Regional Differences in Biceps Femoris Long Head Stiffness during Isometric Knee Flexion

Area of Science:

• Biomechanics
• Muscle Physiology
• Sports Medicine

Background:

• The biceps femoris long head is crucial for knee flexion.
• Muscle injuries are common in this hamstring muscle, often in the proximal region.
• Understanding active muscle stiffness heterogeneity is vital for injury prevention and rehabilitation.

Purpose of the Study:

• To investigate regional differences in biceps femoris long head active stiffness.
• To examine the influence of contraction intensity and muscle length on this stiffness.
• To determine the proximal-to-distal stiffness ratio during isometric knee flexion.

Main Methods:

• Twelve healthy participants underwent isometric knee flexion at varying intensities (20% and 60% MVC) and knee flexion angles (15° and 45°).
• Ultrasound-based shear wave elastography assessed muscle stiffness in proximal and distal regions of the biceps femoris long head.
• The proximal-to-distal stiffness ratio was calculated to evaluate heterogeneity.

Main Results:

• Biceps femoris long head demonstrated greater stiffness in the distal region, at higher contraction intensities, and longer muscle lengths.
• A significantly lower proximal-to-distal stiffness ratio (heterogeneity) was observed at lower contraction intensity.
• This heterogeneity was not present at higher contraction intensities.

Conclusions:

• This study is the first to demonstrate active stiffness heterogeneity in the biceps femoris long head.
• Results suggest that muscle stiffness varies regionally and is influenced by contraction intensity and muscle length.
• Caution is advised when interpreting muscle stiffness studies that assess only a single muscle region, especially concerning injury risk in the proximal hamstring.