When i grip an object (eg: bike handlebars) for a short time, my fingers seem to seize up and are painful when i try to release the object. why?

Myotonias, genetic: Disorders that cause inability to release a grip, are dystrophic (with slowly progressive weakness) or non-dystrophic (not progressive). Dystrophic types have autosomal dominant inheritance, so one of your parents would have it. Non-dystrophic types are from point mutations on specific genes. A neurologist & geneticist can determine if you have either type. Many are annoying, but not disabling.