View Full Version : Use of the phrase "mechanical efficiency"

01-25-2009, 03:32 AM
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

Rich Marsh

Richard L. Marsh, Professor
email: r.marsh@neu.edu
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