Table 2. Characteristics of interventions for the researches included.
| Investigator | Intervention duration | Intervention parameters | Outcomes |
|---|---|---|---|
| Daniel, 2020 | 4 weeks, 5 sessions/week, 60 min/session | Frequency: 20–35 Hz; pulse duration: NR, intensity: 15–90 mA; duty cycle: NR | SGRQ, mMRC, spirometry, PImax, PEmax, 6MWT, bio-electrical impedance |
| Mekki, 2018 | 24 weeks, three times per week, 45 min underwent the same endurance training and 20 min of NMES | Frequency: 50 Hz; pulse duration: 400 μs; intensity: 15–60 mA; duty cycle: 5 s on/15 s off during the first 3 months and 10 s on/30 s thereafter | A stabilometric platform, time up and go, Berg balance scale tests, 6MWT, the maximal voluntary contraction |
| Valenza 2018 | 8-week: 1 h twice a week (2 h/week). Ten minutes of warm-up; 30 min of NMES superimposed on to voluntary muscular contraction; 5 min of relaxation/cool down | Frequency: 50 Hz; pulse duration: 400 ms; the contraction time was 8 s with 20 s of relaxation | SGRQ; 6MWT; 5STS; controlling heart rate (HR), respiratory rate (RR), dyspnea; leg fatigue; FIM; LCADL scale |
| Bonnevie, 2018 | 8 weeks: 5 times per week; 30 min of stimulation | Pulse duration: 400 ms; 10 min of warm-up at 6 Hz, the intensity was individually adjusted to just under the pain threshold. The frequencies used were 35 Hz for the contractions and 4 Hz for the active rest phases, with a duty cycle of 0.5 and 1.5 s for 25 min. Three-minute recovery period at 3 Hz | 6MWT; VO2 peak; maximal workload during CPET; Dyspnea; BMI; airflow obstruction; exercise capacity index, and HRQoL, including SGRQ subscores |
| Maddocks, 2016 | 6 week × 7 sessions/week; 30 min/per session | Frequency: 50 Hz; pulse duration: 350 μs; intensity: max tolerable; duty cycle: 2 s on/15 s off to 10 s on/15 s off | 6MWT; quadriceps muscle strength (MVC); fat-free mass via bioimpedance physical activity; SGRQ |
| Kucio, 2016 | Not provided | NMES of the quadriceps and gastrocnemius using symmetric rectangular impulses with pulse width of 0.30 ms at a frequency of 35 Hz for 2 s on and 4 s off for 36 min | 6MWT; airflow obstruction; arterial oxygen and carbon dioxide concentrations |
| Tasdemir, 2015 | 10 week × 2 sessions/week; 20 min/per session | Frequency: 50 Hz; pulse duration: 300 μs; intensity: max individual tolerance (29.43–35.81 mA); duty cycle: 10 s on/20 s off | SWT; dyspnoea; quadriceps muscle strength; exercise endurance; MRC; SGRQ LCADL; feelings of anxiety and depression |
| Vieira, 2014 | 8 week × 5 sessions/week; 60 min/per session | Frequency: 50 Hz; pulse duration: 300–400 μs; intensity: max tolerable (15–20 to 100 mA); duty cycle: 2 s on/18 s off to 10 s on/30 s off | Thigh circumference; 6MWT; VO2 peak and endurance time; dyspnea; muscle activity; SGRQ; fat-free mass; respiratory muscle strength |
| Sillen, 2014 | 8 week × 5 sessions/week; 18 min/per session | Frequency: 75 Hz; pulse duration: 400 μs intensity: max tolerable; duty cycle: not provided | Quadriceps muscle strength (isokinetic quadriceps muscle strength); constant work-rate cycling endurance test; 6MWT; exercise endurance; dyspnea; SGRQ |
| Vivodtzev, 2012 | 6 week × 5 sessions/week; 35 min of stimulation of the quadriceps followed by 25 min of stimulation of the calf/per session | Frequency: 50 Hz; Pulse duration: 400 μs; Intensity: NR; Duty cycle: 2 s on/16 s off | Quadriceps strength and endurance; incremental and endurance shuttle walk test with cardiorespiratory monitoring; muscle signaling pathways, enzyme activity, fiber type and size, and capillarization via biopsy |
| Vivodtzev, 2006 | 4 week × 4 sessions/week; 30 min/per session | Frequency: 35 Hz; pulse duration: 400 μs; intensity: max tolerable (21–46 mA); duty cycle: 47% | Airflow obstruction via spirometry; BMI; Quality of life and dyspnoea; Respiratory failure questionnaire; Quadriceps muscle strength; Quadriceps muscle composition |
| Neder, 2002 | 6 week × 5 sessions/week; 15 min/per session for the first week, after 30 min/per session | Frequency: 50 Hz; pulse duration: 300–400 μs; intensity: 10–20 to 100 mA; duty cycle: 2 s on/18 s off to 10 s on/30 s off | Quadriceps strength (peak torque), exercise endurance; cardiopulmonary exercise test; HRQoL; lung volumes and airflow obstruction |
| Bourjeily-Habr, 2002 | 6 week × 3 sessions/week; 20 min/per session | Frequency: 50 Hz; pulse duration: NA; intensity: 56.7–95 mA; duty cycle: 0.2 s on/1.3 s off | SWT; quadriceps strength (isokinetic peak torque); cardiopulmonary exercise test |
Abbreviations: cPR, comprehensive PR; FEV1, forced expiratory volume in 1 s; FIM, functional independence measure; MRC, Medical Research Council scale; MVC, maximum voluntary contraction; SWT, shuttle-walk test; UR, usual rehabilitation; 5STS, five-times-sit-to-stand test; 6MWT, 6-min walk test.