The idea of mind-muscle connection is that while doing exercises you should intentionally focus to squeeze and isolate the muscle that you want to target while doing exercises for weight training.
There were studies done to prove this theory right or wrong as with anything, we all want to know the answers. Historically speaking there are two sides of this debate.
Firstly you have bodybuilders, of course, who support the idea to isolate the muscle while doing any type of movement as you should be able to get better activation from that as well as to force it to grow.
Secondly you have more academic approach that have been saying that mind-muscle connection does not matter or yet have such a huge impact as it is believed, they are saying that as long you are doing weight lifting with full range of motion in the gym, since anyway the muscles will be firing so that you will be able to lift the weight up. According to them your main goal is to think about the way you are moving your body and to think about how well are you doing your movement properly. You don’t need to worry about activating specific muscles because they will be activated since you are lifting weight.
So that we should focus on external cuing – which results to doing proper movement and lifting the weight.
For stability, balance and endurance what academic data tells us all together external focus beats the internal focus https://www.researchgate.net/publication/271992035_Attentional_focus_and_motor_learning_A_review_of_15_years. There has been number of studies done that tells us about various variables such as EMG activation and performance, there is not many studies that can tell us about long-term internal or external focus produce better results in terms of the muscle gains and size at the end of the day.
Looking at this study : Differential effects of attention focus strategies during long-term resistance training it tells us that final results showed higher thickness in biceps for Internal group, while the External group saw bigger results in isometric knee extension. Keep in mind that both good sides for External and Internal have not reached any significant statistic level. Where muscle hypertrophy can be enhanced by using mind-muscle connection.
We can draw a question from this study : How come that mind-muscle connection works better for the biceps than for working out quads?
In this paper, provided above, this question is explained because people usually have better coordination of their upper body part than lower extremities. When you think about it it surely does make some sense because upper limbs are in most cases better at performing fine motor tasks which require precision, where lower limbs are not that good with precision. Looking at it in this way we could say that lower body part can be trained over some period of time in order to see better muscle results in lower extremities, because we often don't have good control when it comes to precision with lower body part, we don't use it for the things we do with our hands .
Conclusion
This study was done with single join isolation exercises within moderate rep ranges. Which means that it was done with no compound moments. According to the known data my view is that the External Cuing (focus on proper movement with weight lifting) is better for compound movements such as press, squats and deadlifts. While Internal Cuing (focus on mind-muscle connection) is better for isolation exercises such as leg extensions and curls, biceps curls etc.
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