Numerous studies have established that high levels of lower-body strength and explosiveness are key physical qualities in maximizing athletic performance (Meylan et al., 2012; Secomb et al., 2014; Sheppard et al., 2008). Furthermore, it has been highlighted how strength and explosiveness increases throughout maturity (De Ste Croix et al., 2003), and as such, it is necessary to understand how best to develop these qualities from adolescence to adulthood. Previous cross-sectional research has identified that the structural arrangement, and size of the fascicles within a muscle are associated with the expression of strength and explosiveness in adult populations (Brechue et al., 2002; Earp et al., 2010; Secomb et al., 2015b). As muscle structure has been noted as highly adaptive to training (Blazevich et al., 2007; Earp et al., 2010; Nimphius et al., 2012), it is necessary to understand the relationships between muscle structures and the expression of strength and explosiveness in adolescent athletes. Through identification of the specific muscle structures that may be related to improved strength and explosiveness, the ability to effectively prescribe training to best develop these qualities during an athlete's development may be enhanced (Secomb et al., 2015b). In addition, such data would provide the basis from which longitudinal studies can be performed, to determine whether changes in muscle structure are associated with concomitant changes in strength and explosive qualities, in adolescent athletes.
Although previous research has identified that vastus lateralis (VL) thickness and lateral gastrocnemius (LG) pennation angle are related to enhanced lower-body strength and explosiveness in adults (Brechue et al., 2002; Earp et al., 2010; Nimphius et al., 2012; Secomb et al., 2015b), to our knowledge, no research to date has investigated whether similar associations exist in adolescents. Extensive research has reported that both muscle size, and strength and explosiveness increase throughout maturation (Lloyd et al., 2011; Meylan et al., 2013; Philippaerts et al., 2006), and also that the force producing capability differences between adults and adolescents are largely due to differences in muscle size (Barrett et al., 2002; O'Brien et al., 2009; 2010). Furthermore, it has been suggested that when performing an isometric contraction, adults are able to more efficiently utilize the force-velocity and force-length relationships within a muscle, when compared to adolescents, potentially due to stiffer aponeurosis tissue (Kannas et al., 2010). As such, it may be that the associations between lower-body muscle structure and, lower-body strength and explosive qualities in adolescents, are different to those previously observed in adult populations.
It has been noted that leg stiffness may largely affect the tendons ability to store and redistribute elastic strain energy, and hence, will influence the performance of dynamic movements involving a stretch-shortening cycle (SSC) (Fukashiro et al., 2006; Foure et al., 2010; Secomb et al., 2015b). This contention is supported by Secomb et al. (2015b), which recently identified that eccentric leg stiffness exhibited large relationships with the dynamic strength deficit (DSD) ratio, countermovement jump (CMJ) performance, and LG pennation angle in elite adult surfing athletes. These results suggest that greater eccentric leg stiffness allowed the athletes to apply a greater magnitude of force in a dynamic movement, in relation to their maximal strength, which may be related to increased pennation within the LG muscle (Secomb et al., 2015b). Importantly, it has been extensively demonstrated that both absolute and relative leg stiffness increase throughout maturation (Grosset et al., 2007; Lloyd et al., 2011; 2012). Additionally, Lloyd et al. (2012) reported that although older children (12 to 15 years old) exhibited greater leg stiffness than younger children (9 years old), they were not able to produce greater ground reaction forces when hopping.
Numerous studies (Earp et al., 2010; Nimphius et al., 2012; Secomb et al., 2015b?) have identified that significant relationships are present between specific lower-body muscle structures, strength and explosive qualities, and the mechanical properties of the lower-body. Whilst these cross-sectional studies provide an enhanced understanding of the relationships between muscle structure and physical qualities, and a basis for longitudinal studies, it is necessary to note that these studies were performed with adult populations. As such, the application of such results to adolescents should again be made with caution, as the effects of maturation may alter any such relationships. Therefore, the purpose of this study was to investigate whether any significant relationships were present between the lower-body muscle structure, and the strength, explosive, and eccentric leg stiffness qualities of the lower-body, in adolescent athletes.
Thirty junior competitive male (n = 23) and female (n = 7) surfing athletes (14.8 [+ or -] 1.7 y; 1.63 [+ or -] 0.09 m; 54.8 [+ or -] 12.1 kg) participated in this study. Inclusion criteria involved the following: (i) member of the surfing sports excellence squad at a local high school, (ii) currently competing at a state level or higher, and (iii) currently free of any injury or medical condition, as per a health screening questionnaire. The study and procedures were approved by University Human Ethics Committee (approval number: 10228), and conducted according to the Declaration of Helsinki. All participants and their parents/guardians were provided with information detailing the study prior to providing informed consent and were screened for medical contraindications prior to participation.
This study utilized a cross-sectional analysis, whereby subjects had their VL and LG muscles assessed with ultrasonography, and performed a; CMJ, squat jump (SJ), and isometric mid-thigh pull (IMTP), in a single session (Secomb et al., 2015a).
Ultrasound: Real-time B mode ultrasonography (SSD-1000; Aloka Co., Tokyo, Japan), with a 7.5MHz linear probe was used to assess VL and LG muscle structure (Kawakami et al., 1993; 1995; Kubo et al., 2000). To measure VL muscle thickness and pennation angle, subjects were placed in a supine position, with measures taken at 50% of the distance between the greater trochanter and lateral epicondyle of the femur (Earp et al., 2010; Nimphius et al., 2012; Secomb et al., 2015b). In addition, for assessment of muscle thickness and pennation angle of the LG, subjects were placed in a prone position, with measures taken at two-thirds of the distance between the lateral epicondyle of the femur and lateral malleolus (Earp et al., 2010; Nimphius et al., 2012; Secomb et al., 2015b). Two images were recorded of the VL and LG of both legs, with analysis performed as previously described in Secomb et al. (2015b). Furthermore, to calculate the fascicle length of the VL and LG, the equation reported by Fukunaga et al. (1997) (fascicle length = muscle thickness x [(sin pennation angle).sup.-1]) was utilized. For analysis, the results for both the left and right leg were combined and averaged. The Intraclass Correlation Coefficient...