Antenatal anticipation of postnatal muscle function.

The semitendinosus muscle of the sheep was used as an experimental model to test antenatal anticipation of muscle postnatal function using the alkali-stabile myosin ATPase technique. Areas of variable fibre type population density were seen from 80 days of gestation. The highest population density of ATPase-low fibres was observed in the craniomedial aspect of the muscle (the ATPase-low dense area). The lowest ATPase-low fibre population density area (the ATPase-low sparse area) was seen in the caudolateral aspect of the muscle. Simultaneous electromyographic studies indicated that the ATPase-low dense area is preferentially used for posture and during quiet co-ordinated activity, while the ATPase-low sparse area is recruited only intermittently when the hip and stifle are less co-ordinated in movement. The ATPase-low fibre percentage within the ATPase-low dense area of the muscle increased from 10 to 30% from 80 days of gestation to adulthood. The part of this change occurring antenatally is expected to be the result of genetic anticipation.