The only application of isometrics to sprint and agility development would be isometric training for the core in order to improve running and bounding efficiency. However this is a somewhat esoteric aspect of sprint or agility training because core training is addressed through other traditional methods of strength and power development. Weightlifting (I refer to Olympic Weightlifting, everything else is resistence training, weight training, Powerlifting or body building.), plyometric drills and sprint and agility drills all involve isometric tensioning of the core to some extent, in addition to ab training and deadlifts, etc., etc.
Consider how worked your upper body felt after your last heavy squat session. From the waist up you are the lifting platform. To keep from collapsing under the bar you must set your back and isometrically tension your entire upper body. In Weightlifting, the lifter takes a deep breath and holds (valsalva manuever) it until the lift is completed. Consider that the majority of Weightlifting training movements and both competition lifts end with the bar overhead, it is easy to see why the breath is held. The lifter has to turn his upper body into a high pressure lifting cylinder just like an airshock on a Lincoln. If you have several hundred pounds overhead it's not a good idea to exhale until you are ready to drop the weight.
Isometric training yields the most strength of any single type of resistence training. However, it is effective within only a few degrees of the joint angle. Wall sits with knees bent at 90 deg. are very effective at generating tremendous strength, but only within that tiny range of motion of a couple of degrees either way.
Plyometrics are leaping and bounding movements done to enhance the elastic properties/energy storage component of muscles/joints. Proper plyometric training allows one to sustain forces of several times body weight during ground contact because of the brevity of contact, .65 of a second or less if memory serves. Basically the snap of a finger. For example, jumping rope is a supplemental plyometric exercise.
Combined with efficient technique, it is the amount of force that one puts into the ground with each foot contact combined with leg turnover speed that determines how quick you are. A properly trained core allows little dissapation of force through the body, sending all the athlete is able to generate into the ground. These are some of the training methods that develop the power that helps sprinters to become very fast, Weightlfters to lift big weights and jumpers to leap much higher.
Make sense?
Ubercoach
, so I'll try it...
