# Intra-Individual Variation in the Jump Smash for Elite Malaysian Male Badminton Players
## 1 Introduction  
The forehand smash was reported as the last shot played in 29% and 22% of the men's and women's singles rallies at the 2008 Beijing Olympic Games [1]. It is widely regarded as the most effective and aggressive stroke in badminton. The success of this stroke lies in creating a fast downward trajectory of the shuttlecock towards the desired location on the opposite court and putting the opponent in a defensive situation [2]. Unsurprisingly, the post-impact shuttlecock speed is often used to define the quality of the shot. As a result, substantial attention has been paid by researchers to understand the development of speed in badminton smash [3, 4, 5], as well as in other similar projectile-hitting sporting motions, such as the tennis serve [6, 7, 8] and volleyball jump spike [9, 10, 11]. At the elite level, the badminton smash shuttlecock speed may vary due to body positioning, technique, and the timing (impact location) of the smash, among other factors [12]. However, to the authors' knowledge, there is no study to date that has conducted any intra-individual analysis investigating why a given player's shuttlecock speed may vary between smash strokes. A higher velocity in the smash is associated with specific characteristics at the moment of racket-shuttlecock contact, including a more internally rotated shoulder [3,4], as well as reduced elevation of the shoulder and less extension of the elbow [4]. Additionally, a faster smash is linked to greater angular velocity of the internal rotation of the shoulder, a factor contributing significantly (66%) to the development of racket head speed [13]. Other key elements contributing to the acceleration phase of the smash include forearm pronation [14,15] and wrist flexion [14]. Notably, the rotational motion of the trunk during both the backswing and forward acceleration phases also plays a substantial role in achieving a faster smash speed [5,16]. Furthermore, a consistent impact location of the shuttlecock on the racket face is critical to minimise reductions in smash speed due to the impact location [12]. Previous research has considered differences in technique for overhead sports such as badminton, tennis, and volleyball. In the badminton smash, significantly greater shoulder internal rotation, horizontal abduction, and wrist flexion torque were found for skilled players compared to unskilled players [17]. Furthermore, participants who produce faster smash speeds have been shown to have greater shoulder internal rotation upon impact and a greater pelvis-thorax separation angle in the transverse plane at the end of retraction compared to participants with slower smashes [3]. In the tennis serve, shoulder internal rotation has been shown to be a major contributor to the differences in horizontal racket head velocity between fast and slow servers [8]. In the volleyball spike, differences in upper-limb angular velocities during successful versus faulty spikes were found for different phases, covering 35% to 45% and 76% to 100% of the motion [18]. The parameters that contribute to intra-individual variations in badminton smash speeds are unknown when the goal is to maximise shuttlecock speed. This does not include cases where a reduction in speed may be the goal in favour of a different trajectory, e.g., a steeper smash ('stick smash'). Previously, several key determinants have been identified to differentiate between shuttlecock speeds across a cohort of players; however, these results should not be generalised to an individual [3]. While cohort-based studies provide valuable insights into group trends and biomechanical factors associated with high-performance smashes, they often overlook intra-individual differences. These differences can arise due to unique player-specific techniques, variations in motor control, and adaptations developed through training or anatomical constraints. For instance, individual players may employ distinct smash techniques, including variations in arm kinematics, shoulder elevation, and trunk rotation [5].