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Single-joint isometric and isokinetic knee strength assessment plays an important role in strength and conditioning, physical therapy, and rehabilitation. The literature, however, lacks absolute reference values. We systematically reviewed the available studies that assessed isometric knee strength. Two scientific databases (PubMed and PEDro) were searched for the papers that are published from the inception of the field to the end of 2019. We included studies that involved participants of both genders and different age groups, regardless of the study design, that involved isometric knee extension and/or flexion measurement. The extracted data were converted to body-mass-normalized values. Moreover, the data were grouped according to the knee angle condition (extended, mid-range, and flexed). A meta-analysis was performed on 13,893 participants from 411 studies. In adult healthy males, the pooled 95% confidence intervals (CI) for knee extension were 1.34–2.23Nm/kg for extended knee angle, 2.92–3.45Nm/kg for mid-range knee angle, and 2.50–3.06Nm/kg for flexed knee angle, while the CIs for flexion were 0.85–1.20, 1.15–1.62, and 0.96–1.54Nm/kg, respectively. Adult females consistently showed lower strength than adult male subgroups (e.g., the CIs for knee extension were 1.01–1.50, 2.08–2.74, and 2.04–2.71Nm/kg for extended, mid-range, and flexed knee angle condition). Older adults consistently showed lower values than adults (e.g., pooled CIs for mid-range knee angle were 1.74–2.16Nm/kg (male) and 1.40–1.64Nm/kg (female) for extension, and 0.69–0.89Nm/kg (male) and 0.46–0.81Nm/kg (female) for flexion). Reliable …