98

girls, informed consent was also obtained from their parents. The experimental protocol of this study was approved by the Ethics Committee of Human Research at Waseda University (approval number; 2009-137).

**Data collection **

** **Each subject lay in a supine position on the bed of a magnetic resonance imaging
(MRI) scanner (Signa HDxt, 1.5T, GE medical systems, USA). The right foot and the
right lower leg of the subject were fixed on a custom-made apparatus constructed by the
foot plate and the leg frame. For the fixation, the long-axis of the right foot was
carefully aligned to the foot plate of the apparatus and the right foot was securely
fastened to the foot plate by using three non-elastic bands. The long-axis of the right
lower leg was also carefully aligned to the long-axis of the leg frame, with the knees
fully extended, and the lower extremity was securely fastened to the bed and the leg
frame by using a Velcro tape. The foot position was described as the angle between the
long-axis of the foot plate and the long-axis of the leg frame, and the neutral position of
the foot was defined as the foot position of 90º. A series of right foot images were
obtained at 10ºof dorsiflexion, neutral position and 10º of plantarflexion for the coronal
plane and at neutral position for the transverse plane by using the MRI system with a
head coil. The scan parameters for the coronal plane and the transverse plane scans are
listed in Table 3-2.

**Data reduction **

99

The method proposed by Section 1 of Chapter 2 was used to determine the Achilles tendon moment arm. The coronal MR imaging plane was aligned to the medio-lateral axis of the foot plate and the long-axis of the leg frame, so that the depth of the scanned images was aligned to the long-axis of the foot plate. The position of any given voxel in the series of coronal MR images was converted directly to the three-dimensional coordinates in a right handed orthogonal coordinate system. Selected bony landmarks of the tibia and the talus were visually identified from the obtained MR images and the three-dimensional coordinates of these points were recorded by using the computer-aided diagnosis system (PLUTO, Nagoya University, Japan). Four points for tibia (a1- a4) and three points for talus (b1- b3) were selected as the bony landmarks.

The point a1 is the most proximal point of one reference marker attached to the tibia, the
point a2 is the most distal point of the other reference marker attached to the tibia, the
point a_{3} is the most distal tip of the medial malleolus, and the point a_{4} is the most distal
point of the posterior bony projection forming the incisure fibularis. The point b1 is the
most posterior tips of the lateral tubercle of the talus, the point b_{2} is the center of the
posterior edge of the talus sulcus, and the point b3 is the most lateral tips of the lateral
processus of talus. The right handed orthogonal coordinate systems embedded to the
tibia and the talus were defined by using these bony landmarks. The z** _{tibia}** was defined as
the unit vector directed from a2 to a1. The y

**tibia**was defined as the cross product of the

**z**

**tibia**and the temporal unit vector directed from a

_{4}to a

_{3}. The x

**was defined as the cross product of the y**

_{tibia}**and the z**

_{tibia}**. The y**

_{tibia}**was defined as the unit vector directed from b1 to b2. The z**

_{talus}**was defined as the cross product of the temporal unit vector**

_{talus}**Chapter 4 **

100

directed from b3 to b2 and the y** _{talus}**. The x

**was defined as the cross product of the**

_{talus}**y**

**talus**and the z

**. The finite helical axis for the neutral position was calculated from the elements of the angular displacement of the tibia-embedded coordinate system relative to the talus-embedded coordinate system that occurred over the range from 10º of dorsiflexion and 10º of plantarflexion. The position of the point that the finite helical axis passes through was determined by using the algorithm proposed by Woltring et al (1985). The line of action of the Achilles tendon force at neutral position was determined as the straight line passing through the centers of cross-sectional areas of the Achilles tendon at the proximal insertion site to the soleus and the distal insertion site to the calcaneus. The shortest distance between the talocrural joint axis to the line of action of the Achilles tendon force projected to the orthogonal plane of the talocrural joint axis was determined as the Achilles tendon moment arm for neutral position.**

_{talus}The moment arm of the GRF was estimated with the assumptions that the GRF acted through the most anterior point of the first metatarsal bone and that the GRF was directed normal to the sole of foot. The shortest distance between the talocrural joint axis to the line of action of the GRF projected to the orthogonal plane of the talocrural joint axis was determined as the moment arm of the GRF for neutral position. The mechanical advantage was, then, calculated as the ratio of the Achilles tendon moment arm to the moment arm of the GRF (Biewener 1989).

