Arm Abduction Angle and Contraction Intensity Effects on EMG Median Frequency During Abduction Isometric Contractions

Abstract

The electromyogram (EMG) median frequency (MDF) has been observed to be muscle/length and contraction intensity (CI) dependent in particular muscle groups. However, the interacting effect of these aforementioned variables on the EMG MDF of the scapular stabilizer and deltoid muscles has yet to be conclusively established. PURPOSE: The purpose of this investigation was to determine the effect of arm abduction (AA) angle and CI on the EMG MDF of the deltoid and scapular rotator muscles. METHODS: Twelve healthy, young adults (8 men, 4 women) performed maximal and sub-maximal isometric arm abduction contractions in the scapular plane on a Biodex isokinetic dynamometer. Subjects performed five isometric maximal voluntary contractions (MVC), followed by randomly ordered ten-second sub-maximal contractions at 10–90% MVC (10% increments), at the following AA angles: 15, 30, 45, 60, 75 and 90 degrees. During each contraction EMG was obtained from the deltoid (DEL), upper trapezium (UT), lower trapezium (LT) and serratus anterior (SA) muscles. EMG signals were collected at 2000 Hz and bandpass filtered (20–500 Hz). A 3 second analysis window was chosen during the point of maximal torque for the MVC's and during the point at which the subject was producing the target torque. The power spectrum MDF was calculated during 0.5 sec overlapping windows throughout the three second contraction window. RESULTS: The results demonstrated a significant muscle (P<0.001), and CI (P<0.001) main effects on MDF. The highest MDF was calculated for DEL (98.01 Hz) and LT (91.36 Hz), while a lower MDF was calculated for UT (82.52 Hz) and SA (74.19 Hz). The MDF for all muscles decreased with increasing CI. The AA by CI interaction was statistically significant (P<0.001). The 15 degree AA position showed the greater decrease (77.93–106.02 Hz) in MDF than the other AA positions (30 deg: 82.5–94.9 Hz, 45 deg: 94.3–81. 1Hz, 60 deg: 80.9–94.2Hz, 75 deg: 82.2–93.1 Hz, 90 deg: 85.8–96.0 Hz). A significant three way AAby CI by muscle interaction (P<0.001) was found. DEL and UT MDF increased as AA increased at equal CI, LT and SA MDF were unaffected by increasing AA at equal CI. CONCLUSIONS: The major findings of this investigation demonstrate a decrease in the MDF with increasing CI for each of the four muscles. Increasing AA increased the MDF of the DEL and UT but did not affect the MDF of the LT and SA.