Biomedical Engineering Reference
In-Depth Information
movement have rested exclusively on a computational approach, up until
the past decade. This theoretical orientation postulates the existence of a
central control of voluntary movement by the nervous system. This sets the
organization of all of the body's degrees of freedom, which are stored in
the central nervous system (e.g. Schmidt, 1975).
However, this approach has been questioned by the sudden entrance
of models derived from the analysis of dynamic nonlinear systems
(e.g. Kelso, 1995). Such models postulate, based on the ideas of Bernstein
(1967), that the central nervous system is incapable of controlling all the
degrees of freedom of the body when it is producing a complex movement.
Following this perspective, a complex movement is viewed as emerg-
ing from a network of constraints. Motor commands are not centralized
within the nervous system, but reside within the dynamics of interaction
between the individual and his or her physical environment. Depending
on the level of constraints, many preferential patterns may emerge. For
example, when imposing a low speed of locomotion to a subject on a
treadmill, this subject may adopt one of two motor solutions: walking
or trotting along. When the level of constraint rises, motor solutions tend
to become more restrictive. Thus, when a greater speed is imposed, the
preferential pattern that emerges is running. The subject may only adopt
a different motor pattern, such as walking, at a very high cost in energy
(e.g. Brisswalter and Mottet, 1996).
This example leads us to argue that one of the first levels of action for
ergonomists is to identify and act upon the constraints that affect move-
ment, causing the emergence of types of coordination that are poten-
tially ineffective or detrimental. It is thus possible to identify levels of
constraints that are likely to reduce the variability of possible types of
coordination and to augment the harmful effects of repetition of move-
ment. Indeed, the rhythm imposed by a task, as well as the unforeseen
events that may occur in any work situation, coupled with workstation
design, is likely, beyond a certain threshold, to limit the possibilities of
coordination adopted by the operators.
An example from the food processing industry can be used to illus-
trate this point. The pace of a processing line for deveining
foie gras
(removing the veins from the liver) is set by considering the time required
to carry out operations on the product, but considering all of the prod-
ucts as being identical. Yet, products such as animal livers are variable:
some are tougher than others, larger or smaller, etc. Thus, the operations
that need to be carried out do not require the same time for each liver.
Operators remove the veins using a knife, holding it midway between the
handle and the blade. All of them do not wear gloves, so as to reduce
the sliding of veins between their thumb and the blade of the knife. While
they are removing the veins using the knife with one hand, the other
