Abstract: Nineteen studies met the inclusion criteria of which 16 contributed data on 267 participants with COPD (mean age 56 to 76 years and 67% were men). Of these 16 studies, seven explored the effect of NMES versus usual care and nine explored the effect of NMES plus conventional exercise training versus conventional exercise training alone. Six studies utilised sham stimulation in the control group. When applied in isolation, NMES produced an increase in peripheral muscle force (SMD 0.34, 95% CI 0.02 to 0.65; low-quality evidence) and quadriceps endurance (SMD 1.36, 95% CI 0.59 to 2.12; low-quality evidence) but the effect on thigh muscle size was unclear (MD 0.25, 95% CI -0.11 to 0.61; low-quality evidence). There were increases in six-minute walk distance (6MWD) (MD 39.26 m, 95% CI 16.31 to 62.22; low-quality evidence) and time to symptom limitation exercising at a submaximal intensity (MD 3.62 minutes, 95% CI 2.33 to 4.91). There was a reduction in the severity of leg fatigue on completion of an exercise test (MD -1.12 units, 95% CI -1.81 to -0.43). The increase in peak rate of oxygen uptake (VO2peak) was of borderline significance (MD 0.10 L/minute, 95% CI 0.00 to 0.19). For NMES with conventional exercise training, there was an uncertain effect on peripheral muscle force (SMD 0.47, 95% CI -0.10 to 1.04; very low-quality evidence) and there were insufficient studies to undertake a meta-analysis on the effect on quadriceps endurance or thigh muscle size. However, there was an increase in 6MWD in favour of NMES combined with conventional exercise training (MD 25.87 m, 95% CI 1.06 to 50.69; very low-quality evidence). In people admitted to either in an intensive care unit or a respiratory high dependency centre, NMES combined with conventional exercise reduced the time taken for participants to first sit out of bed by 4.98 days (95% CI -8.55 to -1.41; very low-quality evidence), although the statistical heterogeneity for this analysis was high (I2 = 60%). For both types of studies (i.e. NMES versus usual care and NMES with conventional exercise training versus conventional exercise training alone), there was no risk difference for mortality or minor adverse events in participants who received NMES. NMES, when applied in isolation, increased quadriceps force and endurance, 6MWD and time to symptom limitation exercising at a submaximal intensity, and reduced the severity of leg fatigue on completion of exercise testing. It may increase VO2peak, but the true effect on this outcome measure could be trivial. However, the quality of evidence was low or very low due to risk of bias within the studies, imprecision of the estimates, small number of studies and inconsistency between the studies. Although there were no additional gains in quadriceps force with NMES plus conventional exercise training, there was evidence of an increase in 6MWD. Further, in people who were the most debilitated, the addition of NMES may have accelerated the achievement of a functional milestone, that is, the first time someone sits out of bed.

Plain-language summary: Nineteen studies met the inclusion criteria for the review, of which 16 had data on 267 participants that could be included in the analyses. The average age of people in each of the studies ranged from 56 to 76 years and 179 (67%) were men. Seven studies explored the effect of applying electrical stimulation alone and nine studies explored the effect of adding electrical stimulation to an exercise programme. Electrical stimulation was applied in a range of settings, such as at home, in an outpatient hospital department, on a hospital ward or in an intensive care unit. Most studies stimulated the thigh muscles once or twice a day for 30 to 60 minutes on four to seven days each week for four to eight weeks. Studies that explored the effect of applying electrical stimulation alone showed an increase in strength and endurance of the thigh muscles. They showed an increase in some, but not all, measures of exercise capacity and a decrease in the severity of leg fatigue after exercise. Studies that explored the effect of adding electrical stimulation to an exercise programme showed a small increase in the distance walked in six minutes. In people who were most unwell (e.g. in an intensive care unit), adding electrical stimulation to an exercise programme helped people to spend fewer days confined to bed. Electrical stimulation did not increase the risk of side effects. The quality of evidence provided by this review was low. This is because most studies had design problems. The inclusion of future studies into this review is likely to change the results.