Triceps surae muscle force potential and force demand shift with altering stride frequency in running

Area of Science:

• Biomechanics
• Human Physiology
• Sports Science

Background:

• Preferred stride frequency (PSF) in running correlates with metabolic efficiency, but the mechanisms remain unclear.
• Altering stride frequency impacts joint kinematics, muscle-tendon unit lengths, and muscle efficiency.

Purpose of the Study:

• To investigate how fascicle kinematics and forces of the triceps surae muscle change at different running stride frequencies.
• To understand the relationship between stride frequency, muscle mechanics, and whole-body energy expenditure.

Main Methods:

• Twelve runners performed running trials at five different stride frequencies (PSF; PSF ± 8%; PSF ± 15%) on a force-measuring treadmill.
• Measurements included joint kinematics, whole-body energy expenditure, triceps surae muscle activity, and soleus (SOL) and gastrocnemius medialis (GM) fascicle kinematics.
• Dynamic optimization was used to estimate SOL and GM muscle forces.

Main Results:

• SOL and GM mean fascicle length during stance showed an inverted U-relationship with PSF, being longest at PSF and shortest at lower frequencies.
• Average SOL force was higher at PSF-15% compared to PSF.
• Triceps surae muscle kinematics and force production were less affected at higher stride frequencies.

Conclusions:

• Reduced SOL and GM fascicle lengths at lower-than-PSF frequencies decrease muscle force potential, contributing to higher energy expenditure.
• Increased energy expenditure at higher stride frequencies is likely due to greater leg swing costs and force production demands.
• These findings elucidate the complex interplay between stride frequency, muscle mechanics, and energetic costs in running.