Task failure reveals range-dependent neuromuscular fatigue in shoulder muscles during arm elevation

Repeated upper limb performance fatigues periscapular and/or rotator cuff muscles, which may alter humeral and scapular kinematics leading to increased subacromial impingement syndrome risk. However, the specific changes in neuromuscular patterns across different phases of a functional movement performed to failure remain unclear. This study investigated neuromuscular fatigue during a repeated arm elevation task to failure in asymptomatic individuals. Participants elevated their dominant arm while holding ∼ 30 % of their maximal isometric shoulder flexion load until task failure. High-density surface electromyography (HD-sEMG) was recorded from the pectoralis major (PM), biceps brachii (BB), and trapezius (UT, MT, LT) to accurately identify innervation zones and to enhance signal validity, while bipolar sEMG collected from the deltoid (AD, LD, PD), serratus anterior (SA) and infraspinatus (IN). Root mean square (RMS, %MVIC) and mean power frequency (MPF) were analyzed between three ranges of arm elevation (bottom, middle, top) at baseline and failure. During the middle and top ranges of arm elevation at task failure increased sEMG amplitude and/or decreased MPF of the PM, AD, BB, SA, LD, IN, UT, and MT were present. These results reveal that neuromuscular fatigue was highly dependent on the range of motion, with significant interactions showing that fatigue-related increases in RMS and decreases in MPF were most prominent in the middle and top ranges. These findings suggest that muscle function is altered precisely within the ranges where impingement risk is greatest, providing a potential neuromuscular basis for fatigue-related injury. Building off of these results, future work should use advanced techniques like high-density EMG and shear wave elastography to define how these fatigue-related changes in neuromuscular responses influence motor unit recruitment strategies, muscle force production, and subsequent alterations to SAS and injury risk.