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. 2022 Jun 6;241(3):683–691. doi: 10.1111/joa.13711

Effects of growth on muscle architecture of knee extensors

Yasuyoshi Mogi 1,✉, Taku Wakahara 2,3
PMCID: PMC9358743  PMID: 35666144

Abstract

This study aimed to investigate the effects of growth on the muscle architecture of knee extensors. The present study included 123 male children and adolescents. The muscle thicknesses of the rectus femoris (RF) and vastus intermedius (VI), and pennation angles and fascicle lengths of RF were measured in three regions using ultrasonography technique at rest. The relative muscle thickness was calculated by dividing the absolute muscle thickness by body mass1/3. The years from age at peak height velocity were estimated for each participant, and used as a maturity index. The maturity index was significantly correlated with the relative muscle thicknesses of RF and VI in all regions. The slope of the relationship between the maturity index and the relative muscle thickness did not differ significantly between muscles within the same region or between regions within the individual muscles. The fascicle length and pennation angle of RF were significantly correlated with the absolute muscle thickness in all regions. In the proximal RF region, the coefficient of correlation between the muscle thickness and fascicle length was significantly greater than that between the muscle thickness and pennation angle. The present results showed that growth changes in muscle thickness were uniform between and within RF and VI. Our findings suggest that growth changes in the muscle thickness of RF depend on the increases in both pennation angle and fascicle length, but their contributions to the growth of muscle thickness differ among muscle regions.

Keywords: fascicle length, maturation, muscle thickness, pennation angle, rectus femoris, vastus intermedius

Short abstract

Our results indicated that the growth‐related changes in muscle thickness occurred uniformly along the length of rectus femoris and vastus intermedius and between the two muscles. Knowledge of growth changes in muscle architecture will provide fundamental and physiological information regarding muscle development, which may improve the prediction of parameters for muscle models during the growth period.

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1. INTRODUCTION

It is widely accepted that muscle architecture (geometrical arrangement of fascicles [Lieber & Fridén, 2000]) is related to muscle functions. For example, muscle thickness and pennation angle are closely related to muscle force production (Lieber & Fridén, 2000; Strasser et al., 2013). In addition, the fascicle length plays an important role in determining the shortening velocity of the muscle (Sacks & Roy, 1982) and force–length relation of the muscle (Lieber & Fridén, 2000). Therefore, it is quite important to investigate growth changes in muscle architecture to explain the development of human movements and sports performance during the growth period.

Previous studies have demonstrated that muscle architecture changes with growth (Binzoni et al., 2001; Jacobs et al., 2013; Kubo et al., 2001, 2014; Mogi, 2019; Morse et al., 2008). For example, the combined muscle thickness of the rectus femoris (RF) and vastus intermedius (VI) muscles has been shown to be positively correlated with chronological age (Jacobs et al., 2013), suggesting that muscle thickness increases with chronological age. Almost all of these studies measured the architectural parameters in a single anatomical region to investigate their growth effects. Meanwhile, Deighan et al. (2006) reported that the region of maximal anatomical cross‐sectional area of the elbow extensors changed with growth. Moreover, Kanehisa et al. (2006) investigated growth changes in the cross‐sectional area of the quadriceps femoris muscle in adolescent tennis players and showed that predominant muscle hypertrophy was observed at a level proximal to the knee joint. These findings suggest that the growth changes in muscle architecture differ along the length of the muscles. Therefore, measurement of the muscle architecture in a single region only may be insufficient to gain a comprehensive understanding of the growth changes in the muscle. Only one previous study (O'Brien et al., 2010) has compared the fascicle length and pennation angle of the quadriceps femoris muscle between children and adults in multiple regions. The results showed that the fascicle length of the adults was significantly longer than that of children in the distal and middle regions, and the pennation angle was not significantly different between children and adults in any region. However, as they did not measure the muscle architecture in adolescents, and given that the number of children was relatively small (n = 20), the relationship between growth and muscle architecture remains inconclusive, especially in adolescents.

