title: Muscle heat: a window into the thermodynamics of a molecular machine.
abstract: The contraction of muscle is characterized by the development of force and movement (mechanics) together with the generation of heat (metabolism). Heat represents that component of the enthalpy of ATP hydrolysis that is not captured by the microscopic machinery of the cell for the performance of work. It arises from two conceptually and temporally distinct sources: initial metabolism and recovery metabolism. Initial metabolism comprises the hydrolysis of ATP and its rapid regeneration by hydrolysis of phosphocreatine (PCr) in the processes underlying excitation-contraction coupling and subsequent cross-bridge cycling and sliding of the contractile filaments. Recovery metabolism describes those process, both aerobic (mitochondrial) and anaerobic (cytoplasmic), that produce ATP, thereby allowing the regeneration of PCr from its hydrolysis products. An equivalent partitioning of muscle heat production is often invoked by muscle physiologists. In this formulation, total enthalpy expenditure is separated into external mechanical work (W) and heat (Q). Heat is again partitioned into three conceptually distinct components: basal, activation, and force dependent. In the following mini-review, we trace the development of these ideas in parallel with the development of measurement techniques for separating the various thermal components.
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand; Department of Physiology, The University of Auckland, Auckland, New Zealand ds.loiselle@auckland.ac.nz.
authors.forename: Denis Scott
authors.initials: DS
authors.lastname: Loiselle
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand;
authors.forename: Callum Michael
authors.identifier: http://orcid.org/0000-0002-6773-9884
authors.initials: CM
authors.lastname: Johnston
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand;
authors.forename: June-Chiew
authors.initials: JC
authors.lastname: Han
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand; Department of Engineering Science, The University of Auckland, Auckland, New Zealand; and.
authors.forename: Poul Michael Fønss
authors.initials: PM
authors.lastname: Nielsen
authors.affiliation: Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand; Department of Engineering Science, The University of Auckland, Auckland, New Zealand; and.
authors.forename: Andrew James
authors.initials: AJ
authors.lastname: Taberner
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D004734:Energy Metabolism; D056966:Excitation Contraction Coupling / Q000502:physiology*; D006801:Humans; D006868:Hydrolysis; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology; D009130:Muscle, Smooth / Q000378:metabolism / Q000502:physiology; D009132:Muscles / Q000378:metabolism* / Q000502:physiology; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000378:metabolism; D010725:Phosphocreatine / Q000378:metabolism*; D013816:Thermodynamics; D022722:Thermogenesis / Q000502:physiology*
chemical_list: D010725:Phosphocreatine; D000255:Adenosine Triphosphate
keywords: Microcalorimetry; muscle heat production; myothermy; thermopiles