Most fitness studies usually only last for a few weeks, meaning that participants usually don’t have time gain a lot of measurable strength or muscle mass.
Researchers get around this by using INDIRECT MEASUREMENTS, so called “surrogate markers”. They measure something that is likely to predict the thing we ACTUALLY care about. EMG activity is a good example of this. More EMG activity is many times assumed to lead to more strength or muscle mass. At the same time there are many situations where more strength DOESN’T show more EMG activity.
MPS Is another one, more muscle protein synthesis 12 hours after a workout doesn’t always mean more muscle in the long run.
In our field, medicine, blood lipids are used to predict if someone is at risk of heart disease. This prediction is pretty accurate but it is unfortunately therefore also also assumed that lowering blood lipids lowers risk of cardiovascular disease. There are however many conflicting results as some medications lower blood lipids and decrease risk of heart disease while others lower blood lipids and INCREASE risk of heart disease… Hmmm…
Bottom line: Always value studies measuring outcomes of interest like muscle mass and strength DIRECTLY over studies which use INDIRECT surrogate markers as these cannot always be trusted to correlate to the results we actually care about.