title: Maximal dynamic expiratory pressures with fast and slow inspirations.
abstract: Maximal dynamic expiratory pressures are higher when forced expiration is preceded by a fast inspiration to total lung capacity (TLC) than when preceded by a slow inspiration and a few seconds pause at TLC. We hypothesized that these pressure differences are due to the stretch-shorten cycle (SSC), which refers to enhancement of muscle force when a concentric muscle contraction is immediately preceded by an eccentric contraction. Seven volunteers [36 (2) years; mean (SEM)] performed maximal forced expirations against minimal resistance with fast (F) or slow (S) maneuvers. F maneuvers consisted of a fast inspiration to TLC followed immediately by a fast expiration, whereas S consisted of a slow inspiration to TLC and a 4- to 5-s pause at TLC prior to forced expiration. We measured esophageal pressure ( P(es)), peak expiratory flow rate (PEFR), and the EMG activity of the transversus abdominis (Tr) by means of intramuscular fine-wire electrodes. The subjects performed several runs of each maneuver in a random order, and runs with the greatest expiratory P(es) were analyzed. In comparison with S, F yielded greater P(es) [182 (15) versus 167 (15) cm H(2)O; P=0.003)] but similar PEFR [9.8 (0.7) versus 9.6 (0.7) l/s, P>0.05] and EMG activity of the Tr during forced expiration [221 (31) versus 208 (34) a.u., P>0.05]. Further analysis revealed significant EMG activity of Tr during end-inspiration (eccentric contraction) with F maneuvers only [73 (22) versus 32 (17) a.u., P<0.05]. We conclude that the ability of expiratory muscles to generate greater P(es) with F maneuvers is related to the sequence of an eccentric contraction, which is followed immediately by concentric contraction in a manner analogous to SSC described in skeletal muscles.
authors.affiliation: John D. Dingell Veterans Affairs Medical Center, Detroit, MI 48201, USA.
authors.forename: Ashraf
authors.initials: A
authors.lastname: Altarifi
authors.forename: M Safwan
authors.initials: MS
authors.lastname: Badr
authors.forename: George E
authors.initials: GE
authors.lastname: Tzelepis
mesh_terms: D000328:Adult; D004576:Electromyography; D005541:Forced Expiratory Volume / Q000502:physiology; D006801:Humans; D007320:Inspiratory Capacity / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D011312:Pressure; D015656:Respiratory Mechanics / Q000502:physiology*; D012132:Respiratory Muscles / Q000502:physiology*; D014109:Total Lung Capacity / Q000502:physiology