Dynamic Movement Synergies
In the previous section we noted three challenges that go beyond
that of postural stability: dealing with inertial dynamics,
kinematic/actuator redundancy, and environmental contact. Indeed, there is also considerable
sensorimotor redundancy within the CNS. This includes sensory
redundancy, and structural redundancy in terms of brain structures
that can
work in paralle to influence movement.
There are also a considerable
synergy patterns within neurocircuitry. Here we briefly provide
several classic examples for dynamic tasks:
- For rhythmic movements such as walking, there is considerable
evidence of "central pattern generator" neurocircuitry,
including within the spinal cord. In small animals, all or
near all of neurons within a neurocircuit have been worked
out. In humans, we see indications of such circuitry, interwoven
into the neural fabric of the spinal cord and brain stem.
Such neural oscillators tend to exhibit a rhythmic frequency
that is synergistic
with then mechanical
need, such as roughly 1 Hz for walking. But this is also
connectivity with higher brain centers and sensors that
help enable important modulation, such as scaling
rhythm, movement magnitude, and re-setting after perturbation.
Also, postural mechanisms work closely within rhythmic generators.
This integration takes time, which helps explain why a small
child employs a wider base of support during walking. and
thus. But ultimately many of us routinely walk
on many surfaces and terrains.
- For less intrinsic tasks such as throwing and striking
(e.g., hitting with a bat, racquet, club), and kicking, there
is
normally
a proximal-to-distal sequencing,
with the proximal segment(s) transmitting power (force
times velocity) to the more distal
segments. Often a proximal base such as a twisting torso
provides more than have the power of the release or strike.
Thus one key mechanical principle is proximal-to-distal
segmental power transfer. But there are three others that
are at work, all of take advantage
of muscle properties:
- Muscles can generate more force when they
are lengething at moderate rates, and thus it makes sense to contract
a (previously relaxed antagonist) muscle while it
is lengthening so that it help turn around a joint
as it starts to shorten.
- Muscles that are stretched can store elastic
energy that can be subsequently be released (e.g., this helps
a kangaroo hop or a cheetah sprint), and a "stretch-shortening
cycle" can assist with power release (and transfer),
if timed well.
- Muscles are normally great at "turning off" quickly,
and muscle that is deactivated at the right time can
become nearly invisible during phases of a dynamic
movement, thus not resisting the evolving movement.
Such synergies take time to
develop. Indeed,
a small
child tends
to start
with
a
simple
distal
throw
involving
the
arm and wrist, and gradually brings in
more proximal segments and the movement pattern matures
via practice.
- We'd seen in the previous section that many arm-hand
movements can be viewed as coordinated torso-shoulder-arm-hand
transitions
in posture. But what if the movements are fast? Many research
groups have studied goal-directed fast arm movements, especially
between targets. A key finding for single-joint movement
such as elbow or wrist is that there is first a "burst"
in the agonists that serves to accelerate the limb segment
toward
the intended target, much like we'd previously
seen in eye movements.
There
is
then
a "decelerating"
burst in the antagonists, with a timing, duration and intensity
that is a function of the mechancial need (e.g., magnitude
of the inertia to be decelerated, properties of the muscle
being activated). Finally, there is often a second pulse
in the original agonist, and/or co-activation between agonists
and antagonists, that helps "clamp" the movement
to the new position. This gradually dies down as the muscles
relax
to whatever postural "step" levels are necessary
to hold the new position. In the next section we will consider scaling strategies for such movements. Extending to whole-arm movements, there
are variants
on this strategy that depend on the type of movement. Upper
extremity arm movements tend to classified into two types: "reaching" where
the angular velocity of the upper and lower arms are in
opposite directions, and "whipping" where
the velocities have the same sign. These delineations are
important, because terms such as the Coriolis component
(a function of the product of angular velocities of the
connected links) will change sign! Thus a boxer can "jab"
(reaching movement) more quickly that throw a looping punch
(whipping movement), and uses pretty different coordination
patterns. Nonetheless, there is a remarkable amout of built-in synergy between pulse sequencing. Finally, as
we saw in the previous section, many arm movements
start with contractions of torso and
shoulder
stabilizing
muscles. Thus posture and dynamic movement are intertwined.
We saw in a previous section that neuromotor impairment
such as following trauma often affects mechanisms that are
normaly responsible for posture: We also saw that many movements
can be considered as transitions in posture, including most
ADLs and many IADLs. Yet with disability, neural dysfunction
can also affect coordinated volitional movements, both novel movements and skilled movemented. In the
next section we briefly describe some of these influences on performance.
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