Tendon properties and muscle architecture for knee extensors and plantar flexors in boys and men
Introduction
The number of children who participate in competitive sports is increasing. As a result, injuries from overuse (e.g., Osgood–Schlatter disease, calcaneal apophysitis, Little League elbow) are now being recognized in pre-adolescent athletic populations (Hawkins and Metheny, 2001, Micheli and Klein, 1991). The main reason for this problem is that the immature musculoskeletal system is less able to cope with repetitive biomechanical stress during various activities (Gerrard, 1993, Kujala et al., 1985). Furthermore, previous researchers pointed out that the longitudinal growth of bone was faster than that of muscle, so that the muscles became progressively tighter during a period of rapid growth (Malina, 1974, Micheli, 1983). Considering these points, more compliant and larger tendon and longer muscle fiber (fascicle) are considered to contribute for preventing these injuries in growing children. In lower limb, on the other hand, Osgood–Schlatter disease was commonly involved followed by calcaneal apophysitis (Lau et al., 2008, Omey and Micheli, 1999). Therefore, the difference in frequency of occurrence between Osgood–Schlatter disease and calcaneal apophysitis would be related to the difference in growth changes in the elastic properties and size of tendon structures and fascicle length between knee extensors and plantar flexors.
Recent studies using ultrasonography have demonstrated that the elastic properties of human tendinous structures change by aging, training, and immobilization (Arampatzis et al., 2007, Kubo et al., 2007, Kubo et al., 2009, Reeves et al., 2005). However, little is known about growth changes in the human tendon properties in vivo (Kubo et al., 2001, Neugebauer and Hawkins, 2012, O’Brien et al., 2010, Waugh et al., 2012, Waugh et al., 2013). Among them, the tendinous structures for knee extensors (Kubo et al., 2001) and patellar tendon (O’Brien et al., 2010) were more compliant in children (pre-pubertal; about 10 yrs) than in adults. To be concrete, the tendon strain at a given stress of children was significantly greater than that of adults (Kubo et al., 2001, O’Brien et al., 2010). On the other hand, Waugh et al. (2012) did not find the difference in the maximal strain of the tendinous structures in plantar flexors between children and adults. Although the reasons for the discrepancy among these reports are unknown, there is a possibility that the difference in the tendon properties between children and adults would differ among the sites. At present, however, few studies have simultaneously investigated the age-related differences in the elastic properties of tendinous structures for knee extensors and plantar flexors.
Growth changes in the elastic properties of tendinous structures as mentioned above are considered to be due to changes in the size (including thickness and cross-sectional area), material properties, or both of tendinous structures. Hence, it is important to know growth changes in size as well as their elastic properties for grasping the detailed mechanism of changes in tendinous structures during the period of growth. Many previous studies have demonstrated that muscle size (thickness, cross-sectional area, and volume) increased rapidly during and after puberty (e.g., Kanehisa et al., 1994, Kanehisa et al., 1995). However, few studies have been available so far regarding comparison of the size of tendons in vivo between children and adults (O’Brien et al., 2010, Waugh et al., 2012). Magnusson et al., 2003a, Magnusson et al., 2003b reported that the cross-sectional area of the Achilles tendon was greater in elderly individuals than in the young ones, which may reduce the risk of injury to the tendon in the elderly. If this finding applies to the immature tendinous structures in children, it is likely that the tendon size of children is relatively greater than that of adults to reduce the imposed stress and thereby prevent injuries.
According to the animal experiments (Heslinga and Huijing, 1990, Woittiez et al., 1989), whole muscle length (corresponding to bone length) increased to a greater extent than muscle fiber length during growth. Indeed, we found that the ratio of the fascicle length of human vastus lateralis muscle to thigh length (corresponding to bone length) was significantly lower in children than in adults (Kubo et al., 2001). However, Morse et al. (2008) reported that there was no difference in the ratio of the fascicle length of gastrocnemius muscle to whole muscle length between early pubescent boys and adult men. Therefore, it may be that the growth rate of fascicle length for knee extensors (vastus lateralis muscle) is different from that for plantar flexors (gastrocnemius muscle).
In the present study, we aimed to compare the elastic properties (maximal strain) and size (thickness) of human tendinous structures and muscle architecture (fascicle length) in knee extensors and plantar flexors between early pubescent boys and young adult men. We hypothesized that the differences in extensibility and size of tendinous structures and fascicle length between boys and men were different between knee extensors and plantar flexors.
Section snippets
Subjects
Twenty-two early pubescent boys (9.6–12.7 yrs) and 23 young adult men (19.8–26.2 yrs) participated in this study. The ages and physical characteristics of each group are shown in Table 1. Adult men were either sedentary, or mildly to moderately active. Early pubescent boys were not involved in any specific physical training program beyond their normal school curriculum activities. The procedures, purpose, and risks associated with the study were explained to all subjects and their parents (for
Results
The measured variables of muscle are presented in Table 2. The relative MVC (to body mass) for knee extensors was significantly lower in boys than in men (P < 0.001), although there was no difference in that for plantar flexors between the two groups (P = 0.280). For both knee extensors and plantar flexors, the relative muscle thickness (to body mass1/3) was significantly lower in boys than in men (P = 0.015 for knee extensors, P < 0.001 for plantar flexors). The relative fascicle length (to limb
Discussion
The present study showed that the amount of changes in human muscle and tendon from early pubescent to maturity differed between the sites examined. To our knowledge, this is the first study to compare the elastic properties and size of tendinous structures and fascicle length between boys and men for knee extensors and plantar flexors simultaneously.
The present result on plantar flexors showed that the maximal strain of tendinous structures was greater in boys than in men (Fig. 2B). Previous
Conclusions
In conclusion, the present results suggested that the amount of changes in the elastic properties and sizes of tendinous structures and in the fascicle lengths from early pubescence to maturity differed between knee extensors and plantar flexors.
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This study was supported by a Grant-in-Aid for Young Scientists (A) (21680047 to K. Kubo) from the Japan Society for the Promotion of Science. The authors would like to thank Mr. Shinkawa K. and Mr. Ohkawa M. for their conscientious work in this project and the subjects and their parents who participated in this study.
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