Kunimasa et al.
Journal of Physiological Anthropology(2023) 42:9
https://doi.org/10.1186/s40101-023-00326-3

Journal of
Physiological Anthropology

Open Access

SHORT REPORTIs the muscle–tendon architectureof non‑athletic Kenyans different from that ofJapanese and French males?
Yoko Kunimasa1,2,3,4* , Kanae Sano1,5, Caroline Nicol2, Joëlle Barthèlemy2 and Masaki Ishikawa1

Abstract
Background In endurance running, elite Kenyan runners are characterized by longer thigh, shank, and Achilles tendon (AT) lengths combined with shorter fascicles and larger medial gastrocnemius (MG) pennation angles than elite
Japanese runners. These muscle-tendon characteristics may contribute to the running performance of Kenyans. Furthermore, these specific lower-leg musculoskeletal architectures have been confirmed not only in elite Kenyan runners but also in non-athletic Kenyans since early childhood. However, it remains questionable whether the differencesin muscle-tendon architecture between Kenyans and Japanese differ from those of European Caucasians. Therefore,
this study aimed to compare anthropometry and muscle–tendon architecture of young non-athletic Kenyan maleswith their Japanese and French counterparts.
Methods A total of 235 young non-athletic males, aged 17–22 years, volunteered. The anthropometric measures,
thigh, and shank lengths, as well as AT and MG muscle architecture, were measured using ultrasonography and atape measure. Inter-group differences in anthropometry and muscle-tendon architecture were tested using one-way
ANOVA and ANCOVA analyses controlling for shank length and muscle thickness.
Results The anthropometric and muscle-tendon characteristics of the non-athletic French were closer to those of
the Kenyans than to those of the Japanese. However, the ultrasonography analysis confirmed that the non-athletic
Kenyans had the longest AT as well as the shortest MG fascicles and the largest pennation angle compared to the
French and Japanese, even after controlling for shank length and muscle thickness with ANCOVA, respectively.
Conclusions These results confirmed the specificity of the muscle-tendon architecture of the triceps surae in Kenyans in comparison to their Japanese and French counterparts in non-athletic adults. This study provides additional
support to the fact that Kenyans may have musculotendinous advantages in endurance running.
Keywords Muscle-tendon unit, Achilles tendon, Muscle architecture, Ultrasonography, Kenyans*Correspondence:
Yoko Kunimasa
yoko.k@ed.niigata-u.ac.jp
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Kunimasa et al. Journal of Physiological Anthropology(2023) 42:9

BackgroundIn bipedal locomotion, musculoskeletal architecture andits mechanical behavior are crucial for economical movements and power production [1]. Several studies reported
that inherent advantages of the lower-limb architecture
can affect athletic performance [2–5]. In endurance running, the specific skeletal and muscle-tendon architectureof lower-leg muscles of the Kenyan runners may contribute to their running performance [6, 7], such as a long
Achilles tendon (AT) as well as a thin medial gastrocnemius (MG) muscle with short fascicle length and large
pennation angle. These specific lower-leg muscle-tendon
architecture has been confirmed not only in elite Kenyan runners but also in non-athletic Kenyans comparedto Japanese [8]. Furthermore, this study found that thedecrease in MG fascicle length and increase pennation
angle observed for the adult Japanese with the increase inrunning performance level resulted in a lack of difference
between elite Kenyans and Japanese. These findings suggested that the unique muscle-tendon architecture of the
Kenyan population may be genetically determined andcontribute to the dominance of Kenyans in middle- andlong-distance races. However, to further test this hypothesis, it would be interesting to investigate whether themuscle-tendon architecture of the triceps surae is characteristic of Kenyans compared to other non-Japanese
populations, even after controlling for anthropometricdifferences.
Previous anthropometric studies have reported that
Kenyan endurance runners and young western Kenyans
are characterized by long and light lower legs comparedto their Danish counterparts [9, 10]. As the AT lengthis approximately proportional to the tibia bone lengthand body height [11], it was expected that the long AT
length of Kenyans may still hold even when compared toother European Caucasians than the Danes. Since bodymass index reference values of Danes aged 0–45 years
best match the French reference values, especially formen [12], we thought it would be interesting to compare

Page 2 of 5

the anthropometry and muscle-tendon architecture ofyoung Kenyan non-athletes with those of the Japanese
population and the Caucasian French population. We
hypothesized that similar to the elite Kenyan runners, thenon-athletic Kenyans would have longer AT and thinner
MG muscles with shorter fascicle length and larger pennation angle compared to not only the Japanese but also
the French. This hypothesis could hold even after controlling for the shank length and muscle thickness with
ANCOVA.

