Key words
Achilles tendon - reliability - cross-sectional area - compliance - strain - gender
Introduction
The Achilles tendon (AT) is the strongest tendon in the human body and is exposed
to high forces during daily activities and athletics [8]
[11]
[12]. In consequence, the AT is particularly susceptible to both chronic and acute injuries
such as tendinopathy or tendon ruptures, which are often assumed to be related to
the repetitive high loads [20]
[23]. Those types of tendon pathologies have been shown to be gender-specific with the
ratio of AT rupture in men to women to be 5:1 [37] . Among the factors that might be responsible, tendon stiffness is of particular
interest [10]. Stiffness refers to the degree of resistance offered by the tissues in response
to stretching during loading. In the case of the AT, the displacement of the medial
gastrocnemius myotendinous junction during loading is most often used to determine
its stiffness [31].
Previous studies have shown that males have stiffer tendons compared to females [4]
[5]
[9]
[22]
[29]
[40], which could partly explain the higher rate of injuries in males [6]
[15]
[23]
[37]. These gender-related differences found in tendon properties could be attributed
to the increased body mass and force production capabilities observed in males [28] and increased estrogen levels found in females [3]
[26]. Increased body mass requires weight-bearing tendons to tolerate higher loads whereas
increased muscle mass, which is directly associated with force production capabilities,
requires tendons to transfer high loads. Those requirements lead to specific adaptations
of the tendon by increasing its CSA dimensions and/or its material properties, thus
leading to increased stiffness [28]
[39].
Quantification of tendon structural dimensions and especially that of CSA is of importance
in the research field as it is a variable needed for the calculation of tendon stress.
The stress (N/mm2) imposed on the tendon during muscle contraction is calculated as the force transmitted
to the tendon divided by its CSA dimensions at rest [25]. From an injury standpoint, larger CSA dimensions in asymptomatic tendons are advantageous
in reducing the stress imposed on the tendon for a given load [25]. Strain [%] is another important variable in understanding tendon behavior describing
the relationship between tendon length deformation during contraction and its resting
length dimensions [14]. According to in-vitro models, strain levels above 8% causes tendon microruptures
and at 12% a complete tendon ruptured [38]. However, those values were recently underestimated during functional tasks with
strains reaching levels of up to 16% during a single leg jump without causing tendon
injury [18].
When subjected to tensile load imposed by muscle contraction, tendons undergo a longitudinal
deformation by changing their length. However, as some collagen fibres run also transversely
[19]
[35], CSA deformation is also expected. Currently, no study has investigated the degree
of deformation in CSA under contraction. Therefore the aim of the present study was
to investigate the gender-specific AT-CSA deformation from rest to maximal voluntary
isometric contraction. As females demonstrate a higher compliance compared to males
at the longitudinal level by differences in length, it is hypothesised that females
will also demonstrate a higher compliance at the transverse level by a higher AT-CSA
deformation during contractions compared to males.
Material and Methods
Participants
Thirty healthy recreationally active participants (15 males and 15 females) volunteered
in the present study ([Table 1]). Sample size was determined via power analysis done on preliminary data. An effect
size of 2.00 was calculated for CSA deformation based on an α-level of 0.05 and power
values of 0.80, requiring a minimum number of 6 participants. A standardized clinical
examination, including ultrasonography, was performed by a sports orthopaedic physician.
Participants were excluded if they reported any acute or chronic musculoskeletal injury
of the lower limb and/or signs of tendinopathy on ultrasound imaging [7]. All participants completed an informed consent form.
Table 1 Anthropometric characteristics of participants.
|
Variables
|
Females (N=15)
|
Males (N=15)
|
P Value
|
|
Age [yr]
|
28±3
|
30±4
|
0.063
|
|
Height [cm]
|
167±5
|
182±7
|
0.000*
|
|
Weight [kg]
|
62±8
|
81±7
|
0.000*
|
Values are means±SD and depict group average of data. * Significant group differences
(P≤0.01)
Study design
A cross-sectional design was used in the present study. The measurements and their
further analysis were supported by a single investigator with three years of experience.
The study was approved by the local ethics committee and met the ethical standards
of the International Journal of Sports Medicine [13].
