2. Characteristics of studies that contributed data to meta‐analyses (NMES + exercise versus exercise alone).

Study	Setting	Lower limb stimulation	Clinical stability	Dose	Frequency (Hz)	Exercise intervention received by control group
Abdellaoui 2011	Intensive care unit	Bilateral quadriceps and hamstrings	Acute exacerbation	1 hour per day, 5 days per week for 6 weeks at maximum tolerated current	35	Both groups received education (once per week) and daily active‐passive mobilisation (control group also received sham stimulation).
Akar 2017	Intensive care unit	Bilateral quadriceps	Respiratory failure	5 days per week (total of 20 sessions) at maximum tolerated current	50	Both groups received active exercise, which comprised active joint range of motion exercise for upper and lower limbs. Participants who could not manage active exercise received active‐assisted or passive range of motion exercise.
Akinlabi 2013	Home	Bilateral quadriceps and hamstrings	Stable	2 days per week for 8 weeks (total of 16 sessions)	10‐50	Low‐intensity symptom‐limited exercise
Dang 2011	Outpatient	Bilateral quadriceps	Stable	36 min, 3 sessions per week for 12 weeks (total of 36 sessions) at maximum tolerated current	8‐45	Usual respiratory rehabilitation (no other details given)
Kucio 2016	Inpatient rehabilitation	Bilateral quadriceps and calf	Stable	36 min, presumably 6 supervised sessions per week for 3 weeks, intensity not specified	35	Both groups received breathing exercises, treadmill walking and resistance exercise.
Tardif 2015	Home	Bilateral quadriceps	Stable	Presumably 30 min per day, 5 days per week, presumably for 8 weeks at maximum tolerated current	35	Both groups received pulmonary rehabilitation.
Tasdemir 2015	Outpatient	Bilateral quadriceps	Stable	20 min, 2 days per week for 10 weeks at maximum tolerated current	50	Both groups received pulmonary rehabilitation (control group also received sham stimulation).
Vivodtzev 2006	Inpatient rehabilitation	Bilateral quadriceps	Stable, but shortly following acute illness	> 30 min per session, 4 times per week, for 4 weeks at maximum tolerated current	5‐35	Both groups received active limb exercises. The strongest participants also performed walking on a treadmill together with 5‐10 min of resistance arm exercises. They also completed health education sessions 1 day per week.
Zanotti 2003	Respiratory high dependency unit for inpatient rehabilitation	Bilateral quadriceps and gluteals	Stable, but shortly following acute illness	Up to 30 min per session, 2 times per day, 5 days per week for 4 weeks presumably at maximum tolerated current	8‐35	Both groups received rehabilitation that comprised active limb exercises.

min: minute; NMES: neuromuscular electrostimulation.