Neural adaptations are the primary mediator of strength gains, while total training volume within the 4–30 repetition range is the primary mediator of muscle hypertrophy.

Study: Resistance Training Volume Enhances Muscle Hypertrophy, but Not Strength in Postmenopausal Women: A Randomized Controlled Trial

More workouts made muscles bigger, but didn’t make people stronger than fewer workouts—meaning muscle growth comes from doing more reps, but strength comes from your brain and nerves getting better at using your muscles.

Study: Resistance exercise load does not determine training-mediated hypertrophic gains in young men.

This study found that lifting lighter weights until you can't do more gives you about the same muscle growth as lifting heavy weights — so it's the total work that matters for size. But for getting stronger, lifting heavier weights helped more, which means your brain and nerves play a big role in strength.

Study: Velocity Specific Adaptations to Three Widely Used Strength Training Methods: A Randomized Controlled Trial.

This study found that lifting heavy weights or doing more reps (within a normal range) makes you stronger and bigger, not just moving fast. So strength comes from your brain and nerves learning to use muscles better, while muscle growth comes from doing more total work, not from how fast you lift.

Study: Biceps femoris long-head motor unit discharge rates and recruitment thresholds vary across 9 weeks of eccentric training

This study found that when people start doing hamstring exercises, their muscles get stronger at first because their brain and nerves get better at telling muscles to work harder. After a while, the muscles themselves grow bigger and help make them even stronger.