) and a linear 
array probe (SL 15-4, Supersonic Imagine, Aix-en-Provence, France) were used to measure the mechanical and 
morphological properties of PF. SSI is a valid and reliable technique to evaluate the stiffness of skeletal muscles, 
tendons, and fasciae in vivo14,39–41. In principle, SSI uses multiple push pulses to generate the shear waves propa-
gating within the soft tissues and measures their velocity (i.e., SWV). Since SWV is related to Young’s modulus 
and shear modulus of the soft tissues, it can be used as an index of  stiffness42,43.
Details of SSI measurement and data processing were based on our previous published  work14,15. During 
ultrasound measurements, participants were requested to rest in a supine position on the examination bed with 
their knee fully extended. Additionally, their ankle and toe digits were secured to a custom-made fixture at the 
neutral position. PF was scanned at three different sites along the longitudinal line between the medial calcaneal 
tubercle and the second toe. The locations of measurement sites were that at the proximal (in the proximity to 
the calcaneus), middle (the level of navicular tuberosity), and distal (proximity to the second metatarsal head) 
(Fig. 3). The longitudinal line of the foot and the locations of the transducer were marked on the skin surface 
using a waterproof marker. The scanning head of the probe was coated with transmission gel. An acoustic standoff 
pad (Gelpad for StatUS, Enraf–Nonius, Rotterdam, Netherland) was used to avoid applying excessive compres-
sion on the skin surface. Three images were obtained at each measurement site, and used for further analysis.
After data collection, SWV at each measurement site was measured as the mean value within the region of 
interest (ROI) which was manually traced over the fascial boundaries of PF using a measurement tool included 
in the Aixplorer software (i.e., Q-box Trace). PF thickness at each measurement site was measured the distance 
between the superficial and deep fascial boundaries was measured to determine thickness using a measurement 
Figure 2.  Flow diagram depicting participant selection. 12 runners were eligible for participation in this study. 
Two runners met the exclusion criteria of history of plantar fasciitis and operative treatment of the lower limb. 
Thus, 10 runners and 10 untrained men who matched the baseline physical characteristics with those of runners 
were included in this study.
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tool (i.e., Distance). For SWV and thickness at each measurement site, three images were analyzed at each meas-
urement site, then the three values were averaged to obtain the representative value.
Measurements of the foot dimensions. 
A foot scanner (JMS-2100CU, Dream GP, Osaka, Japan) was 
used to obtain three-dimensional foot shape data. Details of measurement and data processing were based on 
our previous study using the same  system15. Participants were requested to stand in a relaxed position with their 
feet approximately shoulder-width apart. The longitudinal axis of their feet, which is the line connecting between 
the most posterior point of the heel and the head of the second toe, aligned parallel with the guidelines drawn 
on the footplate in the foot scanner. A laser scanner moved around the foot in an oval trajectory, measuring the 
foot dimensions and the anatomical marker positions based on laser line triangulation. After the scanning, foot 
length, dorsal height, and navicular height were measured. The foot length was defined as the length projected 
on the longitudinal axis between the most posterior point of the heel and the head of the first or second toe, 
whichever was longer. The dorsal height was defined as the height of the highest point from the floor at 55% 
of the length of the foot from the heel. The navicular height was defined as the height of the most medial point 
of the navicular bone from the floor. Additionally, the arch height ratio was calculated as the navicular height 
normalized to the foot length.
Statistical analysis. 
The normality of the data was assessed using a Shapiro–Wilk test. After the normal-
ity was confirmed, the difference in physical characteristics between groups were compared using an unpaired 
t-test. The fraction of dominant legs within each group was compared with a Pearson chi-squared test. Com-
parisons of measured variables between left and right feet in each group were performed using a paired t-test. 
The L/R ratios were calculated for the measured variables, and were compared using an unpaired t-test between 
groups. Cohen’s d was calculated as a measure of effect size. For the within-subject factor, it was corrected for 
dependence between mean values using the following equation: d = Mdiﬀ/SDpooled
√2(1 − r) , where Mdiﬀ is 
mean difference between conditions, SDpooled is pooled SD, and r is correlation between mean  values44. Effect 
size is interpreted as trivial (d < 0.2), small (0.2 ≤ d < 0.5), medium (0.5 ≤ d < 0.8) and large effect (d ≥ 0.8)45. Sta-
tistical significance was set at α = 0.05. Statistical analysis was performed using SPSS software (SPSS Statistics 25, 
IBM, Armonk, USA).
Received: 17 November 2020; Accepted: 19 April 2021
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Acknowledgements
This study was part of research activities of the Human Performance Laboratory, Comprehensive Research 
Organization, Waseda University. This study was supported by JSPS KAKENHI Grant Numbers 19J14912 and 
16H01870. The authors express their gratitude to Dr. Pavlos Evangelidis, Dr. Takaki Yamagishi and Mr. Hidetaka 
Hayashi for grammatical corrections of the manuscript.
Author contributions
H.S., R.Y., T.M. and Y.K. designed the research. H.S., R.Y. and T.M. corrected the data. H.S. analyzed the data. 
H.S., R.Y., T.M., N.S., M.N. and Y.K. interpreted the results of experiments. H.S. wrote the main manuscript 
text and prepared Figures and Tables. H.S., N.S., M.N. and Y.K. revised the manuscript. All authors read and 
approved the final version of the manuscript.
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Competing interests 
The authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to Y.K.
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© The Author(s) 2021

Track distance runners exhibit bilateral differences in the plantar fascia stiffness.
Shiotani, Hiroto,Yamashita, Ryo,Mizokuchi, Tomohiro,Sado, Natsuki,Naito, Munekazu,Kawakami, Yasuo