RATIONALE: Muscle weakness is a common impairment after stroke. Electrical stimulation that contracts muscles strongly (cyclical electrical stimulation (CES)) may improve voluntary strength, and electrical stimulation during the practice of an activity (functional electrical stimulation (FES)) may improve the performance of that activity. OBJECTIVES: To evaluate the benefits and harms of electrical stimulation of the motor system for adults with stroke. Specifically, the objectives were to evaluate whether: CES is effective at increasing voluntary strength compared with nothing/sham intervention, and if this carries over to the performance of an activity; CES is effective at increasing voluntary strength compared with another strengthening intervention, and if this carries over to the performance of an activity; FES is effective at improving activity compared with nothing/sham; FES is effective at improving activity compared with practice of the same activity without stimulation. For each comparison, we also wanted to examine the effects on participation and quality of life, and to describe potential harms (e.g. adverse events). SEARCH METHODS: We searched CENTRAL, MEDLINE, Embase, four other databases, and two trial registries, plus reference checking to identify trials for inclusion in the review. The latest search date was December 2024. ELIGIBILITY CRITERIA: We included randomised controlled trials (RCTs) and randomised cross-over trials that compared CES versus nothing/sham or another strengthening intervention, or FES versus nothing/sham or practice of the same activity without stimulation, in adults with stroke. OUTCOMES: Our critical outcome of interest was voluntary strength (i.e. without electrical stimulation) for CES, and activity trained (i.e. without electrical stimulation) for FES. Our important outcomes were participation restrictions, quality of life, and adverse events. RISK OF BIAS: We assessed risk of bias in the included studies using the Cochrane RoB 1 tool. SYNTHESIS METHODS: We synthesised results for each outcome using meta-analysis, where possible, by calculating standardised mean differences (SMD) with 95% confidence intervals (CI) for continuous outcomes. Where this was precluded by the nature of the data, we summarised the results narratively. We used the GRADE approach to assess the certainty of the evidence. INCLUDED STUDIES: We included 83 RCTs and 6 randomised cross-over trials involving a total of 2905 participants. Thirty-four trials investigated CES (1176 participants), and 55 trials investigated FES (1729 participants). Most trials were small, with only nine enrolling more than 50 participants. The risk of bias for most items was low; however, many were unclear (especially regarding allocation concealment and selective reporting), and notably, numerous trials did not use blinded assessors. SYNTHESIS OF RESULTS: CES VERSUS NOTHING/SHAM: CES showed a moderate effect on strength immediately post-intervention (SMD 0.47, 95% CI 0.32 to 0.61; 23 trials, 741 participants; high-certainty evidence) and on activity (SMD 0.50, 95% CI 0.34 to 0.65; 20 trials, 662 participants; high-certainty evidence) compared to nothing/sham. These effects were likely maintained beyond the intervention for both strength (SMD 0.48, 95% CI 0.28 to 0.69; 11 trials, 384 participants; moderate-certainty evidence) and activity (SMD 0.57, 95% CI 0.37 to 0.78; 11 trials, 385 participants; moderate-certainty evidence). We found no usable data for participation or quality of life outcomes. CES VERSUS ANOTHER STRENGTHENING INTERVENTION: The evidence is very uncertain for the effect of CES on strength immediately post-intervention (SMD -0.07, 95% CI -0.70 to 0.57; 1 trial, 38 participants; very low-certainty evidence) and beyond the intervention (SMD 0.23, 95% CI -0.41 to 0.87; 1 trial, 38 participants; very low-certainty evidence) compared to another strengthening intervention. The evidence is also very uncertain for the effect of CES on activity immediately post-intervention (SMD 0.47, 95% CI -0.18 to 1.11; 1 trial, 38 participants; very low-certainty evidence) and beyond the intervention (SMD 0.49, 95% CI -0.16 to 1.14; 1 trial, 38 participants; very low-certainty evidence). FES VERSUS NOTHING/SHAM: FES showed a likely moderate effect on activity immediately post-intervention (SMD 0.42, 95% CI 0.16 to 0.68; 12 trials, 235 participants; moderate-certainty evidence) and may have a large effect beyond the intervention (SMD 0.87, 95% CI 0.29 to 1.44; 3 trials, 53 participants; low-certainty evidence). We found no usable data for participation or quality of life outcomes. FES VERSUS PRACTICE OF THE SAME ACTIVITY: FES showed a small effect on activity immediately post-intervention (SMD 0.22, 95% CI 0.11 to 0.34; 35 trials, 1193 participants; high certainty evidence). FES was probably more effective than practice of the same activity beyond the intervention, but the CI for this result includes the possibility of no effect (SMD 0.22, 95% CI -0.02 to 0.46; 9 trials, 268 participants; moderate-certainty evidence). The evidence is uncertain for the effect of FES on participation (SMD 0.20, 95% CI -0.68 to 1.08; 1 trial, 20 participants; very low-certainty evidence) and quality of life (SMD 0.06, 95% CI -0.03 to 0.43; 1 trial, 119 participants; low-certainty evidence). Across trials of CES and FES, adverse events were not commonly reported, with few overall events related to electrical stimulation, and no serious events described (low- to moderate-certainty evidence). AUTHORS' CONCLUSIONS: High-certainty evidence shows that CES has a moderate effect on strength compared with nothing/sham, and that this carries over to activity. There is moderate-certainty evidence that FES likely has a moderate effect on activity compared with nothing/sham, and high certainty evidence that FES has a small effect compared with practice of the same activity. CES can therefore be used to increase strength in those with weakness after stroke. However, there is limited evidence to inform decisions about the effect of CES compared to other interventions to increase strength. As the person after stroke regains movement and can attempt everyday activities, there is only a small benefit from adding electrical stimulation during the performance of that activity.

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