MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 163

6.3.1.2 Center-of-Mass Approach. Cavagna and Margaria proposed in
(1966) and in many subsequent papers a technique that is based on the
potential and kinetic energies of the body's center of mass. Their data were
based on force platform records during walking and running, from which the
translational kinetic and potential energies were calculated. Such a model
makes the erroneous assumption that the body's center of mass reflects the
energy changes in all segments. The body's center of mass is a vector sum
of all segment mass-acceleration products, and, as such, opposite-polarity
accelerations will cancel out. But energies are scalars, not vectors, and,
therefore, the reciprocal movements that dominate walking and running
will be largely canceled. Thus, simultaneous increases and decreases in
oppositely moving segments will go unnoticed. Also, Cavagna's technique is
tied to force platform data, and nothing is known about the body's center of
gravity during non-weight-bearing phases of running. Thus, this technique
has underestimation errors and limitations that have been documented
(Winter, 1979). Also, the center-of-mass approach does not account for the
energy losses from the simultaneous generation and absorption of energy at
different joints.

6.3.1.3 Sum of Segment Energies. A major improvement on the previ-
ous techniques was made by Ralston and Lukin (1969) and Winter et al.
(1976). Using displacement transducers and TV imaging techniques, the
kinetic and potential energies of the major segments were calculated. A sum
of the energy components within each segment recognized the conservation
of energy within each segment (see Section 6.2.1) and a second summation
across all segments recognized energy transfers between adjacent segments
(see Section 6.0.9). The total body work is calculated (Winter, 1979) to be:

N

W b =

1 | E b |

J (6.24)

i =

However, this calculation underestimates the simultaneous energy genera-
tion and absorption at different joints. Thus, W b will reflect a low estimate of
the positive and negative work done by the human motor system. Williams
and Cavanagh (1983) made empirical estimates to correct for these underes-
timates in running.

6.3.1.4 Joint Power and Work. In Sections 6.0.6 and 6.0.7, techniques for
the calculation of the positive and negative work at each joint were presented.
Using the time integral of the power curve [Equation (6.6)], we are able to
get at the “sources” and “sinks” of all the mechanical energy. Figure 6.17 is
an example to show the work phases at the knee during slow running. The
power bursts are labeled K 1 , , K 5 , and the energy generation/absorption

resulting from the time integral of each phase is shown (Winter, 1983). In

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