Functional muscle synergies constrain force production during postural tasks

• Type of Material: Article
• Authors: J. Lucas McKay, Emory University; Lena Ting, Emory University
• Language: English
• Date: 2008-01-01
• Publisher: Elsevier: 12 months
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2007 Elsevier Ltd. All rights reserved.
• License: Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1016/j.jbiomech.2007.09.012
• Title of Journal or Parent Work: Journal of Biomechanics
• ISSN: 0021-9290
• Volume: 41
• Issue: 2
• Start Page: 299
• End Page: 306
• Grant/Funding Information: Funding for this study was provided by NIH grant HD46922.
• Keywords: muscle synergies; COORDINATION; Technology; ARM MOVEMENTS; NATURAL MOTOR BEHAVIORS; Science & Technology; STANDING PERSONS; musculoskeletal model; Engineering, Biomedical; STRATEGY; feasible force set; endpoint force; Biophysics; Life Sciences & Biomedicine; MODELS; PATTERNS; LOCOMOTION; HINDLIMB; muscle noise; Engineering; QUADRUPEDAL STANCE; balance; cat
• Research Categories: Engineering, Biomedical

Abstract

We recently demonstrated that a set of five functional muscle synergies were sufficient to characterize both hindlimb muscle activity and active forces during automatic postural responses in cats standing at multiple postural configurations. This characterization depended critically upon the assumption that the endpoint force vector (synergy force vector) produced by the activation of each muscle synergy rotated with the limb axis as the hindlimb posture varied in the sagittal plane. Here, we used a detailed, 3D static model of the hindlimb to confirm that this assumption is biomechanically plausible: as we varied the model posture, simulated synergy force vectors rotated monotonically with the limb axis in the parasagittal plane (r2=0.94±0.08). We then tested whether a neural strategy of using these five functional muscle synergies provides the same force-generating capability as controlling each of the 31 muscles individually. We compared feasible force sets (FFSs) from the model with and without a muscle synergy organization. FFS volumes were significantly reduced with the muscle synergy organization (F=1556.01, p≪0.01), and as posture varied, the synergy-limited FFSs changed in shape, consistent with changes in experimentally measured active forces. In contrast, nominal FFS shapes were invariant with posture, reinforcing prior findings that postural forces cannot be predicted by hindlimb biomechanics alone. We propose that an internal model for postural force generation may coordinate functional muscle synergies that are invariant in intrinsic limb coordinates, and this reduced-dimension control scheme reduces the set of forces available for postural control.