Background The relationship between muscle function in critically ill patients assessed using bedside techniques and clinical outcomes has not been systematically described. We aimed to evaluate the association between muscle weakness assessed by bedside evaluation and mortality or weaning from mechanical ventilation, and the capacity of each evaluation tool to predict outcomes. Methods Five databases (PubMed, EMBASE, CINAHL, Cochrane library, Science Direct) were searched from January 2000 to December 2018. Data were extracted and random effects meta-analyses were performed. Results Sixty studies were analysed, including 4382 patients. ICU-related muscle weakness was associated with an increase in overall mortality with odds ratios ranging from 1.2 [95% CI 0.60 to 2.40] to 4.48 [95% CI 1.49 to 13.42]. Transdiaphragmatic twitch pressure had the highest predictive capacity for overall mortality, with a sensitivity of 0.87 [95%CI 0.76 to 0.93] and a specificity of 0.36 [95%CI 0.27–0.43]. The area under the curve (AUC) was 0.74 [95%CI 0.70 to 0.78]. Muscle weakness was associated with an increase in mechanical ventilation weaning failure rate with an odds ratio ranging from 2.64 [95% CI 0.72 to 9.64] to 19.07 [95% CI 9.35 to 38.9]. Diaphragm thickening fraction had the highest predictive capacity for weaning failure with a sensitivity of 0.76 [95%CI 0.67 to 0.83] and a specificity of 0.86 [95%CI 0.78 to 0.92]. The AUC was 0.86 [95%CI 0.83 to 0.89]. Conclusion ICU-related muscle weakness detected by bedside techniques is a serious issue associated with a high risk of death or prolonged mechanical ventilation. Evaluating diaphragm function should be a clinical priority in the ICU.