There is no experimental evidence for non-linear myofilament elasticity in skeletal muscle.

Information on the stiffness of the actin and myosin filaments is a key parameter in the study of muscle contraction. For example, the number of motors attached to actin in each half-sarcomere of a muscle fibre can be obtained from measurements of the half-sarcomere stiffness, provided that the compliances of the actin and myosin filaments are known (Piazzesi et al., 2007). The stress–strain relationship in myofilaments has generally been considered to be linear (Kojima et al., and data suggesting that the myofilament stiffness can vary with force could also find alternative explanations (Huxley et al., 1994; Higuchi et al., 1995). The argument has been recently reinvestigated by Edman (Edman, 2009), who concluded that myofilament stiffness exhibits a strong non-Hookean behaviour. In Edman's paper (Edman, 2009), stiffness is measured by the change in force in response to 2–4 kHz length oscillations imposed on intact fibres isolated from the tibialis anterior muscle of Rana temporaria (2–2.5°C). Measurements are performed at two different sarcomere lengths (sl) in isometric contraction and during isotonic shortening at different pre-set loads. The changes in stiffness with sarcomere length for different loads are estimated from the ratio of the fibre stiffness at sl2.6 µm (S 2.6) to that at sl2.2 µm (S 2.2 ; the condition for full overlap between actin and myosin filaments). The pre-set loads are in the range of 0.4–0.7 the isometric tetanic force (T 0) at sl2.2 µm. The elements considered to contribute to the half-sarcomere stiffness are the array of actin-attached cross-bridges and the portions of both myosin and actin filaments beyond the overlap region. The stiffness of the cross-bridge array is proportional to the number of attached cross-bridges, which depends solely and linearly on the overall level of force, independent of how force is modulated, either with a different degree of overlap (Gordon et al., 1966) or with isotonic shortening at different loads. This latter assumption has been recently found valid in the force range 0.5–1T 0 (Piazzesi et al., 2007). The contribution of myofilaments to the half-sarcomere compliance is assumed proportional to the length of their non-overlapping regions. The compliance per unit length is considered the same for both actin and myosin. This is an arbitrary assumption but is acceptable as a first approximation for the purpose of determining whether or not the myofilament stiffness is Hookean. Other acceptable approximations are that the changes in the length of the fibre are …