用力 means 'to use physical force' — it's about applying muscle power or strength to a concrete action like pushing, pulling, or kicking. It differs from 尽力 and 使劲: 尽力 is about doing your best overall (not just physically), while 使劲 is a close synonym but slightly more colloquial and often used for a sudden burst of effort. 出力 is broader, meaning to contribute effort or labor in general.