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mechanical and metabolic work

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  • mechanical and metabolic work

    Herman's invitation to comment on the use of net joint moments
    and powers has prompted me to broaden the present debate somewhat
    to include the (f)utility of relating mechanical energy measures
    to metabolic energy costs, and more generally still, the choice
    of performance criteria to determine `good' gait.

    There have been many reports in the literature of investigations
    into the energy requirements and `efficiency' (although for most
    cyclic, level movements true efficiency must be zero) of various
    activities, typically walking and running, by analysis of
    mechanical work (See Kaneko [J. Biomech 1990, pp 57-63]).
    Burdett et al. [J. Orthop. Res. 1983, pp 63-72] compared various
    mechanical energy measures with metabolic energy cost and
    achieved a close correlation with work done per second on the
    centre of gravity. Aleshinsky [J. Biomech. 1986 a..e, pp 287-
    315] suggested energy measures based on work done at the joints
    rather than segmental energies.

    Although these investigations may be interesting, it is surely
    misguided to attempt to relate whole body mechanical energy to
    metabolic energy, they are measuring very different things.
    Mechanical energy measures ignore the effects of co-contraction,
    the metabolic costs of an isometric contraction, the extra
    metabolic requirements related to increased cardio-vascular
    workload, changes in the contractile efficiency caused by
    chemical changes in the muscle, elastic energy storage e.t.c.. In
    short they represent an ideal energy cost figure, which bears a
    complicated, time varying relationship to metabolic energy costs.
    If one wishes to measure metabolic energy costs then one should
    surely use a more direct measure such as oxygen consumption or
    increase in heart-rate.

    However, this brings me to the area of interest for me, which is
    the establishment of appropriate performance indicators for
    assessing the `quality' of an activity (whether the activity is
    actually measured from a subject or is the result of a computer
    simulation where the quality measure forms the cost function for
    an optimisation routine). These performance indicators should be
    both RELEVANT to the constraints on the activity being studied
    and also straightforward to measure and interpret.

    Net metabolic energy consumption is relatively easy to measure
    and interpret, but has the following disadvantages when assessing
    pathological gait:

    1. It requires a period of time (minutes) for a steady state
    to be reached, this may be longer than some subjects can
    remain walking. (are studies that get around this by
    measuring oxygen debt valid?)

    2. It can not isolate the metabolic costs of movements at
    individual joints.

    3. It can not isolate the metabolic costs of different
    phases of the movement.

    4. It often requires the use of cumbersome equipment which
    may disturb the gait (can a child or elderly subject be
    expected to walk normally on a treadmill whilst breathing
    into a Douglas Bag?).

    As an example, I wish to assess swing through crutch aided gait
    in paraplegics, the limiting factor in the use of the gait is
    often not the overall energy cost, but specific fatigue of the
    shoulders and arms due to high loads during the body-swing phase.

    Various researchers have examined net moments at the appropriate
    joints, raised to various powers and/or their time integrals
    [Opila, J. Biomch. Eng. 1987 285-290],[Crowninshield and Brand, J
    Biomch. 1981 793-801], others have looked at mechanical work
    [Wells, J. Biomch. 1979, pp 579-585].

    The first approach ignores the fact that a moment applied during
    an eccentric contraction have a lower metabolic cost than one for
    an isometric contraction, which is lower still than a concentric
    contraction (Of course the relationship depends on
    shortening/lengthening speed rather than just direction of
    movement). The second ignores the metabolic cost of sustaining
    an isometric force. They are probably both limited predictors of
    joint fatigue.

    If the actual muscular force distribution around a joint were
    reliably known, together with the speed of lengthening/shortening
    of the muscle fibres (do all the fibres in a muscle change length
    at the same rate along all their length?), then one could
    approximate the energetics of each muscle from anatomical
    information and experiments performed on isolated muscles that
    have been reported in the physiology literature.

    However it is more likely that we will only have net joint
    torques, so my question is (finally):

    Can a relationship be established between the net joint
    torque, angle, and angular velocity and the metabolic energy
    cost at that joint (or better still the tendency to
    fatigue)? The relationship should be valid for positive,
    negative and zero values of angular velocity. An exact
    relationship would not be needed, a monotonically increasing
    one would suffice to show if a particular change in the gait
    pattern was beneficial or not.

    My thought is to isolate the movement of interest in a
    dynamometer that is capable of driving a limb eccentrically (such
    as a KIN/COM). Then to calibrate the increase in metabolic cost
    (measured t hrough oxyg en consumpt ion) agains t
    shortening/lengthening speed and load. These values would be
    used in a look-up table to approximate the metabolic cost at the
    same conditions in the actual movement. I realise that there are
    many assumptions and shortcomings in this technique, can anyone
    suggest a better measure?

    My apologies for straying away from the high ground of
    theoretical mechanics to the boggy depths of rehabilitation
    engineering and exercise physiology!

    Ben Heller, Bioengineering Unit, Strathclyde University, Glasgow,Scotland.
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