Yesterday, I ran 11km and this morning I woke up with a stiff neck. I can no longer move my neck.

Yes, if you are experiencing muscle stiffness or spasms in your neck and shoulders, a muscle relaxant treatment like Robax Platinum could help alleviate your symptoms. It may not shorten the duration of the issue, which typically takes around 5-7 days to heal, but it can make you feel more comfortable. Be aware that it may cause drowsiness during the day. If you need to stay alert, you can use the treatment at bedtime only and apply Voltaren Emulgel X-Fort during the day. Applying heat can also have a relaxing effect on your muscles, so I highly recommend trying this method.