A: I hope this doesn’t sound too hackneyed; however, I’ll say it none the less - you cannot isolate a muscle. With apologies to readers who may have already seen the following quote from Deane Juhan (Job’s Body, available through http://www.amazon.com), read and learn:
"...let us imagine ourselves observing a person who is standing erect and executing the simple gesture of raising their straight right arm to the side until it is horizontal. The fibers in the deltoid, the supraspinatus, and the upper trapezius will contract to produce the primary motion, while the fibers of the pectoral major, the pectoral minor, and lattisimus dorsi must simultaneously extend to allow it. But the contraction of the right trapezius will not only raise the right arm, it will also tend to pull the neck toward the right; therefore the left trapezius, along with the other muscles of the neck, will have to contract as well in order to stabilize it. Furthermore, the extended right arm will overbalance the torso to the right, so the erector spinae muscles on the left side of the spine must contract to brace the whole torso and keep it erect. And since this contraction of the left erector spinae set will tend to pull the left side of the pelvis up as well, the gluteus medius and minimus of the left side must also brace to hold the pelvis level. Since not only the torso, but the body as a whole is threatened with tipping by the overbalancing weight of the extended arm, the right leg must brace as well, using fibers in the hip, the thigh, the calf, the feet, the toes."
Perhaps you now you can understand why I find it so irritating when personal trainers, equipment manufacturers, and other fitness professionals talk about "isolating" a muscle with a certain exercise or technique!
Of course, you CAN perform exercises which place greater emphasis on a particular muscle and recruit less help from the muscles designed to assist the prime mover. So-called isolation exercises can be very useful for muscle development, but they will not create a shape that wasn’t predetermined by your skeletal structure. Cuts, rips, striations, peaks, lines, etc... are not results of adaptations to various exercises, but rather the result of decreasing your bodyfat and increasing muscle mass.
Incidentally, the reason that a muscle cannot be shaped is that they are "non-contiguously innervated." In laymens terms, that means that the fibers assigned to "fire" under certain intensities (loads) are spread out throughout the muscle, not confined to a certain region of the muscle.
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