The CSA of the Achilles tendon insertion representing the transverse attachment site of the Achilles tendon which the Achilles tendon force would act as tensile force was measured by using the transverse MR image of the foot. The most proximal image

101

that the Achilles tendon inserts to the calcaneal epiphysis was chosen, and the CSA of the Achilles tendon insertion was measured. Finally, an index representing the tensile stress at the attachment site of the Achilles tendon (σ) was calculated as follows;

σ= [BW × (MA_{}/MA_{})]/CSA

where BW, MAGRF and MAAT are body weight [N], the moment arm of the GRF [mm]

and the Achilles tendon moment arm [mm], respectively. The determined value indicates the magnitude of tensile stress that should be applied to the attachment site of the Achilles tendon for the intensity of physical activity equivalent to a single-leg tip toe standing.

**Statistical analysis **

Descriptive data are presented as means ± standard deviations (SDs). Two-way analysis of variance (ANOVA) (2 sexes × 3 age groups) with Bonfferonie’s post hoc test was conducted to assess the differences in each parameter among each sex and age groups. The level of significance was set at 0.05.

**4-3. Results **

Significant interaction was not observed for each parameter. The significant main effects of sex and age were observed for the Achilles tendon moment arm and the moment arm of the GRF. The mean values of both moment arms were significantly

**Chapter 4 **

102

greater for males than females in each age group and greater for older groups than
younger groups (Figure 4-1-a, b). The main effects of sex and age were not observed for
the mechanical advantage (Figure 4-1-c). A significant main effect of age was observed
for the CSA of the Achilles tendon insertion, and a significantly greater CSA of the
Achilles tendon insertion was observed in young adult group (males: 117 mm^{2}, females:

105 mm^{2}) than 7-9 years old (boys: 69 mm^{2}, girls: 67 mm^{2}) and 10-13 years old (boys:

78 mm^{2}, girls: 74 mm^{2}) groups (Figure 4-2). A significant main effect of age was
observed on the index of the tensile stress, and a significantly greater index of the
tensile stress was observed in 10-13 years old group (boys: 16.8 N/mm^{2}, girls: 14.1
N/mm^{2}) than 7-9 years old group (boys: 12.2 N/mm^{2}, girls: 12.8 N/mm^{2}).

**4-4. Discussion **

The present study examined the sex difference in the structural parameters of foot and ankle that influence the tensile stress at the attachment site of the Achilles tendon for selected age groups. The main finding of this study was no sex difference in the index of the tensile stress for each different age group. This result suggests that the tensile stress at the Achilles tendon insertion was similar between boys and girls if the physical activity level was assumed to be the same.

Our results showed the no sex difference in the index of the tensile stress for each age group and the significantly greater value for this index for 10-13 years old group than 7-9 years old group for both sexes. In general, physical activity is higher for 10-13 years old group than 7-9 years old group (Malina and Bouchard 1991; Raustorp et al.,

103

2004) and higher for given age of boys than corresponding age of girls (Aarnio et al., 1997; Gordon-Larsen et al., 2002; Vilhjalmsson and Kristjansdottir 2003; Hume et al., 2005; Troiano et al., 2008; Ridgers et al., 2007). On the basis of these previous findings, physical activity may be the highest for boys aged 10-13 among other three boys and girls groups in the present study. Furthermore, the index of the tensile stress is higher for boys and girls aged 10-13 than those aged 7-9. These may be the reasons that Sever's disease occurs the most frequently for boys aged 10-13. Raustorp et al (2004) reported the daily step counts representing the physical activity level for each age of boys and girls. Their result showed that the daily step counts was smaller for girls aged 10-13 than boys aged 10-13 and this sex difference is greater than the corresponding difference between boys and girls aged 7-9. Furthermore, the index of the tensile stress is higher for boys and girls aged 10-13 than those aged 7-9 but not different between boys and girls in each age group. Our result on the index of the tensile stress and Raustorp's result on the daily step counts suggest that the large tensile stress may be applied at the Achilles tendon insertion for boys than girls and this sex difference may be larger for 10-13 years old groups than 7-9 years old groups. This may be the reason that the higher incidence of Sever's disease is observed for boys than girls and this sex difference is notably higher in boys and girls aged 10-13 than those aged 7-9.

