Table 2 .

Summary data of included studies (n = 27)

Study	Intervention description I, Intervention; C, control	FES settings	Study period	Outcome measures	Level of evidence	Results
Therapeutic studies
Houghton et al.35	I: HVPC applied to the wound bed plus SWC program; C: SWC only	HVPC with 50 µs pulse duration, 50–150 V intensity was applied for 20 minutes with 100 Hz, 10 Hz and off each hour for 8hours/day.	3 months	WSA	Grade IA	1. The percentage decrease in WSA was greater in the EST +SWC group (mean ±SD, 70 ±25%) than in the SWC group (36 ± 61%; P = 0.048). 2. The proportion of stages III, IV, or X PUs improving by at least 50% WSA was significantly greater in the EST +SWC group than in the SWC group
Griffin et al.33	I: HVPC of wound; C: Sham HVPC given 1 hour a day for 20 consecutive days, both group received standard nursing care	Stimulation frequency and intensity was 100 pps, 200 V, respectively	20 days	WSA	Grade IB	Percentage reduction in WSA achieved by the HVPC group was greater than the sham treatment group at day 5 (32 vs. 14%, P = 0.03), day 15 (66 vs. 44%, P = 0.05), and day 20 (80 vs. 52%, P = 0.05)
Adegoke and Badmos32	I: IDC of wound; C: Sham IDC for 45-minutes 3 days/week, both group received SWC	ES intensity gradually increased until a minimal muscle contraction, then kept just below contraction	4 weeks	WSA	Grade II	1. WSA decreased by 22.2% in the IDC group vs. 2.6% in the sham treatment group. 2. Most of the decrease in WSA occurred during the first 2 weeks of the study (15.8 vs. 1.9% change in the DC group vs. the sham DC group, respectively)
Baker et al.22	I: asymmetric biphasic ES; II: symmetric biphasic ES; III: microcurrent ES; C: no stimulation. Each treatment last 1.5 hours, 5 days/week	ES intensity increased until a minimal muscle contraction, then decreased until no muscle contraction	4 weeks	Healing rate and WSA	Grade II	1. No statistical differences in healing rates and wound areas among the four groups. 2. Subgroup analysis showed the healing rate by ES in the control group was greater after the control period (43.3 ± 12.5 vs. 9.7 ± 3.4% change/week)
Karba et al.37	I: ES were delivered using the 1 positive stimulation electrode and 4 negative electrodes (DC+). II: same ES program with one positive and one negative pad. C: sham group, no ES delivered	Constant direct electric current of 0.6 mA	Not reported	Relative healing rate (%/day)	Grade II	The relative healing rates of PU treated by direct current with electrode overlaid wound was higher than those with electrodes placed on intact skin, or treated by sham ES
Jercinovic et al.36	I: SWC plus ES edge of PU for 2 hours; C: SWC and standard rehabilitation. Crossover group after 4 weeks	ES was applied with 40 Hz, 250 µs, amplitude adjusted up to 45 mA individually to achieve minimal contraction	1 year	Wound healing rate	Grade II	1. Mean healing rate for ES group in first 4 weeks was greater comparing to the control group. 2. ES group have 1.5–2 times shorter healing period
Stefanovska et al.39	I: conventional treatment plus direct currents with low density (DC); II: conventional treatment plus direct currents with low frequency (AC) were applied across wound for 2 hours daily; III: conventional treatment only	Direct currents with low density an amplitude of 600 µA; AC currents with a pulse duration of 0.25 ms, low frequency of 40 Hz, amplitude 15–25 mA	4 weeks or till wound closure	Healing rates	Grade II	Healing rate in the AC group was significantly better than the DC and control group (P = 0.003) after excluding those with very deep, superficial, or long-term wounds
Trontelj et al.50	I: ES delivered with two electrodes placed on health skin at the edge of each wound for 2 hours daily; C: conventional treatment only	ES with pulse duration of 1.25 ms, frequency of 40 Hz was delivered 4 on 4seconds off. (15–25 mA) adjusted individually to achieve minimal muscle contraction	8 weeks	Wound healing rate	Grade II	ES-treated wounds healed at almost twice the rate of those in the control group. Mean relative healing rate of the ES group was higher than the control group (4.89 ± 3.80 vs. 2.6 ± 2.59)
Recio et al.47	HVES to the wound bed for 60-minute sessions 3–5 times per week	ES was delivered by twin peaked, monophasic, 10 µs pulse width	12 months	PU status	Grade III	WSA decreased (11.5 cm2 at baseline vs. 0.4 cm2 at end of treatment). 2. The long-standing PUs were completely healed after 7–22 weeks
