Comparison of energy output during ramp and staircase shortening in frog muscle fibres.

Abstract

1. We compared the rates of work and heat production during ramp shortening with those during staircase shortening (sequence of step releases of the same amplitude, separated by regular time intervals). Ramp or staircase shortening was applied to isolated muscle fibres (sarcomere length, 2-2 ,sm; temperature, 1 00) at the plateau of an isometric tetanus. The total amount of shortening was no greater than 6 % of the fibre length. 2. During ramp shortening the power output showed a maximum at about 0-8 fibre lengths per second (Lo C1), which corresponds to 1/3 the maximum shortening velocity (VO). For the same average shortening velocity during staircase shortening (step size, -0 5% Lo) the power output was 40-60% lower. The rate of heat production for the same average shortening velocity was -45% higher during staircase shortening than during ramp shortening. 3. The relation between rate of total energy output and shortening velocity was well described by a second order regression line in the range of velocities used (0 1-2 3 Lo s-1). For any shortening velocity the rate of total energy output (power plus heat rate) was not statistically different for staircase (step size, 0 5 % Lo) and ramp shortening.4. The mechanical efficiency (the ratio of the power over the total energy rate) during ramp shortening had a maximum value of 0-36 at 1/5 VO; during staircase shortening, for any given shortening velocity, the mechanical efficiency was reduced compared with ramp shortening: with a staircase step of about 0 5 % Lo at 1/5 VO the efficiency was 0 2. 5.The results indicate that a cross-bridge is able to convert different quantities of energy into work depending on the different shortening protocol used. The fraction of energy dissipated as heat is larger during staircase shortening than during ramp shortening.

High mechanical efficiency of the cross-bridge powerstroke in skeletal muscle

“…4). This may be consistent with the result that a roughly proportional relationship exists between the rate of ATP utilization and the fibre shortening velocity (Reggiani et al, 1997;He et al, 2000;Potma and Stienen, 1996), but not with the biphasic relationship between the rate of energy liberation (heat + work) and the shortening velocity in whole muscle (Hill, 1964;Linari and Woledge, 1995). The approximately linear dependence of the amount of ATP utilized for work production on the distance of shortening (Fig.…”

Chemo‐mechanical energy transduction in relation to myosin isoform composition in skeletal muscle fibres of the rat

“…These increases were larger than those (2 times in EDL and even less in soleus) observed by Heglund & Cavagna (1987) and opposite to those (5 times in the soleus and 1·6 times in the EDL) reported by Barclay et al (1993). Similar increases have been observed in frog single fibres at low temperature: for example, Linari & Woledge (1995) found that, when fibres shortened at about 60% Vmax, the rate of total energy output was 6 times higher than in isometric conditions. The comparison between different studies is, however, made difficult by possible influences of temperature and sarcomere length (for a review see Rall, 1982;and Woledge, Curtin & Homsher, 1985) and by the presence of non-myofibrillar sources of energy consumption in whole muscles as well as in intact single fibres.…”

“…described in detail in frog muscles and single fibres (Fenn, 1924;Hill, 1938;Woledge, 1968;Irving & Woledge, 1981;Linari & Woledge, 1995). Contrasting results have been obtained from the calculation of efficiency as the ratio of mechanical power output to the rate of energy consumption.…”

Toward a Unified Theory of Muscle Contraction. II: Predictions with the Mean-Field Approximation

“…Muscle power vT(v) per myofilament (Fig. 6d) 34 ; it is not clear that a maximum rate for v < v o has been observed in frog sartorius muscle. As a function of v, the predicted efficiency of energy conversion has the expected shape, rising to a maximum of 0.6 at v = 300 nm/s.…”

“…These slower phases can be associated with cycling heads. 7,34,49 Predictions for the phase-2 response of the model are presented in Fig. 8.…”