Load compensating responses of human abdominal muscles

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Abstract
Abdominal muscle responses to positive pressure loads applied suddenly to the external airway, while subjects held a constant lung volume against steady pressure, were studied. The preloading holding pressure was 6 cm H2O and the loading pressures were 6 or 12 cm H2O lasting for 2 s. Surface electromyograms (e.m.g.) were recorded over the internal oblique, external oblique and rectus abdominis muscles. The latency and pattern of the e.m.g. reactions were studied by measuring the raw record for each loading trial and by averaging the rectified e.m.g. for many trials. No responses were obtained if subjects were instructed not to respond to the loads. When instructed and trained to maintain their preload position in spite of the load, a 2-phase compensatory response was found. The initial response (phage I) was 100-300 ms in duration; it was followed by a continuous e.m.g. discharge (phase II) which continued to the offset of the pressure load. Subjects were also trained to make a single respiratory effort as quickly as possible after the load onset, or after just an auditory stimulus. These were simple reaction time tasks of a traditional kind and the e.m.g. responses elicited were single, brief bursts. For all subjects and experimental conditions, the e.m.g. response of the internal oblique occurred first, followed by the external oblique and then the rectus abdominis. For the maintain-position task, phase I latencies (internal oblique) averaged 66-90 ms for individual subjects but, for single trials with optimal conditions of practice and preparatory intervals, these ranged from 42-110 ms with 1/3 of the reactions occurring within 50-60 ms. The latencies for reaction time responses to loading were .apprx. 6 ms shorter than the phase I latencies. The latency distributions for the 2 types of responses were similar, and both were affected to the same degree by practice and changing the length and variability of the preparatory interval. Because of these similarities, phase I reaction was probably a learned response of a simple reaction time type and not comparable to late stretch reflexes such as the M2 of limb muscles. Arm muscle responses to respiratory pressure changes were measured in 3 subjects under identical conditions used to obtain load reaction times of abdominal muscles. These simple reaction times averaged 61-71 ms and ranged from 42-97 ms. Very short proprioceptive reaction times are not peculiar to the respiratory system or to load perturbations of the responding muscle. Mechanical adjustments of airway pressure and lung volume in response to loading are first expressed at the mouth .apprx. 90 ms after abdominal muscle activity begins. In the maintain-position task, the accuracy with which pressure and volume adjustments were made during the last 1.5 s of the loading period approximated that reported in the literature for trained singers holding a tone using constant expiratory pressure.