Arnold said that resistance training was more than just lifting a weight from point A to point B. In his words: "The weights are just a means to an end. How well you contract the muscles is what training is all about."
To maximize muscle development, he talked about developing a strong "mind-muscle connection" where he'd visualize the muscle being trained and feel it working through a complete range of motion during each rep. While it may sound hokey, research shows that the mind-muscle connection can significantly improve muscle recruitment.
Researchers had a group of subjects perform two sets of lat pulldowns with only basic instruction. Then, after a period of rest, the subjects performed a couple of additional sets, only this time they received instruction on how to emphasize the latissimus dorsi while de-emphasizing the biceps.
The results? Muscle activity in the lats, as measured by EMG, was significantly increased in sets performed with a mind-muscle connection. In other words, simply concentrating on the target musculature resulted in greater activation of this muscle.
How to Develop It
Here's how to apply the technique for optimal effect. Rather than thinking about where you feel a muscular stimulus, think about where you're SUPPOSED to feel the stimulus. In the example of the lat pulldown, you must focus on pulling the weight down using only the muscles in your back. Continue with this thought process until you reach the bottom phase of the movement and then squeeze your shoulder blades together, feeling a distinct contraction in your lats.
On the eccentric (negative) portion of the rep, force your lats to resist the gravitational force of the weight so that the muscles lengthen in a controlled fashion. When you approach the starting point of the exercise, you should feel a complete stretch in the lats, and, without hesitation, proceed to the next rep by repeating the process. Keeping your mental focus channeled in this manner will direct the majority of stress to the target muscles of your back, thereby maximizing stimulation.
Multiple studies have emerged for other muscle groups such as the abs and glutes, all showing the same thing – concentrated effort increases neural activation to the intended musculature. Don't get discouraged if it takes longer to develop a mental link with certain muscles than others. It's generally easier to mentally connect with the muscles of the arms and legs than it is with those of the torso. With practice and patience, you'll develop a connection with all the muscles in your body.