Simulation

EC Kuska, N Mehrabi, MH Schwartz, KM Steele (2022) “Number of synergies impacts sensitivity of gait to weakness and contracture”

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Journal Article in Journal of Biomechanics

Muscle activity during gait can be described by a small set of synergies, weighted groups of muscles, that are theorized to reflect underlying neural control. For people with neurologic injuries, like cerebral palsy or stroke, even fewer synergies are required to explain muscle activity during gait. This reduction in synergies is thought to reflect altered control and is associated with impairment severity and treatment outcomes. Individuals with neurologic injuries also develop secondary musculoskeletal impairments, like weakness or contracture, that can impact gait. Yet, the combined impacts of altered control and musculoskeletal impairments on gait remains unclear.

A two-dimensional sagittal plane musculoskeletal model and synergy simulation framework tracked unimpaired gait kinematics. The model had nine degrees of freedom, including right and left leg hip, knee, and ankle flexion, actuated by eight muscles per leg. Fixed sets of synergies constrained control, forcing the direct collocation algorithm to solve for synergy activations. The objective function minimized deviations from unimpaired kinematics and the sum of muscle activations squared (neural demand). Weakness, simulated by a reduction in maximum isometric force, and contracture, simulated by a reduction in tendon slack length, were progressively increased for each muscle or muscle group until the simulation failed to replicate unimpaired gait. Kinematic deviations and convergence determined the success of the simulation. The primary outcomes were (1) musculoskeletal impairment thresholds, defined by the amount of weakness or contracture before failure, and (2) neural demand of each gait cycle.Aim: In this study, we use a two-dimensional musculoskeletal model constrained to synergy control to simulate unimpaired gait.

Methods: We vary the number of synergies, while simulating muscle weakness and contracture to examine how altered control impacts sensitivity to musculoskeletal impairment while tracking unimpaired gait.

Results: Results demonstrate that reducing the number of synergies increases sensitivity to weakness and contracture for specific muscle groups. For example, simulations using five-synergy control tolerated 40% and 51% more knee extensor weakness than those using four- or three-synergy control, respectively. Furthermore, when constrained to four- or three-synergy control, the model was increasingly sensitive to contracture and weakness of proximal muscles, such as the hamstring and hip flexors. Contrastingly, neither the amount of generalized nor plantarflexor weakness tolerated was affected by the number of synergies.

Interpretation: These findings highlight the interactions between altered control and musculoskeletal impairments, emphasizing the importance of measuring and incorporating both in future simulation and experimental studies.

2020 Center for Translational Muscle Research

Dr. Steele MotorControl, Orthoses, Outreach, Presentation , , , , ,

How can we decipher human movement?

CTMR: White text on purple background, UW Center for Translational Muscle ResearchOur skeletal muscles have amazing structure. They provide elegant and efficient actuation to move and explore our worlds. But how do we understand how muscles produce movement?

Dr. Steele presents at the inaugural research symposium for the University of Washington Center for Translational Muscle Research. Her presentation shares examples for how we can use musculoskeletal simulation as a tool to connect muscle biology, dynamics, and mobility.

Slides | Transcript

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

Dr. Steele JournalArticle, Orthoses, Publications , , , , ,

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

Dr. Steele JournalArticle, Orthoses , , ,
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?

Example of gastrocnemius operating length from one subject with different AFOs.Background: Many individuals with cerebral palsy wear ankle foot orthoses during daily life. Orthoses influence joint motion, but how they impact muscle remains unclear. In particular, the gastrocnemius is commonly stiff in cerebral palsy. Understanding whether orthoses stretch or shorten this muscle during daily life may inform orthosis design and rehabilitation.

Objectives: This study investigated the impact of different ankle foot orthoses on gastrocnemius operating length during walking in children with cerebral palsy.

Study design: Case series, within subject comparison of gastrocnemius operating length while walking barefoot and with two types of ankle foot orthoses.

Methods: We performed gait analyses for 11 children with cerebral palsy. Each child was fit with two types of orthoses: a dynamic ankle foot orthosis (Cascade dynamic ankle foot orthosis) and an adjustable dynamic response ankle foot orthosis (Ultraflex ankle foot orthosis). Musculoskeletal modeling was used to quantify gastrocnemius musculotendon operating length and velocity with each orthosis.

Results: Walking with ankle foot orthoses could stretch the gastrocnemius more than barefoot walking for some individuals; however, there was significant variability between participants and orthoses. At least one type of orthosis stretched the gastrocnemius during walking for 4/6 and 3/5 of the Gross Motor Functional Classification System Level I and III participants, respectively. AFOs also reduced peak gastrocnemius lengthening velocity compared to barefoot walking for some participants, with greater reductions among the Gross Motor Functional Classification System Level III participants. Changes in gastrocnemius operating length and lengthening velocity were related to changes in ankle and knee kinematics during gait.

Conclusion: Ankle foot orthoses impact gastrocnemius operating length during walking and, with proper design, may assist with stretching tight muscles in daily life.

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.

Dr. Steele Abstract, MotorControl , ,

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.