Influence of extent of muscle shortening and heart rate on work from frog heart trabeculae.

Shortening, lengthening, and net work done by frog (Rana pipiens) heart trabeculae were measured over a range of strain amplitudes (length change) and cycle frequencies. Net work, the product of muscle strain and force over a full lengthening/shortening cycle, increased with strain to strains well over 25% of the muscle's rest length, a value greater than optimum strains reported for most skeletal muscles. A distinct optimum strain for net work was not found. Maximum net work per cycle averaged 7.5 J/kg for ventricular muscle and 2.0 J/kg for atrial muscle. Isometric twitch stress was maximal at 0.4-0.6 Hz twitch frequency in ventricular trabeculae (average 51 kN/m2) and 0.6-1.4 Hz in the atrium (average 14 kN/m2). The twitch duration decreased with increasing twitch frequency. Shortening and net work were maximal at 0.7-Hz cycle frequency in ventricular trabeculae and 0.9 to 1.4 Hz in the atrium. The decline in work per cycle at slower frequencies was due in part to a decline in twitch force. Maximum power for the ventricle was approximately 5 W/kg and occurred at 0.8 Hz and 26% strain, and was 1/3 to 1/4 the power of most skeletal muscles studied at similar temperatures.