The

maximum velocity of sarcomere shortening is a useful parameter for estimating the contractile function of muscle fibres (Moss, 1992; Josephson & Edman, 1998; Gordon et al. 2000). This parameter can be obtained experimentally either by extrapolating force–velocity curves to zero load (Hill, 1938) or by using the ‘slack test ’ method (Edman, 1979). In such experiments, care is usually taken to avoid a large influence of passive (or resting) tension (PT) on shortening velocity, as elastic recoil of non-contractile structures can accelerate the speed of shortening if PT is significant (Edman, 1979; Claflin et al. 1989; Galler & Hilber, 1994). Many experiments demonstrating the effect of PT level on unloaded shortening velocity (Vo) had been performed some time before the origin of PT in vertebrate skeletal muscle was established. Today it is clear that most of this PT originates in the intrasarcomeric titin (also known as connectin) filaments (Wang, 1996