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Changes in stretch reflexes and muscle stiffness with age in prepubescent children

¹Département de Génie Biologique, Université de Technologie, Compiègne cedex; and ²EA 3300, Laboratoire Activités Physiques et Sportives et Conduites Motrices, Université de Picardie Jules Verne, Amiens, France

Submitted 12 September 2006 ; accepted in final form 6 March 2007

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 (M_max). Sinusoidal perturbations of passive or activated muscles were used to evoke stretch reflexes and to measure passive and active musculoarticular stiffness. The children's H_max-to-M_max ratio did not change with age and did not differ from adult values. The T-to-M_max ratio increased with age but remained significantly lower than in adults. Passive stiffness also increased with age and was correlated with the T-to-M_max 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.

prepubescent children; Hoffmann reflex; T reflex; stretch reflex; musculoarticular stiffness