Moving, Touching, and Manually Manipulating Objects

This section focuses on the more narrow topic of the conceptual role of biomechanics and muscles synergies skill development, especially as related to neurorehabilitation. One of the reasons that the neurorehabilitation field cares about developmental stages is that it is well documented that after neural trauma (e.g., SCI, stroke, TBI), basic motor activity patterns normally associated with stages of motor development in young children, such as certain reflexes and synergy patterns, often become expressed.

Gift of Touching - Coordinated Reaching and Contact

• Goal-directed skilled movements and learning from two-way shared experiences
• Early development: simple reaching as a transition in posture and a grasp that is largely reflexive, core synergies
• Mature capabilities: trajectory planning, multi-segmental coordination, "motor patterns" and synergies, formation of skills (and associated "motor programs").

Dynamics of Movement

• Beyond Posture: Dealing with Inertial Dynamics (e.g., tangential, centripetal, coriolis components)
• Redundancy
  • Sensor - more sensors in muscles, joint and skin than seen "necessary"
  • Kinematic - more "joint DOFs" than end-point coordinates
  • Actuator - more muscle motors than necessary to rotate all joint DOFs
• Changes with Environmental Contact
• "Motor Programs" that "Learn" and Thus "Know" Dynamics?

Neuromotor Dynamic Movement Patterns

• Brain Sensorimotor Structures: More Redundancy (e.g., Multiple Pathways)
• Built-in Central Pattern Generators (e.g., for walking) and Synergy Patterns (e.g., extensor, flexor limb synergies)
• Stages of Motor Development: Motor Patterns and Skill Acquisition
  • Bi- and tri-phasic EMG patterns for fast movements
  • Full-limb patterns for tasks such as "reaching" and "whipping"
• Examples:
  • Stages of walking and the "Central Pattern Generator"
  • Stages of power throwing/striking: proximal-to-distal sequencing
    • "learning mechanics"
    • taking advantage of muscle properties (higher force during lengthening, stretch-shortening cycle, turning antagonistic muscles off)
  • Stages of Goal-Directed Arm Movements
    • single-joint: antagonistic pulse strategies, speed-accuracy tradeoffs
    • multilink: reaching/whipping
    • adding in coordinated grasping to a reach

Performance Assessment and "Performance Criteria"

• Goal-Direct Behavior
• Scaling Strategies for Dynamic Movements

Disability Challenges and Accommodations (Neurorehab Focus)

• General Principles of Neurorehab for Skill Acquisition
• Stable Mobility (e.g., Walking)
• Stable Manipulation (e.g., Reaching/Grasping)
• Examples: Challenges of Functional Neuromuscular Stimulation Systems
  • Lower Extremity: Standing and Walking
  • Upper Extremity: Reaching, Grasping and Manipulation

Gift of Touching - Coordinated Reaching and Contact

Goal-directed skilled movement encompasses more than just transitions in posture, or use of various reflexes. We call this section "touching" in honor of those early first goal-directed neuromotor movement experiences performed by infants, exploratory activities that quickly evolve into two-way shared experiences between individuals and between an individual and their environment. An infant starts with a reach that is really a transition in posture and a grasp that is largely reflexive (and painful for fathers with beards). But soon a new level of function emerges: trajectory planning, multi-segmental coordination (e.g., between torso, arm, hand, head and eyes) that include fundamental "motor patterns" (e.g., proximal-distal movement sequencing) and use of muscle synergies or task templates, and finally the formation of skills or motor programs. While the basic stages of motor development in children have been well documented through careful observation, our scientific knowledge of internal mechanisms remains limited.

This section focuses on the more narrow topic of the conceptual role of biomechanics and muscles synergies in skill development, especially as related to neurorehabilitation. There are several reasons why the neurorehabilitation field cares about developmental stages. One is that with neural trauma, it is well documented that motor activity normally associated with stages of motor development are often expressed: certain reflexes, for example.

Dynamics of Movement

We've previously developed the basic principles behind postural mechanics, and mechanical stability. During activities such as brisk walking, throwing, or playing video games, limb segments are accelerating and decelerating at rates that cause inertial dynamics to be significant. Indeed, for periods of time, inertial terms can dominate. These terms depend on segmental accelerations (tangential component) and products of velocities (centripetal and coriolis components). Given the skill with which individuals often perform movements, the brain clearly does a good job of understanding inertial dynamics.

Another issue is that the musculoskeletal system has several types of redundancy:

• sensor redundancy (more sensors than are needed to sense muscle length, velocity or force, joint angle, finger pressure, etc);
• kinematic redundancy (more joint degrees of freedom than kinematic degrees of freedom of the endpoint); and
• actuator redundancy (more muscles than would be theoretically needed to rotate every joint degree of freedom).

