Lightweight, safe and affordable wearable devices are needed for effective robot-assisted rehabilitation, to reduce compound pressures on hospitals and social care. Despite recent developments in soft wearable robots, many of these devices restrict motion, are difficult to wear and lack quantitative assessment of the moment transfer to the wearer. The decoupled design of our wearable device for upper limb rehabilitation successfully delivers full range of motion to the user of 132 (SD = 13) degrees, can be attached to the wearer in less than a minute, and needs only a single point of adjustment to fit different anatomies. In this paper we quantitatively assess the transfer of the actuator moment, showing that 82% of the moment produced by the actuator is applied to the wearer, significantly higher than shown previously in upper-limb wearables. The human participant study also shows that the device successfully assists the flexion movement and quantitatively assesses the reduction in muscle activity during use. This research is a step towards safe pneumatic soft robotic wearable devices that are adaptable to large cohorts, a necessary prerequisite for their widespread adoption in healthcare.