## 4 Discussion
This also helps systematically protect the joint tissue structure and cushion the force based on specific action experiences [30,33]. The increased pre-activation of the lower limb muscles on the sand surface in the current study contradicts previous findings of reduced pre-activation of lower limb muscles (soleus, gastrocnemius, and tibialis anterior) on unstable/foam surfaces [17,18]. However, the outcome of increased pre-activation of the lower limb muscles at a platform height of 60 cm is in agreement with previous findings [18]. This may be because the sand surface is much more unstable than the foam surface, and, thus, anticipation of the difficulty of the drop jump task may have enhanced the pre-activation of lower limb muscles. This study's findings show that the sandy surface not only enhances the pre-activation of lower limb muscles but also prolongs the effect of the short-latency response. Stretch reflex signals can be categorized into short, medium, and long-latency responses and long-latency response 2 [18, 19, 20]. Among these, the short-latency response is the earliest and is a short muscle reflex [19,21]. Generally, short-latency muscle responses in lower limb muscles do not appear during normal walking, but they appear during intense activities such as running or jumping, especially in the gastrocnemius and soleus muscles [20]. In the current study, drop jumping on sandy turf, PU, and wood surfaces is intensive enough to elicit short-latency muscle responses. A sandy surface induced a stronger pre-activation of lower limb muscles than turf, PU, or wood surfaces, and this difference continued into the short-latency response. Training can lead to neuromuscular adaptation and improved neuromuscular coordination, which can enhance muscle strength; however, the instability of the material surface can affect the acute response of the muscles and neuromuscular adaptation [12]. This study found that the impact of heights of 50 and 60 cm on lower limb muscle activation mainly continued to the short-latency response. The pre-activation period was followed by muscle activation in the latency period. The short-latency response is considered to be the earliest and is a short muscle reflex among muscle responses. Moreover, the level of muscle activation at this stage was positively correlated with action execution mode and intensity of action. Previous studies have revealed that an increase in strength or height, such as jump distance, jump height to landing, or jump difficulty, induces the muscles in the short-latency period to recruit more motor units to complete the action [18,21]. Therefore, the increase in muscle activity in the short-latency period is important for better muscle stiffness during the eccentric phase at the initial ground contact. To estimate muscle activation in the short-latency period from the pre-activation period, Lesinski et al. [18] proposed that the increase in muscle activation during the pre-activation period may trigger a subsequent stretch reflex. In particular, stretch load during the short-latency period may regulate muscle activation. The activation levels of the lower limb muscles from the pre-activation period and short-latency period may increase muscle stiffness in the lower limbs to respond to the high impact load generated by the landing height, as well as to store elastic potential energy more efficiently upon landing. Therefore, it can be inferred that the muscle activation levels during the pre-activation period and short-latency period are highly correlated. From a functional point of view, such a connection is considered to be able to fully regulate muscle stiffness [21]. Taube et al. [19] pointed out that the purpose of glyometric training was to promote SSC, while the stretch reflex can promote muscle stiffness to benefit SSC. Muscle activation during the latent period is the earliest muscle response in the SSC mechanism.