3737 INTRODUCTION Maximum muscular power production during short-term exercise is essential for the successful performance of many motor tasks. As children grow older they improve their ability to generate muscular power as a result of both growth and maturation (Larsson et al., 1979; Martin et al., 2000). As power is a function of force and velocity, and force is well related to muscle mass (Fukunaga et al., 2001; Ikai and Fukunaga, 1968; Knuttgen, 1978), the age-related increase in muscle mass (muscle cross sectional area in particular) is a significant contributor to the age-related increase in force and power production (Kanehisa et al., 1994; Neu et al., 2002). However, it has been consistently reported that changes in muscle mass do not fully account for changes in muscular power production (De Ste Croix et al., 2001; De Ste Croix et al., 2003). Several authors compared the peak power produced during vertical jumping in pre-adolescent children with that of adults (Davies and Young, 1984; Ferretti et al., 1994). These studies demonstrated that the large observed difference in peak power between age groups could not be solely explained by differences in muscle mass. Previous research suggests that maximum power production in jumping is related to lower limb stiffness in adults (Arampatzis et al., 2001). Stiffness is an important parameter because we take advantage of the storage and release of elastic energy in the musculotendinous unit to improve muscle power and jump height (Bobbert, 2001). However, evidence from the literature is inconclusive. When performing a counter-movement jump, those with a stiffer musculotendinous system might benefit from a faster elastic recoil during the upward, concentric, phase of the jump (Arampatzis et al., 2001), as well as a more efficient transfer of force to the skeleton (Wilson et al., 2003). However, elastic energy storage is likely to be greater in those with more compliant muscle–tendon units, which seems important for jump success (Bobbert, 2001). Rabita et al. speculated that in skilled humans, the neuromuscular system adopts strategies to find the optimal balance between these conflicting requirements (Rabita et al., 2008). In a developmental context then, the question arises as to whether the relationship between maximum power production and lower limb stiffness changes as a function of age. It has been reported that the stiffness of the musculotendinous unit increases with age during childhood (Lambertz et al., 2003). Moreover, Wang et al. (Wang et al., 2004) speculated that lower limb stiffness may be a contributor to developmental changes in jumping performance; however, they did not specifically quantify this relationship. The first aim of the present investigation was to determine to which extent lower limb stiffness would contribute to maximum power production in pre-adults during a lower limb multi-joint task. First, in order to seek support for the notion that age-related differences in peak power production during maximum vertical jumping cannot solely be explained by differences in body mass, we hypothesised age-related differences in peak power production even when body mass is accounted for (aim 1A). Second, we tested the hypothesis that the relationship between lower limb stiffness and the peak power measured during a maximum vertical counter- movement jump would be age-dependent (aim 1B). The Journal of Experimental Biology 212, 3737-3742 Published by The Company of Biologists 2009 doi:10.1242/jeb.033191 Development of lower limb stiffness and its contribution to maximum vertical jumping power during adolescence Thomas Korff 1, *, Sara L. Horne 1 , Sarah J. Cullen 1 and Anthony J. Blazevich 2 1 Centre for Sports Medicine and Human Performance, Brunel University, Uxbridge, Middlesex UB8 3PH, UK and 2 School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup Campus, 100 Joondalup Drive, Joondalup, WA, 6027, Australia *Author for correspondence (thomas.korff@brunel.ac.uk) Accepted 11 August 2009 SUMMARY Maximum power production during multi-joint tasks increases as children grow older. Previous research suggests that in adults, maximum power production in jumping is related to lower limb stiffness. In a developmental context, the question arises as to whether the relationship between maximum power production and lower limb stiffness is age-dependent. The purpose of this study was to investigate the relationship between lower limb stiffness and peak power production in adolescents (AD) and pre- adolescents (PA). With institutional approval, two groups of pre-adults (pre-adolescents: 11–13 years of age, N43; adolescents: 16–18 years of age, N30) performed 30 two-legged hops at their preferred frequency and three maximum counter-movement jumps. AD produced significantly greater peak power during the counter-movement jump than PA (t 71 –5.28, P<0.001) even when body mass was accounted for. Lower limb stiffness was significantly correlated with peak power production during the counter- movement jump in AD (R0.62, P<0.001) but not in PA (R0.26, P0.10). When normalised to body mass, the relationship between lower limb stiffness and peak power also differed between the two age groups (R0.30, P0.11 for AD and R0.02, P0.88 for PA). In addition, we found that during hopping, both PA and AD behaved like a simple spring-mass system. Our findings highlight the importance of lower limb stiffness in the context of muscular power production during multi-joint tasks. They let us speculate that during adolescence, children acquire the ability to take greater advantage of elastic energy storage in the musculotendinous system when performing maximum counter-movement jumps. Key words: development, coordination, biomechanics. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY