From Bikesportmichigan:
"Not covering your legs.
It's the first sunny day of the season. 60 degrees feels like 80 and the wind is still. You head out on the bike for your first outdoor ride of the season and decide to catch a few rays also. You wear shorts. Huge mistake.
When I lived at the Olympic Training Center in Colorado Springs we were not allowed to ride outside under 70 degrees with bare legs. It was mandatory to wear tights, leg warmers or knee warmers. We also used "hot cream" on our legs- an analgesic or warming lotion that improves circulation and keeps joints warm.
Riding under 70 degrees with bare legs predisposes you to a variety of problems including patellar tendonitis, Achilles tendonitis, problems with other connective tissues (ACL, PCL, MCL, LCL) and increased muscle soreness. You must keep your knees warm.
To understand why, place the open palm of your hand over your quadriceps muscle and leave it there for a moment. Notice it feels slightly warm- your muscles and body in general are warm. Now place your palm over your bony kneecap. Notice it is substantially cooler to the touch. There is minimal muscle tissue with its attendant warm blood supply over the patellar (kneecap) region. Your knee area is cooler than the surrounding area. On the bike there is a constant flow of cold air (even at 65 degrees Fahrenheit) surrounding the knee. This causes tendons (particularly the vulnerable patellar tendon surrounding the knee cap) to become stiff and prone to injury.
To avoid this keep your knees covered. When I moved to Europe to race bicycles for a Belgian team after leaving the Olympic Training Center the Belgian Director Sportif (Team Director) was even more strict. He told us we must never wear short pants off the bike, to always keep our legs covered off the bike. He told us, "You must protect your legs, they are your tools."
You may see European professional racers doing Spring Classic races such as Het Volk, Ghent-Wevelgem, Tour of Flanders and Paris-Roubaix with shorts on despite freezing, damp conditions. Why aren't their legs covered? They are, you just can't see it. Thick layers of hot cream covered by a protective film that dries to a hard layer. Several manufacturers sell a "weatherguard" balm that covers the legs and is layered over hot cream. This keeps the legs warm and free from wind while allowing freedom of movement. Using hot cream and weatherguard is an acceptable alternative during cold races, but not for training.
Bottom line: Below 70 degrees keep your legs covered. Wear tights, leg warmers or knee warmers. Use hot cream. Protect your knees.
On early season rides when it is cold in the morning but the day heats up wear knee or leg warmers and remove them as the temperature climbs over 70- that's what those pockets on the back of your jersey are for."