Neuromechanics & Motor Control - Lecture 12

Musculoskeletal system

Includes bones, ligaments, joints, and muscles

Neural system

Includes sensors, proprioceptors, vision, and the Central Nervous System (CNS)

Muscle input

Nerve pulse

Muscle output

Muscle force

Muscle stiffness increases with

Greater muscle force, smaller displacements, higher frequencies

Henneman’s Size Principle

Motor units are recruited from smallest to largest as contraction increases

Motor noise

Increases with higher isometric force levels

Muscle spindles

Detect stretch and stretch velocity

Golgi Tendon Organ (GTO)

Detects muscle force

Vestibular organ

Detects linear acceleration (otoliths) and angular acceleration (semicircular canals)

Vision

Provides body position relative to the visual world

Postural control

Stabilization of joint dynamics to maintain posture or position

Joint dynamics influenced by

Muscle visco-elasticity, visco-elasticity of tissues, limb inertia, proprioceptive feedback

Afferent feedback

Contribution of proprioceptors and vision to joint dynamics

Muscle stretch reflex

Response to passive stretch detected by muscle spindles and GTO

Excitation of system

Only forces (F) can excite a system, leading to motion (X)

Admittance

System’s motion response to force perturbations, measured as position or angle per unit force or torque

Admittance equation

H(ω) = θ(ω)/T(ω)

Stability

System's ability to return to position after perturbation

Impedance

Inverse of admittance; force response to position deviation

Impedance control

Minimizes position variation; regulated by muscle co-contraction and reflexes

Co-contraction

Increases stiffness and damping; energy costly

Reflexes

Increase stiffness and damping with low energy cost but have delay

Drilling task admittance

High admittance in drilling direction, low in others

Position task

Focus on motion or posture; best described by mechanical admittance

Force task

Focus on contact force; best described by mechanical impedance

EMG co-contraction task

Maintain muscle activation level; best described by mechanical admittance

Position task strategy

High stiffness, low admittance to minimize displacements

Force task strategy

Low stiffness, high admittance to minimize force variation

EMG task strategy

Constant muscle stiffness, low reflex stiffness

Co-activation of muscles

Increases stiffness and viscosity; effective across frequencies; energy expensive

Proprioceptive feedback

Reduces admittance at low frequencies; energy efficient

Effect of co-contraction

Reduces admittance; energy demanding; stiffness and viscosity increase with activation

Effect of muscle spindle feedback

Reduces admittance; energy efficient; limited by delay

Effect of GTO feedback

Increases admittance

Neural time-delay effect

Proximal joints benefit more than distal due to shorter delay

Manipulator in experiments

Applies perturbations; type depends on experiment goal

Position-controlled manipulator

Controls position; measures reaction force

Force-controlled manipulator

Controls force using human reaction force; forms closed-loop system

Task dominance

Task has more effect on postural control than perturbation

Predictable perturbation

Causes voluntary human actions

Unpredictable perturbation

Triggers intrinsic or reflexive feedback

Transient signals

Easy visual analysis; hard to retrieve dynamics

Continuous signals

Suitable for frequency domain analysis; good for LTI systems

Human adaptation

Postural tasks trigger distinct natural motor behavior