Based on these previous findings mentioned above, it was hypothesized that the muscle thickness would be positively correlated with the maturity index in any region but that the slope of the correlations would be greater in the distal region than in the other regions. It was also hypothesized that fascicle length would be positively correlated with the maturity index in the distal and middle regions but that no significant correlation would exist between the pennation angle and maturity index. This study aimed to test these hypotheses. Increased knowledge of growth changes in muscle architecture will provide fundamental and physiological information regarding muscle development, which may improve the prediction of parameters for muscle models during the growth period.

2. METHODS

2.1. Participants

This study included 123 Japanese children and adolescents voluntarily participated in this study (age: 6.2–17.9 years, stature: 1.164–1.843 m, body mass: 19.8–78.9 kg). The participants were consisted of non‐athletes (n = 20) and baseball (n = 34), soccer (n = 26), basketball (n = 13), track and field (n = 17), judo (n = 6), and wushu (n = 7) athletes. All the participants were healthy males with no disability and/or disorder in their lower extremities. Prior to the experiments, the purpose of this study and the possible risks associated with the measurements were explained to the participants and their parents, and the written informed consent was obtained from the participants and their parents. This study approved by the Institutional Human Research Ethics Committee of Japan Society of Human Growth and Development (approval number: 02).

2.2. Estimation of maturity

During the growth period, chronological age does not always indicate biological maturation (Tanner, 1981), which is associated with muscle growth (Fukunaga et al., 1992; Fukunaga et al., 2014). Therefore, biological indicators of maturity, such as age at peak height velocity (PHV), appear to be more appropriate than chronological age for detecting the effects of growth on muscle architecture. Thus, in our study, the years from age at PHV were estimated based on, stature, body mass, the leg length, and sitting height (calculated by subtracting stature from leg length [Malina et al., 2004]) using the maturity offset method (Mirwald et al., 2002) and were used as a maturity index.

2.3. Measurements and analyses of muscle architecture

The muscle thickness, pennation angle, and fascicle length of RF, and the muscle thickness of VI were measured using Brightness‐mode (B‐mode) ultrasonography (HS‐2000; HONDA ELECTRONICS, Japan) with an electronic linear array probe (HLS‐475, 7.5 MHz wave frequency, 50 mm widths; HONDA ELECTRONICS, Japan). Ultrasound images (Figure 1) were recorded on a secure digital memory card through an analog‐to‐digital recorder (GV‐VCBOX; I‐O DATA, Japan). In the measurement of the muscle thickness, all participants were asked to stand with their legs fully extended (Abe et al., 1994; Miyatani et al., 2004). During the measurements of the pennation angle and fascicle length, the participants were asked to lay supine on a bed with their legs fully extended. Each participant was instructed to relax and not contract their muscles during the measurements. To collect clear ultrasound images, the angle of the probe relative to the skin was carefully adjusted during scanning of the fascicles, especially for the lateral region of RF, according to a previous finding (Ema, Wakahara, Mogi, et al., 2013). Scans were taken from the right leg at 30% (proximal), 50% (middle), and 70% (distal) of the thigh length from the greater trochanter to the popliteal crease.

FIGURE 1.

FIGURE 1

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Ultrasound images for measuring the muscle architectureThe upper image (a) is a transverse section of the thigh for measuring the muscle thickness of the rectus femoris (RF) and vastus intermedius (VI). The lower image (b) is a longitudinal section for determining the fascicle length and pennation angle of RF.

