Inevitable joint angular rotation affects muscle architecture during isometric contraction.

Area of Science:

• Biomechanics
• Human Physiology
• Musculoskeletal System

Background:

• Accurate in vivo measurement of skeletal muscle architecture is essential for understanding muscle function.
• Isometric contractions are commonly used to study muscle mechanics, but joint motion can introduce confounding factors.
• The gastrocnemius medialis (GM) muscle's architecture changes during contraction, influencing force production.

Purpose of the Study:

• To quantify the effect of ankle joint motion on gastrocnemius medialis (GM) muscle architecture changes during isometric contractions.
• To determine if ankle joint motion influences the loading and unloading phases of isometric contractions differently.
• To assess the reliability of muscle architecture measurements without accounting for ankle joint dynamics.

Main Methods:

• Subjects performed isometric maximal voluntary contractions and electrostimulated contractions on a dynamometer.
• Real-time ultrasonography captured GM muscle architecture (fascicle length, pennation angle).
• Synchronous data acquisition included ankle joint angle, plantarflexion moment, and center of pressure.

Main Results:

• Ankle joint motion significantly altered measured changes in GM muscle architecture.
• At maximal tendon force, fascicle length was overestimated by 1.53 cm and pennation angle by 5.5 degrees due to ankle motion.
• After correcting for ankle joint angle, no differences were observed between loading and unloading phases at equivalent tendon forces.

Conclusions:

• Ankle joint motion is a critical confounder in measuring GM muscle architecture during isometric contractions.
• Failure to account for ankle joint motion leads to unreliable fascicle length-force and pennation angle-force relationships.
• Accurate assessment of muscle architecture requires incorporating joint angle compensation during dynamic isometric tasks.