Objectives: To test the reliability of a new muscle strength testing instrument (the Strength Measuring Chair [SMC]) designed to quantify isometric strength in the lower extremities, and to determine the agreement between the SMC and an isokinetic dynamometer (Biodex). Design: Isometric strength tests were performed in plantar-flexors with 2 different knee positions (60 degrees, 30 degrees). Measurements were taken at 3 different sessions. Setting: Strength testing laboratory. Participants: Twenty-three able-bodied adults and 15 able-bodied children. Interventions: Not applicable. Main Outcome Measure: Isometric plantarflexor strength. Results: The reliability of isometric strength measurements of plantarflexors taken in the SMC was excellent for both the adult and children groups (intraclass correlation coefficient range,.84-.87). A Bland-Altman 95% limit of agreement test showed no systematic variation in 3 of the 4 SMC test observations; systematic variation was only observed in the adult group at a knee position of 30 degrees. There was no systematic difference in the adult group between the SMC and the isokinetic dynamometer, but there was a systematic variation in the children's group. Conclusions: The SMC reliably measured isometric plantarflexor strength in the tested populations.