Motor Control

ASEN 5016 Lecture 6a: Motor Control

OBJECTIVES

1. Describe effects of weightlessness on posture, locomotion and related functions

1. Effects of weightless on posture, locomotion and related functions

Standing still is more difficult than walking, neurobiologically speaking

Standing occurs mostly by activity of extensor muscles in back of calf and front of upper leg – acquired almost entirely by muscle properties, not signal driven

Center of mass is projected slightly forward, corrected at ankle via small signals usually below threshold of sensation

Mostly low frequency in range of 0.1-0.3 Hz and up to 8-12 Hz (tremor of tension)

Fluctuations can be recorded by a differential balance

Conventional view that standing is primarily an otolith driven function

Current thinking indicates postural stability may come from deep skin sensors

Postural synergy developed between deep skin sensors (Vater-Pacini corpuscles) in the sole and muscle activities during quiet standing (does not exist in newborns – learned)

Postural Changes in flight - Flexors active, extensors generally not, ‘tired monkey’ posture

Extensor suppression thought to be triggered by weight bearing sensors

Vertical posture only ‘needed’ in a gravity field and tied to sensor system used to project center of body mass

Tonic postural muscles not triggered when weight bearing signal ‘removed’

Locomotion is linked to sole of the foot stimulation

Standing = body weight

Walking ~ 1.5x standing load

Running ~1.8-3x or more

Some factors unique to space flight

Weightlessness à neuromuscular inhibition (proprioceptive feedback)
Reorientation à up and down cues relative to immediate surroundings
Movement through the cabin occurs in a straight-line trajectory
Before adapting, crewmembers tend to "overshoot" objects

Regaining motor control post-flight

Balancing is a difficult feat

Stochastic Resonance – Adding random, subtle (imperceptible) vibrations to sole of foot appears to make human nervous system more sensitive to signals of imbalance - noise can actually enhance weak signals, first proven in crayfish and utilized to improve hearing aids. Less swaying observed with application - human nervous system apparently operates on the edge of noise and instability.

Anthropometrics and Biomechanics à used to quantify human performance

Range of joint motion and torque generation affected by gravity
Some tasks enhanced, others hindered depending on associated inertia
Neutral resting position altered in weightlessness à ~fetal position

Some related research programs

MIT – Astronaut Control and Dynamics - http://mvl.mit.edu/ASTRODYN/html/astrodyn.html

USC Computational Learning and Motor Control Lab - http://www-slab.usc.edu/