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: kinematic redundancy (more joint degrees of freedom than kinematic degrees of freedom of the endpoint); and actuator redundancy (more muscles than would be theretically needed to rotte 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 systems.

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 remarkable 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 fell dozens of muscles dramatically change their activitation levels. There is much happening "beneath the surface" that can impact on not just posture but also on movement strategies.

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" make use of building block muscle synergy patterns. This is the focus of the next section.

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