Gelsy Torres-Oviedo

J Lucas Mckay

Lena H Ting

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NEUROMECHANICAL MODELING OF FUNCTIONAL MUSCLE SYNERGIES FOR 
POSTURAL CONTROL IN THE CAT 
 
J. Lucas McKay1, Gelsy Torres-Oviedo1,2, and Lena H. Ting1,2 
 
1 Georgia Institute of Technology, Atlanta, GA, USA 
2 Emory University, Atlanta, GA, USA 
  E-mail: jlmckay@neuro.gatech.edu 
 
INTRODUCTION 
 
In a recent study, we demonstrated that 
EMG and kinetic data from the automatic 
postural response in cats could be 
decomposed into five “functional” muscle 
synergies, each specifying a pattern of 
hindlimb muscle activation and a correlated 
“synergy force vector” at the ground 
(Torres-Oviedo et al., 2006). The analysis 
showed that these functional muscle 
synergies could explain both muscle 
activation patterns and endpoint forces in a 
range of postural configurations that alter 
the biomechanical conditions of the task. 
However, this generalization was apparent 
only when we expressed the kinetic data in a 
coordinate system that rotated with the 
sagittal hindlimb axis. This suggests that 
muscle synergies provide the nervous 
system with a simple mechanism to generate 
forces consistent in the reference frame of 
the limb. 
 
Here, we used a static 3D musculoskeletal 
model of the cat hindlimb (McKay et al. 
2006) to test the hypothesis that muscle 
synergies could produce endpoint force 
vectors across a range of postural 
configurations corroborating our 
experimental observations. We then 
examined the neuromechanical implications 
of the functional muscle synergy 
architecture by examining changes in the 
feasible force set (FFS; Valero-Cuevas 
2006) after a synergy organization was 
imposed. 
 
METHODS 
 
First, we tested whether muscle synergies 
could produce endpoint forces that were 
consistent with respect to the sagittal 
hindlimb axis in a range of postural 
configurations. For each of three animals, 
we generated five simulated muscle 
synergies that could reproduce the 
experimentally derived synergy force 
vectors in the nominal postural 
configuration. We then examined the 
endpoint forces generated by these muscle 
synergies in the three other postural 
configurations using kinematic data from 
each cat (Torres-Oviedo et al., 2006).  
 
We investigated whether the simulated 
muscle synergies limited or spanned the 
force-production capabilities of the 
hindlimb. We calculated FFSs for all 
conditions as in (McKay et al. 2006); first 
assuming individuated control of muscles 
and later assuming individuated control of 
only the simulated synergies. 
 
RESULTS AND DISCUSSION 
 
Consistent with our hypothesis, simulated 
synergies produced force vectors that rotated 
monotonically with the limb axis in the 
sagittal plane as postural configuration 
varied (Figure 1, colored lines). Synergy 
force vector orientation was highly 
correlated to limb axis angle in the sagittal 
plane (r2 = 0.94 ± 0.08, µ ± σ), and less so in 
the dorsal plane (0.75 ± 0.3), confirming the 
quasi-planar variation we observed 
 
 
Figure 1: Rotation of synergy force vectors and feasible force sets with hindlimb axis when 
the animal stands using various postural configurations. 
 
experimentally. This result suggests that 
postural forces can be coordinated by the 
nervous system throughout the workspace 
with a relatively simple polar transformation 
from intrinsic to extrinsic coordinates. This 
is consistent with the encoding of limb 
orientation in sensory signals traveling from 
the spinal cord to higher brain centers 
(Bosco et al. 1996). 
 
When compared with the nominal FFS 
(Figure 1, thick black lines), the synergy-
limited FFS (Figure 1, gray shaded areas) 
varied much more acutely with postural 
configuration, particularly in the sagittal 
plane (r2 = 0.92 ± 0.05 vs. 0.77 ± 0.2). The 
synergy-limited FFS was also considerably 
more elongated at all postures. We have 
previously demonstrated that the nominal 
FFS is a weak determinant of postural force, 
due to its generally isotropic shape (McKay 
et al, 2006) – taken together, these results 
suggest that the synergy-limited FFS may be 
a much stronger determinant of force 
magnitude in the hindlimb, particularly as 
postural configuration changes.  
 
Our results illustrate that the muscle 
synergies recruited for postural control do 
not span the force-producing capabilities of 
the limb, so that only a limited range of 
forces are available for the postural task. 
Similar reductions in FFS volume occur in 
conditions of neural deficit (cf. Valero-
Cuevas 2006). Based on these results, we 
predict that tasks like locomotion will recruit 
additional synergies to reach the remainder 
of the FFS  (e.g., d'Avella and Bizzi 2005). 
 
Muscle synergies appear to limit the 
biomechanical outputs of the limb based on 
a parsimonious control strategy rather than 
musculoskeletal constraints. Our results 
suggest that it may be valuable to consider 
the changes to the FFS associated with 
muscle synergies when predicting muscle 
coordination and biomechanics for some 
natural behaviors, as opposed to “maximal” 
tasks where coordination is determined 
uniquely by biomechanical limitations (e.g., 
Valero-Cuevas 2006).  
 
REFERENCES 
 
Bosco, G., et al. (1996). J. Neurophys, 76, 
715-726. 
d'Avella, A., Bizzi, E. (2005). PNAS, 102, 
3076-3081. 
McKay, J.L., et al. (2006). J. Biomech, 
doi:10.1016/j.jbiomech.2006.10.013. 
Torres-Oviedo, G., et al. (2006). J. 
Neurophys, 96, 1530-1546. 
Valero-Cuevas. F. (2006). Progress in 
Motor Control V, in press. 
 
ACKNOWLEDGEMENTS 
 
Experimental data courtesy of Jane 
Macpherson. Support provided by NIH 
HD46922 


NEUROMECHANICAL MODELING OF FUNCTIONAL MUSCLE SYNERGIES FOR POSTURAL CONTROL IN THE CAT

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NEUROMECHANICAL MODELING OF FUNCTIONAL MUSCLE SYNERGIES FOR POSTURAL CONTROL IN THE CAT

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