I am conducting a meta-analysis of the effectiveness of stretching for reducing pain and improving function in those suffering from plantar heel pain. I grouping all types of stretching together, so am not looking to compare different types of stretching. I want to compare 'stretching' vs control, and 'stretching' Vs other conservative treatment.

I initially thought that if two groups both received plantar fascia stretching for example, but one group ALSO received calf stretching, that I could class this as "stretching Vs Control" as, in a sense, if there was a difference in the between-group means, this could be attributed to the only difference between groups which was that one group also received calf stretching.

I think I have made a mistake on many levels but would like someone to clearly explain this to me.