H Choi, KM Peters, M MacConnell, K Ly, E Eckert, KM Steele (2017) “Impact of ankle foot orthosis stiffness on Achilles tendon and gastrocnemius function during unimpaired gait.” Journal of Biomechanics

Journal article in Journal of Biomechanics:

How does the stiffness of an AFO impact the muscultendon dynamics of the gastrocnemius?

Abstract

Method combining ultrasound and musculoskeletal modeling to evaluate changes in muscle and tendon length.

Ankle foot orthoses (AFOs) are designed to improve gait for individuals with neuromuscular conditions and have also been used to reduce energy costs of walking for unimpaired individuals. AFOs influence joint motion and metabolic cost, but how they impact muscle function remains unclear. This study investigated the impact of different stiffness ankle foot orthoses (AFOs) on medial gastrocnemius muscle (MG) and Achilles tendon (AT) function during two different walking speeds. We performed gait analyses for eight unimpaired individuals. Each individual walked at slow and very slow speeds with a 3D printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25 Nm / °, 1 Nm / °, 2 Nm / °, and 3.7 Nm / °. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify MG and AT lengths with each AFO condition. Increasing AFO stiffness increased peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak AT length decreased, while peak MG length increased with increasing AFO stiffness. Peak MG activity, length, and velocity significantly decreased with slower walking speed. This study provides experimental evidence of the impact of AFO stiffness and walking speed on joint kinematics and musculotendon function. These methods can provide insight to improve AFO designs and optimize musculotendon function for rehabilitation, performance, or other goals.

 

 

H Choi, TL Wren, KM Steele (2016) “Gastrocnemius operating length with ankle foot orthoses in cerebral palsy.” Prosthetics & Orthotics International

Example of gastrocnemius operating length from one subject with different AFOs.

Journal article in Prosthetics & Orthotics International:

How does the operating length of the gastrocnemius vary with different common AFOs in children with cerebral palsy?

Clinical relevance: Determining whether ankle foot orthoses stretch tight muscles can inform future orthotic design and potentially provide a platform for integrating therapy into daily life. However, stretching tight muscles must be balanced with other goals of orthoses such as improving gait and preventing bone deformities.

KM Steele and MH Schwartz, “Do muscle synergies reflect optimal control during gait in unimpaired individuals and individuals with cerebral palsy?” International Symposium on Computer Simulation in Biomechanics (Edinburgh, UK) July 11, 2015.

Kat Steele presented at the International Society of Biomechanics Technical Group on Computer Simulation in Edinburgh, Scotland on July 11, 2015. This study quantifies how traditional methods for estimating muscle activity in musculoskeletal simulation (e.g., minimizing sum of squared muscle activations) fails to accurately predict muscle activity for individuals with cerebral palsy. This research will help to guide the development of new methods to quantify patient-specific changes in neuromuscular control.

KM Steele, MC Tresch, EJ Perreault (2015) “Consequences of biomechanically constrained tasks in the design and interpretation of synergy analyses.” Journal of Neurophysiology

Synergy similarity is reduced with musculoskeletal constraints.

Journal article in Journal of Neurophysiology

Consequences of biomechanically constrained tasks in the design and interpretation of synergy analyses

Matrix factorization algorithms are commonly used to analyze muscle activity and provide insight into neuromuscular control. These algorithms identify low-dimensional subspaces, commonly referred to as synergies, which can describe variation in muscle activity during a task. Synergies are often interpreted as reflecting underlying neural control; however, it is unclear how these analyses are influenced by biomechanical and task constraints, which can also lead to low-dimensional patterns of muscle activation. The aim of this study was to evaluate whether commonly used algorithms and experimental methods can accurately identify synergy-based control strategies. This was accomplished by evaluating synergies from five common matrix factorization algorithms using muscle activations calculated from 1) a biomechanically constrained task using a musculoskeletal model and 2) without task constraints using random synergy activations. Algorithm performance was assessed by calculating the similarity between estimated synergies and those imposed during the simulations; similarities ranged from 0 (random chance) to 1 (perfect similarity). Although some of the algorithms could accurately estimate specified synergies without biomechanical or task constraints (similarity >0.7), with these constraints the similarity of estimated synergies decreased significantly (0.3-0.4). The ability of these algorithms to accurately identify synergies was negatively impacted by correlation of synergy activations, which are increased when substantial biomechanical or task constraints are present. Increased variability in synergy activations, which can be captured using robust experimental paradigms that include natural variability in motor activation patterns, improved identification accuracy but did not completely overcome effects of biomechanical and task constraints. These results demonstrate that a biomechanically constrained task can reduce the accuracy of estimated synergies and highlight the importance of using experimental protocols with physiological variability to improve synergy analyses. PDF

KM Steele and S Lee (2014) “Using dynamic musculoskeletal simulation to evaluate altered muscle properties in cerebral palsy.” Proceedings of ASME Dynamics Systems and Control

KM Steele and S Lee (2014) “Using dynamic musculoskeletal simulation to evaluate altered muscle properties in cerebral palsy.” Proceedings of ASME Dynamics Systems and Control

Paper accepted at ASME Dynamics Systems and Control Conference:

Using dynamic musculoskeletal simulation to evaluate altered muscle properties in cerebral palsy

Abstract: Cerebral palsy is caused by an injury to the brain, but also causes many secondary changes in the musculoskeletal system. Altered muscle properties such as contracture, an increased passive resistance to stretch, are common but vary widely between individuals and between muscles. Quantifying these changes is important to understand pathologic movement and create patient-specific treatment plans. Musculoskeletal modeling and simulation have increasingly been used to evaluate pathologic movement in CP; however, these models are based upon muscle properties of unimpaired individuals. In this study, we used a dynamic musculoskeletal simulation of a simple motion, passively moving the ankle, to determine (1) if a model based upon unimpaired muscle properties can accurately represent individuals with cerebral palsy, and (2) if an optimization can be used to adjust passive muscle properties and characterize magnitude of contracture in individual muscles. We created musculoskeletal simulations of ankle motion for nine children with cerebral palsy. Results indicate that the unimpaired musculoskeletal model cannot accurately characterize passive ankle motion for most subjects, but adjusting tendon slack lengths can reduce error and help identify the magnitude of contracture for different muscles.