Does negative work cost less

Abstract

The Fenn effect states that the cost of producing work by muscle is higher than producing no work at all. Here we evaluate whether negative work costs even less. We measured the cost of force production in contractions where work (concentric), no work (isometric) and negative work (eccentric) were done using a new apparatus for measuring human muscle mechanics and energetics in vivo. Our mechanical apparatus allows measurements at the same force, displacement and duration for all contraction types simultaneous with magnetic resonance (MRI) measures of the ATP cost. This unique suite of controls and measurements in the same individual allows for a direct comparison of the cost of force production (ATP/(kN·s)) between the different types of contractions. Contrary to our original thought negative work during eccentric contractions (20±4 ATP/(kN·s)) had the same cost of force production as no work (18±6 ATP/(kN·s)) in an isometric contraction but both were less than half the cost of force production (53±9 ATP/(kN·s)) in a concentric working contraction. This suggests a common mechanism underlies the cost of force in contractions that produce no work and negative work, which is more economical than force production in contractions that generate positive work. The key difference appears to be that the generation of movement requires more energy than contractions that are static or resist movement during force production. So contrary to the natural extension of the Fenn effect, negative work is not cheaper but costs the same as no work at all.