Lippert-Grüner53	ES of gluteal muscles was delivered using anal probe for 15–20 minutes tid.	No details given	6 weeks	Size of PUs	Grade IV	After 2 weeks of stimulation, the size of ulcers were reduced on both side, within 6 weeks, all ulcers were completed healed
Pollack et al.55	ES of bilateral gluteus hamstring and quadriceps muscles twice weekly	ES with a frequency of 60 Hz and a pulse duration of 400 µs	6.5 months	PU status	Grade IV	After 6.5 months of ES, the PU completed closed
Prevention studies
Kim et al.38	I: bilateral sub-threshold ES of the gluteus muscles was applied using surface electrodes. C: sham ES	Biphasic, charge-balanced stimulation was applied at 10 Hz frequency with a pulse duration of 200 µs	12 weeks after recruitment	TcPO2, muscle thickness, and interface pressure	Grade IB	1. A 78% increase in TcPO2 immediately following ES in the intervention group, but this was not maintained at follow-up. 2. No significant changes in regional TcPO2, gluteal muscle thickness, or pressure distribution pre- and post-treatment using sub-threshold ES
Gyawali et al.34	I: continuous stimulation; II: bursting stimulation, 3 bursts of stimuli were delivered bilaterally to the gluteus maximus muscles	ES with pulse duration of 200 µs and 40 Hz frequency	Dynamic	Interface pressure over the IT	Grade II	1. Both continuous and bursting ES paradigms decreased pressure around IT. 2. Within the continuous paradigm, the 7 seconds of stimulation produced greater pressure reduction than 13 seconds stimulation. 3. ES increased signal intensity by MRI in the atrophied and loaded muscles
Londen et al.26	I: the alternating stimulation of 0.5 seconds ES of one gluteal muscle and a 15 seconds rest, followed by 0.5 seconds stimulation of the other side and a 15 seconds rest. II: the simultaneous stimulation of a 0.5 seconds stimulation of both gluteal muscles followed by a 15-second rest	Rectangular monophasic pulses were applied with 50 Hz stimulation frequency and 80 mA current amplitude	Dynamic	Interface pressure	Grade II	1. Both alternating and simultaneous stimulation caused a significant (P < 0.01) decrease in interface pressure (−17 ± 12 mmHg, −19 ± 14 mmHg) and pressure gradient (−12 ± 11 mmHg, −14 ± 12 mmHg) during stimulation periods compared with rest periods. 2. There was no significant difference in effects between the alternating and simultaneous stimulation
Petrofsky40	ES of quadriceps for 10–15 minutes per day; after 4 weeks, sequence stimulation of the quadriceps, gluteus maximus, and hamstring muscles for 30 minutes, 3 days/week	ES with pulse width of 350 µs, at frequency of 40 Hz, and amplitude varies from 0 to 150 mA	1 year	Incidence of PU	Grade II	The incidence of PU was 5.2% in SCI who had ES, 32% in control population
Smit et al.48	ES to gluteal and hamstring muscles using a custom-made electrode garment with build-in electrodes	1 hour stimulation to gluteal muscles (g) or gluteal +hamstring muscle (g + h).Gluteal muscles were stimulated first and then g + h muscles	On time	Interface pressure over the IT and pressure gradient	Grade III	1. Pressure reduced by 34.5% after g + h muscles activation compared with rest pressure, 2. Pressure reduced by 10.2% after activation of g muscles only. 3. Pressure gradient reduced significantly only after stimulation of g + h muscles (49.3%)
Smit et al.49	ES to gluteal and hamstring muscles was delivered through surface electrodes	ES with a duty cycle of 1 second stimulation and 4 seconds rest for 3 minutes was delivered at standard 150 V, with 50 Hz, amplitude ranging from 55 to 125 mA to induce a titanic contraction	On time 4 hours	Interface pressure over the tuberosities, blood flow, and oxygenation	Grade III	1. Pressure was significantly lower during ES as compared with rest. 2. There were no significant changes of oxygenation during ES as compared with rest. 3. There was a significant difference in peak blood flow during ES as compared with rest (P = 0.007), but no significant change on mean blood flow for ES
Liu et al.44	ES to sacral nerve root was delivered using an sacral nerve root implant or a magnetic stimulator	Sacral ES frequency was 20 pps with pulse with of ranging from 128 to 600 µs	On time	Interface pressure under ischial tuberosities and skin blood flow	Grade III	1. Peak pressure and gradient at peak pressure significantly decreased during FMS as compared with baseline. 2. Peak pressure and gradient at peak pressure significantly decreased during sacral nerve root via SARS implant as compared with baseline. 3. Ischial skin blood perfusion significantly increased during the FMS and SARS
