The Rectus Femoris Muscle Fatigue through Mechanomyographic Time-Frequency Response in Paraplegic Subject

The purpose of this study is the evaluation of mechanomyographic (MMG) time-frequency response of rectus femoris muscle of a paraplegic subject during an isometric electrically-elapsed fatigue protocol. An accelerometer sensor was used to measure the vibration of muscle during voltage-controlled functional electrical stimulation application at 1 kHz pulse frequency (20% duty cycle) 70 Hz modulated frequency (20% duty cycle). A load cell (50 kgf) measured the force signal with the participant seated on a bench with the hip and knee angle set to 90º. During the protocol the electrical output voltage was adjusted to keep the force at 30% of maximal stimulated contraction (MSC). When the electrical stimulation was unable to keep the force above approximately 10% of MSC the protocol was ceased. Ten seconds with unfatigued (initial period) and fatigued (final period) muscle, MMG signal was processed with Cauchy wavelet transformation (bandpass 5-100 Hz). For fatigue conditions of paraplegic subject, MMG signal presents concentration energy to lower frequencies mainly to 11.31 Hz band frequency.