Cathy,

What Kelly says is definitely correct. However, if you are locking your elbows because you are too stretched out, you might need a shorter stem (horizontal or angled part of the stem) or to raise the vertical part up a bit. Your LBS can help you check your fit to the stem and see if your reach is correct.

If that's not a problem, your arms may just be getting tired as you ride, especially if you start off more bent and relaxed and end up with straight elbows towards the end of a ride. That's really common as riders get fatigued - to use your joints to support your weight (bad) rather than your muscles (good). I've found that weight training really helps with arm/shoulder endurance on rides. It doesn't have to be anything heavy-duty, just some dumbell work, or machines at the gym. Triceps and pectorals (shoulders) are the most important in helping support your upper body while you ride. Core strength (abs, lower back, torso) is also important so that you don't put too much weight on your arms. If your core is week, you sorta collapse forward and put too much weight on your upper body when riding.

Hope these ideas help some! Sometimes it's just a matter of reminding yourself not to lock elbows while riding, until it becomes second nature. But sometimes there is a "fix" that really is needed.

Emily