I’ve been harping on core strength forever, it seems. Core strength training has been a hot buzzterm for a while but coaches and athletes have been onto it for years. Core strength training works with all the muscles that stabilize the spine and pelvis.
These muscle groups transfer the energy from large to small body parts during your run, bike, swim, etc and work the spine, pelvis, shoulders so that the limbs (which move you around, actually, right?) can do their job correctly and more powerfully.
This works to increase your speed and power through your nervous system that stabilizes the core (spine, etc) BEFORE the rapid movement of your arms and legs begin; hence the likelyhood of increased energy in the movement.
Core strength is different from a lot of regular weight training by combining many muscles in an exercise so that the whole body is more involved in a routine. Here are the benefits: increased efficiency, better body control, better balance, reduced probability of injury, and increased power. All of these are important for any endurance sport, as well as impulse sports.
Here are a few pictures of general core exercises that I recommend. There are many more online to see, but they all involve pretty much the same muscles. The first is The Plank (also to be done on each side, supported by one arm), The Russian Twist, Superman (self explanatory, ha ha), and The Hollow Rock (pulling your stomach in while rocking). Each of these will pretty well cover the “core” muscles and, over time, will improve your speed in any sport.
There are many more online to see, but they all involve pretty much the same muscles.