Chen et al. 2002, Validity of the Borg RPE scale, a meta-analysis

Meta-analysis of the criterion validity of the Borg rating of perceived exertion. RPE correlates moderately with blood lactate (pooled coefficient about 0.57) and with heart rate and oxygen uptake, with stronger correlations in some submaximal running studies. RPE is a usable but imprecise effort anchor. Moderate.