## 1 Introduction  
Jumping ability is one of the most important skills of basketball players [1]. Blocked shots and lay-ups are common skills in basketball competition, which is performed with a single leg. Differing from standing vertical jumps, the running single leg jump (RSJ) is generally preceded by an approaching and takeoff with one leg, [2; 3] which is similar to the long jump and high jump. Hence, the ability of the RSJ is important for improving performance in sports. Moreover, a lot of the sports involve taking off from a single leg such as high jump, basketball, and handball [4; 5].  
The RSJ is a typical skill in sports competition. For example, such as being able to jump higher, faster, and farther, to break through the hostile or increase offensive opportunity in different circumstances. The ability of energy conversion from the horizontal-to-vertical explains the RSJ skill feature because the major joint power and work were generated by lower extremity [6]. Some studies reported horizontal kinetic energy of the center of mass (CoM) decreases to transformation a higher vertical kinetic energy and potential energy during the long jump and high jump touchdown phase [7; 8] and the takeoff leg is main conversion pivoting. Ankle plantar-flexion, knee, and hip extension torques are believed to be the main contributors that generate the energy for the jumping height in RSJ [8; 9].  
Various studies investigated the biomechanical difference and functional application on RSJs movement through kinematic, kinetic and EMG. They included the training effect of RSJ [4; 5; 10], the difference in long jump [8], high jump [7; 11; 12], and approach drop jump [10; 13]. None of them provide enough specific descriptions on RSJ of lower extremity work. As we know, jumping performance is achieved by the coordination, interaction and self-organization of the ground kinetic, body segments and muscles [14]. Hence, to understand how an external force triggers the stretch-shortening cycle (SSC) and stretch reflex is needed. In addition, we need to know how the body segment and joints respond.  
Some studies reported an approach that increases jumping movement activation level, thus improving the explosive power and coordination of the neuromuscular control of lower limbs [5; 10]. Previous studies reported that the running movement can increase both the ground reaction force [15] and lower extremities' joint torque [7; 10] while maintaining a shorter ground contact time [15; 16]. Therefore, the feature of RSJ causes a faster and more powerful jump than a jump from a standing position. It is crucial to investigate the biomechanical characteristics of RSJ, as it can facilitate in determining the factor that improves jumping capability, decreasing risks of injury, or using RSJ as an optimized training method for explosive power.  
Although studies were conducted based on the biomechanical parameters of RSJ between jumps for height and length, they are primarily used to compare the difference in parameters and are not applicable to athletes or coaches. To the best of our knowledge, none have examined the biomechanical characteristics of RSJ. Therefore, the present study aimed to clarify the biomechanical characteristics and muscle activation condition of the lower extremities during RSJ in collegiate basketball players. We hypothesized that RSJ performance will be affected by running speed and that the lower extremity biomechanical parameters may correspond with muscle activation, in which the ankle joint may be an essential factor.