Table 1.

Functional electrical stimulation (FES) of denervated muscles in animal models

Reference	Purpose	Subjects	Protocol/Parameters used	Results/Conclusion
Tamaki et al., 2017	To test the effects of Electrical Stimulation on bone strength and muscle atrophy	Forty-two males, 7-wk-old Fischer 344 rats with denervated Tibialis Anterior (TA) muscle were used	Three groups with n = 14 each; randomly grouped into Control (Cont), Denervation (Dn) without E stim and Denervation with Electrical stimulation (ES). Dn + ES group stimulation parameters: intensity 16 mA, frequency 10 Hz, 250 μs pulse duration for 30 min per day for 1 wk. ES consisted of 2 s stimulation and 6 s of rest.	Body weight did not significantly differ but the relative muscle weight was significantly higher in in the Dn + ES group compared to the Dn group (P < 0.05); 1.3 ± 0.1 in Dn and 1.6 ± 0.2 in Dn + ES group. Bone strength is determined by parameters like max load, stiffness and elastic modulus; there was a significant increase in Dn + ES group compared to the Dn group.
Bueno et al., 2017	To investigate the effects of electrical stimulation through Russian Currents on cranial tibial muscle of experimentally denervated rats.	Thirty-six young male Wistar rats aged 80 d	Four groups: Initial control group euthanized at 80 d (ICG), Final control group (FCG) euthanized at 125 d, Experimental Denervation group (EDG) denervated at 80 d, no treatment for 45 d and Experimental Denervated Treated Group (EDTG) denervated at 80, treated for 45 d, three weekly sessions, two application cycles each session. Cycle duration: 10 min, cycle frequency was 2500 Hz in periods of 0.4 ms; 3/1 intervals, 9 s of stimulation and 27 s rest period with modulation percent at 50%. First cycle stimulates red fibers at 30 Hz, second one for white fibers at 100 Hz.	Histological observations: the EDTG group has the characteristics of both FCG and EDG groups, most of the muscle fibers observed peripheral nuclei like the FCG, central nuclei were also observed rarely. Morphometric data: significant difference between EDTG (2402 ± 94 µm2) compared to EDG (755 ± 227 µm2); No significant difference when comparing ICG (2441 ± 193 µm2) to EDTG.
Cheetham et al., 2015	To assess the effects of direct intramuscular stimulation with long pulses at low activation frequency; on muscle size and function in denervated laryngeal muscles	Ten adult horses (age range 5–7 yr) with transected recurrent laryngeal nerve	Quadripolar intramuscular electrodes were instrumented in the left posterior cricoarytenoid (PCA) muscle. After 12 wk of denervation, the left PCA was stimulated every day for 8 wk at 50 Hz, 2 ms pulse duration, 2 s ON and 2 s OFF 10 V for 48 min to produce 72,000 impulse/d and daily frequency equivalent 0.83 Hz, biphasic pulses used; 7 animals stimulated (FES + group) and 3 animals implanted but not stimulated, served as control (FES- group).	Quantitative assessment using strength-duration curve to determine the effects of denervation and stimulation, muscle contraction, PCA volume and laryngeal functions were measured 12-wk post denervation and post 8-wk of FES. The PCA function under increasing inspiratory loads drops down severely post denervation, post FES in the FES + group significantly improved at high levels of inspiratory loads (90 and 100 HR max).
Monaco et al., 2015	To investigate the effects of combination of testosterone and electrical stimulation on functional recovery post laryngeal nerve crush	One hundred and forty-eight adult male Sprague-Dawley rats with a crush injury in the left RLN were used	Animals were divided into four experimental groups 1) no treatment (CTL) 2) ES only 3) TP only and 4) ES + TP. These animals were also subdivided into timepoints of 1, 2, 3 and 4 wk. The ES voltage ranged from 200 to 300 mV with the current from 0.1 to 0.2mA at 20 Hz frequency for 30 min.	ES and TP had similar recovery. Testosterone did not significantly shorten the treatment time. The Vocal Fold Mobility (VFM) scores determine the recovery. At no time points were the VFM scores from any of the treated group significantly different from any other treatment groups. At 2 wk the VFM score of ES and ES + TP group was significantly higher than the control group, only TP group had similar effects but was not statistically significant. By wk 3 and 4, the VFM scores were reaching max in all the groups.
Fujita et al., 2011	To study the effects of electrical stimulation on muscle atrophy	Twenty-eight adult male Wistar rats were used. Hindlimb unloading was applied to induce muscle atrophy.	The animals were randomly divided into four groups control (Cont); hindlimb unloading (HU); hindlimb unloading plus electrical stimulation (ES) and hindlimb unloading plus combination of electrical stimulation with forceful isometric contraction (ES + IC)	The mean values of wet weight of the Tibialis Anterior muscle in HU was 353 ± 8 mg; 405 ± 5 mg in the ES group and 434 ± 3 mg in the ES + IC group. This was a stastically significant difference as compared to the HU group. The effect of combination of ES + IC was better than just ES.
