Funny you should mention muscle twitches. I was just thinking about them the other day after I did a run...
I attended a seminar a couple of months ago given by a guy who was a researcher at a physiology lab for sports medicine at a university out west. He was very informative. I asked him about muscle twitches, and he gave a thorough analysis about muscle twitches. We were laughing because I told him sometimes I get muscle twitches in my arm- and I'll just push down on them, and nothing seemed to work. It reminded me of jiggling raw chicken. He liked my analogy.
Twitches can be caused by a number of different factors- very heavy exercise, muscle overuse, heat exhaustion, dehydration, and electrolyte imbalances. More serious causes of muscle twitches can be caused by circulation problems, nerve problems, or some autoimmune problems. Sometimes msucle twitches can be caused by pregnancy too, but I assume you aren't pregnant, so no need to elaborate here (or IS there.... hmmmm... let me know....
Anyway, muscle twitches are caused when a group of muscles in the same location are continuously stimulated by a motor neuron. The muscle fibers are being overloaded, but they will eventually adapt, and the twitching ceases. On the cellular level, there are proteins called actin and myosin that interact with the help of calcium and ATP. Calcium binds to another protein, and when that happens, this calcium/protein complex releases from the actin and myosin muscle proteins, and the myosin releases itself from the actin (muscle relaxation). ATP is needed in order to facilitate this. When ATP is lacking in the body (i.e. in the case of strenuous workout sessions), the calcium cannot release from the actin/myosin site, so the muscle remains in a continuous contracting state.
What you can do to try and prevent this from happening is try GENTLE stretching while the muscle is contracting. You can also try to walk around during the contracting also. This sometimes helps. During your exercise, drink LOTS of fluids- plenty of water and sports drinks. Do a short warm up (like 5 minutes) before exercise, then stretch the muscle before you start your workout, and when you finish working out, make sure you stretch the muscle too. Keep your muscles warm- especially if you're riding in cold weather. Cold weather will just cause the muscle to constrict and can impede blood flow to the muscles. You can also increase some of the vitamins in your diet- vitamin E, calcium, potassium, magnesium and zinc.
If you do all this, and the muscle twitches still persist, there may be a more serious underlying problem. See your doctor, and ask for a serum electrolytes blood test, a CAT scan of the spine, and an angiography of the legs. They may conduct other tests for autoimmune disorders too.
That last part is the serious part, but it probably will not be your problem. I bet you just do some strenuous exercise and the second to last paragraph will probably apply to you.
Keep us updated after a few weeks of trying this.