Observation of human sympathetic nerve activity recorded microneurographically during sustained muscle contraction

Abstract

The purpose of this study was to evaluate directly and quantitatively the muscle sympathetic nerve activity (MSA) in man during exercise and to reveal the effects of static muscle contraction on MSA by applying the microneurographic technique. MSA was recorded by a tungsten microelectrode inserted percutaneously and manually into the muscle nerve fascicle of the tibial nerve at the popliteal fossa of the subject in supine position. The sustained muscle contraction was performed for five minutes using handgrip, at a tension of 20% of maximal voluntary contraction which was determined before the experiment at the same position. Subjects were six healthy male students. MSA could be recorded in all subjects. The average MSA, expressed as burst numbers per minute (BR), were 12.8 (±3.2; SD) bursts/min (ranging 9.3 to 18.6) at rest in supine position. During sustained muscle contraction, MSA increased by 25% at first minute of exercise compared to the value at rest before contraction. The activity increased more and more with the duration of muscle contraction. At fifth minute of the contraction, MSA increased by 165% of the value at rest. At the same time, mean blood pressure became elevated with the duration of muscle contraction. These results indicate that the elevation of blood pressure during sustained muscle contraction may be, at least in part, due to the increase of MSA, which induces vasoconstriction of the inactive muscular beds. In conclusion, microneurographic technique is a valuable method to evaluate the sympathetic nerve activity during exercise. Using this technique, it was demonstrated that the muscle sympathetic activity increased gradually during the muscle contraction until the end of the exercise, even when relatively low tension (20% MVC) of the sustained handgrip was loaded.