Abstract
Neuromusculoskeletal simulation provides a promising platform to inform the design of assistive devices or inform rehabilitation. For these applications, a simulation must be able to accurately represent the person of interest, such as an individual with a neurologic injury. If a simulation fails to predict how an individual recruits and coordinates their muscles during movement, it will have limited utility for informing design or rehabilitation. While inverse dynamic simulations have previously been used to evaluate anticipated responses from interventions, like orthopedic surgery or orthoses, they frequently struggle to accurately estimate muscle activations, even for tasks like walking. The simulated muscle activity often fails to represent experimentally measured muscle activity from electromyographic (EMG) recordings. Research has theorized that the nervous system may simplify the range of possible activations used during dynamic tasks, by constraining activations to weighted groups of muscles, referred to as muscle synergies. Synergies are altered after neurological injury, such as stroke or cerebral palsy (CP), and may provide a method for improving subject-specific models of neuromuscular control. The aim of this study was to test whether constraining simulation to synergies could improve estimated muscle activations compared to EMG data. We evaluated modeled muscle activations during gait for six typically developing (TD) children and six children with CP. Muscle activations were estimated with: (1) static optimization (SO), minimizing muscle activations squared, and (2) synergy SO (SynSO), minimizing synergy activations squared using the weights identified from EMG data for two to five synergies. While SynSO caused changes in estimated activations compared to SO, the correlation to EMG data was not higher in SynSO than SO for either TD or CP groups. The correlations to EMG were higher in CP than TD for both SO (CP: 0.48, TD: 0.36) and SynSO (CP: 0.46, TD: 0.26 for five synergies). Constraining activations to SynSO caused the simulated muscle stress to increase compared to SO for all individuals, causing a 157% increase with two synergies. These results suggest that constraining simulated activations in inverse dynamic simulations to subject-specific synergies alone may not improve estimation of muscle activations during gait for generic musculoskeletal models.
Highlights
Muscle synergies have been used as a method to describe how muscles are commonly activated during tasks such as walking, by identifying a low dimensional space of weighted muscle groupings (Bizzi and Cheung, 2013)
All children with cerebral palsy (CP) were in Gross Motor Function Classification System (GMFCS) Levels I or II
For Static optimization (SO), there was no difference in single-limb stance [CP: 0.39 (0.26), typically developing (TD): 0.34 (0.16)] or swing [CP: 0.38 (0.26), TD: 0.33 (0.21)] for estimates of muscle activity compared to EMG data
Summary
Muscle synergies have been used as a method to describe how muscles are commonly activated during tasks such as walking, by identifying a low dimensional space of weighted muscle groupings (Bizzi and Cheung, 2013). These weighted groups of muscles have been shown to be altered among individuals with neurologic injuries, such as stroke or cerebral palsy (CP) (Cheung et al, 2009; Clark et al, 2010; Steele et al, 2015a; Tang et al, 2015; Shuman et al, 2016). Using an individual’s synergies calculated from experimental data to inform neuromusculoskeletal simulations may improve estimates of an individual’s muscle coordination or response to interventions like assistive devices or rehabilitation
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