In order to determine whether a prestretch potentiation of human leg extensor muscle is affected during explosive type of strength training, the squatting jump, counter movement jump, and rebound jump were performed by 24 subjects which were divided consisted of thue 3 groups of althletes, i.e.,swimmers,jumpers, and weight trainers. We regarded swimmer as untrained subject, jumper as trained subject of being characteristic of ballistic jump movement, and weight trainer as specially trained subject on the strenght of leg muscles. Four parameters, such as height of jump, knee angular velocity, positive force integral which would show the use of prestretch potentiation, and coupling-time which would affect the former three parameters, were obtained from the measurement of force-time curve and goniogram. The results indicated that prestretch potentiation was utilized more effectively in trained subjects compared with untrained subjects. The use of prestretch potentiation may increase the first half of knee extension speed, and the strength training for the muscle contractile component may increase the latter half of knee extension speed during jumping action. The training effect on prestretch potentiation of skeletal muscle plays an important role during explosive type of jumping action. The present findings thus futher support the concept of specificity of the muscle strength training. Elastic component of the muscle is also discussed.