The purpose of this investigation was to describe the relationship between the central activation ratio (CAR) and the percent maximum voluntary effort (% MVE) during isometric quadriceps femoris contractions. Twenty-one healthy, young adults participated in three test sessions. During each session, one of three train types was tested: a 100-HZ 120-ms train, a 100-HZ 250-ms train, or a 50-HZ 500-ms train. Subjects were seated on a force dynamometer and stabilized to perform a 3-5-s isometric knee extension at MVE. Force targets were set at 25, 50, 75, and 100% of the MVE. With 5 min rest between efforts, subjects produced forces at the specified target levels. When each target was reached, the test train was delivered to quantify the amount of central activation. There were no significant differences in CARs across train types during maximal efforts, but during submaximal efforts at 25 and 50%, the 100-HZ 250-ms and 50-HZ 500-ms trains produced significantly lower CARs than the 100-HZ 120-ms train. The relationship between the CAR and the %MVE was curvilinear and best described by a second-order polynomial for all three train types. If tests of central activation are going to be used clinically, it is important to know the relationship between the CAR and voluntary effort; however, further study will be required to extend these results to specific patient populations.