Objective: The aim of this pilot study was to measure the activity of the masseter in young adults treated with clear aligner therapy using ambulatory electromyography. 
Materials and methods: The electromyographic activity in the right masseter muscle of 6 young adults (3 female, 3 male, 18.8 ± 4.3 years) undergoing clear aligner therapy over 4 weeks (week 1 baseline without aligners [T0], week 2 with a dummy passive aligner [T1], week 3 with the first active aligner [T2] and week 4 with the second active aligner [T3]) was recorded using portable electromyographic data loggers. A mixed-effects model was used to test differences in surface electromyographic activity over the four experimental conditions. 
Results: The surface electromyographic activity of the masseter increased significantly with aligners compared to baseline. It also increased significantly from T0 to T1 (P < 0.001), and increased from T1 to T2 (P = 0.024). There was no statistically significant difference between T2 and T3 (P = 0.799). 
Conclusions: Clear aligner therapy in young adults is associated with an increase in masticatory muscle activity, possibly due to an increase in parafunctional tooth clenching.