The index of the tensile stress at the attachment site of the Achilles tendon was determined by three parameters; the CSA of the Achilles tendon insertion, the mechanical advantage and body mass. There is no sex difference in the mechanical advantage and this was attributable to that both the Achilles tendon moment arm and the

**Chapter 4 **

104

moment arm of the GRF were significantly greater by the same ratio for males than females in each age group. For body mass, a significant sex difference was observed for young adult group (males: 64.6 kg, females: 53.5 kg) but not for 7-9 years old (boys:

28.1 kg, girls: 29.1 kg) and 10-13 years old (boys: 43.5 kg, girls: 39.0 kg) groups. The CSA of the Achilles tendon insertion was not different between males and females for each age group. The increases in the CSA of the Achilles tendon insertion between 7-9 and 10-13 years old groups were found to be 19% and 18% of the differences between and 7-9 years old and young adult groups for males and females, respectively (Figure 4-3). The corresponding values in body mass between 7-9 and 10-13 years old groups were found to be 42% and 41% for males and females, respectively (Figure 4-4). Both the relative increase in the CSA of the Achilles tendon insertion and body mass may be similar between males and females in each two period. In general, the onset of the growth spurt of body mass is earlier for girls than boys and this ages were about 10.5 years old for girls and 11.5 years old for boys (Malina and Bouchard 1991). Therefore, 10-13 years old groups for both boys and girls may be undergoing the growth spurt of body mass, and this may be a reason of the no sex difference in the relative increase in body mass. The relative increase in the CSA of the Achilles tendon insertion is also similar between boys and girls, and its relative increase is notably smaller than that of body mass in the period between 7-9 to 10-13 years old, but larger between 10-13 years old to young adult. These suggest that the onset of growth spurt of the CSA of the Achilles tendon insertion may occur over the age of 13 for both boys and girls. In this study, subjects were divided by their age, on the basis of the age-specific sex difference

105

in the incidence of Sever's disease (Micheli and Ireland 1987), and there are no sex differences in the structural parameters in this grouping. Therefore, there is no structural parameter of foot and ankle that account for the age-specific sex difference in the incidence of Sever's disease.

**Chapter 4 **

106
**4-5. Summary **

This study examined the sex difference in the structural parameters of foot and ankle that influence the tensile stress at the attachment site of the Achilles tendon for selected age groups. The result showed no sex differences in the mechanical advantage, the CSA of the Achilles tendon insertion and the index of the tensile stress for each age group. These results suggest that the tensile stress at the attachment site of the Achilles tendon was similar between boys and girls if the physical activity level was the same.

There is no structural parameter of foot and ankle that account for the age-specific sex difference in the incidence of Sever's disease.

**Cha****pter**** 4** 107
Table 4-1 Physical characteristics of each age group of boys and girls.

### 7-9 ye a rs 1 0-13 ye a rs a dul t male fe male male fe male male fe male n u m b e r o f su b jects 16 9 24 14 19 12 ag e ( y e ar s) 7.9 s 0.8 8.4 s 0.9 1 1.8 s 1.1 1 1.7 s 1.1 26.2 s 2.9 22.5 s 1.8 body he ight (m ) 1.28 s 0.08 1.32 s 0.05 1.52 s 0.1 1 1.48 s 0.06 1.71 s 0.05 1.60 s 0.06 body m a ss (kg) 28.1 s 6.7 29.1 s 3.6 43.5 s 10.9 39.0 s 7.0 64.6 s 6.8 53.5 s 7.1

**Chapter 4 **

108

Figure 4-1 The sex differences in the structural parameters of foot and ankle that determine the index of the tensile stress. The graphs (a), (b), (c) and (d) are the Achilles tendon moment arm, the moment arm of the GRF, the mechanical advantage and the CSA of the Achilles tendon insertion, respectively. The closed and opened plots represent the males and females, respectively. * and † denotes the significant difference between different age groups and between sexes, respectively.

