MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 239

no further increase in force. Also, there is viscous friction in the fluid
surrounding the muscle fibers, and this friction force must be overcome
as well. However, the DM (distributed moment) state variable model that
approximates the Huxley-type cross-bridge theories (Zahalak and Ma, 1990)
predicted the drop in tension reported in the eccentric region for constant
velocity stretch.
Experimentally, it is somewhat more difficult to conduct experiments
involving eccentric work because an external device must be available to
do the work on the human muscle. Such a requirement means that a motor
is needed to provide an external force that will always exceed that of the
muscle. Experiments on isolated muscle are safe to conduct, but in vivo
experiments on humans are difficult because such a machine could cause
lengthening even past the safe range of movement of a joint. The excessive
force could tear the limb apart at the joint. Foolproof safety mechanisms
have to be installed to prevent such an occurrence.
9.2.3 Combination of Length and Velocity versus Force
In Section 9.1 and in this section, it is evident that the tendon force is a func-
tion of both length and velocity. Therefore, a proper representation of both
these effects requires a three-dimensional plot like that shown in Figure 9.13.
Figure 9.13 Three-dimensional plot showing the change in contractile element tension
as a function of both velocity and length. Surface shown is for maximum muscle
activation; a new “surface” will be needed to describe each level of activation. Influence
of parallel elastic element is not shown.