I gave blood last week! And directly from the little card they gave me: "Do not lift any heavy objects or participate in any strenuous exercise such as bowling, basketball, volleyball, swimming, golf, yard work, etc., for twelve hours following donation." So, as long as you don't plan on doing a heavy workout the day OF your donation, you should be perfectly fine. Happydaisy is absolutely right, there are critical shortages of blood and any little part you can do to help the situation is worth it! I say do it! (gently, of course...)