Non-uniform distribution of passive muscle stiffness within hamstring

Area of Science:

• Biomechanics
• Musculoskeletal Physiology
• Sports Medicine

Background:

• Limited understanding exists regarding the differential stiffness within and between hamstring muscle constituents (biceps femoris long head, semitendinosus, semimembranosus).
• This knowledge gap hinders effective comprehension of hamstring injuries and the impact of stretching interventions on hamstring stiffness.

Purpose of the Study:

• To determine if passive muscle stiffness varies between the biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) muscles.
• To investigate differences in passive muscle stiffness at proximal, middle, and distal sites within each hamstring muscle.
• To examine the effect of static stretching on the heterogeneity of passive muscle stiffness.

Main Methods:

• Ultrasound shear wave elastography was employed to measure passive muscle shear modulus (a proxy for stiffness).
• Measurements were taken at proximal, middle, and distal sites of the BFlh, ST, and SM in lengthened positions.
• Passive muscle shear modulus of the semimembranosus (SM) was measured before and after static stretching protocols.

Main Results:

• The semimembranosus (SM) exhibited significantly higher shear modulus compared to the biceps femoris long head (BFlh) and semitendinosus (ST).
• Stiffness was consistently higher at the distal site compared to the proximal site across all measured hamstring muscles.
• Post-stretching, the SM maintained significantly greater distal stiffness relative to proximal stiffness (+81% vs. +80% pre-stretching).

Conclusions:

• Passive muscle stiffness is heterogeneous within the hamstring muscles, irrespective of stretching.
• The observed stiffness variations, particularly the distal emphasis in the SM, challenge the notion that stiffness is highest proximally where typical strain injuries occur.