Gen. Physiol. Biophys (1999), 18, 155—163 155 Passive Forces in Mammalian Skeletal Muscle: A Freely-Jointed and Worm-Like Chain J. MUNIZ 1 , J. L. MARÍN 2 , M. HUERTA 1 , J. D E L R I O 1 , A. LARIOS 1 AND X. TRUJILLO 1 1 Centro Universitario de Inveshgaciones Biomédicas, Universidad de Cohma, Apdo. Postal No. 11, 28000 Cohma, Cohma, Mexico 2 Centro de Investigación en Fisica, Universidad de Sonora, Apdo Postal 5-088 83190 Hermosillo, Sonora, Mexico Abstract. The passive mechanical properties of whole muscle in active and non- active states are compared. The experimental results are presented as stress-strain curves, which are analyzed in the framework of the current theoretical background [viz. the freely-jointed chain model (FJCM) and the worm-like chain model (WLCM)] in a semi-quantitative fashion. This analysis shows that both models can explain the mechanical behavior of whole muscle in non-active state. In the ac- tive state, the presence of crossbridges alters the mechanical response, leading to a markedly different behavior, as expected. A discussion of the mechanisms involved and the interpretation of the parameters required for the fitting of the stress-strain curves is also presented. Key words: Mammalian skeletal muscle — Passive forces — Freely-jointed model — Worm-like chain model Introduction Mammalian skeletal muscles can be characterized functionally as either slow or fast- twitch depending on the relative content of slow-twitch (type I) and fast-twitch (type IIA and IIB) fibres. Fibre types are identified and classified according to their contractile and metabolic characteristics (Brooke and Kaiser 1970; Peter et al. 1972). The plantaris muscle of the rat is a fast muscle, containing as it does about 85% of type IIA-B muscle fibres. Skeletal muscles develop both active and passive tension. The first involves the hydrolysis of ATP following actomyosin interaction in the presence of Ca 2+ . Passive Correspondence to Prof Dr. M Huerta, CUIB, Universidad de Cohma, Apdo. Postal No 11, 28000-Colima, Cohma, Mexico E-mail huertam9cgic.ucol.mx