The Relationship Between Short-Range Stiffness and Yielding in Type-
Identified, Chemically Skinned Muscle Fibers from the Cat Triceps Surae
Malamud, Jean G., Robert E. Godt, T. Richard Nichols.
Departments of Physiology, Emory University, Atlanta, GA 30322 and
Physiology and Endocrinology, Medical College of Georgia, Augusta, GA.
APStracts 3:0122N, 1996.
SUMMARY AND CONCLUSIONS
1. Transient, stretch-evoked force responses of chemically skinned muscle
fibers from the cat hindlimb were investigated. The purpose of these
experiments was to determine the exent to which short range stiffness, the
apparent stiffness exerted by the fiber over the first 0.5% of length change,
is higher in type I than type II muscle fibers. Fibers were obtained from (a)
soleus and vastus intermedius muscles, which contain predominantly type I
fibers, (b) the LGm, a compartment of the lateral gastrocnemius muscle, which
contains predominantly type II fibers, and (c) LG3, a compartment of mixed
type. 2. Beyond a short range of approximately 1% of muscle length during a
0.5 muscle length/s (ML/s) stretch, most fibers exhibited an abrupt decrease
in apparent stiffness, or yield. Fibers from the muscles containing
predominantly type S (slow twitch, or type I) fibers, soleus and vastus
intermedius, exhibited such a pronounced decline in apparent stiffness that
force declined as well during continued stretch. Most of the fibers from the
LG3 compartment could be divided into two distinct groups depending upon
whether or not they showed a force yield at the stretch velocity of 0.5 ML/s.
3. The short-range stiffness measured over the first 0.5% of stretch was
greater for fibers showing force yield than for those which did not at matched
initial forces and normalized stretch amplitudes. This result is consistent
with the hypothesis that the same mechanism which endows the fiber with high
short range stiffness is also responsible for a greater extent of yielding. 4.
Fibers from soleus were found to exhibit a force yield over a 200-fold range
of velocities (0.01 - 2 ML/s). In contrast, most fibers from the LGm
compartment showed only an increase in extent of yield with stretch velocity.
Some of these fibers eventually yielded in force, but only when they were
stretched at velocities greater than 2 ML/s. The proposed relationship between
high short-range stiffness and yielding was supported by the finding that
short-range stiffness increased sharply in the range of velocities where the
fiber showed the greatest increase in extent of yield. 5. Following the
physiological experiments, fibers were subjected to SDS gel electrophoresis.
Two distinct patterns of bands in the low molecular weight range were found to
correspond to the two types that were identified on the basis of their dynamic
mechanical properties. Fibers which did not yield at 0.5 ML/s showed a band
pattern very similar to that of rabbit psoas (type II) fibers. 6. These
results support the hypothesis that type I fibers are specialized in
presenting a high short-range stiffness for effective postural control in
advance of reflex mechanisms, and that this property results from intrinsic
properties of the fiber and is not due to differences in the dimensions of
type I and II fibers. Yielding serves to protect the fiber from damaging
levels of force during lengthening contractions. The importance of these
transient properties to the mechanical behavior of muscle during ongoing
movements is suggested by the observation that high stiffness followed by
yielding is repeated with little alteration during successive stretches.
Received 6 November 1996; accepted in final form 29 May 1996.
APS Manuscript Number J749-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 17 June 96