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Fatigue in the shoulder muscles during static work at two different torque levels
1Department of Physical Medicine and Rehabilitation, University of Umeå, Sweden.
Clinical Physiology (Oxford, England)
|September 1, 1993
Summary
This study found that the mean power frequency (MPF) shift in electromyography (EMG) may not accurately reflect peripheral fatigue in shoulder muscles during low-torque endurance tests. Muscle fatigue perception was higher at low torque, but MPF changes were less pronounced.
Area of Science:
- Exercise Physiology
- Neuromuscular Physiology
- Biomedical Engineering
Background:
- The mean power frequency (MPF) shift in electromyography (EMG) is commonly used to assess peripheral fatigue.
- Understanding the relationship between MPF shifts and muscle fiber type activation is crucial for accurate fatigue monitoring.
- Low-torque endurance activities involve different muscle fiber recruitment patterns compared to high-torque activities.
Purpose of the Study:
- To investigate if and when MPF shifts occur during maximal endurance tests of shoulder flexors at low torque levels.
- To compare EMG-derived fatigue measures (MPF and RMS) with perceived fatigue at high (50% MVC) and low (18% MVC) torque levels.
- To determine the utility of MPF shifts in monitoring peripheral fatigue, particularly concerning type-1 and type-2 muscle fibers.
Main Methods:
- Twelve healthy women performed maximal endurance tests of shoulder flexors at 50% MVC and 18% MVC until exhaustion.
- Surface EMG signals were recorded from the trapezius descendens, anterior deltoid, and infraspinatus muscles.
- MPF and RMS of EMG signals were computed, and perceived fatigue was rated on a 10-graded scale throughout the tests.
Main Results:
- Perceived fatigue was significantly higher at the low torque level (18% MVC) compared to the high torque level (50% MVC).
- MPF showed significantly lower values at the end of endurance tests at the high torque level versus the low torque level.
- At low torque, MPF remained constant in the trapezius, while the deltoid showed a decrease mainly in the initial 30-40% of the test duration.
Conclusions:
- The MPF shift is suggested to primarily reflect peripheral fatigue in type-2 muscle fibers.
- The findings question the reliability of using MPF shifts to monitor peripheral fatigue in predominantly type-1 fibers active during low-torque contractions.
- Different mechanisms of fatigue may be involved in low- versus high-torque endurance activities, impacting EMG-based assessments.