Quantifying Active and Passive Stiffness in Plantar Flexor Muscles Following Intermittent Maximal Isometric...

Area of Science:

• Biomechanics
• Exercise Physiology
• Musculoskeletal Adaptations

Background:

• Muscle fatigue significantly alters neuromuscular function and mechanical properties.
• Understanding changes in muscle stiffness post-fatigue is crucial for injury prevention.

Purpose of the Study:

• To assess the impact of isometric exercise-induced fatigue on plantar flexor (PF) active and passive stiffness.
• To explore the relationship between mechanical parameter changes and neuromuscular alterations after fatigue.

Main Methods:

• Healthy participants performed maximal isometric contractions to induce fatigue.
• Plantar flexor tendon complex (PFC) stiffness, gastrocnemius medialis (GM) active/passive stiffness, and PF torque/rate of torque development (RFD) were measured.
• Electromyography (EMG) recorded muscle activity during assessments.

Main Results:

• Fatigue led to decreased active stiffness, peak torque, RFD, and GM EMG activity (p < 0.001).
• A significant correlation was found between reduced PF peak torque and GM active stiffness (r = 0.6; p < 0.05).
• Both PFC stiffness and GM shear modulus decreased significantly (p < 0.001).

Conclusions:

• Isometric fatiguing exercises alter mechanical properties of muscle contractile and elastic elements.
• Reduced passive and active stiffness post-fatigue may elevate injury risk in athletes.