Introduction. It has been shown that the ability to produce maximal strength with respect to muscle size is lower in children than in young adults. That difference may be attributed to the lower ability of children to fully activate their motor units (O’Brien et al. 2009). However, other studies reported comparable levels of voluntary activation (VA) in children and older subjects (Belanger and McComas, 1989). Part of these controversies could be attributed to discrepancies in the mechanical conditions of muscle contraction, i.e. muscle length. Therefore, the aim of the present study was to compare the effect of muscle length on the level of VA at short and long muscle lengths between children and adults. Methods. Thirteen prepubertal boys (10.2 ± 1.1 y) and ten men (23.9 ± 2.9 y) performed isometric 5-s maximal voluntary contractions (MVC) of the knee extensor (KE) muscles at three muscular angles (20°, 90°, 100°; 0° = full extension) interspersed with at least 60-s passive recovery periods. Single magnetic stimulations were delivered to the femoral nerve to calculate the maximal level of VA by means of the twitch interpolation technique. The specific torque was calculated as the absolute torque divided by thigh muscle mass, as assessed using DXA. Finally, the theoretical specific torque that could be produced with a complete (i.e. 100%) activation level (specific torque@100%VA) was estimated from the values of specific torque and VA. Results. Specific torque was higher in adults at 90° (60 ± 11 N.m/kg vs. 48 ± 10 N.m/kg, P<0.05) and 100° (48 ± 8 N.m/kg vs. 41 ± 7 N.m/kg, P<0.05) but not at 20°. Accordingly, VA was significantly higher in adults at 90° (94 ± 4% vs. 88 ± 8%, P<0.05) and 100° (93 ± 6% vs. 86 ± 8%, P<0.05), while no significant difference was observed at 20°. Interestingly, the specific torque@100%VA was not different between groups, whatever the joint angle. Discussion. The results of the present study reveal for the first time that the level of VA is only lower in children at long muscle length. This lower ability of children to fully activate their motor units at long muscle length could account for their lower specific torque since no difference in theoretical specific torque was observed between groups at 90° and 100°. Further studies are however required to identify the mechanisms underlying the effects of muscle length on specific strength and VA in children. References. Belanger AY, McComas AJ. (1989). Eur J Appl Physiol Occup Physiol, 58, 563-7. O'Brien TD, Reeves ND, Baltzopoulos V, Jones DA, Maganaris CN. (2009). Eur J Appl Physiol, 106:849-56.