With advances in prosthetic technology, functional intent has extended past basic support toward providing increased dynamic ability for daily and athletic use. Addressing a disparity between universality and complexity in sport-grade and energy-storage-and-return (ESR) prostheses, this paper presents a pneumatic transtibial ankle prosthesis concept with semi-active control of ankle stiffness to adjust the prosthesis' properties for a wider range of gym exercises. Functional validation of the device falls under specific scenarios including the parallel back squat weightlifting exercise. The prosthesis features 30 deg sagittal ankle range of motion and provides wireless adjustment of static air pressure via a smartphone app to transition between the force and stiffness demands of walking and weightlifting. This pneumatic system includes a self-replenishing feature, providing a practical solution for the variable air pressure demands of athletics and everyday use. The mechanical, pneumatic, and control systems of the prosthesis are therefore described. Biomechanical tests including the back squat were conducted with one transtibial amputee subject. The resultant kinematic analysis validated the functional goals of the device, including an increased range of ankle rotation and variable stiffness across three different cylinder pressure settings. The kinetic profiles of the amputated leg and the natural leg also reveal an improvement in bilateral symmetry compared to a standard ESR prosthesis. This prosthesis concept has the potential to help persons with amputation participate in a wider range of activities, by improving the versatility of current ESR and sport prostheses.