Christopher John Barclay

Mechanical efficiency and fatigue of fast and slow muscles of the mouse.

1. In this study, the efficiency of energy conversion in skeletal muscles from the mouse was determined before and after a series of contractions that produced a moderate level of fatigue. 2. Initial mechanical efficiency was defined as the ratio of mechanical power output to the rate of initial enthalpy output. The rate of initial enthalpy output was the sum of the power output and rate of initial heat output. Heat output was measured using a thermopile with high temporal resolution. 3. Experiments were performed in vitro (25 degrees C) using bundles of fibres from fast‐twitch extensor digitorum longus (EDL) and slow‐twitch soleus muscles from mice. Muscles were fatigued using a series of thirty isometric tetani. Initial mechanical efficiency was determined before and again immediately after the fatigue protocol using a series of isovelocity contractions at shortening velocities between 0 and the maximum shortening velocity (Vmax). Efficiency was determined over the second half of the shortening at each velocity. 4. The fatigue protocol significantly reduced maximum isometric force Vmax, maximum power output and flattened the force‐velocity curve. The magnitude of these effects was greater in EDL muscle than soleus muscle. In unfatigued muscle, the maximum mechanical efficiency was 0.333 for EDL muscles and 0.425 for soleus muscles. In both muscle types, the fatiguing contractions caused maximum efficiency to decrease. The magnitude of the decrease was 15% of the pre‐fatigue value in EDL and 9% in soleus. 5. In a separate series of experiments, the effect of the fatigue protocol on the partitioning of energy expenditure between crossbridge and non‐crossbridge sources was determined. Data from these experiments enabled the efficiency of energy conversion by the crossbridges to be estimated. It was concluded that the decrease in initial mechanical efficiency reflected a decrease in the efficiency of energy conversion by the crossbridges.

Energy turnover for Ca2+ cycling in skeletal muscle.

The majority of energy consumed by contracting muscle can be accounted for by two ATP-dependent processes, cross-bridge cycling and Ca2+ cycling. The energy for Ca2+ cycling is necessary for contraction but is an overhead cost, energy that cannot be converted into mechanical work. Measurement of the energy used for Ca2+ cycling also provides a means of determining the total Ca2+ released from the sarcoplasmic reticulum into the sarcoplasm during a contraction. To make such a measurement requires a method to selectively inhibit cross-bridge cycling without altering Ca2+ cycling. In this review, we provide a critical analysis of the methods used to partition skeletal muscle energy consumption between cross-bridge and non-cross-bridge processes and present a summary of data for a wide range of skeletal muscles. It is striking that the cost of Ca2+ cycling is almost the same, 30–40% of the total cost of isometric contraction, for most muscles studied despite differences in muscle contractile properties, experimental conditions, techniques used to measure energy cost and to partition energy use and in absolute rates of energy use. This fraction increases with temperature for amphibian or fish muscle. Fewer data are available for mammalian muscle but most values are similar to those for amphibian muscle. For mammalian muscles there are no obvious effects of animal size, muscle fibre type or temperature.

The influence of tendon compliance on muscle power output and efficiency during cyclic contractions

SUMMARY Muscle power output and efficiency during cyclical contractions are influenced by the timing and duration of stimulation of the muscle and the interaction of the muscle with its mechanical environment. It has been suggested that tendon compliance may reduce the energy required for power production from the muscle by reducing the required shortening of the muscle fibres. Theoretically this may allow the muscle to maintain both high power output and efficiency during cyclical contraction; however, this has yet to be demonstrated experimentally. To investigate how tendon compliance might act to increase muscle power output and/or efficiency, we attached artificial tendons of varying compliance to muscle fibre bundles in vitro and measured power output and mechanical efficiency during stretch—shorten cycles (2 Hz) with a range of stretch amplitudes and stimulation patterns. The results showed that peak power, average power output and efficiency (none of which can have direct contributions from the compliant tendon) all increased with increasing tendon compliance, presumably due to the tendon acting to minimise muscle energy use by allowing the muscle fibres to shorten at optimal speeds. Matching highly compliant tendons with a sufficiently large amplitude length change and appropriate stimulation pattern significantly increased the net muscle efficiency compared with stiff tendons acting at the same frequency. The maximum efficiency for compliant tendons was also similar to the highest value measured under constant velocity and force conditions, which suggests that tendon compliance can maximise muscle efficiency in the conditions tested here. These results provide experimental evidence that during constrained cyclical contractions, muscle power and efficiency can be enhanced with compliant tendons.