The muscle thickness of RF was defined as the distance between its deep and superficial aponeuroses. The muscle thickness of VI was defined as the distance between the superficial aponeurosis and the muscle–bone interface. The fascicle length of RF was determined as the length of the fascicular path between the intersections of the fascicle and its deep and superficial aponeuroses. The pennation angle of RF was defined as the angle between the fascicle and the deep aponeurosis. All parameters were analyzed using software (ImageJ; National Institutes of Health, USA). The fascicle length of RF was longer than the width of the probe, and the fascicles were not visible in their entirety in some cases. In such cases, the length of the missing portion was estimated by linear extrapolation (Austin et al., 2010; Ema, Wakahara, Mogi, et al., 2013; Erskine et al., 2009). Our previous study reported that an error in fascicle length estimation by the linear extrapolation was <2 mm in RF of adults (Ema, Wakahara, Mogi, et al., 2013). For the two adolescents, RF was not visible in the distal region because of their short muscle length. Thus, the number of datasets (muscle thickness, fascicle length, and pennation angle) was 121 (123–2) in the distal region of RF. The repeatability of the measurements of muscle thickness, pennation angle, and fascicle length was investigated on two separate days in a preliminary study with five children aged between 9.4 and 14.5 years. There were no significant differences between the test and retest values of muscle thickness, pennation angle, or fascicle length. The coefficient of variation was 3.9 ± 3.2% and 5.5 ± 4.6% for muscle thicknesses of RF and VI, respectively, and 6.7 ± 3.2% and 3.8 ± 2.5%, for the pennation angle and fascicle length of RF, respectively. The between‐day errors (root mean squared error) were less than 1.7 mm and 2.4 mm for the muscle thicknesses of RF and VI, respectively, and 2.0° and 4.7 mm for the pennation angle and fascicle length of RF, respectively.

2.4. Statistics

Descriptive data are presented as the mean and standard deviation (SD). Previous studies have reported that muscle thickness is positively correlated with body mass (Linek et al., 2017), and that fascicle length is positively correlated with limb length (Binzoni et al., 2001). The present results also showed significant correlations between muscle thickness and body mass (RF: r = 0.722–0.880 [proximal, middle, distal], p < 0.001, VI: r = 0.708–0.880 [proximal, middle, distal], p < 0.001), and between fascicle length and thigh length (r = 0.482–0.564 [proximal, middle, distal], p < 0.001). To exclude the effects of body size, the relative muscle thickness and relative fascicle length were calculated by dividing the muscle thickness and fascicle length by the body mass1/3 and thigh length, respectively. A simple regression analysis was performed to calculate the Pearson product–moment correlation coefficients for the relationships between the two variables. For these relationships, we also tested the significance of the difference in the slopes between RF and VI in the same region and between regions within the individual muscles. In addition, Fisher's r to z transformation was used to assess significant differences in the coefficients of correlation. The level of statistical significance was set at 0.05.

3. RESULTS

The relative muscle thicknesses of RF (Figure 2(a)) and VI (Figure 2(b)) were significantly correlated with the maturity index in all regions (RF: r = 0.521, p < 0.001 [proximal]; r = 0.695, p < 0.001 [middle]; r = 0.541, p < 0.001 [distal]; VI: r = 0.502, p < 0.001 [proximal], r = 0.477, p < 0.001 [middle], r = 0.378, p < 0.001 [distal]). The slope of the regression line for the relationship between relative muscle thickness and maturity index did not differ significantly between regions within RF (p = 0.145 [proximal vs. middle], p = 0.811 [proximal vs. distal], p = 0.069 [middle vs. distal]), VI (p = 0.465 [proximal vs. middle], p = 0.390 [proximal vs. distal], p = 0.126 [middle vs. distal]), or between muscles within the same region (p = 0.474 [proximal], p = 0.156 [middle], p = 0.438 [distal]).

FIGURE 2.

FIGURE 2

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Relationships between the maturity index and the relative muscle thickness of the rectus femoris (a) and the vastus intermedius (b) The black circles, the gray squares, and the white rhombuses indicate the data of the proximal, middle, and distal regions, respectively. The dotted, the solid, and the dashed lines indicate the regression lines of the proximal, middle, and distal regions, respectively.

