Adding a third group that receives flexibility training would be an appropriate manipulation check in designing this study.
Even if you take precautions, your research may suffer from attrition. Attrition bias can be detected by comparing participants who stay with those who leave your study. Keep comparing participants on all factors in your study using your baseline data. An appropriate manipulation check would be to include a third group that obtains flexibility training.
Learn more on manipulation-
https://brainly.com/question/15573966
#SPJ4