The aims were to assess the reliability of knee extensor and flexor muscle strength measurements using the Strength Measuring Chair (SMC) in children and adults, and to assess agreement between the SMC and an isokinetic dynamometer (ID). 20 healthy children (10/10 boys/girls, 5-13 years) and 23 adults (10/13 men/women, 23-60 years) were included. Muscle strength tests were performed in the SMC and in the ID. The intra-subject reliability was shown to be excellent in both instruments (ICC 0.93-0.99) and an excellent test-retest reliability of measurements in the SMC was found (ICC 0.87-0.93). Agreement between instruments was evaluated with ICC, paired t-test and Bland-Altman 95% limits of agreement plots. In both adults' and children's groups, disagreements were found between the two instruments, with more in the children's group. The SMC, which has previously shown to be valid and reliable for measuring plantarflexor muscle strength, reliably measured knee muscle strength in healthy children and adults. However, the large disagreement found between the instruments, especially in the children's group, warrants care in standardizing measuring positions for different body size and warrants caution in comparing one's muscle strength measured using different assessment methods.