The pennation angles (Figure 3(a)) of RF were significantly correlated with the maturity index (r = 0.263, p < 0.001 [middle]; r = 0.475, p < 0.001 [distal]), except for the proximal region (r = 0.107, p = 0.239). There were significant correlations between the maturity index and fascicle lengths of RF (Figure 3(b)) in each region (r = 0.613, p < 0.001 [proximal]; r = 0.529, p < 0.001 [middle]; r = 0.524, p < 0.001 [distal]). However, the relative fascicle lengths of RF (Figure 3(c)) were not significantly correlated with the maturity index (r = 0.107, p = 0.240 [proximal]; r = −0.150, p = 0.097 [middle]; r = −0.152, p = 0.097 [distal]).

FIGURE 3.

FIGURE 3

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Relationships between the maturity index and the pennation angles (a), fascicle lengths (b), and relative fascicle lengths (c) of the rectus femoris. The black circles, the gray squares, and the white rhombuses indicate the data of the proximal, middle, and distal regions, respectively. The dotted, the solid, and the dashed lines indicate the regression lines of the proximal, middle, and distal regions, respectively.

The pennation angles of RF were significantly correlated with absolute muscle thicknesses (Figure 4(a)) in each region (r = 0.227, p = 0.012 [proximal]; r = 0.442, p < 0.001 [middle]; r = 0.542, p < 0.001 [distal]). Similarly, there were significant correlations between the fascicle lengths and absolute muscle thicknesses of RF (Figure 4(b)) in each region (r = 0.602, p < 0.001 [proximal]; r = 0.578, p < 0.001 [middle]; r = 0.392, p < 0.001 [distal]). However, the coefficient of correlation between the fascicle length and absolute muscle thickness was significantly greater than that between the pennation angle and absolute muscle thickness in the proximal region (p < 0.001); no such difference was found in the middle (p = 0.152) or distal regions (p = 0.138).

FIGURE 4.

FIGURE 4

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Relationships between the absolute muscle thickness and the pennation angle (a) and fascicle length (b) of the rectus femoris. The black circles, the gray squares, and the white rhombuses indicate the data of the proximal, middle, and distal regions, respectively. The dotted, the solid, and the dashed lines indicate the regression lines of the proximal, middle, and distal regions, respectively.

4. DISCUSSION

The main findings of our study were that the relative muscle thicknesses were positively correlated with the maturity index in each region of both RF and VI, and the slopes of the regression line for the relationships did not differ between regions within a muscle or between muscles within the same region. Therefore, the results did not support our hypothesis that the slope of the relationship between the muscle thickness and maturity index would be greater in the distal region than in the other regions. In addition, the present results showed that the fascicle lengths and the pennation angles were positively correlated with the maturity index in each region, except for the pennation angle in the proximal region. The data partially support our hypothesis that fascicle length would be positively correlated with maturity index in the distal and middle regions, but no significant correlation would be observed between the pennation angle and maturity index. These results indicate that growth changes in the pennation angle are not uniform within RF.

Our results showed that the relative muscle thicknesses of RF and VI were positively correlated with the maturity index in all regions. These results indicate that muscle hypertrophy occurs in all regions with growth, even after considering the effects of the growth change in body mass. These results are consistent with a previous finding that the absolute muscle thickness increased as a function of chronological age in the medial gastrocnemius (Binzoni et al., 2001). The present results also demonstrated that the slopes of the regression line between the relative muscle thickness and the maturity index did not differ between RF and VI or between regions within each of RF and VI. These findings are in contrast to those of previous reports, which observed predominant muscle hypertrophy at the level proximal to the knee joint in the quadriceps femoris of adolescent tennis players (Kanehisa et al., 2006) and in VI and vastus lateralis of adolescent Olympic weight lifters (Kanehisa et al., 2003). The discrepancy between the present and previous results may be attributed to the participant characteristics and different techniques used for the measurement of muscle size. The participants in the present study consisted of non‐athletes and athletes engaged in various sports events and thus we did not focus on specific sports players. Meanwhile, the previous findings (Kanehisa et al., 2003, 2006) may have reflected the effects of sports‐specific training on muscle architecture in addition to growth changes. Moreover, the maturity range of participants was relatively narrower in the previous studies (skeletal age: 17.0 to adults [Kanehisa et al., 2003], 12.7 to 14.5 years [Kanehisa et al., 2006]) compared with our study (years from age at PHV: −5.76 to 3.32 years). Furthermore, Kanehisa et al. (2003, 2006) measured the cross‐sectional area of the muscle using magnetic resonance imaging (MRI), while we evaluated muscle thickness using ultrasonography. Although muscle thickness is closely related to its cross‐sectional area (Abe et al., 1997; Martinson & Stokes, 1991), it does not take into account possible transverse hypertrophy of the muscle with growth. Therefore, further studies are needed to confirm whether muscle hypertrophy occurs in the transverse direction with growth.

