DETERMINATION OF THE ACTIVE STATE CURVE OF SKELETAL MUSCLE
Owino Okongo
ABSTRACT
The purpose of the present investigation was to measure the active state of skeletal muscle
during an isometric twitch. The magnitude of the active state at any instant was defined as
the force which the muscle would have developed at that instant had the contractile
component remained at a constant length throughout the preceding duration of its
mechanical activity.
The three-element model of muscle (Hill, 1938) was assumed and the experiments were
performed in frog sartorius muscle at the length of maximum contractility so as to exclude
the effects of the parallel elastic component and minimize the effects of the equilibrium
length tens ion relation.
In each experiment, a family of force and shortening twitch records and their derivatives were
made with springs of different compliances in series with the muscle. The force and shortening
data so obtained were used to construct force shortening curves at intervals of 20 msec. These
curves were best described by an empirical equation of the same form as Aubert's (1956) force
-velocity equation from which the force at zero external shortening (the true isometric force)
could be computed. At any sample point, this equation and the extension of the series elastic
component corresponding to the true isometric force were used to compute the force at zero
internal shortening: the active state force. The same equation was used to determine the
corresponding contractile component shortening at zero load.
Instantaneous force -velocity curves were computed at selected intervals from: 1) force and
shortening records, 2)'the time derivatives of force and shortening, and 3) the derivative of the
force extension curve of the series elastic component.
The results showed that in a twitch the active state curve obtained by analytical elimination of
the series elastic component rises gradually to a peak which is approximately equal to the
maximum tetanic force and is attained earlier than the peak isometric twitch force. This curve
has no plateau and is consistently greater than the isometric force throughout the duration of
mechanical activity. The "active state" curve obtained by the force-velocity method is greater
than the isometric myogram during contraction and is essentially identical with it during
relaxation. This curve is, however, smaller than the isometric force in the zone of transition from
contraction to relaxation. The active state curve obtained by analytical elimination of the
series elastic component is considered the most reasonable active state curve to date.
ACKNOWLEDGEMENTS
I am grateful to Dr. Norman R. Alpert for many useful and critical discussions of the ideas
presented in this thesis and to Dr. William C. Ullrick for encouraging the initial
development of the concept of analytical elimination of the series elastic component. Mr.
Richard Becker, Mr. George Luhr and Mr. Joe Trono of the Instrumentation and Model
Facilities of the University of Vermont were most helpful in the design and construction of
custom-made equipment. Their assistance is gratefully acknowledged.
I am especially grateful to Mrs. Clair Phillips for the generous donation of her time to help
prepare the manuscript. This gift and her experience in the preparation of theses were
invaluable. I am grateful to Mrs. Mary Anne Freedman for expeditious writing of the computer
program used in the final analysis of the data and to Dr. Ronald Schnitzler for proof-reading the
manuscript.