0.000 0.100 0.200 0.300 0.400 0.500

**me****c****h****an****ic****a****l ad****van****tage**

**7-9 **
**years old**

**10-13 **
**years old**

**young **
**adult**
0

10 20 30 40 50

60 * *

*

*

†

†

†

**A****c****h****illes**** t****e****n****d****o****n**** mome****n****t ar****m (mm)**

**7-9 **
**years old**

**10-13 **
**years old**

**young **
**adult**

0 20 40 60 80 100 120 140

†

† †

*

*

*

*

**7-9 **
**years old**

**10-13 **
**years old**

**young **
**adult**

**mome****n****t ****ar****m of th****e**** G****R****F**** (mm)**

0 20 40 60 80 100 120 140 160

*

*

**7-9 **
**years old**

**10-13 **
**years old**

**young **
**adult**
**C****S****A**** o****f t****h****e ****A****c****h****illes**** t****e****n****d****o****n** **in****sert****io****n**** (****m****m****2****)**

(a) (b)

(c) (d)

109

Figure 4-2 The sex difference in the index of the tensile stress at the attachment site of the Achilles tendon. The closed and opened plots represent the males and females, respectively. * and † denotes the significant difference between different age groups and between sexes, respectively.

0 5 10 15 20

25

### *

### *

**in** **d** **e** **x ** **of th** **e** ** te** **n** **si** **le** ** s** **tr** **e** **ss** ** (N** **/m** **m**

**2**

**)**

**7-9 **
**years old**

**10-13 **
**years old**

**young **
**adult**

**Chapter 4 **

110
**Figure 4-3 **

The relative increase in the CSA of the Achilles tendon insertion during the period from 7-9 years old to young adult for boys (opened square) and girls (closed square), respectively. The mean values of these parameter for 7-9 years old group and young adult group were set as 0% and 100%, respectively.

7-9 years old

10-13 years old

young adult

120

100

80

60 140

120

100

80

**CSA of the Achilles tendon** **2** **insertion for boys (mm)** 60

**0 %**
**100 %**

**0 %**
**100 %**

boys girls

**CSA of the Achilles tendoninsertion for girls (mm** **2****)**

111
**Figure 4-4 **

The relative increase in body mass during the period from 7-9 years old to young adult for boys (opened circle) and girls (closed circle), respectively. The mean values of these parameters for 7-9 years old group and young adult group were set as 0% and 100%, respectively.

7-9 years old

10-13 years old

young adult

80

60

40

20

**body ** **m** **a** **ss** ** f** **o** **r boys** ** (** **k** **g)**

**0 %**
**100 %**

**0 %**
**100 %**

boys

60

40

20

**b** **o** **d** **y** ** m** **a** **ss f** **o** **r g** **irls ** **(k** **g** **)**

girls

**Chapter 5 **

112
**Chapter 5 General discussion **

In this chapter, the main findings of this thesis are described firstly. Secondly, the limitations of the experiments of this thesis were discussed. Lastly, on the basis of the present results, the prevention methods of Sever's disease were discussed.

**5-1. Summary of the present thesis **

Sever's disease occurs the most frequently for boys aged 10-13 (Micheli and Ireland 1987; Kvist and Heinonen 1991; Scarfbillig et al., 2011). The presence of the hyaline cartilage, the age-related difference in body mass and the age-related and sex differences in the physical activity were known to be the factors of the age-specific and the sex difference in the incidence of Sever's disease. Additional factors may also account for these specificities of incidence of Sever's disease. Present thesis focused on the structural characteristic of foot and ankle for selected age groups of males and females for clarifying an additional biomechanical risk factor that account for these specificities of incidence of Sever's disease. The general purpose of this thesis, therefore, was to examine the age-related differences and sex differences in the structural parameters of foot and ankle that influence the magnitude of the tensile stress at the attachment site of the Achilles tendon.

