Posture-dependent neuromuscular contributions to three-dimensional isometric shoulder torque generation.

Area of Science:

• Biomechanics
• Neuroscience
• Human Movement Science

Background:

• Activities of daily living rely on coordinated shoulder muscle activation for generating 3D torques.
• Shoulder posture influences individual muscle contributions to torque, but how muscle coordination changes with posture is not well understood.

Purpose of the Study:

• To investigate how neuromuscular coordination for 3D shoulder function changes across different arm postures.
• To characterize variations in muscle synergies during isometric shoulder torque generation.

Main Methods:

• Surface electromyography recorded activations from 16 shoulder muscles in 12 healthy participants.
• Participants generated eight 3D isometric shoulder torques across 20 different arm postures.
• Nonnegative matrix factorization identified muscle synergies, with similarity indices and mixed-effects models analyzing changes.

Main Results:

• Three distinct muscle synergies were identified as fundamental to healthy 3D shoulder function.
• The overall structure of these synergies remained consistent across various shoulder postures.
• The contributions of specific muscles within two synergies were significantly modulated by shoulder elevation and plane of elevation angles.

Conclusions:

• Healthy 3D shoulder function is governed by three stable muscle synergies whose fundamental structure is preserved across postures.
• Neuromuscular control adapts by adjusting individual muscle weights within these synergies based on arm position.
• Findings offer insights for developing targeted rehabilitation strategies for shoulder dysfunction.