From 'Stretching Scientifically' Tom Kurz, Stadion, 0940149303, page 24, third edition:
"Muscles are usually long enough to allow for a full range of motion in the joints. It is the nervous control of their tension, however, that has to be reset for the muslces to show their full length. This is why ten minutes of stretching in the morning makes your full range of motion possible later in the day without a warm up. This is also why repeating movements that do not use a full range of motion in the joints (eg bicycling, certain techniques of Olympic weightlifting, push-ups) can set the nervous control of length and tension in the muscles at the values repeated most often or most strongly. Stronger stimuli are remembered better. Eateren European coaches will not let their gymnasts ride bicycles, for example, even though they seem to have all the flexibility they need. Strenuous workouts slightly damage the fibres of the connective tissue in the muscles. Usually these micro-tears heal in a day or two, but a loss of flexibility is supposedly caused by these fibres healing at a shorter length. To prevent this, some physiologists recommend static stretching after strength workouts. All this sounds very good, but the same gymnasts who are kept from bicycling, run with maximal accelerations to improve their specific endurance. Such running is a strenuous intensive strength effort for leg muscles, but in running, these muscles work through a full range of motion in the hip and knee joints, and because of that there is no adverse effect on flexibility. If connective tissue were a factor, then stretching after a workout would be enough and these gymnasts could ride bicycles with the same result. The situation with pushups is very similar. If you do a couple of hundred a day, on the floor, so the muscles of your chest, shoulders and arms contract from a shortened position, no amount of static stretching will make you a baseball pitcher or javelin thrower."
Running and cycling are both fine ways to train, just have a stretching session afterwards, it's a great warm down and a great way to improve flexibility. Stretching once "warmed up" by cardio work is by far the safest way to stretch.
Skipping is another fine way to get a cardio workout and improve your pliometric fitness (your bouncy-ness).
Swimming will also help, especially if you are recovering from a sprain or strain.
Light (or heavy) gym work can also be good but be carefull not to become too "musclebound", it looks OK but will slow you down as a fighter. Lean and mean is best.
