Every one of us could do our own selves a big favour by riding our trainers in front of a mirror once in a while and watch ourselves ride. You will not believe what you can learn about your own position by simply looking in a mirror
For example, If you see that you have lateral shoulder movement (movement side to side) when you pedal, this could be a sign that you have tight psoas muscles that are causing your SI joints to move laterally in your hips. The poas muscle is a long thick muscle that is attached to the side of the lumbar region of your spine at one end and the top of your femur (lesser trochanter) on the other. This muscle contributes to flexion and external rotaion of your hip joint and is part of your hip flexors
Tightness in this muscle group can also cause lower back pain while cycling.
The movement in your shoulders is your body's way of counter balancing the unwanted movement in your hips, so if you see this in the mirror talk to your Physiotherapist and find out some good stretches for increasing the flexibility in your psoas muscles.
This will ultimately result in a quieter upper body on the bike, decrease lower back pain, save you energy and result in a smoother overall pedal stroke
So be vain... You'll be glad you did