Effect of time-varying load on degree of bronchoconstriction in the dog

It is wellestablished that the degree of airway smooth muscle shortening producedby a given dose of bronchial agonist is greatly affected by lungvolume. The airways are tethered by parenchymal attachments, thetension of which increases progressively with lung volume, therebypresenting a commensurately increasing hindrance to smooth musclecontraction. Earlier studies (P. F. Dillon, M. O. Aksoy, S. P. Driska,and R. A. Murphy. Science 211:495-497, 1981) presented evidence that smooth muscle contractioninitially involves rapidly cycling cross bridges, whichthen change to noncycling (latch) bridges. They also suggested thatmost of the muscle shortening occurs during the early rapidcross-bridge phase. This implies that smooth muscle subject to a givenload early in contraction should shorten less than when it is subjectto the same load later on. An in vitro study (W. Li and N. L. Stephens.Can. J. Physiol. Pharmacol. 72:1458-1463, 1994) obtained support for this notion. To test thishypothesis in vivo, we measured the changes in lung impedance at 1 and6 Hz produced in dogs by a bolus intravenous injection of methacholinewhen lung volume was increased for 10 s at different times afterinjection. We found that the changes in mechanics were greatlyinhibited, whereas lung volume was elevated. However, when lung volumewas returned to its initial level, the lung mechanics continued tochange at a rate unaffected by the preceding volume change. We concludethat temporary mechanical inhibition of airway smooth muscle shorteningin the normal dog in vivo merely delays an otherwise normal course ofcontraction.

     

Energy cost of front-crawl swimming in women

The purpose of this study was to examine the relationship between the energy cost of swimming per unit distance (Cs) at different velocities (v) and performance level, body size and swimming technique in women. A total of 58 females swimmers were studied. Three performance levels (A, B, C) were determined, ranging from the slower (A) to the faster (B, C). At level C and at 1.1 m.s-1, Cs,1.1 was reduced by 7% when directly compared to level B. The Cs,1.1 was reduced by 10% when calculated per unit of height (h) and by 37% when calculated per unit of h and hydrostatic lift (HL). For the whole group of swimmers, the equation regression was Cs,1.1 = 0.27 h-2.38 HL - 7.5 (r = 0.53, P less than 0.01). To evaluate the specific influence of arm length two groups of long- and short-armed swimmers were selected among swimmers of similar h and performance. The Cs was significantly higher (P less than 0.05) by 12%, SD 2.2%, for short-armed than for long-armed swimmers. To evaluate the influence of different types of swimming technique, two other groups of similar performance and anthropometric characteristics were selected. The Cs was significantly higher (P less than 0.05) by 12%, SD 4.5% for swimmers using for preference their legs rather than their arms. The Cs of the sprinters was 15.7%, SD 2% higher than that of the long-distance swimmers. For all groups, Cs increased with v on average by 8% to 11% every 0.1 m.s-1. These findings showed that Cs variations of these women were close to those previously demonstrated for men. The Cs depends on performance level, body size, buoyancy, swimming technique and v.