Methods
A total of 235 young healthy males (age range 17–22),
54 Kenyans, 81 French Caucasian, and 100 Japanese volunteered (Table 1). Japanese were recruited and tested
in different cities (i.e., Osaka, Hyogo, Shiga, Okayama,
Hiroshima, and Fukuoka) between 2012 and 2015. Kenyans were recruited and tested in Nairobi in March 2015
and March 2016 or Kitui (160 km away) in February 2016.
French were recruited and tested in Gap (in the south ofFrance) in September 2014. This study was performed
according to the guidelines of the Declaration of Helsinki
and was approved by the local Ethics Committee of the
Osaka University of Health and Sport Sciences (approval
number: 16–1).
Body mass was measured by electric scale (RD-503,
Tanita Corp., Japan). Body height, thigh, and shanklengths were measured using tape measure and carpenter square [6]. AT and MG muscle architecture
was measured using ultrasonography [Noblus with a
55-mm linear array probe (model: L55, 5–13 MHz),
Hitachi Ltd., Japan] [6, 8]. All parameters were measured bilaterally while standing at rest and averaged forboth legs. Thigh length was defined as the distancefrom the greater trochanter to the proximal head offibula and shank length from the outer femoral condyle to the tip of the lateral malleolus. AT length wasquantified using ultrasonography and tape measuresfrom the point of its distal insertion into the calcaneus

Table 1 Anthropometric parameters for Kenyans, French, and Japanese
Kenyan (n = 54)

French (n = 81)

Japanese (n = 100)Age (years)

18.5 ± 1.3

18.4 ± 1.3

18.5 ± 1.3

Body height (cm)

169.1 ± 5.6

177.5 ± 6.5***

171.6 ± 5.2*†††

Body mass (kg)58.6 ± 6.1

67.8 ± 8.4***

60.6 ± 6.1†††

Thigh length (cm)40.1 ± 1.7

39.3 ± 2.0*

38.7 ± 2.5***

Thigh length (% height)

23.8 ± 0.9

22.2 ± 0.9***

22.6 ± 1.2***†

Shank length (cm)39.0 ± 1.6

37.6 ± 2.1***

35.4 ± 1.9***†††

Shank length (% height)

23.1 ± 0.7

21.2 ± 0.7***

20.6 ± 0.7***†††

Group mean ± SD values*

and *** significant difference between Kenyan vs. French and Japanese (p < 0.05 and p < 0.001, respectively)†and ††† significant difference between French vs. Japanese (p < 0.05 and p < 0.001, respectively)

Kunimasa et al. Journal of Physiological Anthropology(2023) 42:9

to the AT junction between medial and lateral gastrocnemii muscles [8].
MG fascicle length, its pennation angle, and muscle
thickness were measured from midsagittal ultrasoundimages of the MG muscle belly. The length of one fascicle located in the center of the MG ultrasound image
was measured as the distance between its insertions into
the superficial and deep aponeuroses [13] and digitizedusing analysis software (ImageJ, NIH, Bethesda, MD,
USA). The muscle thickness was the largest perpendicular distance between deep and superficial aponeuroses.
The MG pennation angle was measured between the
MG fascicle insertion points in the superficial and deep
aponeuroses and another point on the deep aponeurosis.
Inter-day repeatability of the muscle architecture measured by the same experimenter was confirmed [8].
Statistical analysis was performed using IBM SPSS
Statistics (version 27.0, IBM Corp. Armmonk, NY,
USA). Values in the text are reported as mean ± SD.
Inter-group differences for each parameter were tested
by one-way ANOVA analyses. An additional one-way
ANCOVA was conducted to determine statisticallysignificant differences between Kenyans, Japanese,and French on the AT length controlling for the shanklength and on the MG pennation angle and fascicle
length controlling for the MG muscle thickness, respectively. Further post hoc analyses were conducted with a
Bonferroni adjustment. We used an alpha level of 0.05
for all statistical tests.

