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

We have investigated the etiology of lower-back muscle fatigue using simultaneous recordings of electromyography (EMG), mechanomyography (MMG), and near-infrared spectroscopy (NIRS) in an attempt to shed some light on the electrophysiological, 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. Surface 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 (BV) and oxygenation (Oxy-Hb). The root mean square amplitude value (RMS) of the EMG signal was significantly increased at the initial phase of contraction and then fell significantly, while mean power frequency (MPF) of the EMG signal decreased significantly and progressively as a function of time. There were also significant initial increases in RMS-MMG that were followed by progressive decreases at the end of fatiguing contractions. MPF-MMG remained unchanged. Muscle BV and Oxy-Hb decreased dramatically at the onset of the contraction and then remained almost constant throughout the rest of the contraction. These results, obtained by simultaneous recordings of EMG, MMG, and NIRS, demonstrate that the restriction of blood flow due to high intramuscular mechanical pressure is one of the most important factors in muscle fatigue in the lower-back muscles. In addition, the simultaneous recording system described here can be used to obtain more reliable information regarding the mechanism(s) of lower-back muscle fatigue.