Jane M. Macpherson

Absence of postural muscle synergies for balance after spinal cord transection.

Although cats that have been spinalized can also be trained to stand and step with full weight support, directionally appropriate long-latency responses to perturbations are impaired, suggesting that these behaviors are mediated by distinct neural mechanisms. However, it remains unclear whether these responses reflect an attenuated postural response using the appropriate muscular coordination patterns for balance or are due to fundamentally different neural mechanisms such as increased muscular cocontraction or short-latency stretch responses. Here we used muscle synergy analysis on previously collected data to identify whether there are changes in the spatial organization of muscle activity for balance within an animal after spinalization. We hypothesized that the modular organization of muscle activity for balance control is disrupted by spinal cord transection. In each of four animals, muscle synergies were extracted from postural muscle activity both before and after spinalization with nonnegative matrix factorization. Muscle synergy number was reduced after spinalization in three animals and increased in one animal. However, muscle synergy structure was greatly altered after spinalization with reduced direction tuning, suggesting little consistent organization of muscle activity. Furthermore, muscle synergy recruitment was correlated to subsequent force production in the intact but not spinalized condition. Our results demonstrate that the modular structure of sensorimotor feedback responses for balance control is severely disrupted after spinalization, suggesting that the muscle synergies for balance control are not accessible by spinal circuits alone. Moreover, we demonstrate that spinal mechanisms underlying weight support are distinct from brain stem mechanisms underlying directional balance control.

Muscle Synergies Robustly Control Forces for Balance Control

We demonstrate that a limited number of muscle synergies can be used to reconstruct complex muscle activation patterns and forces produced during two types of postural perturbations. Our results suggest that low-dimension neural command signals may activate muscle synergies to produce high-dimension muscle activation patterns. Furthermore, muscle synergies may be used to coordinate multijoint muscle actions for the control of high-level task variables. Muscle synergies may represent natural building blocks for motor behaviors and provide insight into neuroengineered strategies for motor rehabilitation