Cardiac hypertrophy due to physical exercise--an example of hypertrophy without decrease of contractility: unreliability of conventional estimation of contractility by simple parameters.

In 100 young, male Sprague-Dawley rats, a long term swimming training (2 hr/day for 8-12 weeks) produced an increase in heart weight of 10 percent, and an increase of about 15 percent in the relation of heart weight to body weight compared with control rats of the same age and initial weight. In examinations of the mechanical properties of the whole ventricle as well as of trabecular preparations, there was no evidence of impaired myocardial contractile ability because of the swimming training. Some parameters for the estimation of "contractility" increased, whereas others decreased. At a muscle length near lmax, the developed force and the maximal rate of force development were slightly augmented. The results reveal the limited value of some indices of contractility. Alterations in the shape of the contraction curve must to be considered adequately in order to avoid misinterpretations.