Liu et al.45	ES to the second sacral nerve root (S2) was delivered using an sacral anterior nerve root implant	Sacral ES frequency was 20 pps with pulse with of ranging from 64–600 µs	On time	Interface pressure under ischial tuberosities	Grade III	1. Peak pressure and gradient at peak pressure significantly decreased during sacral nerve root via a SARS implant as compared with baseline
Bogie and Triolo41	ES of gluteal muscles, leg, and back muscles was delivered by NMES implant	The exercise regime included 3 different stimulation patterns with frequency 16 or 30 Hz. Ramp up “2 seconds”, on time “5 seconds” or “10 seconds”, ramp down “2 seconds” or “4 seconds” off time “‘10 seconds”	8 weeks	Interface pressure and TcPO2	Grade III	1. There was no significant difference in overall mean interface pressure between baseline and post-exercise. 2. Mean region interface pressure statistically decreased post-conditioning as compared with baseline. 3. Baseline mean unloaded TcPO2, increased by 1–36% at post-exercise assessment for five participants, but showed a decrease in other three participants. 3. Differences between baseline and post-exercise TcPO2, levels were not statistically significant
Mawson et al.46	HVPGS was applied using electrodes taped on the spine when participants were supine or prone	HVPGS of 50 V and 10 Hz, then at 75 V and 10 Hz was applied to the back T6 during prone. HVPGS of 75 V and 10 Hz was delivered during prone	On time	Sacral transcutaneous oxygen tension TcPO2	Grade III	Sacral TcPO2 was increased during HVPGS and the results were reproducible
Levine et al.43	ES of gluteus maximus began with a 20 minutes rest, followed by 12 minutes stimulation	50 Hz with a duty cycle of 2 on 4 seconds off	On time	Ischia region muscle blood flow	Grade III	All participants showed an increase in muscle blood flow during ES
Wu et al.51	ES to bilateral lumber spinal muscle and gluteal muscle was delivered by NMES implant	20 Hz, 20 mA pulse amplitude	On time	Interface pressure under tuberosities and region TcPO2	Grade III	1. Maximum interface pressure gradient showed a variable response overall. 2. Subgroup analysis for sacral sitters, sacral interface pressure, and maximum interface pressure gradient tend to decrease on ES application; mean TcPO2 increased during ES and remained elevated after the intervention
Ferguson et al.42	ES of quadriceps was applied bilaterally and simultaneously 30 minutes per day for at least 5 days/week	Pulse width 300 µs, frequency 20 Hz and amplitude 100 mA. The stimulation was applied for 10 seconds intervals with 20 seconds rest period, which was repeated after a one minute rest	On time	Pressure at ischia	Grade III	1. Mean pressure across all participants at both ischia reduced during the stimulation as compared with resting (55 vs. 99 mmHg on the right, 49 vs. 76 mmHg on the left, respectively). 2. Two participants had an increase in left pressure during quadriceps stimulation. 3. In general, the greatest reductions occurred in participants with large knee movement
Bogie et al.52	ES of gluteal muscles was delivered using an NMES implant	Alternating left and right gluteal stimulation at 20 Hz, 15 seconds on and 15 seconds off to each muscle for a 3-minute period on and 17-minute interval for up to 10 hours/day.	5 years	Seated interface pressure, tissue oxygen, gluteal muscle thickness, and sitting tolerance	Grade IV	1. Seating interface pressure was reduced significantly at 6 weeks, 6 months, and 40 months follow-up. 2. Tissue oxygen level improved over the study time. 3. Gluteal muscle thickness was increased at 1 year and 5 years. 4. Sitting tolerance had increased from 6 hours a day to more than 12 hours a day
Rischbieth et al.54	ES of gluteal muscles for 15 minutes tid between 0 and 4 months, 30 minutes bid between 7 and 24 months	Frequency was 30 pps, duty cycle was 10:15 seconds between 0 and 1 months, 10:8 between 4 and 24 months; intensity was 54% at start, 80% at 1 month and 100% between 4 and 24 months	24 months	Dimension of buttocks and sitting tolerance	Grade IV	The circumferential dimensions across the buttocks were increased 21%
Vanoncini et al.56	ES of erector spine through surface electrodes	A train of square pulses with a frequency of 50 Hz and a fixed pulse width of 450 µs and manually altered pulse amplitude	On time	Seated interface pressure	Grade IV	1. The pressure decreased on the side opposite to the stimulation. 2. Sitting tolerance increased from 30 minutes to more than 2 hours