Biomedical Engineering Reference
In-Depth Information
( a )
( b )
Figure 6.5 Negative power as defined by net muscle moment and angular velocity.
( a ) An external force causes extension when the flexors are active. ( b ) An external
force causes flexion in the presence of an extensor muscle moment. (Reproduced by
permission of Physiotherapy Canada .)
6.0.5 Negative Work of Muscles
Negative work is work done during an eccentric contraction when the muscle
moment acts in the opposite direction to the movement of the joint. This
usually happens when an external force, F ext , acts on the segment and is such
that it creates a joint moment greater than the muscle moment. The external
force could include gravitational or ground reaction forces. Using the polarity
convention as described, we can see in Figure 6.5 a that we have a flexor
moment (positive) with an extensor angular velocity (negative). The product
yields a negative power, so that the work done during this angular change
is negative. Similarly, when there is an extensor moment (negative) during a
flexor angular change (positive), the product is negative (Figure 6.5 b ). Here,
the net work is being done by the external force on the muscles and represents
a flow of energy from the limbs into the muscles (absorption).
6.0.6 Muscle Mechanical Power
The rate of work done by most muscles is rarely constant with time. Because
of rapid time-course changes, it has been necessary to calculate muscle power
as a function of time (Elftman, 1939; Quanbury et al., 1975; Cappozzo et al.,
1976; Winter and Robertson, 1978). At a given joint, muscle power is the
product of the net muscle moment and angular velocity,
P m =
M j ω j
W
(6.1)
where: P m =
muscle power, watts
M j
=
net muscle moment, N
·
m
ω j
=
joint angular velocity, rad/s
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