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  • EFFICIENCY QUESTION

    19 October 1993

    Dear BIOMCH-L Readers,

    We have just completed a study of the energy requirements
    of the Canadian Aerobic Fitness Test (CAFT). This is a
    cadence-controlled, bench-stepping test of fitness where
    the participant ascends two 20.3cm steps forwards and
    descends the two steps backwards. There are 6-steps per
    ascent-descent cycle. Our study involved testing a
    group of younger females (< 35 yrs) and older females
    (>65 yrs) at Level 1 (11 cycles per minute), The CAFT
    predicts aerobic fitness on the basis of regression
    equations. Recently, Shephard has calculated that
    the efficiency of a person performing the CAFT is
    approximately 14%. He calculated this using (a) the
    so-called 'net' metabolic power (subtracting baseline);
    and (b) estimating mechanical power based on a point-
    mass model.

    Our study involved collecting O2 data along with kinematics
    of a 12-segment sagittal plane 2D model of the subjects.
    Mechanical power was calculated in several ways:

    1. Mechanical power assuming that energy can be exchanged
    within and between segments

    2. Mechanical power assuming only energy exchanges within
    segments

    3. Mechanical power assuming no exchanges of energy of any
    kind

    [The above are adapted from Pierrynowski et al. (1980)]

    4. Mechanical power of the centre of mass assuming that an
    exchange of energy types can occur (KE PE)

    5 Mechanical power of the centre of mass assuming no exchanges
    of energy types

    6 Mechanical power calculated by the following simple equation:

    (Mass x (2 x 0.203m) x 9.81)/ time of one cycle
    [This is basically Shephard's method]

    We also have the metabolic power data in both gross and net
    formats.

    >From the above 6 mechanical power values (numerator) and 2
    metabolic power values (denominators), we can calculate
    12 efficiency values (Oh no, not the dreaded "E" word!). We
    believe that our "best" estimate of the efficiency of the
    CAFT uses #1 above as our mechanical power and the gross
    metabolic power, for an efficiency of about 28%. The data
    look something like this:

    Mechanical Power Gross O2 Power Net O2 Power
    Wwb (#1 above) 28% 35%
    Ww (#2) 30% 40%
    Wn (#3) 38% 50%
    CG-exchange not available yet not available yet
    CG-no exchange not available yet not available yet
    CG-simple calculation 12% 22%

    After all this, our query to our learned colleagues is this:

    In the absence of any gold standard that we can ascertain, what
    is the concensus about the best way to calculate efficiency?
    We believe we have something quite valuable to add to the body
    of literature in this area. Our data refutes Shephard's assumptions
    about the efficiency of the CAFT, and probably has implications for
    other activities. However, we would appreciate the input of others
    with more experience in this area.

    We will be happy to post a synopsis of the responses we receive.

    Thank you.

    Drew Smith PhD Elaine Aimone MSc LYNDHURST HOSPITAL
    Scott Thomas PhD Sara McConnell MSc UNIVERSITY OF TORONTO

    ************************************************** ************************
    * Andrew (Drew) Smith, Ph.D. | INTERNET: awsmith@utcc.utoronto.ca *
    * Director of Research | BITNET: awsmith@utorgpu *
    * Lyndhurst Spinal Cord Centre | *
    * 520 Sutherland Drive | Home Address: *
    * Toronto ONT Canada M4G 3V9 | 33 Ponymill Drive *
    * (416) 422-5551 x3041,3040 | Scarborough ONT Canada M1V 2X9 *
    * (416) 422-5216 FAX | (416) 609-0469 *
    * "Damn the torpedoes....full steam ahead!" *
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