TWITCH POTENTIATION FOLLOWING VOLUNTARY DIAPHRAGMATIC CONTRACTION

The purpose of this study was to determine whether twitch potentiation (a transient augmentation of twitch tension following Vigorous muscul ar contraction) occurs in the human diaphragm invivo. Six healthy subj ects were studied. To determine whether twitch potentiation occurs, th e subjects attempted to maximally activate their diaphragm by performi ng the combined Mueller-expulsive maneuver with visual feedback (Pdi(m ax) maneuver). Twitches were obtained before, at 10 s after the transd iaphragmatic pressure (Pdi) maneuver, and at intervals over the ensuin g 10 min. We also determined whether twitch potentiation would occur f ollowing submaximal voluntary diaphragmatic contractions (33 and 66% o f Pdi(max)). In addition, we examined whether repeated voluntary contr actions would result in greater twitch potentiation compared with that observed after a single voluntary contraction. Twitch potentiation wa s observed in every subject. The number of maximal voluntary contracti ons (MVC) (one, two, or four) had no significant effect on the degree of twitch potentiation. The increase in twitch amplitude (expressed as a percentage of the control value) averaged 63 +/- 35% (SD) (pooled d ata from one, two, and four contraction trials). Twitch potentiation d ecayed in a monoexponential fashion (r = 0.99) with a time constant of 125 s (95% Cl = 100 to 160 s). Twitch potentiation was also observed after submaximal voluntary diaphragmatic contractions. Again, the numb er of voluntary contractions (one or four) had no significant effect o n the degree of twitch potentiation. After submaximal diaphragmatic co ntractions of 66% of Pdi(max), the degree of twitch potentiation was n ot significantly different, 61 +/- 36% (pooled data from one and four contraction trials) from that observed following the maximal voluntary contraction maneuvers. However, twitch potentiation was significantly less, 19 +/- 17% (p < 0.0001) (pooled data from one and four contract ion trials) after submaximal diaphragmatic contractions of 33% of Pdi( max) compared with that observed at higher levels of contractions.