This thesis presents a novel passive dynamic system for energy returning in transtibial prostheses, aiming to improve the mechanical efficiency and energy recovery of lower limb prostheses. The research focuses on developing a customizable ankle-foot prosthesis that can store and return energy during the gait cycle, particularly during the initial contact and final stance phases. The study analyzes gait data from trans-femoral amputees and able-bodied individuals to understand the dynamics of the ankle joint and identify key design variables for optimizing the prosthesis. A dynamic model is proposed based on the ISO 22675 standard, incorporating shape, size, and laminate thickness parameters for the ankle-foot design. A global sensitivity analysis is conducted to determine the most influential variables in the model's performance. A surrogate-based optimization algorithm is then used to find the optimal design variables that maximize mechanical work during different gait speeds. The results show that the proposed passive dynamic system can achieve higher energy return compared to traditional Energy Storage and Return (ESR) prostheses, offering a more efficient and cost-effective solution for users. The study also highlights the importance of customizing prostheses to individual needs, leveraging additive manufacturing techniques to produce low-cost, high-performance ankle-foot prostheses. The research contributes to the field of biomechanics and prosthetic design by providing a framework for optimizing the dynamic behavior of transtibial prostheses through computational modeling and optimization techniques. Keywords: Energy Storage and Return, Ankle Dynamics Joint Stiffness, Bayesian optimization, Surrogate modeling, Ankle-foot, prosthesis, quasi-stiffness.This thesis presents a novel passive dynamic system for energy returning in transtibial prostheses, aiming to improve the mechanical efficiency and energy recovery of lower limb prostheses. The research focuses on developing a customizable ankle-foot prosthesis that can store and return energy during the gait cycle, particularly during the initial contact and final stance phases. The study analyzes gait data from trans-femoral amputees and able-bodied individuals to understand the dynamics of the ankle joint and identify key design variables for optimizing the prosthesis. A dynamic model is proposed based on the ISO 22675 standard, incorporating shape, size, and laminate thickness parameters for the ankle-foot design. A global sensitivity analysis is conducted to determine the most influential variables in the model's performance. A surrogate-based optimization algorithm is then used to find the optimal design variables that maximize mechanical work during different gait speeds. The results show that the proposed passive dynamic system can achieve higher energy return compared to traditional Energy Storage and Return (ESR) prostheses, offering a more efficient and cost-effective solution for users. The study also highlights the importance of customizing prostheses to individual needs, leveraging additive manufacturing techniques to produce low-cost, high-performance ankle-foot prostheses. The research contributes to the field of biomechanics and prosthetic design by providing a framework for optimizing the dynamic behavior of transtibial prostheses through computational modeling and optimization techniques. Keywords: Energy Storage and Return, Ankle Dynamics Joint Stiffness, Bayesian optimization, Surrogate modeling, Ankle-foot, prosthesis, quasi-stiffness.