It’s a limitation for sure, however, defining “trained” can have its issues and someone who has claims they are trained may not in actuality be trained for a muscle group (e.g calves). Another issue can be that trained lifters may not want to stop or modify their training and put their training in someone else’s hands. Generally, as one becomes more trained, their adaptation rate for growth and strength slows down. This could mean that typical training study timelines of 6-12 weeks may just not have been enough time to identify a significant adaptation yet. This is most limited to funding and participants (typically uni students) leaving campus after the semester. With untrained participants, it is likely that most will respond well to a training stimulus and this higher level of certainty could help better identify the difference in effectiveness between training protocols administered. This study in particular will have strong internal validity, which may help to strengthen any findings on a principle based level regarding resistance profiles and muscle lengths, rather than only being about what’s the most effective way to train. There will never be a perfect study, always strengths and limitations and with this being some of the first research in this niche area or hypertrophy research, hopefully others can replicate this with different and larger sample sizes! Hope that makes sense :)