Table 3.

Population, intervention, comparison, outcomes, and study design of each study included in the systematic review

Ref.	Interventions by group	Frequency	Session duration	Intervention Duration	Outcomes	Main results	Adverse events
Fischer et al[19]	NMES: biphasic rectangular pulses at 66 Hz, pulse duration 0.4 ms, duty cycle 3.5 s on and 4.5 s off to quadriceps muscle bilaterally. CG: stimulator electrodes were applied but no electricity was delivered	2 times/d for 7 d/wk	30 min	From POD 1 until ICU exit or POD 14	Muscle layer thickness, Muscle strength; Functional capacity	No significant effect on MLT. ↑ 4.5 times in recovering muscle strength to NMES group during ICU stay. Positive correlation between change in MLT and cumulative fluid balance (r = 0.43, P = 0.01) the first 3 PODs. No significant effect on functional ability	5 patients in the NMES group mentioned a feeling of discomfort
Schardong et al[20]	1FES: symmetric biphasic rectangular pulses at 15 Hz, pulse duration 0.5 ms, duty cycle 5 s on and 10 s off to vastus medialis and lateralis muscle bilaterally. CG: Stimulator electrodes were applied but no electricity was delivered	2 times/wk	40 min	8 wk	Functional capacity; Muscle strength; Muscle endurance; Muscle mass	↑ Distance to 6MWT in the FES group by 11.0% (49.6 m, 95%CI: 15.9-83.3) and in the CG by 10.4% (41.5 m, 95%CI: 7.8-75.2) with no significant between-groups. ↑ muscle strength (7.2 kg, 95%CI: 0.2-14.2). ↑ Muscle endurance (2.2 repetitions, 95%CI: 1.0-3.4)	No complications
Kitamura et al[21]	NMES: Symmetric biphasic square pulses, duty cycle 0.4 s on and 0.6 s off, 10 pulse trains (10 s) with 30 s intervals to quadriceps femoris and triceps surae muscle bilaterally. Usual postoperative rehabilitation program. CG: Usual postoperative rehabilitation program	1 time/d	30 min	3 d before surgery and from POD 1 to POD 5 (8 sessions)	The mean concentration of 3-MH/Cre; Physical function; Walking speed; Grip strength	No significant difference in the mean 3-MH/Cre from POD 1 to POD 6 between groups (225.3 [204.0-248.3] μmol/g vs 227.3 [206.3-259.9] μmol/g, P = 0.531). No significant difference in the KEIS on POD 7 between groups (0.44 ± 0.13 kgf/kg vs 0.41 ± 0.12 kgf/kg, P = 0.149. No significant difference in walking speed between groups (1.04 ± 0.24 m/s vs 0.99 ± 0.23 m/s, P = 0.294). No significant difference in grip strength between groups (29.1 ± 10.5 kg vs 26.9 ± 8.7 kg, P = 0.213)	1 patient mentioned muscle soreness
Cerqueira et al[22]	NMES: Stimulation at 50 Hz, duration 400 ms duty cycle 3 s on and 9 s off, to quadriceps and gastrocnemius muscle bilaterally. Regular physiotherapy care. CG: Usual physiotherapy care twice a day	2 times/d	60 min	from POD 1 to POD 5	Ambulation ability; Muscle strength; Functional independence; Quality of life	No significant difference in distance walked (95%CI: -64.87 to 65.97) and walking speed (95%CI: -0.55 to 0.57) between groups. No significant difference in muscle strength in the upper- limb, lower limb, and total MRC values, functional independence, and quality of life between groups	2 patients reported hypotension, and 1 patient complained of pain
Cerqueira et al[23]	NMES: Stimulation at 50 Hz, duration 200 ms duty cycle 3 s on and 9 s off, to quadriceps and gastrocnemius muscle bilaterally. Regular physiotherapy care. CG: stimulator electrodes were applied but no electricity was delivered	Once during the first 48 h of ICU stay	60 min	60 min	Hemodynamic responses; Respiratory responses	No difference in heart rate, systolic blood pressure, diastolic blood pressure, mean blood pressure respiratory rate, and oxygen saturation between groups	No complications
