MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 150

neurological inefficiency in the control of that energy. Metabolic energy is
converted to mechanical energy at the tendon, and the metabolic efficiency
depends on the conditioning of each muscle, the metabolic (fatigue) state
of muscle, the subject's diet, and any possible metabolic disorder. This
conversion of energy would be called metabolic or muscle efficiency and
would be defined as follows:

mechanical work done by all muscles
metabolic work of muscles

metabolic (muscle) efficiency =

(6.8)

Such an efficiency is impossible to calculate at this time because it is
currently impossible to calculate the work of each muscle (which would
require force and velocity time histories of every muscle involved in the
movement) and to isolate the metabolic energy of those muscles. Thus, we
are forced to compromise and calculate an efficiency based on segmental work
and to correct the metabolic cost by subtracting estimates of overhead costs
not associated with the actual mechanical work involved. Thus, an efficiency
would be defined as:

mechanical work (internal + external)

mechanical efficiency = (6.9)

metabolic cost − resting metabolic cost

The resting metabolic cost in bicycling, for example, could be the cost
associated with sitting still on the bicycle.
A further modification is work efficiency , which is defined as:

external mechanical work
metabolic cost − zero-work metabolic cost

work efficiency = (6.10)

The zero-work cost would be the cost measured with the cyclist

freewheeling.
In all of the efficiency calculations described, there are varying amounts of
positive and negative work. The metabolic cost of positive work exceeds that
of equal levels of negative work. However, negative work is not negligible
in most activities. Level gait has equal amounts of positive and negative
work. Running uphill has more positive work than negative work, and vice
versa for downhill locomotion. Thus, all of the efficiency calculations yield
numbers that are strongly influenced by the relative percentages of positive
and negative work. An equation that gets around this problem is:

metabolic cost (positive work)
+

=

metabolic cost (negative work) metabolic cost

positive work
η +

negative work
η −

or + = metabolic cost (6.11)

where η + and η − are the positive and negative work efficiencies, respectively.