Region of interest
In the present study, the dominant limb was identified by asking the participants
to electively use either the left or right foot to kick a ball, the elected side was
considered as the non-dominant [30]. Participants were positioned prone on the examination table with hip and knee extended
and ankle flexed at 90°. A diagnostic ultrasound device (Vivid q; GE Healthcare, Tirat
Carmel, Israel) with a 7.5 MHz continuous linear ultrasound array (4–13 MHz) was used.
Presets were standardized at a frequency of 13 MHz and a depth of 3 cm. The ultrasound
video clips were recorded at a rate of 40 frames per second. The AT distal insertion
was sonographically detected by use of metal fine wires placed between the skin and
the transducer, overlaying the corresponding structure and providing an acoustic shadow
visual to the ultrasound. This method was used to accurately mark the location of
the AT distal insertion on the skin [16]
[17]. The region of interest was defined at a distance of 6 cm from the AT distal insertion
by the use of a measuring tape and marked on the skin [17]. The region of 6 cm from the distal insertion is the most reliable site to measure
AT-CSA at rest [17]. Furthermore, injuries, e. g. tendinopathy and ruptures, most often occurred at
this so- called “mid-portion” [24].
Ultrasonographic assessment of AT-CSA
Participants were positioned prone on the isokinetic dynamometer (Con-trex MJ, Physiomed,
Germany) with the hip and knee extended and ankle flexed at 90° ([Fig. 1]). The axis of rotation was carefully aligned with the lateral malleolus. The foot
was strapped securely to the footplate by use of Velcro straps. AT-CSA was initially
assessed during rest. The probe was place on the defined point on the AT by the investigator
and three image scans were taken. For warm-up and familiarisation with the measurement
procedure, a standardized warm-up protocol was chosen consisting of three submaximal
and two maximal isometric plantar flexion contractions of 5 s, with 1 min rest in
between [1]
[17]
[18]. After these practice trials, participants performed 3 maximal isometric plantar
flexion contractions of 3 s with 1 min rest. AT-CSA during the contractions was recorded
simultaneously with the investigator holding the probe on the defined location ([Fig. 1]).
Fig. 1 Participants position on the isokinetic dynamometer and probe placement by the investigator
over the region of interest.
Reliability of the methodology used
In the context of a pilot study, the reliability of the ultrasonographic methodology
and image analysis used in the present study for the assessment of AT-CSA was also
evaluated at rest and under contraction. Inter-rater reliability of AT-CSA was assessed
in a randomised order within the same day by use of the same equipment by two investigators
(experienced investigator (three years) and inexperienced investigator (one month
focused training prior the study)). Intra-rater reliability was assessed by the experienced
investigator within an interval of one week. Reliability was assessed by Intraclass
Correlation Coefficient (ICC, 2.1) with a 95% confidence interval (CI: 95%). An ICC
value ≤0.50 was considered low, 0.50 to 0.75 was considered moderate, ≥0.75 was considered
good and ≥0.90 was considered excellent [32]. The agreement between the measurements was verified qualitatively using Bland-Altman
analysis (Bias±Limits of Agreements, [LoA]) and was calculated by the following equation:
Table 2 Reliability of assessing Achilles tendon cross-sectional area (CSA) at rest and maximal
voluntary isometric contraction (MVIC).
|
Intra-rater
|
Inter-rater
|
|
N=42 (24 females; 18 males)
|
ICC [2,1]
|
TRV [%]
|
Bias±LoA [mm2]
|
SEM [mm2]
|
ICC [2,1]
|
IRV [%]
|
Bias±LoA [mm2]
|
SEM [mm2]
|
|
CSA at rest [mm2]
|
0.94
|
4±4
|
–1±5
|
2
|
0.84
|
8±4
|
3±7
|
3
|
|
CSA at MVIC [mm2]
|
0.95
|
4±3
|
–1±2
|
2
|
0.87
|
6±4
|
2±7
|
3
|
Measures of reliability: ICC=Intraclass correlation coefficient, TRV/IRV=test-retest/inter-rater
variability, Bias±LoA=95% limits of agreement, SEM=standard error of measurement
Bias±1.96×SD
Variability was calculated as the absolute differences between the two investigators
(inter) and between the two measurements (intra) divided by their average and expressed
as percentage [%]. Additionally, to provide an estimate of the precision of measurement,
the standard error of measurement (SEM) was calculated by the following equation:
The reliability values for both intra- and inter-rater are presented in [Table 2]. Analysis demonstrated a good to excellent reliability with low levels of variability
for CSA assessment at rest and contraction both intra- as well as inter-rater.
