Assessment of lower-back muscle fatigue using electromyography, mechanomyography, and near-infrared spectroscopy

We have investigated the etiology of lower-back muscle fatigue using simult aneous recordings of electromyography (EMG), mechanomyography (MMG), and ne ar-infrared spectroscopy (NIRS) in an attempt to shed some light on the ele ctrophysiological, mechanical, and metabolic characteristics, respectively. Eight male subjects performed isometric back extensions at an angle of 15 degrees with reference to the horizontal plane for a period of 60 s. Surfac e EMG, MMG and NIRS signals were recorded simultaneously from the center of the erector spinae at the level of L3. NIRS was measured to determine the level of muscle blood volume (V) and oxygenation (Oxy-Hb). The root mean sq uare amplitude value (RMS) of the EMG signal was significantly increased at the initial phase of contraction and then fell significantly, while mean p ower frequency (MPF) of the EMG signal decreased significantly and progress ively as a. function of time. There were also significant initial increases in RMS-MMG that were followed by progressive decreases at the end of fatig uing contractions. MPF-MMG remained unchanged. Muscle BV and Oxy-Hb decreas ed dramatically at the onset of the contraction and then remained almost co nstant throughout the rest of the contraction. These results, obtained by s imultaneous recordings of EMG, MMG, and NIRS, demonstrate that the restrict ion of blood flow due to high intramuscular mechanical pressure is one of t he most important factors in muscle fatigue in the lower-back muscles. In a ddition, the simultaneous recording system described here can be used to ob tain more reliable information regarding the mechanism(s) of lower-back mus cle fatigue.