MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 143

cyclist must do his or her own internal work plus the internal work on the
negative-work cyclist plus any additional negative work of that cyclist. Thus,
if the internal work of each cyclist were 75 W, the positive-work cyclist
would have to do mechanical work at 150-W rate just to “freewheel” both
cyclists. Then as the negative-work cyclist contracted his or her muscles, an
additional load would be added. Thus, if the negative-work cyclist worked at
150 W, the positive-work cyclists would be loaded to 300 W. It is no wonder
that the negative-work cyclist can cycle with ease while the positive-work
cyclist rapidly fatigues. They are simply not working at the same mechanical
work rate, plus the metabolic demand of positive work far exceeds that of
negative work.

6.0.4 Positive Work of Muscles
Positive work is work done during a concentric contraction, when the muscle
moment acts in the same direction as the angular velocity of the joint. If a
flexor muscle is causing a shortening, we can consider the flexor moment to
be positive and the angular velocity to be positive. The product of muscle
moment and angular velocity is positive; thus, power is positive, as depicted
in Figure 6.4 a . Conversely, if an extensor muscle moment is negative and an
extensor angular velocity is negative, the product is still positive, as shown
in Figure 6.4 b . The integral of the power over the time of the contraction is
the net work done by the muscle and represents generated energy transferred
from the muscles to the limbs.

( a ) ( b )

Figure 6.4 Positive power as defined by the net muscle moment and angular velocity.
( a ) A flexion moment acts while the forearm is flexing. ( b ) An extension moment acts
during and extensor angular velocity. (Reproduced by permission of Physiotherapy
Canada .)