If you’re a serious athlete it's essential to build strength in the winter if you intend to compete at consistent levels throughout the season. Another words don't waist the winter if you plan to compete in endurance sports. Due to the rigors of racing and muscle-catabolism for energy, you lose strength as the season progresses. Come late Aug.-Sept it’s not uncommon to have only 75 - 80% of the strength you had at the beginning of the season. It mostly depends upon the weight-loss you experience, most serious athletes end up losing about 2-5lbs of muscle during the course of a season. This is especially true if you do a lot of long distance running events or compete in cycling events in the 75 - 100 mile+ range. It might not be that noticeable because you don't utilize max-strength often, but the percentage of max determines the efficiency. Early in the season, you may only be using 30-40% of max-strength during most of a race. By the end of the season, to maintain that kind of speed, you have to use 50-60% of max-strength. This won't be as efficient and will consume more oxygen for the same power-output and muscle fatigue will set in faster as well.