An example of the former is that for most of the workspace of the arm, the end point position of the hand (3 DOF in each of translation and rotation) can be achieved by a collection of arm configurations. Similarly, many different muscle patterns (e.g., cocontraction levels) can produce the same body kinematic trajectory. It has often been suggested that the neuromotor system takes advantage of this "complexity," though we really don't have a good understanding of such control system strategies.

Still another issue is what happens to "motor programs" and skills once there is mechanical contact with the environment (e.g., bi-causal contact via the hand). The environment in contact can take many forms, from a stiff wall to a ball with inertia to a power drill to a sports racquet to a keyboard. The human is remarkably effective at using tools and manipulating objects. This hints at principles such as impedance control and theories of dynamic stability that are beyond the scope of this class. But notice that as you choose to lean on a wall while standing or rest your arms on a surface while sitting, you can feel dozens of muscles dramatically change their activation levels. There is much happening "beneath the surface" that can relates not just to posture but also to our movement strategies for reaching, touching and manipulating objects.

Researchers have developed many theories on how the brain learns such mechanics. In essence, our knowledge is limited, and we commonly use the term "motor programs" to refer to coordinated movement patterns related to a skill. These "programs" can make use of building block muscle synergy patterns. This is the focus of the next section.

Dynamic Movement Patterns

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 parallel to influence movement.

There are also a considerable synergy patterns within neurocircuitry that are coded mostly within the spinal cord. 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 to develop, which helps explain why a small child employs a wider base of support during walking. 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 lengthening 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 especially elbow extension. This gradually evolves into bringing in more proximal segments, and the movement pattern matures via practice until there is a coordinated proximal-to-distal sequencing.

• We'd seen in the previous section on positioning 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: 1) 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; 2) there is then a strategic "decelerating" burst in the antagonists, with a timing, duration and intensity that is a function of the mechanical need (e.g., magnitude of the inertia to be decelerated, properties of the muscle being activated); 3) 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; and 4) the muscles gradually 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 fast movement concepts 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: 1) "reaching" where the angular velocity of the upper and lower arms are in opposite directions, and 2) "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 amount of built-in synergy between pulse sequencing.
• Finally, many arm movements start with contractions of torso and shoulder stabilizing muscles. Thus posture and dynamic movement are intertwined. A classic example comes from studies of very fast upward arm movements, made on the ground and in space by astronauts. Even if the command is to move as soon as possible, the first muscles normally active are postural muscles in the back and the calves; the "agonists" (e.g., anterior deltoid) doesn't get the burst until the stabilizers are first activated. But the optimal "stabilizing" strategy differs whether or not one is within the gravitational field of the earth, resulting in some interesting results.

We saw in a previous section that neuromotor impairment such as following trauma often affects mechanisms that are normally 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 movement. In the next section we briefly describe some of these influences on performance.

Human Performance and Goal-Directed Tasks

We are goal-directed creatures. Often there are clear assessment metrics that relate to performance (e.g., hitting a tennis shot where desired; successfully trading off speed versus accuracy). The process of learning skills is often viewed as an optimization process, and detailed discussion in beyond the scope of this class. Here we focus on one small aspect that is often of importance for dynamic movements, and helps us develop an appreciation for the remarkable capacity of the brain to learn, and perform many tasks well: scaling of movements.

Goal-directed skilled movement often requires scaling of dynamic movements. For instance, a fast tracking movement can be made different distances. In such cases neuromotor pulse widths and/or heights must scale. Similarly, there is scaling of a golf or tennis swing, a kick, etc. Thus a "motor program" associated with the skill cannot always be retrieved and implemented directly, without first being adjusted to current aims and circumstances. Typically the cerebellum and basal ganglia are involved in planning such movement scaling, and typically there are "success" criteria related to performance. In an optimization problem, such "performance criteria" are used by an algorithm (e.g., within the brain) to "sculpt" and scale a planned movement.

As a concrete example, consider the classic neuromotor implementation of a simple point-to-point target tracking movement involving one joint, such as an elbow flexion-extension movement. From a neurocontrol perspective, moving quickly from one point to another is initiated by strongly activating the agonists (prime mover) while relaxing any drive to the antagonists. This causes a muscle-induced moment across the joint, which will cause a movement to be initiated toward the new, desired target position. The acceleration will depend on the inertia of the segment to be rotated (Newton's law) and the velocity will increase as this acceleration is integrated. After reaching roughly the half-way point, there is a need to start decelerating the limb segment, i.e. to start planning how to stop. One approach might be to simply turn off the agonist drive at just the right time and "coast" into the new position (taking advantage of some natural "viscosity" (velocity-dependent friction) within the muscle-joint apparatus. But in general, there is a need for active braking, or "clamping" of the movement through a temporary change in sign of the moment. This is accomplished by activating the antagonist muscles while lowering the neuromotor drive to the agonists. The movement rapidly slows toward zero. But since the new moment is not functioning as a frictional brake, there is often a need for an additional agonist clamping pulse. We've just described the classic "tri-phasic burst" pattern (agonist-antagonist-agonist) for very fast goal-directed tracking movements that has been observed across many joints (e.g., seen in EMG's for isolated movements of the elbow, shoulder, wrist, horizontal head, ankle). Typically the first (agonist) burst is large in both pulse magnitude and width (e.g., over half of the movement time), the second (antagonist) burst starts just before the first agonist burst ends and is nearly as high in magnitude but for less time, and the third (agonist second burst) overlaps some with the end of the second. Also, there is often a degree of coactivation around and shortly after the end of the movement (temporarily "stiffening" the joint) that gradually dies down. Finally, there needs to be a sustained "step" increase, typically small, in the agonist relative to the antagonist so as to maintain the new joint position and not drift back towards the old position.

