Objective assessment of mobility and effectiveness of interventions remains an open issue. Timed Up and Go (TUG) and 30 Second Chair Stand (30SCS) tests are routinely used in assessing mobility of subjects, but they provide a single parameter. Instrumenting subjects with wearable sensors enables a detailed mobility assessment. Specifically, we argue that instrumented sit-to-stand (S2ST) posture transitions during the TUG and 30SCS tests can be used to assess the strength and balance of subjects. In this paper we develop a personalized three-segment model that quantifies torques/forces on the body and assesses optimality of each sit-to-stand transition. To characterize a S2ST transition we calculate action defined as an integral of mechanical energy over time. The theoretical optimal transition time can thus be determined for each person by finding the minimum action necessary for a S2ST transition. Our model assesses action during the S2ST transition using inputs from smartphone's inertial sensors, and calculates optimum S2ST transition time for a given body composition of a subject. Our experimental evaluation shows that healthy young subjects have posture transition times close to the optimal transition time generated by the model. We hypothesize that the optimality of posture transition provides an objective and potentially more accurate estimation of the mobility. We tested the model by evaluating optimum action and optimum S2ST transition time for 10 geriatric patients undergoing a mobility improvement program by comparing their performance with the optimum performance generated by the model. This paper presents the model and possible use of the results to assess long-term changes in mobility of users.