Spatiotemporal organization of muscle activity throughout human postural responses Open Access
Safavynia, Seyed A. (2012)
Abstract
Abstract
Spatiotemporal organization of muscle activity throughout human
postural
responses
Falls are the leading cause of morbidity and mortality among the
elderly and result from a failure of the nervous system to
appropriately coordinate muscles to maintain balance. Because
muscle activity represents the output of the nervous system,
examining muscle activation may reveal differences in neural
mechanisms underlying falls; however, there is enormous spatial and
temporal variability in muscle activation patterns, making them
difficult to functionally interpret. Recent work has demonstrated
that the spatial and temporal features of muscle activity can be
functionally yet separately explained by muscle synergies and
task-level feedback, respectively. The spatial coordination of
muscles has been functionally characterized in a variety of motor
tasks using muscle synergies, or groups of muscles with fixed
ratios of coactivation. However, the temporal recruitments of such
muscle synergies as well as the underlying neural mechanisms have
largely been uninvestigated. Conversely, temporal activation of
individual muscles throughout postural responses has been
functionally characterized using task-level feedback of center of
mass (CoM). However, CoM feedback has only been applied to
perturbations where the body starts from rest and CoM kinematics
(displacement, velocity, acceleration) are highly correlated.
I hypothesize that the nervous system continuously uses
task-level feedback of CoM to recruit muscle synergies throughout
standing balance tasks. Here, I unified the muscle synergy
hypothesis with task-level feedback to functionally explain the
spatiotemporal features of muscle activity throughout human
postural responses. I first demonstrated that the temporal
recruitment of muscle synergies throughout discrete
sagittal perturbations could be well-reconstructed using delayed
feedback of CoM. I then developed complex perturbations to test the
robustness of delayed CoM feedback on muscle activity and muscle
synergy recruitment. Delayed feedback of CoM was shown to robustly
modulate muscle activity throughout continuous sagittal
perturbations that decouple CoM kinematics in magnitude. Moreover,
delayed feedback of CoM was shown to robustly modulate muscle
synergy recruitment throughout multidirectional discrete and
biphasic perturbations that decouple CoM kinematics from each other
in direction. These results suggest that a consistent spatial and
temporal structure of motor outputs exists across static and
dynamic states. Such an organization may aid in functionally
identifying pathologic strategies for maintaining balance.
Table of Contents
TABLE OF CONTENTS
CHAPTER 1:INTRODUCTION
.........................................................................................................
1
1.1 LIMITATIONS OF CLINICAL ASSESSMENT OF MOTOR FUNCTION AND
CONTROL ................................. 1
1.2 SPATIAL COORDINATION OF EMG: MUSCLE SYNERGY CONCEPT
.......................................................... 6
1.3 TEMPORAL ACTIVATION OF EMG: FEEDBACK CONTROL OF TASK-LEVEL
VARIABLES ................... 12
1.4 THESIS OVERVIEW
......................................................................................................................................
17
1.4.1 Study 1
.....................................................................................................................................................
20
1.4.2 Study 2
.....................................................................................................................................................
21
1.4.3 Study 3
.....................................................................................................................................................
21
CHAPTER 2: TASK-LEVEL FEEDBACK CAN EXPLAIN TEMPORAL
RECRUITMENT OF
SPATIALLY-FIXED MUSCLE SYNERGIES THROUGHOUT POSTURAL PERTURBATIONS
23
2.1 INTRODUCTION
............................................................................................................................................
23
2.2 METHODS
.....................................................................................................................................................
28
2.2.1 Summary
................................................................................................................................................
28
2.2.2 Data collection
.....................................................................................................................................
29
2.2.3 Data processing
...................................................................................................................................
30
2.2.4 Muscle synergy extraction
..............................................................................................................
32
2.2.5 Determination of number of muscle synergies
......................................................................
35
2.2.6 Muscle synergy analysis
...................................................................................................................
37
2.2.7 Feedback model
..................................................................................................................................
38
2.3 RESULTS
.......................................................................................................................................................
42
2.3.1 Summary
................................................................................................................................................
42
2.3.2 Postural responses throughout perturbations
......................................................................
42
2.3.3 EMG reconstructions using SF versus TF muscle
synergies .............................................
43
2.3.4 SF muscle synergies have similar structure across
perturbation epochs .................. 50
2.3.5 Consistent SF muscle synergies across subjects
....................................................................
51
2.3.6 CoM kinematic feedback reconstructs SF muscle
synergy recruitment ..................... 54
2.3.7 SF muscle synergies recruited by CoM feedback
reproduce temporal variations in
muscle activity
....................................................................................................................................................
58
2.4 DISCUSSION
..................................................................................................................................................
62
2.4.1 Summary
................................................................................................................................................
62
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