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Activity patterns of wrist extensor muscles during wrist extensions and deviations
L Finsen1, K Søgaard, T Graven-Nielsen
1Department of Physiology, National Institute of Occupational Health, Lersø Parkalle 105, DK-2100 Copenhagen, Denmark.
Muscle & Nerve
|November 16, 2004
Summary
Muscle activation patterns in wrist extensors like the extensor carpi radialis (ECR) and extensor carpi ulnaris (ECU) may influence musculoskeletal disorders. Higher ECR activity in pronation, not detected by surface EMG, could link to lateral epicondylitis.
Area of Science:
- Biomechanics
- Musculoskeletal Disorders
- Electromyography
Background:
- Wrist extensor muscles, particularly the extensor carpi radialis (ECR) and extensor carpi ulnaris (ECU), are susceptible to focal musculoskeletal disorders.
- Activation patterns of ECR and ECU muscles are potential risk factors for these conditions.
Purpose of the Study:
- To investigate the electromyographic (EMG) activation patterns of ECR and ECU muscles during isometric wrist extension.
- To compare muscle activity in different forearm positions (semipronation and pronation) and during wrist deviation.
- To explore the relationship between muscle activation and the risk of lateral epicondylitis.
Main Methods:
- Surface and intramuscular EMG recordings were obtained from ECR and ECU muscles.
- Isometric low-force wrist extension and deviation tasks were performed in semipronation and pronation.
- EMG data were analyzed using root mean square (RMS) and motor unit action potential (MUAP) decomposition methods.
Main Results:
- Intramuscular EMG and MUAP amplitudes were higher in pronation than semipronation, despite shorter ECR muscle length in semipronation.
- These pronation-related changes in muscle activity were not detectable using surface EMG.
- ECR exhibited higher activity during wrist extension compared to ulnar/radial deviation.
- Significant differences in motor unit properties were observed between ulnar deviation, radial deviation, and wrist extension.
- ECR MUAPs were approximately twice as large as ECU MUAPs.
- ECR muscle activity levels were generally higher than predicted by biomechanical models.
Conclusions:
- The ECR muscle's activation pattern in pronation may not align with simple biomechanical predictions, potentially contributing to its higher-than-expected activity.
- The discrepancy between intramuscular and surface EMG findings highlights limitations in surface recordings for assessing deep muscle activation.
- Elevated ECR activity, particularly in pronation, may be a contributing factor to the common association of ECR tendon issues with lateral epicondylitis.