Background: the study determined the validity and reliability of measurements obtained using the portable traction dynamometer (PTD) (E-Lastic, E-Sports Solutions, Brazil) and the reproducibility between evaluators (precision) in the evaluation of the isometric muscle strength of the knee extensors of healthy male adults, compared to measurements obtained with the “gold standard” computerized dynamometer (CD) (Biodex System 3, Nova York, NY, USA). Methods: we evaluated sixteen recreationally active men (29.50 ± 7.26 years). The test–retest reliability of both equipment to determine quadriceps strength, agreement analysis, and the minimal important difference were verified. Results: excellent test–retest interrater reliability was observed for absolute and relative measurements, with a low absolute error for both sets of equipment and excellent validity of the PTD against the CD, as verified by linear regression and Pearson’s correlation coefficient. Conclusions: PTD is a valid and reliable instrument for assessing the isometric strength of knee extensors, with results similar to the isometric CD “gold standard”.