Variability of postural "reflexes" in humans.

The functional role of spinal and supraspinal EMG-responses for the maintenance of upright human posture was investigated in ten healthy subjects standing on a force measuring platform, which could be rotated in pitch around an axis aligned with the subject's ankle joint. Voluntary changes of body posture prior to the platform movement by leaning forward or backward led to a change in the amplitude and temporal organization of EMG-responses as compared to platform movements starting from a neutral position. Tilting the platform toe-up while leaning backward led to an increase of the latency of the short- and medium-latency responses in the triceps surae muscle and to a decrease of the latency of the stabilizing response in the anterior tibial muscle. Functionally, a cocontraction of both antagonistic muscles could be observed which partly compensated for the destabilizing action of the "reflex" response in the stretched triceps surae muscle. In analogy, leaning forward and tilting the platform toe-down led to a cocontraction of the two antagonistic muscles. The observed changes of latencies of short-, medium-, and long-latency response show the functional variability of segmental and suprasegmental "reflex" mechanisms. EMG-activities, which are functionally destabilizing posture, can be suppressed or compensated by reflexive cocontractions of antagonists.