RR-PO-0675

Monday 15:00, Palau de Congressos, Exhibition Hall  [Display No. 583]

THE INFLUENCE OF AN IMPROPERLY AND PROPERLY ADJUSTED DESK ON MOTOR UNIT ACTIVITY. Klipstein A, Zennaro D, Krebs D, Uebelhart D, Läubli T; Department of Rheumatology and Institute of Physical Medicine, University Hospital Zurich and Institute of Hygiene and Applied Physiology, Swiss Federal Institute of Technology, Zurich, Switzerland. PURPOSE: To compare temporal changes of motor unit activation of the trapezius muscle during tapping using an appropriate and inappropriate ergonomic desk adjustment. RELEVANCE: Constrained postures, repetitive work tasks, mental demands and improper workstation ergonomics may contribute to muscular shoulder-neck pain. A hypothesis postulates, that the continuous activity of specific Motor Units (MUs) due to lack of recovery, causing muscle pain and strain (Cinderella hypothesis). Adjustment of desk height is often proposed to reduce muscle pain tension. Still, there is no clear evidence if these recommendations may affect pain and strain of neck muscles. SUBJECTS: 16 subjects (8 intensive and 8 non intensive computer users with and without muscular neck pain) executed a 5-minute tapping task (right index finger, 4 strokes a second) on an appropriate and an inappropriate desk height (5 cm higher then appropriate level). METHODS: Intramuscular EMG (I-EMG) signals were recorded from two localizations in the right trapezius descendens muscle using fine-wire electrodes, surface EMG (S-EMG) by using bipolar electrodes. I-EMG signals had been analysed by newly developed fully automated decomposition software. RESULTS: All I-EMG recordings fulfilled thresholds for the quality parameters (Cohen's Kappa 0.7 to 1.0, percentage of correct classification 60 to 97%). Increase of S-EMG parameters with higher desk (median f=8.2,p=0.01) independently of pain and exposure. In I-EMG: Higher number of detectable motor units on two positions in occasional computer users and pain subjects (F=9.7,p=0.01) showing a lower degree of motor control, further more, had been related to neck pain (F=7.2,p=0.02). Longer average length of MU activity with desk elevation (log F=9.3,p=0.02). Number of constantly active units ("Cinderellas") doubled in elevated desk. CONCLUSIONS: An incorrectly adjusted desk results in unnecessary biomechanical motor unit activity, which was also related to neck pain. This activity can be explained by Co-activation of trapezius muscle. The Cinderella hypothesis is supported by the findings. Method has shown to be sensitive to detect small changing effects, as well as to describe particular characteristics of motor control, therefore, may be useful to study physiological changes of muscle.