Muscle short-range stiffness behaves like a maxwell element, not a spring: Implications for joint stability

Area of Science:

• Biomechanics
• Human Physiology
• Computational Modeling

Background:

• Muscles are vital for joint stability during daily activities, partly due to short-range stiffness (SRS).
• SRS generates greater force when active muscle is stretched beyond its predicted force-length relationship.
• SRS has been theorized as a key mechanism for joint stabilization.

Purpose of the Study:

• To investigate if Huxley-type muscle models exhibiting SRS can stabilize a joint under constant activation.
• To test the hypothesis that SRS alone provides joint stability using forward dynamic simulations.

Main Methods:

• An inverted pendulum model (moment of inertia: 2.7 kg m2) incorporated Huxley-type muscle models.
• Simulations analyzed joint stability against a 5 Nm square-wave perturbation (50 ms duration).
• Muscle forces for stabilization were calculated based on minimizing potential energy.

Main Results:

• The inverted pendulum model demonstrated instability and failed to maintain an upright posture.
• This occurred despite initial predictions of stability through antagonist and agonist muscle co-activation.
• The model remained unstable even with fully activated muscles.

Conclusions:

• Short-range stiffness alone is insufficient for joint stability, even with minor perturbations.
• SRS dynamics differ from a typical spring; modeling it as a Maxwell element (series spring-damper) is more appropriate.
• Damping from SRS may allow central nervous system reaction time, and other factors like reflexes likely contribute to joint stability.