Something that's constrained is forced, unnatural and not very spontaneous — kind of like a linebacker trying to perform the ballet Swan Lake.
Constrained contains the word strained for a reason. Mostly because anyone that's constrained is feeling a fair amount of strain because of it. In addition to referring to unnatural or uncharacteristic behavior, being constrained can also mean being held back or forced to do something you don't want to do. If your budget has taken a hit, for example, you may feel constrained financially. If you are on a diet, you may feel constrained by the Weight Watchers point system.