Tissue Oxygenation and EMG during Computer Mouse Work and Submaximal Isometric Wrist Extensions

Abstract

Sufficient tissue oxygenation (TO₂) is essential for muscle function in general. However, it is not known to which extent TO₂ is influenced during different low-level contractions. The aim was to study the interaction between EMG activity and TO₂ during computer mouse tasks and submaximal isometric contractions at load levels corresponding to levels that occur during repetitive work in industry.

Eight healthy subjects performed multidirectional computer mouse tasks (40 and 60 clicks/min) with the right arm. In addition, 1 -minute isometric wrist extensions at 5, 15 and 25 %Maximal Voluntary Contraction (MVC) were performed. Further, a 15 %MVC wrist extension until exhaustion was performed. Near-infrared spectroscopy was used to measure TO₂ of m. extensor carpi radialis. EMG was recorded from m. extensor carpi radialis and m. extensor digitorum.

During mousing the EMG_RMS activity was in generel higher during the high speed compared to the low speed task. Muscular load of the forearm has isometric characteristics to a high degree. No significant difference in TO₂ was found between the two mousing speeds. During the _{isometric contractions} EMG_RMS increased and TO₂ decreased with increasing contraction level. The mean endurance time at the 15 %MVC wrist extension was 22.6 min. The average exerted force remained constant though fluctuations in the force increased as a function of time, indicating impaired force control due to development of fatigue. During the endurance task the EMG_RMS activity increased and the EMG_MPF decreased, indicating electromyographic signs of muscle fatigue. Within less than one minute TO₂ decreased significantly relative to resting values. No further decrease was seen during the endurance contraction. Point of exhaustion was not related either to a gradual or a sudden decrease in TO₂. Although the average exerted force remained constant, sudden changes in EMG_RMS was seen in some subjects especially in the last part of the contraction, indicating changes in force distribution between muscles. This is assumed to be a fatigue-preventing mechanism. These changes in extensor carpi radialis muscle activity were reflected as opposite changes in TO₂-levels, A cross correlation analysis showed a correlation between in extensor carpi radialis muscle activity and TO₂, time displacement=3.4 sec, r=−0.31. This indicates that the changes in EMG activity precede the changes in TO₂-level.

In conclusion, TO₂ was inversely related to muscle activity and exerted force. A steady state level below resting level of TO₂ was found during the prolonged fatiguing isometric contraction. Point of exhaustion could not be explained either by a gradual or a sudden decrease in TO₂.