There are many challenges in the development of pneumatically powered soft robots due to the non-linearities of the materials used, non-linear pneumatic flow, complex non-standard designs, and high compliance that leads to loading-specific mechanical behaviour. As a result, there is still a significant gap towards understanding these robots, as compared to standard well-known actuators such as motors, rigid pneumatic actuators, or voice-coils. This thesis develops a comprehensive system-wide understanding of soft wearable robots through investigation and quantification of their soft actuator mechanical outputs, dynamic performance, and biomechanical effects. Towards this goal, we explore methods for accurate and consistent experimental characterization, modelling of SPA pressure and flow dynamics, and musculoskeletal modelling and analysis of soft wearable robots. First, we present a novel experimental protocol and setup for recreating application-specific loading conditions, in order to accurately capture the mechanical behaviour of soft robots. This concept is based on emulating the physical constraints acting on the soft robot at its contact points, and measuring the mechanical outputs. Then, we analyse the dynamic behaviour of SPAs and its dependency on the PSS and loading conditions. Via simulations and experimental testing, we map a direct relationship between properties of the PSS and SPA, and dynamic performance metrics. Using these relations, we present design optimization of PSSs for simultaneously meeting requirements of performance, portability and additional user-defined metrics such as mass or volume. By further investigating the effect of loading conditions on SPA dynamic behaviour, we introduce a previously unknown property of SPAs, self-sensing, that allows estimation of force and displacement without dedicated sensors. Finally, we investigate the biomechanical effect of soft wearable devices via the design and modelling of a soft exosuit for the human torso. The research in this thesis can be broadly categorized into three sections: quantifying mechanical behaviour of soft robots, modelling pressure dynamics of soft pneumatic actuators, and evaluating biomechanical effect of soft exosuits.