1. This study utilized a skinned myocyte preparation with low end comp
liance to examine force-velocity and power-load curves at 12 degrees C
in myocytes from rat hearts. 2. In maximally activated myocyte prepar
ations, shortening velocities appeared to remain constant during load
clamps in which shortening took place over a sarcomere length range of
similar to 2.30-2.00 mu m. These results suggest that previously repo
rted curvilinear length traces during load clamps of multicellular pre
parations were due in part to extracellular viscoelastic structures th
at give rise to restoring forces during myocardial shortening. 3. Duri
ng submaximal Ca2+ activations, the velocity of shortening at low load
s slowed and the time course of shortening became curvilinear, i.e. ve
locity progressively slowed as shortening continued. This result impli
es that cross-bridge cycling kinetics are slower at low levels of acti
vation and that an internal load arises during shortening of submaxima
lly activated myocytes, perhaps due to slowly detaching cross-bridges.
4. Reduced levels of activator Ca2+ also reduced maximal power output
and increased the relative load at which power output was optimal. Fo
r a given absolute load, the shift has the effect of maintaining power
output near the optimum level despite reductions in cross-bridge numb
er and force generating capability at lower levels of Ca2+.