Interlimb postural coordination in the standing cat.

The dorsal-side-up body posture in standing quadrupeds is maintained by coordinated activity of four limbs. We studied this coordination in the cat standing on the platform periodically tilted in the frontal plane. By suspending different body parts, we unloaded one, two, or three limbs. The activity of selected extensor muscles and the contact forces under the limbs were recorded. With all four limbs on the platform, extensors of the fore- and hindlimbs increased their activity in parallel during ipsilateral downward tilt. With two forelimbs on the platform, this muscular pattern persisted in the forelimbs and in the suspended hindlimbs. With two hindlimbs on the platform, the muscular pattern persisted only in the hindlimbs, but not in the suspended forelimbs. These results suggest that coordination between the two girdles is based primarily on the influences of the forelimbs upon the hindlimbs. However, these influences do not necessarily determine the responses to tilt in the hindlimbs. This was demonstrated by antiphase tilting of the fore- and hindquarters. Under these conditions, the extensors of the fore- and hindlimbs appeared uncoupled and modulated in antiphase, suggesting an independent control of posture in the fore- and hindquarters. With only one limb supporting the shoulder or hip girdle, a muscular pattern with normal phasing was observed in both limbs of that girdle. This finding suggests that reflex mechanisms of an individual limb generate only a part of postural corrections; another part is produced on the basis of crossed influences.