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Human muscle efficiency: what do we know, how do we know it, and what we need to know

Martin J. Kushmerick1, Frank E. Nelson1, J. Ortega2, S. Jubrias1 and Kevin E. Conley1
University of Washington1 and Humbolt State University2

Recently measurements in the human first dorsal interosseous muscle (FDI) of working contractions by 31P NMR spectroscopy showed the contractile efficiency of ATP use is 68%.   What we know about muscle efficiency suggests this value should be between 30 and 50% because efficiency of O2 uptake is 20-30% and the efficiency of metabolic ATP generation is estimated to be 60%.  To explore the possibility of this high efficiency, we examine three factors involved in determining the contractile efficiency: efficiency of the cross bridges, loss of cross bridge work due to anatomical structures, and energy costs for activation. Recent estimates of efficiency of the single cross bridge (50%) are below our measurement.  However, filament elasticity and an array of cross bridges actually increase cross bridge efficiency.  Elastic structures and fascicle pennation dissipate cross bridge work by ~5% based on analyses of ultrasound images of the FDI.  ATP costs for activation in the FDI are small (~ 30%) compared to measurements in sound producing muscle.  This suggests our measurement is possible theoretically, but to validate it further we made additional measurements of contractile efficiency with working and isometric contractions of the FDI.  Costs for working contractions are ~ 1.2 fold greater than for isometric contractions per unit tension time integral (Fenn effect).  From these new experiments we gained the insight that because the efficiency of ATP use is very high, the energy costs in contractions of the FDI are primarily due to cross bridge interactions. Therefore, our measurement of contractile efficiency in vivo is valid, and sets a new high value, but not too high.  In future work we need to quantify activation costs and the efficiency of metabolic ATP generation in vivo.