BIOMECHANICS AS AN INTERDISCIPLINE - Biomechanics and Motor Control of Human Movement - page 8

Figure 1.6 Four levels of integration in the neuromusculoskeletal system provide con-
trol of human movement. The first is the neural summation of all excitatory/inhibitory
inputs to the α motoneuron . The second is the summation of all motor twitches from
the recruitment of all active motor units within the muscle and is seen as a tendon
force . The third is the algebraic summation of all agonist and antagonist muscle
moments at the joint axis . Finally, integrations are evident in combined moments
acting synergistically toward a common goal

.

final common pathway, has its synapse on the motor end point of the muscle
motor unit. A second level of convergence is the summation of all twitches
from all active motor units at the level of the tendon . This summation

results from the neural recruitment of motor units based on the size principle
(cf. DeLuca et al., 1982; Henneman and Olson, 1965). The resultant tension
is a temporal superposition of twitches of all active motor units, modulated
by the length and velocity characteristics of the muscle. A third level of
musculoskeletal integration at each joint center where the moment-of-force

is the algebraic summation of the force/moment products of all muscles
crossing that joint plus the moments generated by the passive anatomical
structures at the joint. The moments we routinely calculate include the net
summation of all agonist and antagonist muscles crossing that joint, whether
they are single- or double-joint muscles. In spite of the fact that this moment
signal has mechanical units ( N m ), we must consider the moment signal as

a neurological signal because it represents the final desired central nervous
system (CNS) control. Finally, an intersegment integration may be present
when the moments at two or more joints collaborate toward a common goal.
This collaboration is called a synergy. One such synergy

·

, referred to as
the support moment, quantifies the integrated activity of all muscles of the
lower limb in their defense against a gravity-induced collapse during walking
(Winter, 1980, 1984).