Residual Force Enhancement is Preserved for Conditions of Reduced Contractile Force

AbstractIntroductionThe isometric muscle force attained after active stretch is greater than that attained in a purely isometric contraction. This property is referred to as residual force enhancement (RFE). Since RFE is thought to be caused by a titin-based passive force, it should be preserved in reduced contractile force states. Therefore, we evaluated the magnitude of RFE in normal and reduced contractile force states.MethodsSkinned fibers of rabbit psoas and soleus (N = 60) were used in all experiments. Reduced contractile force states were induced by (i) using a low Ca2+ concentration (pCa6.0) (N = 30), (ii) by adding 20 mM butanedione monoxime (BDM) (N = 15), and (iii) by lowering the pH (pH6.0) level (N = 15). Force enhancement and reference isometric tests were conducted for each condition. In the force enhancement tests, fibers were actively stretched from an average sarcomere length of 2.4 μm to 3.0 μm. The isometric force attained 15s after the end of stretching was used for analysis. In the isometric reference tests, fibers were activated isometrically at an average sarcomere length of 3.0 μm, and the force at steady-state was used for analysis. The absolute and relative magnitudes of RFE were calculated.ResultsThe absolute RFE was the same for the normal and reduced contractile force states. Since the isometric reference force was smaller in the reduced contractile force states, the relative RFE was greater in the reduced contractile force than the normal states for all conditions.ConclusionRFE was preserved in the reduced contractile force states. Introduction The isometric muscle force attained after active stretch is greater than that attained in a purely isometric contraction. This property is referred to as residual force enhancement (RFE). Since RFE is thought to be caused by a titin-based passive force, it should be preserved in reduced contractile force states. Therefore, we evaluated the magnitude of RFE in normal and reduced contractile force states. Methods Skinned fibers of rabbit psoas and soleus (N = 60) were used in all experiments. Reduced contractile force states were induced by (i) using a low Ca2+ concentration (pCa6.0) (N = 30), (ii) by adding 20 mM butanedione monoxime (BDM) (N = 15), and (iii) by lowering the pH (pH6.0) level (N = 15). Force enhancement and reference isometric tests were conducted for each condition. In the force enhancement tests, fibers were actively stretched from an average sarcomere length of 2.4 μm to 3.0 μm. The isometric force attained 15s after the end of stretching was used for analysis. In the isometric reference tests, fibers were activated isometrically at an average sarcomere length of 3.0 μm, and the force at steady-state was used for analysis. The absolute and relative magnitudes of RFE were calculated. Results The absolute RFE was the same for the normal and reduced contractile force states. Since the isometric reference force was smaller in the reduced contractile force states, the relative RFE was greater in the reduced contractile force than the normal states for all conditions. Conclusion RFE was preserved in the reduced contractile force states. Corresponding Author: Atsuki FUKUTANI, Ph.D. Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga, 525–8577, Japan. E-mail address: atsukifukutani@gmail.com. Tel: +81 77 561 2791, Fax: +81 77 561 3761 This study was supported by the Grant-in-Aid for Challenging Exploratory Research (16K13009) and Postdoctoral Fellowship for Research Abroad (183), Nakatomi Foundation, CIHR Foundation Scheme Grant, NSERC of Canada Discovery Grant, The Canada Research Chair Programme, and the Killam Foundation. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Conflicts of interest: None. Accepted for Publication: 19 January 2018 © 2018 American College of Sports Medicine

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