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

Area of Science:

• Biomechanics
• Neuromuscular Physiology
• Sports Medicine

Background:

• Repeated upper limb activities can fatigue shoulder muscles, potentially altering movement mechanics and increasing subacromial impingement syndrome (SIS) risk.
• Understanding neuromuscular changes during fatigue is crucial for injury prevention, yet specific patterns across movement phases remain unclear.

Purpose of the Study:

• To investigate neuromuscular fatigue patterns in asymptomatic individuals during a repeated arm elevation task to failure.
• To identify how fatigue affects muscle activation and neural drive across different arm elevation ranges.

Main Methods:

• Participants performed repeated dominant arm elevations with ~30% maximal isometric shoulder flexion load until task failure.
• High-density surface electromyography (HD-sEMG) and bipolar sEMG recorded muscle activity from key upper limb muscles.
• Root mean square (RMS) and mean power frequency (MPF) were analyzed in bottom, middle, and top arm elevation ranges at baseline and failure.

Main Results:

• At task failure, increased sEMG amplitude (RMS) and/or decreased MPF were observed in multiple muscles (PM, AD, BB, SA, LD, IN, UT, MT).
• These fatigue-related changes were most prominent in the middle and top ranges of arm elevation.
• Neuromuscular fatigue was significantly dependent on the range of motion.

Conclusions:

• Muscle function is altered in specific arm elevation ranges, coinciding with the ranges where SIS risk is elevated.
• These findings provide a potential neuromuscular basis for fatigue-related upper limb injuries.
• Future research should explore advanced techniques to understand motor unit recruitment and force production changes related to fatigue and injury risk.