Sumin et al[24]	NMES: biphasic rectangular pulses at 45 Hz, duty cycle 12 s on and 5 s off to quadriceps muscle bilaterally. CG: Usual postoperative rehabilitation program	1 time/d	90 min	from POD 3 to exit the hospital (12 sessions or more)	Knee extensors strength; Handgrip strength; Knee flexor strength CSA of quadriceps femoris	↑ Knee extensors strength in the NMES group [28.1 (23.8; 36.2) kg on the right and 27.45 (22.3; 33.1) kg on the left] vs CG [22.3 (20.1; 27.1) and 22.5 (20.1; 25.9) kg, respectively; P < 0.001]. No difference in handgrip strength, knee flexor strength, quadriceps CSA, and 6MWT at discharge between groups	Non mentioned
Rengo et al[25]	2NMES: biphasic rectangular pulses at 25 Hz, pulse duration 400 ms, duty cycle 10 s on and 30 s off to quadriceps muscle bilaterally. CG: no intervention	1 times/d for 5 d/wk	45 min	4 wk	Physical function Mental and physical health	From discharge to 4-wk post-discharge: No significant interaction effect for total SPPB score (P = 0.11; ηp2 = 0.073; CG: 2.89 ± 0.50 vs NMES: 4.11 ± 0.54 units). Time effects for 6MWT distance (P < 0.01; ηp2 = 0.207; CG: 194 ± 18 vs NMES: 267 ± 16 m) and 6MWT power output (P = 0.01; ηp2 = 0.168; CG: 0.4 ± 0.1 vs NMES: 0.6 ± 0.1 W; P = 0.01)	No complications
Cerqueira et al[26]	NMES: Stimulation at 50 Hz, duration 400 ms duty cycle 3 s on and 9 s off, to rectus femoris and gastrocnemius muscle bilaterally. Regular physiotherapy care twice a day. CG: Usual physiotherapy care twice a day	2 times/d	60 min	From POD 1 to POD 5	Distance walked; Gait speed; Lactate levels Muscle strength Electromyographic activity of the rectus femoris; Functional Independence Measure	No significant difference in the distance walked (P = 0.650) between NMES group (239.06 ± 88.55) and CG (254.43 ± 116.67) as well as gait speed (P = 0.363), lactate levels (P = 0.302), knee extensor strength (P = 0.117), handgrip strength (P = 0.882), global muscle strength (P = 0.104), electromyographic activity (P = 0.179) and Functional Independence Measure (P = 0.059)	No complications
Takino et al[27]	NMES: Biphasic symmetric square pulses at 20 and 200 Hz, duty cycle 0.4 s on and 0.6 s off to vastus lateralis, vastus medialis and triceps surae muscle bilaterally. Standard post- surgical rehabilitation. CG: Standard post- surgical rehabilitation.	1 time/d	60 min	from POD 1 to POD 7	% change in isometric knee strength; % change in usual and maximum walking speed; % change in grip strength	↓ %ΔIKES in the NMES than CG [NMES: Mean -2%, 95% confidence interval (CI) -6 to 1; CG: -13%, 95% CI -17 to -9, P < 0.001]. ↓ %ΔMWS (P = 0.04). ↓ %ΔUWS and %ΔGS in the NMES compare to CG but not statistically significant	Non mentioned
Sumin et al[28]	NMES: rectangular pulses at 45 Hz, duty cycle 12 s on and 5 s off to quadriceps muscle bilaterally. Standard preoperative rehabilitation program; CG: Standard preoperative rehabilitation program	1 time/d	90 min	from the 2nd day of hospital stay until the day before surgery (7–10 sessions)	Exercise capacity; Muscle strength	↑ in KES, KFS, and 6MWT distance (all P < 0.001) in the NMES group compared to the CG. Slight ↑ in HS to the NMES group and slight ↓ to the CG but not statistically significant (P = 0.054 on the right hand and P = 0.062 on the left)	No complications

¹Patients were discharged from hospital and submitted to phase I of cardiac rehabilitation.

²Home-based intervention after hospital discharge.

CG: Control group; CSA: Cross-sectional area; FES: Functional electrical stimulation; ICU: Intensive care unit; KEIS: Knee extensor isometric strength; MLT: Muscle layer thickness; NMES: Neuromuscular electrical stimulation POD: Post operative day; SPPB: Short physical performance battery; 3-MH/Cre: 3-methylhistidine concentration corrected for urinary creatinine content; 6MWT: Six minutes walking test; KFS: Knee flexor strength; KES: Knee extensor strength; NA: Not available.