Clearly the experienced human without functional impairments relevant to this type of movement task can easily adjust movement speeds, if desired. Also, the human can adjust their strategy to targets of different magnitudes. Thus both time-scaling (or speed-scaling) and magnitude-scaling are possible. How does such scaling occur?

Let's first consider time-scaling, starting from the fastest movement. While the details depend on biomechanical and neuromotor factors for the given system, in general as the desired movement time increases (speed lowers), the initial agonist burst will have a lower magnitude (for some systems making large movements it may first remain saturated at maximum and start by scaling its pulse width). The second (antagonist) burst will also scale down in magnitude and often change its on-off timing. The third burst will gradually disappear, and for moderate-to-fast speeds the neurocontrol becomes "bi-phasic." For moderate-speed movements, the agonist and antagonist bursts continue to scale down in magnitude, and the antagonist may even end before the movement is completed. With moderate-to-slow movements, at some stage the antagonist clamping burst is no longer mechanically needed and the entire movement (and its magnitude) becomes governed by a "pulse-step" strategy of the agonist. Thus the shape of the neurocontrol signals differs dramatically between a tracking movement performed quickly and performed at a moderate-to-slow pace; but the strategy is very much that which would be predicted based on analysis of Biomechanical factors. To some extent the brain "learns" mechanics.

To scale movement magnitude during fast movements, say for a larger-magnitude movement, the pulse height (magnitude) of the initial agonist burst is adjusted scaled up (unless it is already saturated at maximum neuromotor drive), causing a higher acceleration and velocity during the early phase of movement. Often the pulse width is also increased. Researchers have also studied other scaling phenomena, such as with added inertia (e.g., balls of different sized in the hand). In all cases, the neuromotor solutions make biomechanical sense.

Of note is that the above neuromotor scaling strategies assume that the neuromotor system has perfectly learned mechanics. This is typically close to the case. However, in reality the human does "many activities well" yet none perfectly - even NBA basketball players routinely miss free throws. The more challenging the task and the greater the mechanical sensitivity to internal neurocontrol "noise" or error in formulating these neurocontrol signals, the more likely that accuracy and performance will suffer. One form of this reality is the classic "speed-accuracy trade-off" that is captured under what is called "Fitt's Law" - faster-speed movements generally will exhibit greater target error. From a rehabilitation perspective, a key point is that there is a tremendous degree of neural-mechanical coupling in goal-directed performance. Thus your biomechanical background is very relevant to understanding and addressing the role of functional impairments in goal-directed performance and skill acquisition.

Disability

This is a difficult section to write, as functional impairments and movement deficits take many forms, from the obvious to very subtle. The therapist learns some general principles of neurorehab for, in particular, skill acquisition and synergy patterns. The pathology that relates to the disability can include Parkinson's, stroke, SCI, TBI, various cerebellar pathology, and so on. But here are a few highlights that relate to touching and manual manipulation:

• With SCI and stroke, often certain muscles are affected more than others at the same level. For instance, with stroke, often it is easier to make a fist and grip objects than it is to extend the fingers and let go of objects. Often stroke is hemiplegic, and persons will have a tendency to perform tasks with the unaffected (or less affected) arm, which unfortunately can lead to "learned disuse" in the involved extremity.
• Pathological tremor can come from many sources, including involvement from the brain stem structures (e.g., Parkinson's) and the cerebellum. Some tremors are worse while maintaining a posture, others mostly while making a movement. This helps determine their source, and possible therapy and coping strategies.
• Stable manipulation (e.g., reaching then grasping then manipulating an object) requires more than just a muscle force generation pattern. Co-activation of antagonistic muscles can change the joint "stiffness" which turns out to be very important.
• Co-contraction of proximal joints, making then temporarily stiffer, can help create a "virtual base" for a distal system (e.g., elbow-wrist-hand). If this capability is impaired, there can be challenges even when the distal motor coordination capability is largely intact.