Transtibial prostheses, which replace the lower part of the leg below the knee, enable users to regain mobility and independence. Throughout history, the development of transtibial prostheses has gone through various phases of innovation, but there remains significant room for improvement, particularly in the area of adapting the prosthesis characteristics to the individual user. As the topic of this thesis is the constructional improvement of transtibial prostheses, the work consists of a theoretical part, a constructional improvement part and a practical part. The theoretical part covers topics necessary to create a foundation for understanding the problem and thus determining the constructional requirements needed for improving the below-knee prosthesis. Through the theoretical section, topics such as the classification of prostheses based on their use and the type of amputation are addressed. Additionally, since 3D printing technology was utilized, additive manufacturing is described as an efficient process for creating prototypes for testing. In the constructional improvement section, based on the review of existing solutions on the market and the definition of functional and constructional requirements, a concept representing the constructional improvement of the below-knee prosthesis was developed. In the final part of this work, three prototypes were created based on the concept using 3D printing technology. The prototypes were then evaluated to determine which one was the best solution. According to the chosen prototype, an entire below-knee running prosthesis was manufactured using additive manufacturing. The result of this work is a cost-effective transtibial prosthesis with adjustable foot stiffness.