Da. Syme et Rk. Josephson, INFLUENCE OF MUSCLE LENGTH ON WORK FROM TRABECULAR MUSCLE OF FROG ATRIUM AND VENTRICLE, Journal of Experimental Biology, 198(10), 1995, pp. 2221-2227
The work capacity of segments of atrial and ventricular muscle from th
e frog Rana pipiens was measured as a function of muscle length using
the work loop technique. Both the work done during shortening and the
work required to re-lengthen the muscle after shortening increased wit
h muscle length. Net work increased with length up to a maximum, beyon
d which work declined. The optimum sarcomere length for work output wa
s 2.5-2.6 mu m for both atrial and ventricular muscle. Isometric force
increased with muscle length to lengths well beyond the optimum for w
ork output. Thus, the decline in work at long lengths is not simply a
consequence of a reduction in the capacity of heart muscle to generate
force. It is proposed that it is the non-linear increase in work requ
ired to re-lengthen muscle with increasing muscle length which limits
net work output and leads to a maximum in the relationship between net
work and muscle length. Extension of the results from muscle strips t
o intact hearts suggests that the work required to fill the ventricle
exceeds that available from atrial muscle at all but rather short vent
ricular muscle lengths.