The present results showed that the fascicle lengths and the pennation angles of RF were positively correlated with the maturity index in each region, except for the pennation angle in the proximal region. These results are inconsistent with a previous finding of a study on RF, which showed that the fascicle length in the proximal region, and the pennation angles in the proximal, middle, and distal regions were not significantly different between boys and adult males (O'Brien et al., 2010). This inconsistency may be due to the small sample size of the previous study (n = 6 for RF in boys). The current results, based on a larger sample size, indicate that the fascicles of RF elongate along the whole muscle during growth, but that their increases in pennation angle differ among the regions. These results suggest that the regional variability of pennation angles within RF observed in adults (Blazevich et al., 2006) may be, at least in part, due to biological maturation.

The coefficients of determination (R 2) between the maturity index and the relative muscle thicknesses, the fascicle lengths, and the pennation angles were not so large (relative muscle thicknesses of RF: 0.271–0.483; relative muscle thicknesses of VI: 0.142–0.257; fascicle lengths: 0.274–0.375; pennation angles: 0.01–0.225). These results suggest that other factors than the maturity influence the changes in muscle architecture. A possible factor for the weak association is the different levels of physical activity among participants. It is well known that physical activity such as resistance training affects the muscle thickness (Kawakami et al., 1995; Takai et al., 2013), the fascicle length (Franchi et al., 2014; Seynnes et al., 2007), and the pennation angle (Ema, Wakahara, Miyamoto, et al., 2013; Kawakami et al., 1995). In the present study, the participants included non‐athletes and junior athletes of various sports, and thus the levels of physical activity including physical training varied among participants. The different levels of physical activity among participants may be related to the low coefficients of determination between the maturity index and the muscle thicknesses, the fascicle lengths, and pennation angles. Another possible factor for the weak association is a change in the aponeurosis area with growth. The aponeurosis serves as an attachment site for muscle fibers and its area affects the pennation angle of the muscle fibers (Wakahara et al., 2015). Previous studies reported an increase in the aponeurosis length with growth (Bénard et al., 2011; Weide et al., 2015) and an increase in the aponeurosis width with resistance training (Wakahara et al., 2015). Such changes in the aponeurosis length and/or width could explain the low coefficients of determination between the maturity index and the pennation angle.

The relative fascicle lengths were not correlated with the maturity index in our study, indicating that the proportion of bone to fascicle length is almost constant throughout the growth period. It is widely believed that the muscles cannot keep up with the longitudinal growth of bones and are passively stretched as a result (Frisch et al., 2009; Krivickas, 1997; Micheli & Fehlandt Jr, 1992; Micheli & Klein, 1991; Sever, 1912). If so, one can expect a decrease in the pennation angle with growth because passive stretching of a muscle is accompanied by a decrease in the pennation angle (Kawakami et al., 1998; Wakahara et al., 2005). However, no such trend was observed in our study; rather, we found positive correlations between the pennation angles and maturity index (r = 0.107–0.475, Figure 3(a)). These data suggest that the fascicles of RF were not passively stretched by bone growth in the growth period covered by our study. Nonetheless, there still remains a possibility that the pennation angles did not apparently decrease in cases wherein the growth‐related increase in pennation angle exceeded its stretch‐induced decrease. Evaluation of muscle stretch using elastography (Koo et al., 2014) and/or quantification of the transition point of the tendon using an ultrasound technique (Mogi et al., 2018) may be needed in addition to measurements of the relative fascicle length to determine whether the muscle is passively stretched during the growth period.

