The specific relationship between force and length is one of the most important characteristics of vertebrate muscle. The only accurate method to measure the force-length characteristics is to generate a set of isometric force-time plots at different muscle lengths. In humans, such force-length characteristics mostly are based on indirect measurements that have their limitations. A method of direct, in vivo measurement of force-length characteristics of the human flexor carpi ulnaris muscle using relatively simple equipment during transposition surgery is presented. The method is proven reproducible, with an overall estimated error of 2.8%.