BIOMECHANICS AS AN INTERDISCIPLINE - Biomechanics and Motor Control of Human Movement - page 9

Bernstein (1967) predicted that the CNS exerts control at the level of the
joints or at the synergy level when he postulated the “principle of equal
simplicity” because “it would be incredibly complex to control each and
every muscle.” One of the by-products of these many levels of integration
and convergence is that there is considerably more variability at the neural
(EMG) level than at the motor level and more variability at the motor level
than at the kinematic level. The resultant variability can frustrate researchers
at the neural (EMG) level, but the positive aspect of this redundancy is
that the neuromuscular system is, therefore, very adaptable (Winter, 1984).
This adaptability is very meaningful in pathological gait as a compensation
for motor or skeletal deficits. For example, a major adaptation took place
in a patient who underwent a knee replacement because of osteoarthritic
degeneration (Winter, 1989). For years prior to the surgery, this patient had
refrained from using her quadriceps to support her during walking; the resul-
tant increase in bone-on-bone forces induced pain in her arthritic knee joint.
She compensated by using her hip extensors instead of her knee extensors and
maintained a near-normal walking pattern; these altered patterns were retained
by her CNS long after the painful arthritic knee was replaced. Therefore, this
moment-of-force must be considered the final desired pattern of CNS con-
trol, or in the case of pathological movement, it must be interpreted either
as a disturbed pattern or as a CNS adaptation to the disturbed patterns. This
adaptability is discussed further in Chapter 5, on kinetics.

1.3 SCOPE OF THE TEXTBOOK

The best way to outline the scope of any scientific text is to describe the topics
covered. In this text, the biomechanics of human movement has been defined
as the mechanics and biophysics of the musculoskeletal system as it pertains
to the performance of any movement skill. The neural system is also involved,
but it is limited to electromyography and its relationship to the mechanics of
the muscle. The variables that are used in the description and analysis of any
movement can be categorized as follows: kinematics, kinetics, anthropometry,
muscle mechanics, and electromyography. A summary of these variables and
how they interrelate now follows.

1.3.1 Signal Processing
A major addition to this fourth edition is a chapter on signal processing.
Some aspects of signal processing were contained in previous additions; it
was decided that all aspects should be combined in one chapter and be given
a more rigorous presentation. Why signal processing? Virtually all the vari-
ables we measure or analyze come to us in the time domain: EMG, forces,
displacements, accelerations, energies, powers, moments, and so on. Thus,
they are signals and must be treated like any other signal. We can analyze