Abstract
The aim of this study was to investigate force depression in Type I and Type II muscle fibers. Experiments were performed using skinned fibers from rabbit soleus and psoas muscles. Force depression was quantified after active fiber shortening from an average sarcomere length (SL) of 3.2µm to an average SL of 2.6µm at an absolute speed of 0.115fiber length/s and at a relative speed corresponding to 17% of the unloaded shortening velocity (V0) in each type of fibers. Force decay and mechanical work during shortening were also compared between fiber types. After mechanical testing, each fiber was subjected to myosin heavy chain (MHC) analysis in order to confirm its type (Type I expressing MHC I, and Type II expressing MHC IId). Type II fibers showed greater steady-state force depression after active shortening at a speed of 0.115fiber length/s than Type I fibers (14.5±1.5% versus 7.8±1.7%). Moreover, at this absolute shortening speed, Type I fibers showed a significantly greater rate of force decay during shortening and produced less mechanical work than Type II fibers. When active shortening was performed at the same relative speed (17% V0), the difference in force depression between fiber types was abolished. These results suggest that no intrinsic differences were at the origin of the disparate force depressions observed in Type I and Type II fibers when actively shortened at the same absolute speed, but rather their distinct force–velocity relationships.
Highlights
The steady-state isometric force after active shortening of skeletal muscle is smaller than the purely isometric force at the same level of activation and the corresponding length
The remaining psoas fibers tested for force depression (n=12 shortened at a speed of 0.115 fiber length/s, n=7 shortened at a relative speed of 17% V0) expressed myosin heavy chain (MHC) IId and were labelled
Our main finding in this study is that Type II fibers when actively shortened at the same absolute speed showed greater isometric steady-state force depression after active shortening compared to
Summary
The steady-state isometric force after active shortening of skeletal muscle is smaller than the purely isometric force at the same level of activation and the corresponding length. This phenomenon, referred to as force depression, was first described by Abbott and Aubert in 1952 and has been observed consistently in whole and isolated muscle preparations (Rassier and Herzog, 2004). Force depression has been associated with the development of sarcomere length non-uniformities upon active shortening (Morgan et al, 2000). Force depression has been associated with a stress-induced inhibition of cross bridges in the newly formed actin-myosin overlap zone following shortening owing to actin angular deformation (Marechal and Plaghki, 1979)
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