How fast are feedforward postural adjustments of the abdominal muscles?

Abdominal muscles can be activated during rapid but complex motor responses known as feedforward postural adjustments. Exactly how the latency of feedforward adjustments compares with other responses of varying levels of voluntary input and motoneurone excitability remains unclear. Surface electrodes were placed bilaterally over abdominal muscles and over right anterior deltoid (AD) and orbicularis oculi (OO) muscles in eight healthy volunteers. Latencies were obtained during feedforward postural adjustment, acoustic startle reflex, and voluntary activation (respiratory and nonrespiratory) to soft and startling acoustic stimuli. Results showed voluntary activation was fastest for respiratory tasks during mid- or end-expiration. Voluntary activation was also faster when triggered by startling stimuli compared with soft stimuli. These responses were slower than feedforward postural adjustments in all subjects. Feedforward adjustments were slower than acoustic startle reflexes but were somewhat flexible as their amplitude depended on amplitude of AD activation and therefore matched parameters of the movement task. The latency for feedforward postural adjustments of abdominal muscles suggests organization at a low level of the nervous system with limited central processing to match postural responses to movement demands.