This study investigated the activation of major lower limb muscles during isometric squats (ISs) at different knee angles and their effects as a conditioning activity (CA) on squat jump (SJ) performance at varying initial knee angles. Twelve collegiate sprinters were randomly assigned to three conditions: control (CC), 90°IS condition (90°ISC), and 120°IS condition (120°ISC), using a balanced crossover design. EMG data from five lower limb muscles were collected during ISs, alongside the peak center of mass velocity (V peak), peak force (F peak), peak power (P peak), average power (P avg), and the force-velocity ratio at peak power (SFV) during subsequent SJs. Results showed that the 90°IS condition elicited higher rectus femoris (p = 0.035, d = 0.729) activation, while the 120°IS condition led to greater soleus (p = 0.030, d = 0.759), and tibialis anterior (p = 0.022, d = 0.812) activation. Both the 90°ISC and the 120°ISC significantly increased peak velocity (90°ISC: p < 0.001, d = 1.379; 120°ISC: p < 0.001, d = 0.979) compared to the CC, but showed no significant improvements in F peak or P peak. Notably, the SFV during the 90° and 120°SJ was significantly lower in the 90°ISC and the 120°ISC compared to the CC (90°ISC: p < 0.001, d = −2.658; 120°ISC: p < 0.001, d = −1.785). These findings suggest that the IS at different knee angles selectively activates lower limb muscles and enhances velocity-related performance metrics. Specific IS angles can alter the force-velocity relationship, increasing the velocity component at peak power during subsequent movements. Coaches and athletes in speed-dependent sports are encouraged to select IS angles tailored to their specific training goals and performance needs.
