This paper introduces a model to estimate knee joint extension moment during isokinetic contraction exercise. It is possible to estimate knee joint extension moment combining Hill-type muscle model, analytic results on moment arm, musculotendon length and muscle activation if muscle parameters are exactly known. However, the muscle parameters are all different to each person. And it is almost impossible to know about exact values of those. Thus Delp's data is used except for tendon slack length which is the most critical parameter among Hill-type muscle model parameters. Each tendon slack length of muscles consisting of quadriceps is numeric optimized at MVC and applied to isokinetic contraction condition to evaluate the model by comparison between experimental and estimated results.