Results
Group means and statistical comparisons of anthropometry of Kenyan, French, and Japanese are shownin Table 1. The body mass of the Kenyans and Japanese
was lighter than that of French. The body height wasPage 3 of 5

shorter in the Kenyans than in the French and Japanese. The absolute and relative (% body height) valuesof thigh and shank lengths were significantly longer inthe Kenyans than in their French and Japanese counterparts, respectively. In the comparison between French
and Japanese groups, the relative thigh length as wellas the absolute and relative shank lengths was longer in
the French.
Regarding muscle-tendon architecture, the Kenyans
had longer AT (Fig. 1A) and smaller MG muscle thickness (Fig. 1B) than the French and Japanese, respectively. The relative AT length to the shank length was
65.9 ± 3.7, 62.6 ± 5.1, and 61.4 ± 4.3% for the Kenyans,
French, and Japanese, respectively. The ANCOVA analyses confirmed that there was a significant effect of groupon the AT length after controlling for the shank length[F(2, 232) = 15.567, p < 0.01]. Post hoc tests showed significantly longer AT in Kenyans than in Japanese andFrench (p < 0.001, respectively) (Fig. 2A). The MG fascicle length was shorter in the Kenyans than in the French
and Japanese in both absolute (Fig. 1C) and relative scales(12.9 ± 1.4 vs. 15.5 ± 1.6 and 16.5 ± 2.0%, p < 0.001 forboth comparisons). The ANCOVA analyses confirmed
a significant group effect on fascicle length and pennation angle after controlling for the MG muscle thickness[fascicle length: F(2232) = 11.668, p < 0.001, pennation
angle: F(2232) = 11.129, p < 0.001]. Post hoc tests showedthat Kenyans had significantly shorter MG fascicle lengththan Japanese and French (p < 0.01 and p < 0.001, respectively) and larger pennation angle (p < 0.001, respectively)
than Japanese and French (Fig. 2B, C). In the comparison
between French and Japanese groups, the French had
longer absolute AT (Fig. 1A), smaller MG muscle thickness (Fig. 1B), and relatively shorter MG fascicle length(p < 0.001).

Fig. 1 Comparison of muscle-tendon architecture between Kenyans (KEN), French (FR), and Japanese (JPN). Results are shown as mean ± SD.
Inter-group differences for each parameter were tested by one-way ANOVA analyses. *** and * indicate significant differences (p < 0.001 andp < 0.05, respectively)

Kunimasa et al. Journal of Physiological Anthropology(2023) 42:9

Page 4 of 5

Fig. 2 Achilles tendon (AT) and medial gastrocnemius (MG) muscle architecture of Kenyan (KEN), French (FR), and Japanese (JPN) adult males.
One-way ANCOVA was conducted on the AT length controlling for the shank length (A) and on the MG fascicle length and pennation angle
controlling for the MG muscle thickness (B and C, respectively). *** and ** indicate significant inter-group differences (p < 0.001 and p < 0.01,
respectively)

Discussion
The anthropometric and muscle-tendon characteristicsof the French were closer to those of the Kenyans thanthose of the Japanese. However, the ultrasonography
analysis confirmed that the non-athletic Kenyans had
the longest AT as well as the shortest MG fascicle lengthand the largest pennation angle compared to the French
and Japanese, even after controlling for shank length andmuscle thickness with ANCOVA.
The dominance of Kenyans in endurance running wasthought to be due to genetic factors, as first suggested by
Bengt Saltin’s group [9, 10, 14, 15]. Not only elite Kenyan
runners but also Kenyan boys have better running economy than their Danish counterparts, which is explainedby their thinner calf muscles [10]. Consistent with thisfinding, our previous study showed that Kenyan runners
and non-athletes, including boys, had a thinner MG muscle with a longer AT and shorter MG fascicles comparedto Japanese [8]. The relationships between these uniquestructural and functional characteristics suggest that
Kenyans have an inherent muscle-tendon advantage for
endurance running [6–8]. The present results provide
additional support for the hypothesis that Kenyans havesome unique muscle-tendon characteristics, compared
not only to the Japanese but also to the French population. Interestingly, this uniqueness remained even after
controlling for anthropometric differences between Kenyans, French, and Japanese. In contrast to the greater
contribution of MG fascicle pennation angle to greater
MG muscle thickness in European Caucasians [16, 17],
Kenyans had the shortest MG fascicle length and thegreatest pennation angle, although their MG muscle
thickness was the lowest. These results bring additional
support to the genetic origin of the unique muscle-tendon architecture of Kenyans [8].

