MECHANICAL WORK, ENERGY, AND POWER - Biomechanics and Motor Control of Human Movement - page 164

this runner, it is evident that the energies absorbed early in stance (53 J) by
the knee extensors and by the knee flexors in later swing (24 J) dominate
the profile; only 31 J are generated by the knee extensors in middle and late
stance.
It should be noted that this technique automatically calculates any external
work that is done. The external power will be reflected in increased joint
moments, which, when multiplied by the joint angular velocity, will show an
increased power equal to that done externally.

6.3.1.5 Muscle Power and Work. Even with the detailed analysis
described in the previous section, we have underestimated the work done
by cocontracting muscles. Joint power, as calculated, is the product of the
joint moment of force M j and the angular velocity ω j .M j is the net moment
resulting from all agonist and antagonist activity, and therefore, cannot
account for simultaneous generation by one muscle group and absorption
by the antagonist group, or vice versa. For example, if M j = 40 N · m and
ω j = 3rad / s, the joint power would be calculated to be 120 W. However, if
there were a cocontraction, the antagonists might be producing a resisting
moment of 10 N · m. Thus, in this case, the agonists would be generating

at the rate of 50 × 3 = 150 W.

energy while the antagonists would be

× =

absorbing energy at a rate of 10 30 W. Thus, the net power and work

calculations as described in Section 6.3.1.4 will underestimate both the
positive and the negative work done by the muscle groups at each joint. To
date, there has been very limited progress to calculate the power and work
associated with each muscle's action. The major problem is to partition the
contribution of each muscle to the net moment, and this issue has been
addressed in Section 5.5.1. However, if the muscle force F m and the muscle
velocity V m were known, the muscle work W m would be calculated as:

3

t 2

W m = F m · V m dt (6.25)

t 1

Morrison (1970) analyzed the power and work in four muscles in normal
walking, and some later work (Yack, 1986) analyzed the muscles forces and
powers in the three major biarticulate muscle groups during walking.

6.3.1.6 Summary of Work Calculation Techniques. Table 6.1 summarizes
the various approaches described over the past few decades and the different
energy components that are not accounted for by each technique.