What internal focus does during a rep
The mind-muscle connection refers to deliberately concentrating on contracting a specific target muscle during an exercise, an internal attentional focus. Electromyography studies in trained lifters have found that this kind of focus can increase measured electrical activity in the targeted muscle during a movement, with reported increases often in a modest range rather than a dramatic one. The effect appears more reliable at lighter to moderate loads, where there is room to redirect effort.
Untrained people sometimes struggle to do this, which is one reason it is described as a skill that develops with practice.
What it does and does not change
Greater activation during a set does not automatically guarantee more long-term muscle growth, though some studies suggest internal focus can favor growth in the targeted muscle under certain conditions. At very heavy loads, an external focus on moving the weight efficiently is often better for performance, so the technique is context dependent.
Reported activation increases vary by study and muscle and should be read as approximate. This is general educational information about a training technique, not medical advice.