Tl. Allinger et al., FORCE-LENGTH PROPERTIES IN STABLE SKELETAL-MUSCLE FIBERS - THEORETICAL CONSIDERATIONS, Journal of biomechanics, 29(9), 1996, pp. 1235-1240
Differences in the force-length (F-L) properties between sarcomeres an
d fibers have been associated with the supposed unstable nature of the
sarcomere F-L relation on the descending limb (i.e. at sarcomere leng
ths greater than optimal length). Recently, it has been suggested that
sarcomere behavior in a fiber is stable during contractions on the de
scending limb of the F-L relation; therefore, a factor other than sarc
omere instability must be responsible for the observed differences in
the F-L relation of sarcomeres and fibers. The purpose of this study w
as to determine theoretically the F-L relation of a muscle fiber when
sarcomeres were at a stable, steady-state length. Three models of musc
le fibers are presented; each model contains sarcomeres with different
mechanical properties which have been observed experimentally. Result
s of these theoretical considerations demonstrate that sarcomeres with
the classic F-L properties as measured by Gordon et al. (J. Physiol.
184, 170-192, 1966) cannot predict the F-L relation exhibited by fiber
s. The addition of cross-bridge stiffness properties to the classic sa
rcomere F-L relation still does not explain the differences between th
e sarcomere and fiber F-L relations. However, if history dependent sar
comere properties are used, the fiber F-L relation exhibits an elongat
ed plateau and greater forces on the descending limb compared to the c
lassic sarcomere F-L relation; and the fiber F-L relation corresponds
qualitatively to experimental findings. Copyright (C) 1996 Elsevier Sc
ience Ltd.