Development of postural control in children: short-, medium-, and long latency EMG responses of leg muscles after perturbation of stance.

Short (SL), medium (ML), and long (LL) latency EMG responses of leg muscles were recorded after perturbation of stance by means of a sudden toe-up tilt of a movable platform. 56 healthy children varying in age between 14 months and 15 years were investigated. All three responses were present when children were able to stand on the recording platform. The SL-response in the triceps surae muscle, which corresponds to the mono- and oligo-synaptic spinal stretch reflex, showed a decreasing latency up to the age of 5 years. This reflects the increasing peripheral nerve conduction velocity. The ML-response in the triceps surae muscle, which as the SL-response has no stabilizing effect in this experiment, showed somewhat delayed maturational changes. The LL-response in the relaxed anterior tibial muscle helps to restore upright posture even in the youngest children. Its maturational changes in terms of latency by far exceed the range that can be explained by the increase of peripheral and spinal conduction velocities. Its mechanisms of maturation, besides the biophysical optimalization of a polysynaptic network, may include learning in terms of selecting the shortest pathways by way of synaptic potentiation within structures involved in the supposedly transcortical pathway of the LL-response. Qualitative observations made during the trials showed that the pattern of postural adaptation changed with age, suggesting the development of additional intersegmental mechanisms.