Data analysis
The ultrasound images and video clips of AT-CSA were stored digitally as DICOM files
and processed on a PC using a public domain NIH image program (imageJ, (http://rsb.info.nih.gov/nih-image/)). The freehand selection tool was used to outline the tendon and measure the CSA
both at rest and during MVIC ([Fig. 2]). AT-CSA during the MVIC was outlined at each maximal deformation by manually tracking
the CSA from rest to MVIC. In order to decrease the variability within subjects, each
image and video clip was digitized three times and the average was taken. The analysis
was performed in a blinded procedure in order to minimize a possible bias in the results
from the investigator preference or expectations. All images and video clips were
stored under a four-digit random number that was assigned prior to testing and stored
in an identification file. As a consequence, the investigator was blinded to the participants
and measurement day. After finalizing the analysis of the data, the results were assigned
to the corresponding participants.
Fig. 2 Achilles tendon Cross-sectional area (CSA) at rest (a, c) and under maximal voluntary
isometric contraction (b, d). The white dotted lines outline the tendinous structure
defining the CSA.
AT-CSA deformation
To describe the change in CSA from rest to MVIC, the following equation was used:
CSA Deformation = CSAMVIC[mm2 ] - CSA Rest [mm2]
Traditionally tendon strain has been used to describe the longitudinal strain (length
change (elongation)/resting length) along the axis of the tendon. In the present study,
this calculation was modified and transferred to the transverse level taking into
account the change in CSA during the contraction by its resting dimensions.
AT-CSA strain
To describe the change in CSA deformation in relation to its CSA dimensions at rest,
CSA tendon strain was calculated by the following equation:
Tendon compliance is usually calculated by dividing the tendon elongation by tendon
force. Both tendon force and torque are indirect ways of expressing the forces acting
on the tendon. In the present study a modification of this equation was made and transferred
to the transverse level taking into account the deformation of the CSA divided by
the peak torque.
AT-CSA compliance
To describe the deformation of the CSA in relation to the peak torque, the following
equation was used:
Statistical analysis
All statistical calculations were performed using SPSS (SPSS Statistics 22, IBM, USA).
Data were initially analysed descriptively (mean±SD). Gender differences for AT-CSA
dimensions at rest and MVIC, deformation, torque, strain and compliance were compared
using an independent sample t-test followed by Bonferroni correction for multiplicity
(α=0.01).
Results
The average values (mean±SD) for anthropometric characteristics, variables measured
and statistical test used, are given in [Tables 1] and [3]. Males demonstrated a statistically significant larger AT-CSA dimensions both at
rest (range=males: 47.9 to 66.3 mm2, females: 33.2 to 55.6 mm2, p=0.001) as well as during MVIC (range=males: 47.2 to 65.0 mm2, females: 30.4 to 53.3 mm2, p=0.000) and a statistically significant higher peak torque (range=males: 74.0 to
159.0 Nm, females: 54.0 to 119.0 Nm, p=0.001). On the other hand, females demonstrated
a statistically significant higher AT-CSA deformation (range=males: –4.3 to 2.6 mm2, females: –4.4 to –1.2 mm2, p=0.000), strain (range=males: –8.1 to 5.0%, females: –8.7 to –2.2%, p=0.000) and
compliance (range=males: –0.044 to 0.030 mm2/Nm, females: –0.050 to –0.022 mm2/Nm, p=0.000).
Table 3 Achilles tendon cross-sectional area (CSA) properties and peak torque between males
and females.
|
Variables
|
Males
|
Females
|
P Value
|
|
CSA at rest [mm2]
|
54.4±5.1
|
46.2±7.0
|
0.001*
|
|
CSA at contraction [mm2]
|
53.7±5.1
|
43.4±6.9
|
0.000*
|
|
CSA deformation [mm2]
|
–0.9±1.8
|
–2.8±0.9
|
0.000*
|
|
Peak torque[Nm]
|
120.1±26.8
|
86.9±21.6
|
0.001*
|
|
CSA strain [%]
|
–1.4±3.3
|
–6.2±2.0
|
0.000*
|
|
CSA compliance [mm2/Nm]
|
–0.007±0.008
|
–0.033±0.018
|
0.000*
|
Values are means±SD and depict group average of data. *Significant group differences
(P≤0.01)
Discussion
The present study aimed to investigate the gender differences of AT-CSA under maximal
isometric contraction. The results indicate that the AT also deforms at its transverse
level by reducing its CSA. Females demonstrated a significantly higher CSA deformation
under MVIC, indicating also a more transversely compliant tendon compared to males.