The results of this study add to the existing literature on
the intrinsic musculoskeletal architecture of the lower leg
in Kenyans, including comparisons with different populations, but some limitations deserve to be mentioned. First,
contrasting with our previous comparison between Kenyans
and Japanese males [8], the current data were collected froma limited number of people and a limited age range of 17–22.
In addition, the current study did not include elite runners,
although Kenyan’s muscle-tendon architecture may be considered a prerequisite for becoming elite runners in endurance running events. While Kenyans would have a specificmuscle-tendon architecture by the age of 8 years old [8], thecurrent age range of 17–22 is after the growth period. Finally,
the comparisons were limited to three male populations,with only the Japanese group included participants from different cities and locations. Future studies should include elite
runners and investigate the relationship between muscletendon architecture, running cost, and performance.

Conclusion
This study provides additional information consistent
with the specifics of the muscle-tendon architectureof the triceps surae in Kenyans by comparing them toJapanese and French non-athletic young adults. Regardless of shank length, Kenyans had the longest AT, andregardless of MG muscle thickness, the shortest muscle
fascicles and the largest pennation angle. These results
suggest again that Kenyans can possess inherent muscletendon advantages for endurance running. Such muscletendon architecture may be considered as a prerequisite
for becoming elite runners in endurance running events.
Further research should include data from elite endurance runners and provide a more comprehensive picture of the relationships between the structural lower-leg
characteristics and the running cost and performance.

Kunimasa et al. Journal of Physiological Anthropology(2023) 42:9

Abbreviations
AT	Achilles tendon
MG	Medial gastrocnemius
Acknowledgements
We sincerely thank the volunteer participants for participating in this study.

Page 5 of 5

6.
7.

Authors’ contributions
YK, CN, and MI conceived and designed research; YK, KS, CN, JB, and MI
performed experiments; YK analyzed data; YK, CN, and MI interpreted resultsof experiments; YK prepared figures; YK drafted manuscript; YK, KS, CN, JB, andMI edited and revised manuscript; and YK, KS, CN, JB, and MI approved final
version of manuscript.

8.

Funding
Y. Kunimasa was supported by MEXT/JSPS KAKENHI Grant Numbers 17H07267
and 19K19998. M. Ishikawa was supported by Open Partnership Joint Projectsof JSPS Bilateral Joint Research Projects (2015–2017).

10.
11.

9.

12.

Availability of data and materials
The data in this study are available from the corresponding author upon
reasonable request.

13.

Declarations

14.

Ethics approval and consent to participate
This study was performed according to the guidelines of the Declaration of
Helsinki and was approved by the local Ethics Committee of the Osaka University of Health and Sport Sciences (approval number: 16–1). All participants
were fully informed of the experimental procedures and possible risks before
obtaining written informed consent.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Graduate School of Sport and Exercise Sciences, Osaka University of Healthand Sport Sciences, Kumatori‑Cho, Sennan‑Gun, Osaka 590‑0496, Japan. 2 ISM,CNRS & Aix-Marseille University, 13288 Marseille, France. 3 Faculty of Education,
Niigata University, Niigata, Niigata 950‑2181, Japan. 4 Faculty of Engineering, Niigata University, Niigata, Niigata 950‑2181, Japan. 5 Faculty of Healthand Well‑being, Kansai University, Sakai, Osaka 590‑8515, Japan.

15.
16.
17.

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Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Received: 6 January 2023 Accepted: 24 May 2023

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