Muscle prestimulation tunes velocity preflex in simulated perturbed hopping

Area of Science:

• Biomechanics
• Human Movement Science
• Muscle Physiology

Background:

• Muscle fibers exhibit visco-elastic properties enabling immediate stabilization against perturbations.
• This rapid response, termed "preflex," bypasses neural transmission delays crucial for locomotion.
• The elastic component of preflex is well-understood, but the role of muscle viscosity remains unclear.

Purpose of the Study:

• To develop a novel method for isolating and quantifying preflex force generated by the muscle force-velocity relation.
• To analyze the contribution of muscle viscosity to preflex responses during simulated vertical hopping perturbations.

Main Methods:

• Utilized musculoskeletal computer simulations to model muscle responses.
• Analyzed the preflex phase (first 30 ms post-impact) of simulated vertical hopping.
• Quantified muscle force, energy dissipation, and the impact of varying stimulation patterns.

Main Results:

• Muscle force and energy dissipation increased with perturbation height, aiding rejection.
• Constant muscle stimulation resulted in only 15% rejection of step-down perturbation energy.
• Open-loop rising stimulation, mimicking experimental observations, increased energy rejection to 68% via the force-velocity relation.

Conclusions:

• Muscle fiber viscosity, through the force-velocity relation, plays a significant role in preflex responses.
• Open-loop neuronal control of muscle activity around impact optimizes viscous capacity.
• This feed-forward mechanism allows muscles to effectively adjust energy handling during unexpected perturbations in locomotion.