Muscle_synergies "Motor
coordination") _ is a neural organization of a multi-element system that (1)
organizes sharing of a task among a set of elemental variables; and (2)
ensures co-variation among elemental variables with the purpose to stabilize
performance variables.  [32]  [33]  The components of a synergy need not be
physically connected, but instead are connected by their response to
perceptual information about the particular motor task being executed.
Synergies are learned, rather than being hardwired like reflexes, and are
organized in a task-dependent manner; a synergy is structured for a particular
action and not determined generally for the components themselves.  Nikolai
Bernstein  famously demonstrated
synergies at work in the hammering actions of professional blacksmiths. The
muscles of the arm controlling the movement of the hammer are informationally
linked in such a way that errors and variability in one muscle are
automatically compensated for by the actions of the other muscles. These
compensatory actions are reflex-like in that they occur faster than perceptual
processing would seem to allow, yet they are only present in expert
performance, not in novices. In the case of blacksmiths, the synergy in
question is organized specifically for hammering actions and is not a general
purpose organization of the muscles of the arm. Synergies have two defining
characteristics in addition to being task dependent; sharing and
flexibility/stability.  [34]

"Sharing" requires that the execution of a particular motor task depends on
the combined actions of all the components that make up the synergy. Often,
there are more components involved than are strictly needed for the particular
task (  see "Redundancy" below  ), but the control of that motor task is
distributed across all components nonetheless. A simple demonstration comes
from a two-finger force production task, where participants are required to
generate a fixed amount of force by pushing down on two force plates with two
different fingers.  [35]  In this task, participants generated a particular
force output by combining the contributions of independent fingers. While the
force produced by any single finger can vary, this variation is constrained by
the action of the other such that the desired force is always generated.

Co-variation also provides "flexibility and stability" to motor tasks.
Considering again the force production task, if one finger did not produce
enough force, it could be compensated for by the other.  [35]  The components
of a motor synergy are expected to change their action to compensate for the
errors and variability in other components that could affect the outcome of
the motor task. This provides flexibility because it allows for multiple motor
solutions to particular tasks, and it provides motor stability by preventing
errors in individual motor components from affecting the task itself.

Synergies simplify the computational difficulty of motor control. Coordinating
the numerous  degrees of freedom 
"Degrees of freedom \(mechanics\)") in the body is a challenging problem, both
because of the tremendous complexity of the motor system, as well as the
different levels at which this organization can occur (neural, muscular,
kinematic, spatial, etc.). Because the components of a synergy are
functionally coupled for a specific task, execution of motor tasks can be
accomplished by activating the relevant synergy with a single neural signal.
[36]  The need to control all of the relevant components independently is
removed because organization emerges automatically as a consequence of the
systematic covariation of components. Similar to how reflexes are physically
connected and thus do not require control of individual components by the
central nervous system, actions can be executed through synergies with minimal
executive control because they are functionally connected. Beside motor
synergies, the term of sensory synergies has recently been introduced.  [37]
Sensory synergy are believed to play an important role in integrating the
mixture of environmental inputs to provide low-dimensional information to the
CNS thus guiding the recruitment of motor synergies.

Synergies are fundamental for controlling complex movements, such as the ones
of the hand during grasping. Their importance has been demonstrated for both
muscle control and in the kinematic domain in several studies, lately on
studies including large cohorts of subjects.  [38]  [39]  [40]  The relevance
of synergies for hand grasps is also enforced by studies on hand grasp
taxonomies, showing muscular and kinematic similarities among specific groups
of grasps, leading to specific clusters of movements.  [41]