In our study, the absolute muscle thicknesses of RF were positively correlated with the fascicle length and pennation angle in all regions. It is well known that an increase in muscle thickness after resistance training is accompanied by an increase in pennation angle in adults (Ema, Wakahara, Miyamoto, et al., 2013; Kawakami et al., 1995). The reason for the increase in pennation angle is that a large pennation angle allows more contractile materials to be attached to a limited aponeurosis area (Maxwell et al., 1974). Meanwhile, previous findings have demonstrated that the aponeurosis length increases with growth (Bénard et al., 2011; Weide et al., 2015), suggesting that the more contractile materials can be attached to the aponeurosis with growth; in this case, an increase in the pennation angle may not be needed with growth. However, the present results showed that the pennation angles were positively correlated with the absolute muscle thicknesses, indicating that growth of aponeurosis length may not be sufficient for increased contractile materials to be attached. Consequently, the pennation angle may have to increase with growth. The increase in the pennation angle contributed to the increase in muscle thickness in conjunction with the elongation of fascicle length.

The current results showed that the fascicle length of RF was more strongly correlated with absolute muscle thickness than the pennation angle in the proximal region, but no such difference was found in the middle or distal regions. These findings suggest that the contributions of fascicle length and pennation angle to the growth of muscle thickness differed among regions. Although the reason(s) for the regional difference is unknown, it might be associated with the anatomical and functional differences within RF. Indeed, different motor nerve branches innervate the proximal region and the other regions of RF (Yang & Morris, 1999). In addition, the muscle activity of RF has been demonstrated to be higher in the proximal region than in the middle‐distal regions during maximal and submaximal isometric hip flexion (Miyamoto et al., 2012; Watanabe et al., 2012). These regional differences might also be related to the growth changes in muscle architecture. However, these discussions are currently speculative and require additional data for clarification.

To gain insights into the effects of sport‐specific training on the changes in muscle architecture during the growth period, the samples were grouped by sports events (non‐athletes: n = 20, baseball: n = 34, soccer: n = 26, and basketball: n = 13). The samples of the other athletes (sprinters [n = 8], wushu [n = 7], judo [n = 6], long distance runners [n = 6], long jumpers [n = 2], and javelin thrower [n = 1]) were too small to group. First, we checked group differences in age among groups. As a result, no significant difference in the age was found between non‐athlete (11.3 ± 4.1 years) and baseball (11.7 ± 1.6 years) groups. Then, we used unpaired t‐tests to test the group differences in variables of muscle architecture between these two groups. The results showed that the baseball group was significantly greater in the relative muscle thickness of RF in the proximal region (7.3 ± 0.7 mm/kg1/3) than that of the non‐athlete group (6.9 ± 0.6 mm/kg1/3, p = 0.037), and was significantly smaller in the relative muscle thickness of VI in the proximal region (5.8 ± 0.9 mm/kg1/3) than that of the non‐athlete group (6.7 ± 1.3 mm/kg1/3, p = 0.007). The current results indicate that sport‐specific training affects the changes in muscle architecture even in the growth period. Baseball players are required to perform hip flexion movements as well as knee extension movements in their habitual activities (e.g., running and batting). During running, the hip flexion moment (about 220 Nm) is generated by the swing leg during the initial swing (Schache et al., 2011), and RF is activated at the same time (Jönhagen et al., 1996). During batting, the hip flexion moment (about 100 Nm) is generated by the stride (lead) leg during the swing phase (Ae et al., 2017) and peak muscle activation of RF for the stride leg is observed at the same time (Nakata et al., 2013). Therefore, habitual baseball training can act as a stronger training stimulus for the biarticular RF (hip flexor and knee extensor) than the monoarticular VI (knee extensor). This may be related to the greater relative muscle thickness of RF in the baseball group than in the non‐athletes. Although the reason for the greater relative muscle thickness of VI in the non‐athlete group than in the baseball group is unknown, it might be associated with the fact that the non‐athlete group included adolescents after PHV (6 of 20) but the baseball group did not. The muscle size in adolescents after PHV is much greater than that of adolescents before PHV (Malina et al., 2004). The higher mean value of the relative muscle thickness of VI in the non‐athlete group might be due to the inclusion of adolescents after PHV.

