Recent reports suggest that training to increase the amount of hamstring activation and hamstring to quadriceps activation ratios (H:Q) may stabilize the knee during jump landings and cutting, and potentially prevent anterior cruciate ligament injuries. However, the potential effect of increased antagonist activation may impair the agonist, including the performance of activities involving knee extension. This study evaluated the relationship between H:Q and countermovement jump (CMJ) height. Subjects included 43 female high school and college students (age = 19.14 ± 1.8 years). All subjects provided informed consent, as well as parental consent for those who were less than 18 years old. The study was approved by the university review board. Subjects performed 2 repetitions of maximum voluntary isometric contractions (MVIC) for the quadriceps and hamstring muscles. Subjects also performed 2 repetitions each of the drop jump from a height equal to their countermovement jump (DJ). Electromyographic (EMG) data were collected for the rectus femoris, vastus lateralis, vastus medialis, lateral hamstring, and medial hamstring. Root mean square (RMS) signal processing was used on all EMG data which were analyzed to assess the magnitude and timing of the muscles' bursts pre and post landing for the DJ using the average of both trials and normalized to MVIC. The timing of the foot contact was synchronized with the EMG data using a switch mat. The H:Q were calculated from the collective average of the hamstring muscles divided by the collective average of the quadriceps muscles. Data were evaluated using a Pearson's correlation coefficient in order to examine the relationship between the subject's H:Q and CMJ height for both the pre and post foot contact phase of the DJ. Results revealed that pre landing H:Q was positively correlated to CMJ height (r = .35, p = 0.02). Post-landing H:Q was not correlated to CMJ height (r = .21, p = 0.18) This study demonstrates that high levels of hamstring activation, relative to the quadriceps, do not impair, and may be positively associated with CMJ performance. The prescription of additional hamstring training for female athletes appears unlikely to impair performance of exercises that involve dynamic knee and hip extension, such as the countermovement jump.