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Normalizing upper trapezius EMG amplitude: Comparison of ramp and constant force procedures
M Attebrant1, S E Mathiassen, J Winkel
1National Institute of Occupational Health, Division of Applied Work Physiology, S-171 84 Solna, Sweden.
Summary
This study compared three upper trapezius surface electromyography (EMG) normalization methods. The two-force procedure showed a more apparent ergonomic effect than the reference voluntary electrical activity (RVE) method due to less inter-individual variation.
Area of Science:
- Biomechanics
- Ergonomics
- Neuroscience
Background:
- Surface electromyography (EMG) normalization is crucial for comparing muscle activity across individuals and conditions.
- Existing normalization methods, including ramp, two-force, and reference voluntary electrical activity (RVE), have varying degrees of precision and repeatability.
- Understanding the impact of different normalization techniques on ergonomic assessments is essential for accurate work-related injury prevention.
Purpose of the Study:
- To compare three upper trapezius EMG normalization procedures: ramp, two-force, and RVE.
- To evaluate the repeatability of the ramp and RVE procedures.
- To assess the ergonomic implications of using the two-force versus RVE normalization methods in work tasks.
Main Methods:
- Fifteen subjects performed maximal voluntary contractions (MVC) for normalization.
- EMG data were normalized using ramp (%MVC), two-force (%MVC), and RVE (%RVE) procedures.
- Repeatability was assessed using coefficient of variation (CV).
- Trapezius EMG during two work tasks was compared using two-force and RVE normalization.
Main Results:
- A conversion equation between ramp and two-force procedures was established (%MVC(2force) = - 0.6 + 0.9 *%MVC(ramp)), but with significant inter-individual imprecision.
- The ramp procedure showed high intra-individual CV at low force levels (45% at 5% MVC) but improved at higher levels (10% at 30% MVC).
- The RVE procedure had a CV of 15%, with reference contractions varying widely (median 33% MVC).
- The ergonomic intervention's load-reducing effect was less evident with RVE normalization compared to the two-force procedure due to greater inter-individual variation.
Conclusions:
- The choice of EMG normalization method impacts the interpretation of ergonomic interventions.
- The two-force procedure may offer a more reliable assessment of ergonomic effects compared to the RVE procedure, particularly when large inter-individual differences exist.
- Further research is needed to optimize normalization strategies for diverse populations and tasks.
