Reduced Active Muscle Stiffness after Intermittent Submaximal Isometric Contractions

Area of Science:

• Biomechanics
• Exercise Physiology
• Musculoskeletal Research

Background:

• Muscle stiffness is a key factor in force transmission and injury risk.
• Traditional methods assess joint-level stiffness, potentially missing localized muscle changes due to fatigue.
• Ultrasound shear wave elastography offers a method to measure muscle-specific stiffness.

Purpose of the Study:

• To investigate the influence of fatigue on active muscle stiffness.
• To determine if intermittent submaximal isometric contractions alter muscle shear modulus.
• To utilize ultrasound shear wave elastography for precise stiffness measurements.

Main Methods:

• Participants (n=19) underwent 15 minutes of intermittent submaximal isometric contractions (4s ON, 4s OFF at 60% MVC).
• Muscle shear modulus was measured on vastus lateralis or abductor digiti minimi pre- and post-fatigue.
• Active muscle stiffness was calculated as the slope of shear modulus vs. joint force.

Main Results:

• Maximal voluntary contraction (MVC) significantly decreased post-fatigue (22-32%).
• Active muscle stiffness decreased significantly post-fatigue: 12% in vastus lateralis and 44% in abductor digiti minimi.
• The abductor digiti minimi showed a more pronounced reduction in stiffness compared to the vastus lateralis.

Conclusions:

• Intermittent submaximal isometric contractions lead to decreased active muscle stiffness.
• The observed decrease in muscle stiffness suggests a potential mechanism influencing performance and injury risk.
• Further research is needed to elucidate the mechanisms and implications of fatigue-induced stiffness changes.