Regular strength exercise training is recommended by the WHO. Usersespecially novices or patientscall for intensive supervision to execute strength exercises correctlyeven if performed with a fitness device. Usually, a trainer or a physiotherapist provide guidance and assist the user. However, increasing costs and missing experts call for new solutions that support the user to perform strength exercises correctly and without supervision of an expert. This paper describes the fitness system SMART-FIT. The system comprises a cable machine, a notebook, and a tablet PC providing visual feedback during strength exercise training. 36 participants volunteered in our user study, designed as within-subject experiment. Performing a shoulder exercise, participants tested 3 different visual feedback interfaces: 2-Pillar, 1-Pillar, and Ribbon. We evaluated the effect of the interfaces on the performance, e.g., Pearson's correlation coefficient. Additionally, we applied the questionnaires Flow-Short-Scale, NASA-TLX, AttrakDiff2, and System Usability Scale among others. The results of our user study will enable designers to make more appropriate design choices for future exercise support systems.