Abstract— The present work investigated the response of triaxial MMG signals acquired from the rectus femoris muscle of spinal cord injured volunteers during fatigue protocol electrically evoked. A custom functional electrical stimulator voltage-controlled was configured as: pulse frequency set to 1 kHz (20% duty cycle) and burst (modulating) frequency set to 70 Hz (20% active period). The triaxial MMG signal of rectus femoris muscle was processed with third-order 5-50 Hz bandpass Butterworth filter and the values were normalized. A load cell was used to register the force. The stimulator output voltage was increased until the maximal electrically-evoked extension (MEEE) of knee joint. After the load cell placement, the stimuli magnitude required to reach the MEEE was applied and registered by the load cell as muscular force-100% response. Stimuli intensity was increased until and during the control to keep the force in force-100%. Four instants with force functional electrical stimulation (FES)-controlled were selected between force-100% and slope down to force-30%. The MMG energy decreased with FES application due neuromuscular fatigue in paraplegic subjects. X-axis between instant I (1 ± 0) and instant IV (0.74 ±0.27), and the same tendency was found to Y-axis between instant II (1.14 ± 0.44) and instant IV (0.91 ± 0.3). I. INTRODUCTION Due to the inability of voluntary lower limb contraction, paraplegics consider a challenge to perform sit-to-stand movements and maintain balance standing [1]. Although there is no technique to naturally generate physiological contraction on their handicapped lower limbs, studies investigated the use of implanted functional electrical stimulation neuroprostheses. With this aim, Guiraud et al. [2] in their case report, showed the effects of nine years using a neuroprosthesis and concluded that muscle fatigue is a major issue for the future. At the end, they suggest more studies focusing on neural stimulation efficiency. In order to get such efficiency it is necessary to avoid muscle fatigue induced by electrical stimulation [2]. However, how to avoid something that is not clear when it happens? E. Krueger and P. Nohama are with CPGEI, Universidade Tecnológica Federal do Paraná (UTFPR), Curitiba, PR, Av. Sete de Setembro 3165, CEP 80230-901, Brazil (phone: +55-41-3310-4679; fax: +55-41-3310-4679; e- mail: kruegereddy@gmail.com). E. M. Scheeren is with PPGTS, Pontifícia Universidade Católica do Paraná, Curitiba, PR, Brazil. G. N. Nogueira-Neto is with Pontifícia Universidade Católica do Paraná, Curitiba, PR, Brazil. Previous studies applying mechanomyography showed the feasibility to measure and monitor mechanical muscle oscillations during electrically elicited contraction during fatigue protocols [3]. Therefore, the goal of the research discussed in this paper is to verify the MMG energy response in rectus femoris muscle of paraplegic participants during electrically-evoked isometric contractions and a fatigue protocol. II. METHODS A. Participants This work was approved by Secretaria de Saúde do Estado do Paraná´s Research Ethics Committee according to the protocol number 189/2010 and in conformity with Helsinki Declaration of 1975, as revised in 1983. The patients inclusive criteria were: spinal cord injury without voluntary contraction on quadriceps muscle. The patients exclusive criteria were: cancer in the lower limb (stimulated area), participants who have been submitted to X-ray examination in the last two weeks (aversion), with metal implanted in the stimulated limb, cognitive impairment or without toleration to FES sensation. The protocol flowchart is shown in Fig. 1. During the clinical research sessions, the environment temperature and moisture were 31.4 ± 2.28 ºC and 43.4 ± 10 %, respectively. Spastic events and medicines to regulate the muscular tonus did not jeopardize the tests. The sample demography is shown in Table I. All of them were ranked on American Spinal Cord Injury Association Impairment Scale A or B, without voluntary contraction in lower limbs. Figure. 1. Protocol flowchart. Mechanomyography energy decreases during muscular fatigue in paraplegics Eddy Krueger, Affiliate, EMBS, Eduardo M. Scheeren, Guilherme N. Nogueira-Neto, and Percy Nohama 978-1-4244-7929-0/14/$26.00 ©2014 IEEE 5824