The current rationale of clinical practice in spastic tendon transfer surgery is based on four assumptions: (1) changes in muscle fiber length (serial number of sarcomeres) determine the available length range and joint excursion, (2) muscle cross-sectional area determines the maximal force output, (3) fiber length and muscle force are invariable functions of muscle length, (4) there is an invariable relation between the elastic force and the active force exerted by the sarcomeres. The validity of these assumptions is discussed. Additionally, some new perspectives in muscle research are discussed and myofascial force transmission is introduced as a co-determinant for the outcome of tendon transfer by presenting some exploratory observations.