Table 1.

Modularity terminology and relationships among descriptions and units

Marr level of description	Increasingly finer resolution elements of action composition and control					Type of data and description
Task  Behavioral task/Kinematic task/Task planning	Behavior (e.g., Hunt, groom etc.)	Task: (run to capture); akinematic oscillation	Subtask: (swing, stance, foot placement, reach to grasp)	Stroke/kinematic segmentation unit = kinematic stroke or kinematic primitive		Kinematics
Algorithms  Controls/algorithms (kinetics necessary)		Timing, rhythm & sequence	d Force pattern for subtask, possible co-articulation	Force pattern for individual kinematic stroke, possible co-articulation	Unit building block for force patterns. Force-field primitives = kinetic primitives	Kinetics
		bRhythm generator system	cPattern shaping under rhythm generator, descending and sensory control influences or		gBurst synergies = motor primitives = force-field primitive. Unitary burst (or “pulse”) of a synchronous muscle synergy	Muscle patterns
		Rhythm generator system	Pattern shaping system	eTime-varying synergies
Implementation  Implementations/circuitry		Neural oscillator or state chaining system	fNeural switching systems or emergent pattern shaping and sequencing processes		Separated neural premotor drives and burst generation	Neural support
		<=> robotic Dynamical movement primitive (oscillator primitives) or finite state machine	<=> robotic Coupling or selection and switching policy, e.g., finite state machine with time outs	<=> robotic Kinematic primitive and PID controller	<=> robotic Kinetic primitive or multi-joint PD control unit	Sample equivalent idea in biomimetic robotics

Note: Missing from the table are the synergy formulations as optimal controlled subspaces, and controlled/uncontrolled manifolds. The missing terms and models of modularity are harder to relate simply and directly to pattern generation and spinal neural circuits in testable hypotheses. In the table, specific relationships or areas need significant work and are important to understand new data on modularity in stroke and spinal cord injury.

^aThe relations of rhythmic and discrete motion at kinematic and production descriptions remain an area of ambiguity.

^bAs might be expected, rhythm/timing generation mechanisms and circuit location remain insufficiently specified.

^cThe structure of pattern shaping remains difficult and controversial. The separation from rhythm generation is controversial. The composition and modularity of pattern is disputed. Modularity can be considered emergent from network dynamics or based on switching and incorporation of explicit modules.

^dEfficient kinematic and kinetic action may involve co-articulation of modules in sequences and mechanisms should support this contextual adjustment for increasing skill levels in tasks.

^eModules in pattern shaping could be fixed sequences of several bursts, also called time varying synergies, that are explicitly related to kinematic modules such as strokes or cycles. Alternatively, pattern shaping modules could be single bursts, that are not uniquely or closely related to kinematic strokes and cycles (e.g., unit bursts/synchronous synergies that instead relate directly to units of biomechanical modularity such as unitary force-fields, and to neural drive (see ^g below).

^fThe structure of neural switching and pattern sequencing is very poorly understood but is probably crucial to understand processes of co-articulation, merging, splitting, and sequence changes after neural damage and during therapy.

^gNeural underpinning of unit bursts and drive structure are also part of the key to these clinically meaningful processes.