title: Effects of fiber type on force depression after active shortening in skeletal muscle.
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 3.2µ m to an average SL of 2.6 µm at an absolute speed of 0.115f iber 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 0.115 fiber 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.
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4. Electronic address: vjoumaa@ucalgary.ca.
authors.forename: V
authors.initials: V
authors.lastname: Joumaa
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4.
authors.forename: G A
authors.initials: GA
authors.lastname: Power
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4.
authors.forename: B
authors.initials: B
authors.lastname: Hisey
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4.
authors.forename: A
authors.initials: A
authors.lastname: Caicedo
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4.
authors.forename: J
authors.initials: J
authors.lastname: Stutz
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada AB T2N 1N4.
authors.forename: W
authors.initials: W
authors.lastname: Herzog
mesh_terms: D000818:Animals; D009119:Muscle Contraction / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D018995:Myosin Heavy Chains; D011817:Rabbits; D012518:Sarcomeres / Q000502:physiology
chemical_list: D018995:Myosin Heavy Chains
keywords: Cross-bridge theory; Fast-twitch; History-dependence; MHC; Mechanical work; Slow-twitch; Speed of shortening; Stiffness; Transient forces