BACKGROUND: Some types of foot orthoses have been researched for their effect on lower limb electromyographic muscle activity during walking. However, foot orthoses with high levels of medial rearfoot wedging ('inverted' foot orthoses) have not been investigated. METHODS: In a cross-sectional study, asymptomatic participants with a pronated foot type (n=15) were each issued with a pair of 0 degrees, 15 degrees and 30 degrees inverted custom-made foot orthoses. After four weeks of habituation to the orthoses, surface electromyography was used to measure the onset and maximum EMG amplitude of tibialis anterior, peroneus longus, medial gastrocnemius and soleus muscles using five conditions [barefoot, shoe-only, 0 degrees, 15 degrees and 30 degrees inverted foot orthoses conditions]. FINDINGS: A statistically significant increase in tibialis anterior maximum EMG amplitude occurred using the shoe only (30% increase), 0 degrees (33% increase), 15 degrees (38% increase) and 30 degrees (30% increase) inverted orthoses conditions compared to walking barefoot (P<0.01). Peroneus longus maximum EMG amplitude increased significantly using the 15 degrees inverted orthosis condition compared to walking barefoot (21% increase, P=0.04). INTERPRETATION: Footwear and orthoses can significantly alter the maximum EMG amplitude of leg muscles during walking. Foot orthoses appear to increase peroneus longus EMG amplitude compared to footwear alone. However, the level of medial rearfoot posting within an orthosis does not appear to significantly alter maximum EMG amplitude. The individual responses to foot orthoses are highly variable. The changes in EMG amplitude with the use of foot orthoses and shoes may have clinical implications.