Layer-specific stiffness dynamics in supraspinatus and trapezius muscles during a sustained isometric contraction

This study assessed layer-specific changes in muscle stiffness within the supraspinatus muscle, upper trapezius, and their fascia during sustained isometric shoulder abduction, thereby characterizing regional tissue adaptation over time. Twenty asymptomatic participants performed sustained 90° shoulder abduction (isometric contraction) with a 1-kg load until task failure. Stiffness (kPa) was continuously assessed using shear wave elastography in the superficial and deep layers of the supraspinatus, upper trapezius, and fascia, and values were normalized to the percentage of time to task failure. All tissues demonstrated significant changes in muscle and fascial stiffness over time. Peak muscle stiffness occurred at 60–70% of time to task failure in the supraspinatus and at 70–80% in the trapezius and fascia. In the supraspinatus, both superficial and deep layers showed a significant increase in muscle stiffness over time followed by a decline near task failure ( p ≤ 0.001). Throughout most of the contraction, the superficial layer consistently exhibited higher muscle stiffness than the deep layer ( p ≤ 0.001). This study revealed that tissue stiffness adapts dynamically during sustained isometric shoulder abduction, with supraspinatus muscle stiffness peaking earlier than that of the trapezius and fascia. Further, the consistently higher muscle stiffness in the superficial compared to the deep supraspinatus layer highlights compartment-specific mechanical responses across muscle and fascial layers during fatiguing isometric contraction.