Aj. Vansoest et Mf. Bobbert, THE CONTRIBUTION OF MUSCLE PROPERTIES IN THE CONTROL OF EXPLOSIVE MOVEMENTS, Biological cybernetics, 69(3), 1993, pp. 195-204
Explosive movements such as throwing, kicking, and jumping are charact
erized by high velocity and short movement time. Due to the fact that
latencies of neural feedback loops are long in comparison to movement
times, correction of deviations cannot be achieved on the basis of neu
ral feedback. In other words, the control signals must be largely prep
rogrammed. Furthermore, in many explosive movements the skeletal syste
m is mechanically analogous to an inverted pendulum; in such a system,
disturbances tend to be amplified as time proceeds. It is difficult t
o understand how an inverted-pendulum-like system can be controlled on
the basis of some form of open loop control (albeit during a finite p
eriod of time only). To investigate if actuator properties, specifical
ly the force-length-velocity relationship of muscle, reduce the contro
l problem associated with explosive movement tasks such as human verti
cal jumping, a direct dynamics modeling and simulation approach was ad
opted. In order to identify the role of muscle properties, two types o
f open loop control signals were applied: STIM(t), representing the st
imulation of muscles, and MOM(t), representing net joint moments. In c
ase of STIM control, muscle properties influence the joint moments exe
rted on the skeleton; in case of MOM control, these moments are direct
ly prescribed. By applying perturbations and comparing the deviations
from a reference movement for both types of control, the reduction of
the effect of disturbances due to muscle properties was calculated. It
was found that the system is very sensitive to perturbations in case
of MOM control; the sensitivity to perturbations is markedly less in c
ase of STIM control. It was concluded that muscle properties constitut
e a peripheral feedback system that has the advantage of zero time del
ay. This feedback system reduces the effect of perturbations during hu
man vertical jumping to such a degree that when perturbations are not
too large, the task may be performed successfully without any adaptati
on of the muscle stimulation pattern.