Force-velocity relationships of guinea pig ureter.

Longitudinal force-velocity relationships of the guinea pig ureter were studied in vitro in 3 age groups: 2 to 3 weeks, 2 to 3 months and 2 to 3 years. Velocity of shortening depends on the load lifted and not on the initial length of the muscle. Force-velocity relationships did not change with age, but the isometric force when the velocity is zero and no shortening occurs, increased with age. The shortening of the muscle, expressed as per cent of in vivo length, was independent of age and of preload. Therefore, work performed by a unit length of ureter which is the product of shortening and load lifted, and power which is the product of velocity and load lifted, are unaffected by age or by initial length. Thus there appears to be no significant age-related change in isotonic parameters of contraction such as velocity, shortening, work and power.