Cyclically producing the same average muscle-tendon force with a smaller duty increases metabolic rate

Area of Science:

• Biomechanics
• Human Physiology
• Muscle Metabolism

Background:

• Ground contact duration and stride frequency influence muscle metabolism and energy expenditure during locomotion.
• These biomechanical parameters are often studied independently, but their product, the duty factor, may impact muscle contractile mechanics.

Purpose of the Study:

• To investigate the distinct metabolic effects of active force production duration, cycle frequency, and duty factor.
• To determine how duty factor influences muscle contractile mechanics and metabolic energy expenditure.

Main Methods:

• Human participants performed cyclic contractions of the soleus muscle at controlled cycle-average ankle moments.
• Ankle moments were produced over varying durations (short, medium, long) while maintaining a constant cycle frequency.
• Measurements included muscle-tendon force, fascicle length, velocity, and net metabolic power.

Main Results:

• A decreased duty factor did not alter cycle-average fascicle force but significantly increased net metabolic power.
• Smaller duty factors led to increased maximum muscle-tendon force, stretching of in-series tendons, and shorter, faster soleus fascicle lengths.
• Increased soleus total active muscle volume was observed with decreased duty factors and strongly correlated with net metabolic power.

Conclusions:

• Cyclically producing the same cycle-average muscle-tendon force with a decreased duty factor increases metabolic energy expenditure.
• This increased expenditure is attributed to less economical muscle contractile mechanics, specifically greater active muscle volume.
• The findings highlight the importance of duty factor in understanding the relationship between locomotion biomechanics and metabolic cost.