The structural parameters of the foot and ankle necessary for determining the Achilles tendon force with inverse dynamics approach were to be measured directly from the participants of the present study. Previously, the two-dimensional center of

113

rotation method was used to determine the Achilles tendon moment arm with the assumption that the selected plane for capturing the bony configurations is orthogonal to the joint axis (Rugg et al., 1990; Fukunaga et al., 1996b; Maganaris et al., 1998). This assumption should induce the systematic error for determining the Achilles tendon moment arm. For overcoming the methodological limitations of the two-dimensional center of rotation methods, the first aim of this thesis was to develop a three-dimensional method for accurately determining the Achilles tendon moment arm from live subjects (Section 1 of Chapter 2). This method determine the Achilles tendon moment arm as the shortest distance between the talocrural joint axis to the line of action of the Achilles tendon force projected to the plane orthogonal to the talocrural joint axis. As a result, this method was found to be highly reliable (trial-to-trial, day-to-day and inter-examiner. The good agreement of the orientation of the talocrural joint axis between present and some previous results validated this method. Present result found that the two-dimensional center of rotation method overestimates the Achilles tendon moment arm by 21% and this error was induced by the assumption of the center of rotation method. The three-dimensional method developed in this study, therefore, is suitable for accurately determining the Achilles tendon moment arm.

Sever's disease occurs under weight-bearing condition, and therefore the influence of the muscle contraction on the Achilles tendon moment arm should be examined. Previous studies, using the two-dimensional center of rotation method, reported that the moment arm of a given muscle-tendon force increased when the muscles contracted to generate force (Maganaris et al., 1998, 1999, 2000, 2004;

**Chapter 5 **

114

Tsaopoulos et al., 2007; Akagi et al., 2012, 2013). This suggests that the Achilles tendon moment arm determined in a rest condition may not represent the Achilles tendon moment arm at weight-bearing condition accurately. The second aim of this thesis was to examine the influence of the muscle contraction on the Achilles tendon moment arm and the relationship between the Achilles tendon moment arms determined in the rest and muscle contraction conditions. The results showed that the Achilles tendon moment arm determined in the muscle contraction condition was greater by 18% than that determined in the rest condition and a significant correlation (r = 0.968) was found between the Achilles tendon moment arms determined in the two conditions. The Achilles tendon moment arm determined in the rest condition, therefore, underestimates but corresponds well to the Achilles tendon moment arm determined in the muscle contraction condition, suggesting that he Achilles tendon moment arm determined in the rest condition can be used to examine the difference in the Achilles tendon moment arm between different groups.

Sever's disease occurs most frequently in boys aged 10-13 (Micheli and Ireland 1987; Kvist and Heinonen 1991; Scarfbillig et al., 2011). Although the presence of the hyaline cartilage at the injured site (Achilles tendon insertion) largely accounts for the age-specificity of incidence of this injury, additional factors, particularly those that influence the magnitude of tensile stress at the Achilles tendon insertion, may also account for the age-specificity. The third aim of this thesis was to examine the age-related differences in the structural parameters of foot and ankle that influence the magnitude of the tensile stress at the attachment site of the Achilles tendon. The index

115

that represents the magnitude of the tensile stress at the Achilles tendon attachment site for a given physical activity was found to be significantly greater in boys aged 10-13 than in boys aged 7-9. The mechanical advantage was not different among the groups.

Body mass was significantly greater in older groups than in younger groups. The CSA of the Achilles tendon insertion was not different between boy's groups and was greater in young adult males than in the boy's groups. These results suggest that the onset of growth spurt for the CSA of the Achilles tendon insertion does not coincide with that for body mass, and this gap might result in the significantly large value in the index for boys aged 10-13. The delayed onset of growth spurt for the CSA of the Achilles tendon insertion may be an additional biomechanical risk factor that accounts for the age-specificity in the incidence of Sever's disease.

The higher incidence of Sever's disease was reported for boys than girls (Meyerding and Stuck 1938; McKenzie et al., 1981; Micheli and Fehlandt 1992). Then, the notable sex difference in the incidence was observed for boys and girls aged 10-13 but for those aged 7-9 (Micheli and Ireland 1987). These suggest that the sex difference in the incidence of Sever's disease is age-specific. In general, the onset of the growth spurt of body dimensions is earlier for girls than boys (Malina and Bouchard 1991).

This suggests that the onset of growth spurt of foot and ankle dimensions should, therefore, be earlier for girls than boys and this results in sex difference in the parameters determining the index of the tensile stress. The fourth aim of this thesis was to examine the sex differences in the structural parameters of foot and ankle that influence the magnitude of the tensile stress at the Achilles tendon attachment site for