MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 151

The interpretation of efficiency is faulty if it is assumed to be simply a
measure of how well the metabolic system converts biochemical energy into
mechanical energy, rather than a measure of how well the neural system
is performing to control the conversion of that energy. An example will
demonstrate the anomaly that results. A normal healthy adult walks with 100 J
mechanical work per stride (half positive, half negative). The metabolic cost
is 300 J per stride, and this yields an efficiency of 33%. A neurologically
disabled adult would do considerably more mechanical work because of his
or her jerky gait pattern, say 200 J per stride. Metabolically, the cost might
be 500 J per stride, which would give an efficiency of 40%. Obviously, the
healthy adult is a more efficient walker, but our efficiency calculation does
not reflect that fact. Neurologically, the disabled person is quite inefficient
because he or she is not generating an effective and smooth neural pattern.
However, the disabled person is quite efficient in the actual conversion of
metabolic energy to mechanical energy (at the tendon), and that is all that is
reflected in the higher efficiency score.

6.1.1 Causes of Inefficient Movement
It is often difficult for a therapist or coach to concentrate directly on efficiency.
Rather, it is more reasonable to focus on the individual causes of inefficiency
and thereby automatically improve the efficiency of the movement. The four
major causes of mechanical inefficiency (Winter, 1978) will now be described.

6.1.1.1 Cocontractions. Obviously, it is inefficient to have muscles cocon-
tract because they fight against each other without producing a net movement.
Suppose that a certain movement can be accomplished with a flexor moment
of 30 N m. The most efficient way to do this is with flexor activity only.

However, the same movement can be achieved with 40 N

·

· m flexion and

10 N · m extension, or with 50 N · m flexion and 20 N · m extension. In the

latter case, there is an unnecessary 20 N m moment in both the extensors and

the flexors. Another way to look at this situation is that the flexors are doing
unnecessary positive work to overcome the negative work of the extensors.
Cocontractions occur in many pathologies, notably hemiplegia and spastic
cerebral palsy. They also occur to a limited extent during normal movement
when it is necessary to stabilize a joint, especially if heavy weights are being
lifted or at the ankle joint during walking or running. At present, the mea-
surement of unnecessary cocontractions is only possible by monitoring the
EMG activity of the antagonistic muscles. Without an exact EMG calibration
versus tension for each muscle, it is impossible to arrive at a quantitative
measure of cocontraction. Falconer and Winter (1985) presented a formula
by which cocontraction can be quantified,

·

M antag
M agon +

%COCON = 2 ×

M antag ×

100% (6.12)