Task-dependent force sharing between muscle synergists during locomotion in turkeys

Area of Science:

• Biomechanics
• Animal Locomotion
• Muscle Physiology

Background:

• Joints exhibit muscle redundancy, allowing multiple solutions for force sharing among synergists.
• Understanding muscle force sharing offers insights into movement mechanics and control.
• The gastrocnemius muscle in wild turkeys presents a model for studying muscle force sharing.

Purpose of the Study:

• To investigate how force is shared between the lateral (LG) and medial (MG) gastrocnemius heads in wild turkeys.
• To determine if force sharing patterns vary with different mechanical tasks, specifically running speed and limb loading.
• To elucidate the role of muscle synergy in locomotion.

Main Methods:

• Strain gauges were attached to gastrocnemius tendons to measure force in LG and MG heads separately.
• Inverse dynamics were employed to calculate the total required force during the swing phase.
• Running speeds (1-3.5 m/s) and added limb loads (30g, 60g) were manipulated to alter mechanical demands.

Main Results:

• Force distribution between LG and MG varied significantly with the stride phase.
• During stance, force was shared almost equally between LG and MG, unaffected by speed or load.
• During swing, only the LG produced force, which matched the total required force, indicating no sharing.

Conclusions:

• Muscle force sharing between gastrocnemius heads is task-dependent, changing between stance and swing phases.
• The lateral gastrocnemius exclusively generates force for limb extension during the swing phase.
• Findings support the theory that muscle synergist recruitment is adapted to specific mechanical requirements.