Spontaneous rest activity was recorded from hind limb flexor muscle bilaterally. Electromyography (EMG) recording was done by 23 gauge needles for 10 s one day prior to sacrifice of animals. The EMG signal was amplified (Grass, Astro-Med, West Warwick, RI), digitized (5 kHz, Digi-data 1322A; Axon instruments, Foster City, CA), and filtered (30–300 Hz; Fouad et al., 2013). After recording, the recruitment index of motor units was acquired via compression of 10 s of recording to 1 s by EMG software. The recruitment index was scored on an ordinal scale (0 to ++++) (Stålberg et al., 1998; Hosseini et al., 2015).