Changes in hamstrings' active stiffness during fatigue tasks are modulated by contraction duration rather than...

Area of Science:

• Biomechanics
• Musculoskeletal Physiology
• Sports Science

Background:

• Ultrasound shear wave elastography is increasingly used to study hamstring stiffness.
• Limited research exists on the active stiffness of biceps femoris long head (BFlh) and semitendinosus (ST) muscles during fatigue at different contraction intensities.

Purpose of the Study:

• To compare the effects of knee flexor isometric contractions to exhaustion at 20% versus 40% of maximal voluntary isometric contraction (MVIC) on the active stiffness of the BFlh and ST muscles.

Main Methods:

• Eighteen recreationally active males participated in two experimental sessions.
• Participants performed isometric knee flexor contractions to exhaustion at either 20% or 40% MVIC.
• Active muscle stiffness of BFlh and ST was measured using shear wave elastography.

Main Results:

• At 20% MVIC, BFlh active stiffness was unchanged, while ST active stiffness significantly decreased. At 40% MVIC, neither BFlh nor ST showed significant intramuscular stiffness changes.
• Intermuscular active stiffness differences persisted longer at 20% MVIC (64% contraction time) compared to 40% MVIC (33% contraction time).
• The BFlh/ST stiffness ratio increased more steeply during the 20% MVIC fatigue task than the 40% MVIC task.

Conclusions:

• Contraction duration appears to be a more critical factor than contraction intensity in altering hamstring mechanical properties during fatigue.
• These findings contribute to understanding muscle fatigue mechanisms and may inform training and rehabilitation strategies for hamstring injuries.