title: Muscle force, work and cost: a novel technique to revisit the Fenn effect.
abstract: Muscle produces force by forming cross-bridges, using energy released from ATP. While the magnitude and duration of force production primarily determine the energy requirement, nearly a century ago Fenn observed that muscle shortening or lengthening influenced energetic cost of contraction. When work is done by the muscle, the energy cost is increased and when work is done on the muscle the energy cost is reduced. However, the magnitude of the 'Fenn effect' and its mirror ('negative Fenn effect') have not been quantitatively resolved. We describe a new technique coupling magnetic resonance spectroscopy with an in vivo force clamp that can directly quantify the Fenn effect [E=I+W, energy liberated (E) equals the energy cost of isometric force production (I) plus the work done (W)] and the negative Fenn effect (E=I-W) for one muscle, the first dorsal interosseous (FDI). ATP cost was measured during a series of contractions, each of which occurred at a constant force and for a constant duration, thus constant force-time integral (FTI). In all subjects, as the FTI increased with load, there was a proportional linear increase in energy cost. In addition, the cost of producing force greatly increased when the muscle shortened, and was slightly reduced during lengthening contraction. These results, though limited to a single muscle, contraction velocity and muscle length change, do quantitatively support the Fenn effect. We speculate that they also suggest that an elastic element within the FDI muscle functions to preserve the force generated within the cross-bridges.
authors.affiliation: Department of Kinesiology & Recreation Administration, Humboldt State University, Arcata, CA 95521, USA Department of Radiology, University of Washington Medical Center, Seattle, WA 98195, USA.
authors.forename: Justus O
authors.initials: JO
authors.lastname: Ortega
authors.affiliation: Department of Biological Sciences, Northern Arizona University, South Beaver Street, Flagstaff, AZ 86001, USA stan.lindstedt@nau.edu.
authors.forename: Stan L
authors.initials: SL
authors.lastname: Lindstedt
authors.affiliation: Department of Radiology, University of Washington Medical Center, Seattle, WA 98195, USA.
authors.forename: Frank E
authors.initials: FE
authors.lastname: Nelson
authors.affiliation: Department of Radiology, University of Washington Medical Center, Seattle, WA 98195, USA.
authors.forename: Sharon A
authors.initials: SA
authors.lastname: Jubrias
authors.affiliation: Department of Radiology, University of Washington Medical Center, Seattle, WA 98195, USA.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Kushmerick
authors.affiliation: Department of Radiology, University of Washington Medical Center, Seattle, WA 98195, USA Department of Physiology & Biophysics, University of Washington Medical Center, Seattle, WA 98195, USA Department of Bioengineering, University of Washington Medical Center, Seattle, WA 98105, USA.
authors.forename: Kevin E
authors.initials: KE
authors.lastname: Conley
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology*; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*
chemical_list: D000255:Adenosine Triphosphate
keywords: 31P MRS; First dorsal interosseous; Muscle energetics; Muscle mechanics