# Dynamic Characteristics of Approach Spike Jump Tasks in Male Volleyball Players
## 1 Introduction  
Spiking is a crucial technique in the sport of volleyball. Volleyball players usually perform volleyball spiking with combined movement of three-step approach running, countermovement jumping, and hitting the ball straight down on the other side of the net [1]. The approach running spike-jump (RSJ) is often practiced in volleyball training programs [1] to enhance jumping performances required for volleyball players. During the RSJ, volleyball players have to approach the target rapidly, jump off the ground, and smash the volleyball vigorously and accurately at the highest position in the air. Fundamental volleyball techniques, such as front-row spikes of a high set and short set ball in an attack tactic, are often practiced with the RSJ [2].  
Previous researchers reported kinematic differences on duration of flight, jump height, and ball velocity in the front-row spike between the short set and high set ball [2] and indicated that the performance of the three-step approach RSJ was mainly influenced by vertical jump ability [1] and technical coordination [3]. The jump height demonstrated a positive correlation with the approach velocity, velocity transfer, upper-body lean, range of motion of the knee and ankle joints [4], and knee flexion angle [1]. Moreover, higher approach running velocity before a jump can enhance the stretch-shortening cycle (SSC) [5], make the subsequent neuromuscular force production and muscular concentric contractionmore effective [6; 7], and further improve the take-off velocity of the center of mass (COM) and jumping performance [8].  
For a tactical purpose of the spike, both fast horizontal and vertical momentum of the COM during the RSJ are needed. The RSJ includes a three-step approach run where the last step is transitional to prepare for transferring horizontal momentum to vertical momentum, and a horizontal run to a vertical jump [9]. The last step of the approach run and take-off movement are important for the RSJ performance. Previous studies showed that the approach run can increase both ground reaction force and joint moment of the lower extremity [10; 11; 12] while maintaining shorter ground contact time [12; 13], and improve explosive power and coordination of the neuromuscular control of the lower extremity [8; 14].  
Two types of RSJs are commonly used for the volleyball spike attack: (1) RSJ with one leg (RSJ-1L) and (2) RSJ with two legs (RSJ-2L). The RSJ-1L is usually performed in a quick spike of a short set ball with quick take-off movement and large body shift, while the RSJ-2L is usually performed in a block and spike of a high set ball with significant countermovement [1]. The coordination of the countermovement was crucial for a two-leg vertical jump performance [15], while [PERSON] et al. [16] reported that an optimal jumping performance was affected by the approach velocity before touchdown. Previous studies found that the RSJ-1L produced larger kinetic feedback and energy generation in simulation [17] but decreased joint angular displacement making the stiff lower extremity [12].  
Although previous researchers have conducted certain biomechanical analyses on the RSJ-2L [1; 4] and analyzed the RSJ-1L using kinematic [12] or simulation methods [16], respectively, limitations exist with no comprehensive comparison. Based on the aforementioned different practical characteristics between RSJ-1L and RSJ-2L, it is worth to further look into academic differences in kinetics combined with kinematics between RSJ-1L and RSJ-2L. Therefore, a comprehensive biomechanical analysis of RSJs is needed to extend knowledge of clarification of RSJ-1L and RSJ-2L. The purposes of the current study were to compare the kinematic and kinetic differences between the RSJ-1L and RSJ-2L, and identify the primary kinematic and kinetic parameters for the RSJ-1L and RSJ-2L performances. We hypothesized that there were significant differences between the RSJ-1L and RSJ-2L, and primary functional determinants for their performances.