MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 162

The following major conclusions can be drawn from this example.

1. When muscles do positive work, they increase the total body energy.
2. When muscles do negative work, they decrease the total body energy.
3. During cyclical activity, such as constant velocity level running, the net
energy change per stride equals zero (neglecting the small air friction
and shoe friction losses). Thus, the internal positive work done per
stride should equal the internal negative work done per stride.

6.3 CALCULATION OF INTERNAL AND EXTERNAL WORK

Internal and external work has been calculated in a multitude of ways by
different researchers. Some assume that the energy changes in the body center
of mass yield the total internal work done by all the muscles. Others look
only at the “vertical work” resulting from potential energy increases of the
body center of gravity, while others (especially in the exercise physiology
area) even ignore internal work completely. It is, therefore, important to look
at all possible sources of muscle activity that have a metabolic cost and keep
those readily available on a “checklist” to see how complete any analysis
really is. This same list serves a useful purpose in focusing our attention on
possible causes of inefficient movement (see Section 6.1.1).

6.3.1 Internal Work Calculation
The various techniques for calculating the internal work have undergone a
general improvement over the years. The vast majority of the research has
been done in the area of human gait, and because gait is a complex movement,
it will serve well as an example of the dos and don'ts.

6.3.1.1 Energy Increases in Segments. A number of early researchers
attempted a calculation of work based on increases in potential or kinetic
energies of the body or of individual segments. Fenn (1929), in his account-
ing of the flow of energy from metabolic to mechanical, calculated the kinetic
and potential energies of each major segment of a sprinter. He then summed
the increases in each of these segment energies over the stride period to
yield the net mechanical work. Unfortunately, Fenn's calculations ignored
two important energy-conserving mechanisms: energy exchanges within seg-
ments and passive transfers between segments. Thus, his mechanical work
calculations were predictably high: the average power of his sprinters was
computed to be 3 horsepower. Conversely, Saunders et al. (1953), Cotes and
Meade (1960), and Liberson (1965) calculated the “vertical work” of the
trunk as representing the total work done by the body. These calculations
ignored the major energy exchange that takes place within the HAT and also
the major work done by the lower limbs.