Thus, these findings confirm the hypothesis that females will demonstrate higher AT-CSA
deformation and compliance during contractions compared to males.
Previously, studies investigating the gender-related differences in tendon properties
have led to the conclusion of higher tendon elongation and smaller structures in females
exhibiting lower tendon stiffness compared to males [4]
[5]
[9]
[22]
[29]
[40]. The present study adds that gender-related differences in CSA during loading can
also be observed. Females demonstrated higher CSA deformation with lower forces indicating
a more compliant tendon. As the ratio of injuries is higher in males compared to females[15]
[37], higher compliance might allow a better adaptation towards loading.
In the present study, males demonstrated significantly larger CSA dimensions both
at rest and during contraction compared to females. These findings are in line with
previous studies reporting that males have larger tendon dimensions [4]
[5]
[9]
[22]
[29]
[40]. These increased CSA dimensions found in males could be an adaptation of AT towards
increased force generation capacity and higher body mass compared to females. As the
stiffness of its structures depends on its dimensions, it could be speculated that
the higher compliance in CSA found in females is attributable to their smaller structures.
However, this relationship is quite ambiguous [27] as there are studies which have shown that stiffness in tendons is independent of
its dimensions [18]
[34]
[36]. Hence other factors such as tendon micro-structure could be assumed to also play
a role, e. g. increased fibril diameter, fibril packing [33], collagen cross-linking [2] and reduced collagen crimping [21].
Calculations of tendon mechanical and material properties are essential for understanding
mechanisms that enable to optimize the functional behavior of the muscle-tendon complex
[1]. Stress (N/mm2) is a variable which is used in order to determine the material properties of the
tendon, the so-called Young’s modulus (stress/strain). Traditionally, stress is calculated
by taking CSA dimensions at rest [25]. However, as those dimensions are changing under loading with females demonstrating
7% and males 2% of CSA reduction also the estimated stress values are deem to change
leading to underestimation between 2% – 7% of this calculated variable. This finding
leads to critical considerations of tendon stress calculations and future studies
should address this issue as this might result in a false estimations of tendon young’s
modulus.
To better understand the changes of CSA during loading in relation to its dimensions,
an attempt was made to calculate its strain (CSAdeformation/CSArest). The result revealed that females have a significantly higher CSA strain compared
to males. Higher tendon strains are thought to cause ruptures in the longitudinal
level of tendons when strain (elongation/length) reaches a level between 8%–12% [38]. However those values were based on in-vitro models and do not represent how tendons
respond to loading under physiological loads [18]. The findings of the present study demonstrated that during isometric contractions,
CSA strain can reach levels of up to –9% without causing any injuries. However, to
be able to classify CSA strain, more investigation is also needed in pathological
tendons in order to understand the possible implications.
Although the methodology was reliable in the assessment of CSA both at rest and during
contraction, the definition of the region of interest should be critically discussed.
In the present study, the region of interest was defined at 6 cm from the distal attachment.
Since CSA dimensions are not the same throughout its length, this region was specifically
selected because it was shown to have the highest reproducibility [17]. As males in the present study were significantly taller than females, it could
be argued that the region of interest in males was defined more distally. However,
a recent study by Intziegianni et al. showed that taller participants do not necessarily
have a longer Achilles tendon [18]. Thus, for a more accurate comparison between participants, a percentage of distance
rather than a standardized point should be selected.
The findings of this study provide important information of tendon response at its
transverse level during loading by a reduction of its CSA. The findings further add
that those changes were gender-specific, with females demonstrating a higher AT-CSA
deformation and compliance compared to males. As the incidence of AT injuries is higher
in males compared to females, higher AT-CSA compliance might play a protective role
towards stress-related injuries, possibly indicating higher adaptability to loading.
Thus, to better understand tendon CSA behaviour and possible implications in performance
and injury, its assessment should also be evaluated under functional tasks and between
different populations where pathologies are present.
