I recently posted an article on the benefits of vitamin D, and the recent realization that a high percentage of people are deficient.  Research is showing that among other functions, vitamin D has the ability to up-regulate and down-regulate genes.  In other words, when we have suboptimal levels of vitamin D our genes are not turned and off appropriately.  Genes are used to create proteins which are the building blocks of nearly everything in our body--from our structure (bone, ligament, tendon, etc) to neurotransmitters, hormones, blood, etc.  

This wide range of effects translates a host of problems linked to vitamin D deficiency including poor immune function, less vitality, and decreased athletic function.   Specific to athletic function, a recent New York Times article highlighted the following:

"In one of the studies, four Russian sprinters were doused with artificial, ultraviolet light. Another group wasn’t. Both trained identically for the 100-meter dash. The control group lowered their sprint times by 1.7 percent. The radiated runners, in comparison, improved by an impressive 7.4 percent." (**note that ultraviolet light causes the formation of vitamin D).

"when researchers tested the vertical jumping ability of a small group of adolescent athletes, Larson-Meyer says, “they found that those who had the lowest levels of Vitamin D tended not to jump as high,” intimating that too little of the nutrient may impair muscle power. Low levels might also contribute to sports injuries, in part because Vitamin D is so important for bone and muscle health. In a Creighton University study of female naval recruits, stress fractures were reduced significantly after the women started taking supplements of Vitamin D and calcium."

I highly recommend having your vitamin D level checked with your next blood work.  If it is low supplement with 2,000IU of vitamin D for 6 weeks and recheck.  With my family I have everyone supplement with about 1,000IU for about three weeks, then we take about a three week break.  WE also get plenty of sunshine!  If you are local, I have an excellent product in my office.