MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 154

Figure 6.11 Example of a point in time during gait that positive work by the pushoff
muscles can be canceled by negative work of the weight-accepting muscles of the
contralateral leg. (Reproduced by permission of Physiotherapy Canada .)
is a major cause of this type of inefficient muscle activity. The only way
to analyze such inefficiencies is to calculate the muscle power at each joint
separately and to quantify the overlap of simultaneous phases of positive and
negative work.
In spite of the inefficiencies inferred by such events, it must be remembered
that many complex movements such as walking or running require that several
functional tasks be performed at the same time. The example in Figure 6.11
illustrates such a situation: the plantarflexors are completing pushoff while the
contralateral muscles are involved in weight acceptance. Both these events
are essential to a safe walking pattern.
6.1.1.4 Jerky Movements. Efficient energy exchanges are characterized
by smooth-looking movements. A ballet dancer and a high jumper execute
smooth movements for different reasons, one for artistic purposes, the other
for efficient performance. Energy added to the body by positive work at one
point in time is conserved, and very little of this energy is lost by mus-
cles doing negative work. The jerky gait of a cerebral palsy child is quite
the opposite. Energy added at one time is removed a fraction of a second
later. The movement has a steady succession of stops and starts, and each of
these bursts of positive and negative work has a metabolic cost. The energy
cost from jerky movements can be assessed in two ways: by work analysis
based on a segment-by-segment energy analysis or by a joint-by-joint power
analysis. Both of these techniques are described later.
6.1.2 Summary of Energy Flows
It is valuable to summarize the flows of energy from the metabolic level
through to an external load. Figure 6.12 depicts this process schematically.
Metabolic energy cannot be measured directly but can be calculated indirectly
from the amount of O 2 required or by the CO 2 expired. The details of these
calculations and their interpretation are the subject of many textbooks and
are beyond the scope of this topic.