Understanding muscle function during perturbed in vivo locomotion using a muscle avatar approach

Area of Science:

• Biomechanics
• Muscle Physiology

Background:

• The work loop technique is crucial for understanding muscle function during locomotion.
• Existing ex vivo methods struggle to replicate the complex, variable strain rates of in vivo muscle activity.

Purpose of the Study:

• To develop and validate an 'avatar' approach for ex vivo muscle studies.
• To investigate guinea fowl lateral gastrocnemius (LG) muscle mechanics during unsteady running using mouse extensor digitorum longus (EDL) ex vivo.

Main Methods:

• Ex vivo work loop experiments on mouse EDL muscles.
• Replication of in vivo strain trajectories from guinea fowl LG during various running conditions (obstacle perturbations, steady running).
• Comparison of forces generated using in vivo strain trajectories versus sinusoidal trajectories.

Main Results:

• Ex vivo forces using in vivo strain trajectories closely matched in vivo LG forces (R2=0.58-0.94).
• Sinusoidal trajectories yielded significantly less accurate force predictions (average R2=0.045).
• In vivo strain patterns revealed shifts in muscle work output based on movement direction (e.g., ascending vs. descending obstacles).

Conclusions:

• The avatar approach effectively simulates in vivo muscle mechanics ex vivo.
• Muscle activation tunes viscoelastic properties, influencing force production under time-varying loads.
• This method enhances the study of muscle dynamics in complex locomotor behaviors.