274 K. W. Ranatunga and others J Physiol 585.1 and before energy liberation. Overall, this would lead to the well-known Fenn effect (Fenn, 1924) that energy liberation in muscle is enhanced during shortening and depressed during lengthening (Infante et al. 1964; Curtin & Davies, 1973; Linari et al. 2003b). Furthermore, the implication is that the high force in lengthening muscle (see Fig. 4) arises from the pre-power stroke crossbridges attaching, getting strained by stretch and detaching without proceeding through the full ATPase cycle, a conclusion that is also generally consistent with previous studies (Lombardi & Piazzesi, 1990; Getz et al. 1998; Pinniger et al. 2006). It is also noteworthy that, whereas the role of muscle tendon in energy storage and its relevance and signiﬁcance in animal locomotion are well recognized (Alexander, 1992), within-muscle energy storage during lengthening is probably not fully appreciated (Linari et al. 2003b). The non-endothermic nature of steady tension during lengthening and, although not characterized here, a more obvious thermal-expansion effect by a T-jump (see Figs 1 and 2) indicate that stretch of non-crossbridge elements within muscle may contribute to the force and energy storage in lengthening muscle (Edman & Tsuchiya, 1996; Pinniger et al. 2006). From isotonic release experiments on intact rat muscle, at different temperatures, it was found that the maximum shortening velocity and the steady state force-shortening velocity curve were very temperature sensitive such that the maximal mechanical power output increased markedly (> 20-fold) in warming from 10 to 35◦C (Ranatunga, 1984, 1998). The present ﬁndings that the steady tension in shortening muscle is particularly sensitive to a T-jump (Figs 2 and 3) and that the tension at a given shortening velocity increases with warming, in a manner similar to the isometric force (Fig. 7), were basically expected from those studies. Additionally, the fact that the lengthening muscle tension is insensitive to a T-jump is also consistent with the eccentric force measurements made in human muscle experiments at high and low temperatures (De Ruiter & De Haan, 2001) and is found in steady state, frog ﬁbre experiments (Fig. 6A of Piazzesi et al. 2003). T-jump tension response in shortening muscle (i) During approach to steady state. When a standard T-jump was applied at different times during a ramp shortening, it appeared that the T-jump tension response becomes more pronounced as the shortening continues and tension during shortening approaches the steady state (Fig. 6A and B). However, when the underling tension decrease due to shortening is removed, the T-jump response remains similar at different times; thus, examination of the difference tension traces (Fig. 6C) Figure 9. Simulated tension responses to a standard T-jump, during steady shortening Simulations carried out using Scheme 1, where the sum of the fractional occupancy of three states (AM∗.ADP.Pi, AM∗ADP and AM∗ .ADP) is taken as force – see Methods (in the plots, it is normalized to the sum of occupancy of the three states in isometric case (i.e. P0 at 10◦C)). In each case, a shortening was simulated by increasing k+4 and, at steady state, a 5◦C T-jump (re-setting k+1) is induced at time zero (see Gutfreund & Ranatunga, 1999). A, simulated tension responses to T-jumps, at ∼10◦C; initial force is reduced by simulated shortening at different velocities (low to high). Note that a T-jump induces a faster tension rise at higher velocities, but the absolute amplitude of tension rise is similar (as in Figs 3B and 5). B, simulated T-jump tension responses, when shortening at the same velocity (constant k+4) at different temperatures (as labelled). The pre- and post T-jump tension and the rate of tension rise to a T-jump are increased at the higher temperatures (as in Figs 7 and 8). C 2007 The Authors. Journal compilation C 2007 The Physiological Society