Contribution of stroke-related changes in neuromuscular factors to gear ratio during isometric contraction of medial gastrocnemius muscle: A simulation study

  • Jongsang Son
    Corresponding author at: 323 Dr Martin Luther King Jr Blvd, Newark, NJ 07102, United States.
    Department of Biomedical Engineering, Newark College of Engineering, New Jersey Institute of Technology, Newark, NJ, United States
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  • William Z. Rymer
    Shirley Ryan AbilityLab (formerly the Rehabilitation Institute of Chicago), Chicago, IL, United States

    Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States
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      • Gear ratio decreases significantly with smaller pennation angle and shorter optimal fiber length.
      • Gear ratio may be primarily determined by initial pennation angle and fiber rotationshortening ratio.
      • Pennation angle may play an important role for efficient muscular contraction.



      It is not clear which neuromuscular factors are most closely associated with the loss of variable fascicle gearing after chronic stroke. The purpose of this simulation study is to determine the effects of stroke-related changes in key neuromuscular factors on the gear ratio.


      A modified Hill-type model of the medial gastrocnemius was developed to determine the gear ratio for a given muscle activation level and musculotendon length. Model parameters were then systematically adjusted to simulate known stroke-related changes in neuromuscular factors, and the gear ratio was computed for each change in the parameters. A Monte Carlo simulation was performed to understand which neuromuscular factors and fiber behavior-related parameters are most relevant to the loss of variable gearing. Dominance analyses were also conducted to quantify the relative importance of fiber behavior-related parameters on the gear ratio.


      The gear ratio decreases significantly with smaller pennation angle and with shorter optimal fiber length. In addition, muscle thickness and pennation angle at optimal fiber length appear to be the most important muscle architectural parameters. Dominance analyses further suggest that primary determinants of gear ratio include initial pennation angle, fiber rotation-shortening ratio, initial muscle thickness, and fiber rotation.


      Our findings provide insight that the pennation angle may play an important role for efficient muscular contraction, implying that maintaining muscle architecture and/or improving fiber/fascicle rotation could a key goal in rehabilitation interventions. Our findings will help us to better interpret altered gearing behavior in aging and pathological muscles.


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