## 1 Introduction
The researchers found out that the natural frequencies increase with stiffer prosthetic categories. They concluded that the foot should not be merely prescribed based on the weight of the amputees but also based on their activity level so as to reach the first bending natural frequency of the component and thus achieve enhanced performances. The first four eigenmodes observed in these studies ([31; 33]) are twisting and bending modes, which could be excited by impacts such as those occurring when jumping, walking, or running. The first bending mode, contributing to vertical displacement, is found to be significant in this process. Twisting modes occur close to or before the first bending mode, indicating an elastic coupling between bending and twisting. Factors like asymmetrical loading, section asymmetry, or shear coupling due to material anisotropy influenced this behaviour. Despite observing non-linearities, the researchers concluded that approximating the stiffness as linear could closely predict the frequency of the first bending mode, irrespective of the mass or weight of the amputee, providing practical applications for selecting prosthetic stiffness based on gait performance requirements. [PERSON] et al. [34] provided insights into the rebound characteristics of running-specific prostheses. By applying an impactor on the blade prosthetic at different impact speeds, in order to simulate jogging, sprinting, and top athletes' contact speeds, it resulted that, as the impact speed increases, both load and displacements also increase. The ratio between the rebound and impact speeds shows significant variation with changes in the model and impact speed, while the ratio between the maximum impact load and the correspondent displacement remains almost constant regardless of the impact speed. Moreover, [PERSON] et al. [25] aimed to examine the feasibility of using Finite Element Analysis (FEA) to standardise the mechanical characterisation of ESAR feet, proposing a methodology that integrates numerical and experimental analysis. Two quasi-static tests were conducted on the prosthetic, exploiting an Instron tensile/compression testing machine in order to find out the Young modulus and the static and dynamic friction coefficient. Afterwards, the simulation of the dynamic loading response was also carried out and the Rayleigh parameters, stiffness, and energy efficiency were evaluated. From the dynamic loading response, it was possible to deduce that ESAR prostheses with high stiffness result in low energy loss but high impulse, potentially increasing the residual limb reaction force. The largest contributing factors to energy loss in dynamically loaded ESAR prostheses are shape and thickness. These studies highlight the significance of conducting dynamic tests on blade prosthetics to gain a deeper understanding of their performance, thereby contributing to the improvement in the manufactured products. Recently, researchers have also explored the potential of harvesting energy from the movement and mechanical forces experienced during walking or running [37; 38; 39]. This could be an important step for advancement in prosthetics technology since the harvested energy could be used to power various components of the prosthesis, such as motors, sensors, or microprocessors. This paper presents the dynamic characterisation of the composite ESAR prosthetic feet Cheeta Xcel, Ossur [40] through Experimental Modal Analysis (EMA). In contrast with previous studies, the experimental roving hammer test is performed in free-free conditions in order to avoid potential uncertainties associated with the applied loads and constraints. As underlined by the literature review, the dynamic characterisation of a prosthetic is generally conducted applying a vertical load and simulating the athlete weight [31; 33] or by constraining the prosthetic area close to the socket [34].