Asensio-Pinilla et al., 2009	To compare the effects of the combination of electrical stimulation and exercise as compared to their individual effects	Five groups of adult rats with sciatic nerve denervation	One group received acute E stimulation (3 V, 0.1 ms at 20 Hz), immediately after denervation (ESa). All the other groups received treatment for 4 wk. The second group received chronic E stimulation (ESc); the third group receives Treadmill Exercise + E stimulation (ES + TR); the fourth group receives only Exercise (TR); the fifth group does not receive treatment and serves as the control (C).	The results demonstrate that the final level of reinnervation of the tibialis anterior (TA) muscle was significantly higher in three groups; ESa group by 11 ± 2%, TR group by 9.4 ± 1% and ES + TR by 10.4 ± 2% compared to the control group C with 6 ± 1%. But it was found to be higher in the ES + TR group at earlier timepoint of 1 wk. There was an increase in the number of myelinated regenerated fibers in groups ESa, TR and ES + TR. The increase in ES + TR group (5124 ± 404) compared to the control group (2403 ± 196) was significant.
Sharma et al., 2009	To investigate the combined therapeutic effects of Electrical stimulation and gonadal steroids on peripheral nerve regeneration	Ninety-eight adult male Sprague-Dawley rats with its right facial nerve crushed near its exit from the stylomastoid foramen	A total of 8 groups; 1) Untreated 2) Treated with E stimulation only 3) Treated with Testosterone Propionate (TP) only 4) E stimulation + TP 5) Dihydrotestosterone (DHT) only 6) E stimulation + DHT 7) Estradiol (E2) only 8) E stimulation + E2; Stimulation was conducted with Supramaximal pulses with 20 Hz frequency; 1-d post axotomy for 30 min every day until sacrifice (4 and 7 d).	The group 4 with E stimulation + TP showed the highest rate of regeneration with the facial nerve outgrowth distance being 11.8 ± 0.7 mm at 4 dpo as compared to untreated animals at 7.3 ± 1.0 mm; and 25.0 ± 0.7 mm at 7 dpo compared to untreated at 19.0 ± 1.6 mm. Increase in regeneration rate (E stimulation) and delay in sprout formation (TP) has beneficial effects. DHT and E2 did not prove to be as effective as TP.
Ashley et al., 2008	To evaluate the effects of stimulation parameters on the extent of regeneration	Twenty-six male New Zealand white rabbits were used	Rectangular bipolar constant-current pulses used. A total of 5 different patterns with different stimulation parameters were used. Varying pulse duration (20, 20, 20, 10 & 10 ms); frequency (20, 20, 20, 40 & 40 Hz); daily stimulation minutes (1 × 60, 1 × 60, 2 × 30, 1 × 300 and 1 × 20 min); no: of impulses per day (24,000, 48,000, 24,000, 480,000 & 24,000) and the duration of stimulation (6, 6, 6, 6 & 10 wk) were used in patterns 1 to 5 respectively.	The group with Pattern 1 proved to me more effective with maximum CSA 62.5 ± 8.8 and fibre number 18,200 ± 885 as compared to denervated group with CSA 36.0 ± 3.1 and fibre number 15,500 ± 1120.
Vivo et al., 2008	To investigate whether electrical stimulation performed immediately after the injury may boost axonal regeneration	Thirty Sprague-Dawley female rats; subjected to complete transection of the sciatic nerve and repaired immediately by epineural suture	Two groups with n = 15 each. The first group received electric stimulation using a Grass S44 stimulation for 1 h immediately after the injury. Parameters used were continuous 20 Hz square pulses of 3 V, 0.1 ms. The second group n = 15 was the control group.	Nerve regeneration, as indicated by higher number of axons distal to the lesion post 1 and 2 mon seen in the E stimulation group. Test performed 1-wk post injury showed denervation of the muscles. Proximal Tibialis Anterior (TA) muscles showed signs of reinnervation 3 wk post injury with indications of small amplitude M waves; final reinnervation values were 28% in the control group and 33% in the E stimulation group.
Demiryurek and Babül, 2004	Effects of vitamin E treatment and electrical stimulation on progression of atrophy in denervated rat gastrocnemius muscle studied	Thirty Wistar albino rats (both sexes) were used	Five groups (n = 6): Protocol 1, only incision; protocol 2, denervation group; protocol 3, E stimulation + denervation; protocol 4, vitamin E + denervation; protocol 5: vitamin E + E stimulation + denervation. Vitamin E: 30 mg/kg per day for 7 d; E stimulation: bipolar, 1 ms, 3–10 mA strong pulses-10 min for 7 d.	Malondialdehyde levels found significantly increased in denervated gastrocnemius muscle (62.6 ± 4.9 nmol/g) compared to the control group (49.7 ± 2 nmol/g). There was no significant difference between E stim or vitamin E treatment. The best results were in vitamin E + E stimulation + denervated muscle group lowest MDA levels (25.8 ± 1.3 nmol/g).