Musculo-endinous (MT) stiffness of the triceps surae (TS) muscle group was quantified in 28 prepubertal children (7 to 10 years) by using quick-elease movements at different levels of submaximal contractions. Surface electromyograms (EMG) of each part of the TS and of the tibialis anterior (TA) were also recorded. A stiffness index, defined as the slope of the angular stiffness - torque relationship (SI_MT-Torque), was used to quantify changes in MT stiffness with age. Results showed a significant decrease in SI_MT-Torque with age, ranging from 4.02±0.29 to 2.88±0.31 rad^-1 for the youngest to the oldest children. Since an increase in stiffness with age was expected due to the maturation of elastic tissues, overactivation of the TS was suspected to contribute to the higher SI_MT-Torque values found in the youngest children. TS EMG - torque analyses confirmed that neuromuscular efficiency was significantly lower for the 7- or 8-year-old children compared to 10-year-old children, notably due to a higher degree of TA co-activation found in the youngest children. Thus, the stiffness index originally defined as the slope of the angular stiffness - EMG relationship (SI_MT-EMG) increased significantly with age towards adult values. The results underlined the necessity to take into account the capacities of muscle activation to quantify changes in elastic properties of muscles, when those capacities are suspected to be altered.