152 Pediatric Exercise Science, 2010, 22, 152-164 © 2010 Human Kinetics, Inc. Medial Gastrocnemius Architectural Properties During Isometric Contractions in Boys and Men Theodoros Kannas, Eleftherios Kellis, Fotini Arampatzi, and Eduardo Saez Saez de Villarreal Aristotle University of Thessaloniki The aim of this study was to examine the differences in muscle architecture during isometric tests between children and adults. Eight boys (age= 11.2 ± 0.26 years) and eight men (age= 22.3 ± 2.01 years) performed plantar lexion isometric efforts at angles of -15°, 0°, 15° at 0%, 40%, 60%, 80% of MVC. Analysis of variance tests indicated that adults showed greater fascicle length from rest to 80% of MVC (p < .05), greater pennation angle at 80% and 100% of MVC (p < .05) and greater aponeuroses displacement at levels of effort greater than 60% of MVC (p < .05). These differences observed in MG would appear to favor better utilization of the force-length and the force-velocity relationships, of the muscle in adults compared with children. It is well known that muscle strength increases as the child grows. This increase often coincides with changes in body weight and lean body mass (36). Other fac- tors that might have an effect on strength development with growth include neural mechanisms (8,27), biomechanical factors such the mechanical advantage (33), and muscle architecture (6.). Speciically, the architectural arrangement of ibers within a muscle is an important determinant of its properties (32). Longer muscle ibers are associated with greater shortening velocities (26,2) while a higher pennation angle affects force production (7). A few studies examined in vivo muscle architectural characteristics in children (6,38). It has been reported that as the child grows, the pennation angle increases, reaching maximal values in early adulthood (6). Further, young males showed signiicant smaller fascicle length than adults (38). However, an interesting inding was that children display a higher speciic force than adults which suggests that children have a higher capacity of force for the same muscle volume (38). This was attributed to various methodological allometric, neural or tendinous factors (38), several of which have not been yet investigated. The force exerted by the muscles is transferred through the tendons to the joints. As the muscle contracts, the muscle ibers shorten by pulling the tendon. In turn, Kannas, Kellis, Arampatzi, and de Villarreal are with the Laboratory of Neuromechanics, Dept. of Physical Education and Sports Science at Serres, Aristotle University of Thessaloniki, Greece.