Musculotendinous (MT) stiffness of the triceps surae (TS) muscle group was quantified in 28 prepubertal children (7-10 yr) by using quick-release movements at different levels of submaximal contractions. Surface electromyograms (EMG) of each part of the TS and of the tibialis anterior were also recorded. A stiffness index, defined as the slope of the angular stiffness-torque relationship (SIMT-Torque), was used to quantify changes in MT stiffness with age. Results showed a significant decrease in SIMT-Torque with age, ranging from 4.02 +/- 0.29 to 2.88 +/- 0.31 rad-1 for the youngest to the oldest children. Because 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 SIMT-Torque values found in the youngest children. TS EMG-torque analyses confirmed that neuromuscular efficiency was significantly lower for the 7- or 8-yr-old children compared with 10-yr-old children, notably due to a higher degree of tibialis anterior coactivation found in the youngest children. Thus the stiffness index originally defined as the slope of the angular stiffness-EMG relationship increased significantly with age toward 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.

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The twitch of the triceps surae muscle (TS), which characterizes the contractile properties independently of volition, differs in amplitude, but not in time course, when evoked in pre or postpubertal children. The aim of the present study was to compare the TS twitch contractile properties in prepubertal children (7 to 11 y). M-wave and twitch were recorded at rest by supramaximal electrical stimulations of the posterior tibial nerve. Twitches were characterized by peak torque (Pt), contraction time (CT), half relaxation time (HRT), and rate of torque development (dPt/dt). Electromechanical delay (EMD) was quantified with regard to the TS M-wave onset. Pt values increased significantly with the age of the prepubertal children but remained lower than that for adult subjects. CT and HRT values did not change with age. Thus, dPt/dt increased significantly between the 7-year-old and the 11-year-old children but remained significantly lower than that for adults. Despite EMD values decreased with age, they remained significantly higher than those of adult subjects. These results confirmed the link between growth processes and the increase in twitch torque for prepubertal children within a limited range of age. However, the time-course characteristics were not affected by age. The increase in dPt/dt and the decrease in the EMD could be used as indirect indicators of changes in contractile kinetics and in musculo-tendinous stiffness with the age of the prepubertal children. It is now well-established that during growth, muscles become stronger (1-4) and this increase in strength is associated with changes in body and muscle sizes (2,5,6).Electrical stimulation techniques, to evoke twitches, can be used to quantify contractile properties independently of volition and skill of the subject. This was performed to compare contractile properties of skeletal muscles between prepubertal and postpubertal children (1,2) or between children (prepubertal and postpubertal) and adult subjects (4,7). It has been shown that twitch force increased with age (1-4) and that this increase was related to the increase in muscle size (2). Furthermore, no significant difference in the time course of the twitch (contraction time and half relaxation time) was reported between prepubertal and postpubertal children (1-4), which suggests that the fiber-type composition of the muscles was already adult-like.In the present study, we analyze contractile properties during prepubertal growth (between 7 and 11 y), not only by the measure of the twitch peak torque and the time-course characteristics of the twitch but also by the quantification of the rate of torque development and of the electromechanical delay.In humans, the rate of torque development has been used to assess changes in muscle contractile kinetics with immobilization (8), electrostimulation training (9), or aging (10).The electromechanical delay (EMD) measured in vivo informs principally about the force transmission through more or less compliant elastic elements rather than about the mech...

Musculo-articular stiffness of the triceps surae (TS) increases with age in prepubescent children, under both passive and active conditions. This study investigates whether these changes in muscle stiffness influence the amplitude of the reflex response to muscle stretch. TS stiffness and reflex activities were measured in 46 children (7–11 yr old) and in 9 adults. The TS Hoffmann reflex (H reflex) and T reflex (tendon jerk) in response to taping the Achilles tendon were evaluated at rest and normalized to the maximal motor response (Mmax). Sinusoidal perturbations of passive or activated muscles were used to evoke stretch reflexes and to measure passive and active musculoarticular stiffness. The children's Hmax-to-Mmax ratio did not change with age and did not differ from adult values. The T-to-Mmax ratio increased with age but remained significantly lower than in adults. Passive stiffness also increased with age and was correlated with the T-to-Mmax ratio. Similarly, the children's stretch reflex and active musculoarticular stiffness were significantly correlated and increased with age. We conclude that prepubescent children have smaller T reflexes and stretch reflexes than adults, and the lower musculoarticular stiffness is mainly responsible for these smaller reflexes, as indicated by the parallel increases in reflex and stiffness.