Fig. 2

Fibre shortening. Force (a), and corresponding fascicle length change (b) for an isometric contraction of the cat medial gastrocnemius (MG) muscle. Isometric here refers to the constant length of the entire muscle-tendon unit. For this particular example, the MG muscle fascicles/fibres shorten from about 24 mm to about 18 mm with increasing force, demonstrating that fascicle/fibre/sarcomere lengths in a muscle do not depend on muscle length alone (the muscle was kept at a constant length), but also depend crucially on the amount of force that the muscle is producing. The interpretation of this finding has been that with increasing force, structural (visco-) elastic elements of the muscle are stretched, allowing muscle fascicles/fibres to shorten. The amount of shortening of muscle fibres depends on the initial muscle length and the force produced (e.g. [4,5,6]) [Reprinted with permission from The Physiological Society, the Journal of Physiology, Griffiths et al. 1991 [3]]