Estimating contraction level using root mean square amplitude in control subject

[Guest guest]

Arch Phys Med Rehabil. 2008 Apr;89(4):711-8.

Estimating contraction level using root mean square amplitude in

control subjects and patients with neuromuscular disorders.

Boe SG, Rice CL, Doherty TJ.

School of Kinesiology, University of Western Ontario, London, ON,

Canada.

OBJECTIVES: To assess the utility of the surface electromyographic

signal as a means of estimating the level of muscle force during

quantitative electromyography studies by examining the relationship

between muscle force and the amplitude of the surface

electromyographic activity signal; and to determine the impact of a

reduction in the number of motor units on this relationship, through

inclusion of a sample of patients with neuromuscular disease. DESIGN:

Cross-sectional, cohort study design.

SETTING: Tertiary care, ambulatory, electromyography laboratory.

PARTICIPANTS: A volunteer, convenience sample of healthy control

subjects (n=10), patients with amyotrophic lateral sclerosis (n=9),

and patients with Charcot-Marie-Tooth disease type X (n=5).

INTERVENTIONS: Not applicable.

MAIN OUTCOME MEASURES: The first dorsal interosseous (FDI) and biceps

brachii muscles were examined. Force values (at 10% increments) were

calculated from two 4-second maximal voluntary contractions (MVCs).

Surface electromyographic activity was recorded during separate 4-

second voluntary contractions at 9 force increments (10% -90% of

MVC). Additionally, a motor unit number estimate was derived for each

subject to quantify the degree of motor unit loss in patients

relative to control subjects.

RESULTS: The relationships between force and surface

electromyographic activity for both muscles (controls and patients)

were best fit by a linear function. The variability about the grouped

regression lines was quantified by 95% confidence intervals and found

to be +/-6.7% (controls) and +/-8.5% (patients) for the FDI and +/-5%

(controls) and +/-6.1% (patients) for the biceps brachii.

CONCLUSIONS: These results suggest that the amplitude of the surface

electromyographic activity signal may be used as a means of

estimating the level of muscle force during quantitative

electromyography studies. Future studies should be directed at

examining if the variability associated with these force and surface

electromyographic activity relationships is acceptable in replacing

previous methods of measuring muscle force.