To date, a range of exercises have been used to improve the function of the lumbar multifidus (LM) and transversus abdominis (TrA) muscles in people with low back pain, but uncertainty remains as to what exactly constitutes meaningful LM and TrA training. We examined the effects of exercising with a new device which combines weight-bearing, an unstable base of support (BOS) (feet), an upright posture with a relatively stable lumbo-pelvic area, and functional lower limb movement, with the aim of exploring which of these elements may be effective, in increasing LM and TrA muscle activity. Twelve non-symptomatic participants had ultrasound images taken of their LM and TrA during a range of conditions, including rest, traditional exercise approaches to LM and TrA recruitment, and exercising on the new device. Our results indicate that an unstable BOS on its own is not enough to increase LM and TrA activity, and that a combination of weight-bearing, an unstable BOS (feet), an upright posture with a relatively stable lumbo-pelvic area, and functional lower limb movement is most effective at increasing LM and TrA activity. This way of exercising appears to recruit LM more effectively than the widely used "swelling" of LM, and to cause automatic TrA and LM recruitment. Importantly, our findings also indicate LM and TrA may have slightly different roles during trunk stabilisation.