Novel, short-lived upper limb tasks performed by young adults have been used to investigate the role of transversus abdominis (TrA) in maintaining postural stability. Little information exists concerning the behaviour of TrA in older adults during protracted postural challenges. The primary aim of this study was to describe the change in thickness of TrA during an incremental upper limb exercise test in older adults. A secondary analysis was undertaken to compare the results between younger and older subjects. Healthy subjects aged between 40 and 70 years performed the Unsupported Upper Limb Exercise Test (UULEX) while activity of TrA was continuously monitored by using rehabilitative ultrasound imaging. Changes in muscle dimensions were measured every minute during the test. The thickness of TrA increased significantly and exponentially between minutes 8 and 13 of the UULEX. Although there was a significant difference between older and younger subjects in absolute measures of TrA thickness, the change in TrA thickness as a percentage of resting thickness during the UULEX demonstrated an identical pattern between younger and older subjects. This study suggests that TrA is continually and increasingly active during an upper limb task in both younger and older asymptomatic subjects.