Evidence for intermuscle difference in slack angle in human triceps surae

Area of Science:

• Biomechanics
• Human Physiology
• Musculoskeletal System

Background:

• The triceps surae muscle group, comprising the gastrocnemius and soleus, is crucial for plantarflexion.
• Understanding the in vivo properties of these synergists is essential for biomechanical analysis and clinical applications.
• Previous research has not fully elucidated the differences in slack angle among these synergistic muscles.

Purpose of the Study:

• To investigate and compare the slack angles of the medial gastrocnemius, lateral gastrocnemius, and soleus muscles in healthy adults.
• To determine if the joint angle at which passive muscle force generation begins varies among these synergists.

Main Methods:

• Ultrasound shear wave elastography was employed to measure the shear modulus of each triceps surae muscle.
• Measurements were taken during passive stretching of the ankle joint in healthy subjects.
• Slack angle was identified as the ankle joint angle at which shear modulus began to increase during stretching.

Main Results:

• A significant difference in slack angle was observed among the triceps surae synergists.
• The medial gastrocnemius exhibited the most plantarflexed slack angle (20.7±6.7°).
• The lateral gastrocnemius (14.9±6.7°) and soleus (2.0±4.8°) had progressively more dorsiflexed slack angles.

Conclusions:

• The study provides evidence that the slack angle varies across the medial gastrocnemius, lateral gastrocnemius, and soleus muscles.
• These differences suggest distinct functional roles in passive force production during ankle movements.
• The medial gastrocnemius generates passive force at more plantarflexed positions compared to the soleus.