Quantification of muscle co-contraction using supersonic shear wave imaging

Area of Science:

• Biomechanics
• Musculoskeletal System
• Medical Imaging

Background:

• Muscle stiffness, estimated via shear wave elastography, can indicate muscle force during isometric contractions.
• Quantifying co-contraction is crucial for understanding muscle function and dysfunction.

Purpose of the Study:

• To investigate the relationship between muscle stiffness and myoelectrical activity in the lateral gastrocnemius (LG) muscle during isometric plantar flexion and dorsiflexion.
• To differentiate active muscle force from passive tension using shear wave elastography and surface electromyography (sEMG).

Main Methods:

• Supersonic shear wave imaging was used to estimate the shear modulus of the LG muscle.
• Surface electromyography (sEMG) measured myoelectrical activity during graded isometric contractions.
• Passive muscle tension was assessed by measuring the LG muscle shear modulus during passive ankle rotation.

Main Results:

• The shear modulus was significantly lower during dorsiflexion compared to plantar flexion at similar sEMG levels.
• Passive tension, estimated from passive ankle rotation, significantly increased from plantar flexion to dorsiflexion.
• After accounting for passive tension, the active muscle shear modulus was significantly greater during plantar flexion than dorsiflexion.

Conclusions:

• Negligible active force contribution by the LG muscle during isometric dorsiflexion was indicated by a negligible shear modulus, despite recorded sEMG activity.
• sEMG activity during isometric dorsiflexion was likely due to cross-talk, not active LG muscle contraction.
• Changes in passive muscle tension can contribute to joint torque during isometric dorsiflexion.