MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 149

Figure 6.8 Reaction forces and velocities at a joint center during dynamic activity.
The dot product of the force and velocity vectors is the mechanical power (rate of
mechanical energy transfer) across the joint.

situation in Figure 6.8 at the joint between two adjacent segments. F j 1 ,the
reaction force of segment 2 on segment 1, acts at an angle θ 1 from the veloc-
ity vector V j . The product of F j 1 V j cos θ 1 is positive, indicating that energy is
being transferred into segment 1. Conversely, F j 2 V j cos θ 2 is negative, denot-
ing a rate of energy outflow from segment 2. Since P j 1 =− P j 2 , the outflow

from segment 2 equals the inflow to segment 1. In an n -joint system, there
will be n power flows, but the algebraic sum of all those power flows will
be zero, reinforcing the fact that these flows are passive and, therefore, do
not add to or subtract from the total body energy.
This mechanism of energy transfer between adjacent segments is quite
important in the conservation of energy of any movement because it is a
passive process and does not require muscle activity. In walking, this has
been analyzed in detail (Winter and Robertson, 1978). At the end of swing,
for example, the swinging foot and leg lose much of their energy by transfer
upward through the thigh to the trunk, where it is conserved and converted
to kinetic energy to accelerate the upper body in the forward direction.

6.1 EFFICIENCY

The term efficiency is probably the most abused and misunderstood term in
human movement energetics. Confusion and error result from an improper
definition of both the numerator and the denominator of the efficiency
equation (Gaesser and Brooks, 1975; Whipp and Wasserman, 1969). In the
next section, four causes of inefficiency are discussed in detail, and these
mechanisms must be recognized in whatever formula evolves. Overlaid
on these four mechanisms are two fundamental reasons for inefficiency:
inefficiency in the conversion of metabolic energy to mechanical energy, and