Original article
Estimating Contraction Level Using Root Mean Square Amplitude in Control Subjects and Patients With Neuromuscular Disorders

https://doi.org/10.1016/j.apmr.2007.09.047Get rights and content

Abstract

Boe SG, Rice CL, Doherty TJ. Estimating contraction level using root mean square amplitude in control subjects and patients with neuromuscular disorders.

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.

Section snippets

Participants

Nine patients (age, 52±12y) with clinically probable or definite amyotrophic lateral sclerosis (ALS) as defined by the revised El Escorial criteria14 and 10 healthy control subjects (age, 27±4y) volunteered to participate in the study. Three of the 9 ALS patients were unable to perform the FDI portion of the study due to severely atrophied muscles characterized by unrecordable M waves. Due to this limitation, 5 additional patients (age, 37±11y), with Charcot-Marie-Tooth disease type X confirmed

FDI MUNE, MVC, and RMS Amplitude

MUNEs differed significantly between control subjects (n=10) and patients (n=11, Charcot-Marie-Tooth disease type X=5, ALS=6) (table 1). Despite this difference, the initial MVC values were similar between groups (see table 1). The addition of the Charcot-Marie-Tooth disease type X patients did not alter this finding, because the MVC values of both the Charcot-Marie-Tooth disease type X (21.4±10.5N) and the ALS (18.5±11.1N) patients did not differ compared with the control subjects

Discussion

The results of this study show that for these experimental conditions, the individual and grouped force−RMS amplitude relationships are best fit by a simple linear function for both the biceps brachii and FDI muscles. These relationships did not differ in comparing a sample of control subjects and patients with neuromuscular disease, in whom substantial motor unit loss and remodelling had occurred. Similar results were found across groups within each of the muscles studied, with higher levels

Conclusions

For the experimental conditions described for this study, linear force and RMS amplitude relationships were observed for the biceps brachii and FDI muscles in control subjects and patients with neuromuscular disease. These results are inclusive of the relationships derived for subjects and groups, although the establishment of the grouped relationships and their corresponding 95% CIs may have greater clinical applicability in estimating the level of contraction because they express the

References (25)

  • J.J. Woods et al.

    Linear and non-linear surface EMG/force relationships in human musclesAn anatomical/functional argument for the existence of both

    Am J Phys Med

    (1983)
  • J.H. Lawrence et al.

    Myoelectric signal versus force relationship in different human muscles

    J Appl Physiol

    (1983)
  • Cited by (0)

    No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated.

    View full text