An innovative apparatus for measuring in vivo efficiency of positive and negative work for human muscle studies

Abstract

Muscle efficiency is emerging as a key factor in both health and disease. Two problems in understanding in vivo muscle efficiency are that 1) indirect calorimetry measurement of energy consumption includes the variable efficiency of O2 conversion to ATP and 2) release of elastic energy could erroneously raise the measured work output. We constructed an apparatus that incorporates a simple lever linked to a pulley system to measure work in the bore of a magnet. This apparatus 1) eliminates the release of stored elastic energy, 2) permits the generation of both positive and negative muscle work and 3) allows for simultaneous determination of muscle work and ATP flux. Mechanical tests indicate that the design of our system limits maximum force fluctuation to 1.01 N (S.D. ±0.64) and limits the maximum amount of elastic energy released to 0.01% (0.05 mJ) of the work per contraction. In agreement with past observations, our measurement of muscle efficiency is 50±6% for contractions producing positive work. In order to compare this efficiency with the costs associated with negative work we determined the energetic cost of generating force (CoF). The CoF for concentric contractions (53±9 ATP/(kN·s)) is more than double the CoF of eccentric contractions (20±4 ATP/(kN·s)). Thus, reduced cost, rather than the release of elastic energy, likely underlies the greater efficiency of eccentric vs. concentric contractions.