Fatiguing exercise reduces cellular passive Young's modulus in human vastus lateralis muscle

Area of Science:

• Skeletal Muscle Physiology
• Exercise Science
• Biochemistry

Background:

• Acute fatiguing exercise is known to decrease whole-muscle stiffness.
• The underlying cellular mechanisms and potential sex differences in this response are not well understood.
• Reduced muscle stiffness may increase injury risk and impair performance.

Purpose of the Study:

• To investigate the intracellular mechanisms responsible for reduced muscle stiffness after fatiguing exercise.
• To determine if these effects differ between males and females at the cellular level.
• To examine the role of titin phosphorylation in mediating changes in muscle stiffness.

Main Methods:

• Muscle biopsies were obtained from healthy males and females after maximal voluntary knee extensions to task failure.
• Cellular passive Young's modulus was measured in muscle fibers using mechanical stretching.
• Titin phosphorylation was analyzed using mass spectrometry.

Main Results:

• A significant reduction in passive Young's modulus was observed in fatigued muscle fibers from males, but not females.
• Fatiguing exercise altered the phosphorylation status of titin in multiple serine residues, particularly within its elastic region.
• No significant changes were found in active tension or sarcomere ultrastructure post-fatigue.

Conclusions:

• Acute fatigue induces sex-dependent reductions in skeletal muscle cellular stiffness, primarily in males.
• Altered titin phosphorylation is a key intracellular mechanism contributing to fatigue-induced changes in muscle compliance.
• These findings provide mechanistic insights into sex-specific differences in soft-tissue injury risk and muscle function.