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  • Use of the phrase "mechanical efficiency"

    Knowing that in a given field people can be passionate about the
    terms in regular use, I post the following with some trepidation.

    In discussing with a colleague the terms to be used for the
    efficiency calculated from joint work, we ended up discussing the
    term "mechanical efficiency". I have come into biomechanics late in
    my career, and the phrase "mechanical efficiency" in the sense used
    in the biomechanics community is not clear to the outsider. Because
    the literature on muscle and movement can be confusing unless one
    always makes clear how efficiency is being calculated, having the
    same terms mean different things in different contexts can be confusing.

    Based on the definitions in Winter's book, "mechanical efficiency" is
    used for a work efficiency estimate (work out)/(energy used), and it
    main function seems to be to distinguish the efficiency estimate
    based on kinetics from the work efficiency of muscle (muscle work
    out)/(energy used by the muscle), termed "muscle efficiency". I do
    not know whether this distinction originated with Winter, or was
    coined previously.

    Viewed as an outsider, the use of "mechanical efficiency" in
    biomechanics is problematic because it has different meanings in
    different fields. I was already familiar with two very different
    uses of "mechanical efficiency". In mechanics, "mechanical
    efficiency", is used for the ratio of (work out)/(work in) for a
    machine, e.g., "the maximum mechanical efficiency of a simple lever
    is 1.0". In muscle physiology, A.V. Hill originated the use of the
    term for values arising from measuring the heat production of muscle
    defining it as the ratio (work out)/(heat out + work out), a measure
    of that should perhaps better be called "enthalpic efficiency".
    Biological systems are not heat engines, however, and under
    measurement conditions excluding recovery metabolism, the enthalpic
    efficiency does not equal the true thermodynamic efficiency. Because
    biomechanics sits between muscle physiology and mechanics having a
    definition of "mechanical efficiency" that agrees with neither field
    seems potentially confusing to the unwary.

    For future publications from my lab I am considering how best to deal
    with this situation. Efficiencies calculated from joint work and
    energy use certainly fit the definition of "mechanical efficiency" as
    used in biomechanics, but I would like to avoid this phrase. I have
    considered using the term "joint efficiency", but this suffers from
    non-specificity, i.e., positive work or integrated work in the
    numerator, and also could be misinterpreted by reading "joint" in the
    sense of "combined". Perhaps the best solution is a longer phrase
    when first introduced, i.e., "the efficiency of positive work at the
    joint", followed by the introduction of an appropriately subscripted
    symbol, which can be repeated to avoid the repetition of an the long
    phrase.

    Thanks,
    Rich Marsh


    ******************************
    Richard L. Marsh, Professor
    email: r.marsh@neu.edu
    Office Location:
    461 Richards Hall

    Mail Address:
    Department of Biology, 134 Mugar
    360 Huntington Ave.
    Northeastern University
    Boston, Massachusetts 02115

    Phone: 617 373-3495
    FAX: 617 373-3724
    ******************************
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