1) The most common cause of a drop in performance in cycling, or any other sport, is overtraining: going hard when you should go easy. Hard-and- easy refers to intensity (speed and pressure on the pedals), not to total mileage. On one day, you ride very fast with your group, feel sore on the next day and go slowly for as many days as it takes for the soreness to go away. When the soreness goes away, you ride fast again. If you take hard workouts while your muscles are still sore you can cause chronic muscle soreness.
2) A second cause is a low-salt syndrome caused by sweat loss. The only mineral that you need in large amounts is sodium, common table salt. All athletes have to salt food heavily and use lots of salt. Your doctor can check for low-salt syndrome by having you take a very hard workout on one day, replenish your fluids, and then draw blood for sodium and chloride on the next morning. If you are worried about developing high blood pressure, check your blood pressure frequently.
3) The third most common cause is lack of vitamin D. Blood levels of vitamin D3 below 75 nmol/L can cause muscles to feel sore, particularly in the wintertime.
4) You can also fail to recover adequately from intense workouts if you do not carbohydrate- and protein-load within a half hour after you finish a workout. Your muscles are maximally sensitive to insulin during exercise and for up to a half hour after you finish exercising. Sugar taken within a half hour after you finish your intense workout will raise your blood sugar level enough to increase insulin levels. Insulin then drives protein into cells to help you recover faster.
5) Another cause of muscle soreness is not getting off your feet after intense workouts. Muscles recover fastest when they are not used. After intense workouts, lie down instead of sitting, standing or walking.
If you have had a marked drop in performance in your sport and none of these causes applies to you, you may need a medical evaluation.