## Article
### 1. Introduction  
The concept of stiffness originates in physics, particularly as a part of Hooke's Law, which defines objects as deformable bodies that are capable of storing and returning elastic energy. Hooke's Law states that the force (F) required to deform a material is related to a proportionality constant (k) and the distance (x) that the material is deformed, provided that its shape is not permanently changed. Mathematically, this relationship is expressed as F = kx. The proportionality constant 'k' is referred to as the spring constant, which describes the stiffness of an ideal spring and mass system. Stiffness in the human body, or in body segments, describes the body's ability to resist displacement when subjected to ground reaction forces or moments. In this context, stiffness can be described at various levels, ranging from a single muscle fiber to creating a model for the entire body as a mass and spring system [1]. For instance, lower limbs are often modeled as springs supporting the mass of the body, and the existing literature suggests that the stiffness of the human body is a combination of all individual stiffness values that are contributed by tendons, ligaments, muscles, cartilage, and bone [2]. Leg stiffness is a critical concept in sport biomechanics, and it has significant implications for both performance and injury prevention. Consequently, finding the right balance of stiffness is crucial as excessively high or low levels can lead to negative outcomes or injuries [1]. Therefore, training programs should aim to determine the optimal range of leg stiffness to achieve the desired athletic effects. Jumping performance, a key aspect of many sports and human activities, is influenced by leg stiffness and related training protocols. Repeated jumps and pylemetric training [3] show analogies with the mass-spring model [4; 5; 6; 7; 8; 9; 10] and are considered crucial for improving leg stiffness. Particularly, pylemetric training has been found to have a substantial impact on joint stiffness during drop jumps, with a significant increase (+63.4%) in stiffness being reported after a 12-week program [11]. Conversely, insufficient stiffness can lead to soft tissue injuries, as demonstrated by a decrease (\(-\)32.7%) in ankle joint stiffness following 7 weeks of pylemetric training using sinusoidal perturbations [11; 12; 1]. Leg stiffness plays a crucial role in the stretch-shortening cycle, which allows for the storage and release of elastic strain energy in the musculotendinous unit [13]. This capability contributes to improved muscle reactive power (R\({}_{\text{P}}\)) and jump height [14], suggesting that maximum power production during jumps is highly related to lower level of limb stiffness in adults [15]. Interestingly, hopping frequency has also been shown to increase leg stiffness, potentially due to an increase in ankle stiffness [16; 17]. However, the relationship between leg stiffness and athletic performance is not solely dependent on the athlete's physiological structure. Factors such as fatigue protocols, methods used to calculate leg stiffness, and the training surface can all influence the findings reported on leg stiffness [18]. One method to alter the surface is through mini trampoline training, which has been demonstrated to simultaneously enhance strength, balance, body stabilization, fitness, lower limb asymmetry, and spatial orientation [19; 20; 21]. The use of an elastic surface during mini trampoline training leads to an increase in leg stiffness, reducing the average force required to execute jumps and thus increasing the mechanical work done by the surface [17]. Despite the potential benefits and multiple effects of mini trampoline training on movement patterns, there is currently a lack of research investigating the acute effects of trampoline training on leg stiffness and R\({}_{\text{P}}\).