This study has some limitations. First, this was a cross‐sectional study, and it did not detect within‐individual changes in the muscle architecture during growth. Therefore, it is possible that our results reflect differences in the underlying properties of participants, such as genetic variation and exercise and dietary habits, rather than the growth changes. Therefore, a longitudinal study is needed to confirm the current findings. Second, the present study did not examine female children or adolescents. Previous studies have reported a sex difference in muscle architecture (Chow et al., 2000), and females differed from males in terms of the tempo and timing of growth in the body, including the musculoskeletal system (Tanner, 1981). Therefore, the present results may not be applicable to changes in the muscle architecture of females. Third, we measured the muscle thickness as an index of muscle size. As mentioned previously, we cannot detect a change in muscle size across the transverse direction with growth from the muscle thickness. Therefore, further research is required to evaluate the growth changes in the muscle cross‐sectional area by using MRI or other techniques. The last limitation is related to the difference in the postures of participants during measurements of the muscle thickness (standing) and the fascicle length and pennation angle (lying). The different postures during measurements may be an issue in interpreting the present data. A previous study (Thoirs & English, 2009) reported that the muscle thickness measured while lying was smaller than that while standing. To evaluate the effects of posture during measurements on muscle thickness, we calculated the thicknesses as the products of the fascicle lengths and sine of the pennation angles in each region. Consequently, the calculated muscle thicknesses were significantly correlated with the measured muscle thicknesses (r = 0.669–0.833, p < 0.001). The calculated muscle thicknesses were also correlated with the pennation angles (r = 0.361–0.746, p < 0.001) and fascicle lengths (r = 0.650–0.743, p < 0.001) as with the case of measured thickness. In addition, the coefficient of correlation between the fascicle length and calculated muscle thickness was still significantly greater than that between the pennation angle and calculated muscle thickness in the proximal region (p = 0.002). These results suggest that, although there is a possibility that the muscle thickness measured while standing differs from that while lying, the main outcome of the present study would not have changed by the postures during measurements.

5. CONCLUSIONS

This study investigated the effects of growth on the muscle architecture of RF and VI. The results indicated that the growth‐related changes in muscle thickness occurred uniformly along the length of RF and VI and between the two muscles. In RF, the growth of muscle thickness was associated with an increase in both the pennation angle and fascicle length, but their contributions to the growth of muscle thickness differed between regions of the muscle.

AUTHOR CONTRIBUTIONS

YM conceived the present study, collected the data, performed the analysis, and drafted the manuscript. TW helped to perform the statistical analysis and helped to draft the manuscript. All authors read and approved the final manuscript.

CONFLICT OF INTEREST

The authors have no conflict of interest.

ACKNOWLEDGMENTS

This work was supported in part by a Grant‐in‐aid from Meiji Yasuda Life Foundation of Health and Welfare and by JSPS KAKENHI (Grant Number; 19K20079Grant‐in‐aid for Early‐Career Scientists).

Mogi, Y. & Wakahara, T. (2022) Effects of growth on muscle architecture of knee extensors. Journal of Anatomy, 241, 683–691. Available from: 10.1111/joa.13711

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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