Want more definition but not larger muscles? read on...

“The perfecting of a movement is achieved by improving the ability of the muscles to generate force during the transition from eccentric to concentric work.”
Mel Siff, Supertraining

Mel Siff was a mathmetician who reviewed the Soviet model for sports training and wrote about the science behind the methodology; his book "Supertraining" is a MUST read for anyone who is serious in interested in learning about fitness, but it's heavy reading, so be ready!

this post is about how to add definition without size, so first it is important to describe (briefly) how muscles work:
there are 2 phases of muscle action: the lengthening (eccentric) phase and the shortening (concentric) phase. the lengthening phase allows the muscle to store elastic energy (potential mechanical energy) which is then released during the shortening phase. this is known as the stretch-shorten cycle (SSC); the faster a muscle can go through the SSC, the more power/energy can be created for muscle action. the concentric/shortening action is how the muscle actually releases mechanical energy for dynamic movement.

muscle pulls can occur when a muscle experiences a rapid lengthening phase and is not able to efficiently convert potential elastic energy to kinetic mechanical energy. most exercises place an emphasis on the concentric phase of action (think "squeeze your butt" or "flex your biceps"); but in order for a muscle to efficiently create mechanical energy the training needs to focus on the rapid lengthening phase of muscle action--this is the purpose behind plyometrics.

plyometric training includes actions such as hops (take off and land on the same foot); jumps (take off and land on both feet); bounding (take off on right-land of left) for the lower body and throws using such implements as medicine balls or sandbags or lifts such as the clean or jerk for the upper body or olympic lifts such and the clean-and-jerk or snatch for the entire body.

the focus of a plyometric exercise is to rapidly load the muscle with potential energy during the lengthening phase in order to create explosive muscle action during the concentric/shortening phase. If you want to boost your definition without adding size or are active and want to increase your strength/power levels for your favorite activity then consider adding some plyometric training to your program

Start with a few exercises like skips or squat jumps in place; do no more than 6-8 reps at a time and allow for at least 45 sec rest between sets; start with a light volume of 3-4 exercises, for 2-3 sets; and progress to a high volume of 6-8 exercises for 3-5 sets. the cool thing is that plyometric training DOES NOT make your muscles larger, but trains them to contract much more efficiently, so if you are looking for muscular definition without adding size, this is the way to go.

muscle "tone" comes from a state of semi-contraction of the muscle. the muscle is activated by the nervous system; the more neural activity in a muscle, the greater the muscle tone and consequently the muscle will get more defined without adding size. Weight training adds muscle size by increasing the size of the muscle fiber or the amount of intracellular fluid in the muscle (the post-lifting "pump") but plyometrics works by increasing the number of motor units in a muscle and the amount of nervous system energy going to the muscle. a motor unit is like a spark plug for the muscle, so the more motor units activated with plyometric training the greater the muscular definition.

3 books which will provide some great exercises for plyo training:
"Jumping into plyometrics" by Don Chu; or
"High Performance Sports Conditioning" edited by Bill Foran; or
"Sports Power" by David Sandler