Inferring crossbridge properties from skeletal muscle energetics

Work is generated in muscle by myosin crossbridges during their interaction with the actin filament. The energy from which the work is produced is the free energy change of ATP hydrolysis and efficiency quantifies the fraction of the energy supplied that is converted into work. The purpose of this review is to compare the efficiency of frog skeletal muscle determined from measurements of work output and either heat production or chemical breakdown with the work produced per crossbridge cycle predicted on the basis of the mechanical responses of contracting muscle to rapid length perturbations. We review the literature to establish the likely maximum crossbridge efficiency for frog skeletal muscle (0.4) and, using this value, calculate the maximum work a crossbridge can perform in a single attachment to actin (33 x 10(-21) J). To see whether this amount of work is consistent with our understanding of crossbridge mechanics, we examine measurements of the force responses of frog muscle to fast length perturbations and, taking account of filament compliance, determine the crossbridge force-extension relationship and the velocity dependences of the fraction of crossbridges attached and average crossbridge strain. These data are used in combination with a Huxley-Simmons-type model of the thermodynamics of the attached crossbridge to determine whether this type of model can adequately account for the observed muscle efficiency. Although it is apparent that there are still deficiencies in our understanding of how to accurately model some aspects of ensemble crossbridge behaviour, this comparison shows that crossbridge energetics are consistent with known crossbridge properties.

Slow skeletal muscles of the mouse have greater initial efficiency than fast muscles but the same net efficiency

The aim of this study was to determine whether the net efficiency of mammalian muscles depends on muscle fibre type. Experiments were performed in vitro (35°C) using bundles of muscle fibres from the slow‐twitch soleus and fast‐twitch extensor digitorum longus (EDL) muscles of the mouse. The contraction protocol consisted of 10 brief contractions, with a cyclic length change in each contraction cycle. Work output and heat production were measured and enthalpy output (work + heat) was used as the index of energy expenditure. Initial efficiency was defined as the ratio of work output to enthalpy output during the first 1 s of activity. Net efficiency was defined as the ratio of the total work produced in all the contractions to the total, suprabasal enthalpy produced in response to the contraction series, i.e. net efficiency incorporates both initial and recovery metabolism. Initial efficiency was greater in soleus (30 ± 1%; n= 6) than EDL (23 ± 1%; n= 6) but there was no difference in net efficiency between the two muscles (12.6 ± 0.7% for soleus and 11.7 ± 0.5% for EDL). Therefore, more recovery heat was produced per unit of initial energy expenditure in soleus than EDL. The calculated efficiency of oxidative phosphorylation was lower in soleus than EDL. The difference in recovery metabolism between soleus and EDL is unlikely to be due to effects of changes in intracellular pH on the enthalpy change associated with PCr hydrolysis. It is suggested that the functionally important specialization of slow‐twitch muscle is its low rate of energy use rather than high efficiency.

Temperature change as a probe of muscle crossbridge kinetics: a review and discussion

Following the ideas introduced by Huxley (Huxley 1957, Prog. Biophys. Biophys. Chem. 7, 255–318), it is generally supposed that muscle contraction is produced by temporary links, called crossbridges, between myosin and actin filaments, which form and break in a cyclic process driven by ATP splitting. Here we consider the interaction of the energy in the crossbridge, in its various states, and the force exerted. We discuss experiments in which the mechanical state of the crossbridge is changed by imposed movement and the energetic consequence observed as heat output and the converse experiments in which the energy content is changed by altering temperature and the mechanical consequences are observed. The thermodynamic relationship between the experiments is explained and, at the first sight, the relationship between the results of these two types of experiment appears paradoxical. However, we describe here how both of them can be explained by a model in which mechanical and energetic changes in the crossbridges occur in separate steps in a branching cycle.

Is the efficiency of mammalian (mouse) skeletal muscle temperature dependent

Myosin crossbridges in muscle convert chemical energy into mechanical energy. Reported values for crossbridge efficiency in human muscles are high compared to values measured in vitro using muscles of other mammalian species. Most in vitro muscle experiments have been performed at temperatures lower than mammalian physiological temperature, raising the possibility that human efficiency values are higher than those of isolated preparations because efficiency is temperature dependent. The aim of this study was to determine the effect of temperature on the efficiency of isolated mammalian (mouse) muscle. Measurements were made of the power output and heat production of bundles of muscle fibres from the fast‐twitch extensor digitorum longus (EDL) and slow‐twitch soleus muscles during isovelocity shortening. Mechanical efficiency was defined as the ratio of power output to rate of enthalpy output, where rate of enthalpy output was the sum of the power output and rate of heat output. Experiments were performed at 20, 25 and 30°C. Maximum efficiency of EDL muscles was independent of temperature; the highest value was 0.31 ± 0.01 (n= 5) at 30°C. Maximum efficiency of soleus preparations was slightly but significantly higher at 25 and 30°C than at 20°C; the maximum mean value was 0.48 ± 0.02 (n= 7) at 25°C. It was concluded that maximum mechanical efficiency of isolated mouse muscle was little affected by temperature between 20 and 30°C and that it is unlikely that differences in temperature account for the relatively high efficiency of human muscle in vivo compared to isolated mammalian muscles.

The energetic cost of activation in mouse fast‐twitch muscle is the same whether measured using reduced filament overlap or N‐benzyl‐p‐toluenesulphonamide

Aim:  Force generation and transmembrane ion pumping account for the majority of energy expended by contracting skeletal muscles. Energy turnover for ion pumping, activation energy turnover (EA), can be determined by measuring the energy turnover when force generation has been inhibited. Most measurements show that activation accounts for 25–40% of isometric energy turnover. It was recently reported that when force generation in mouse fast‐twitch muscle was inhibited using N‐benzyl‐p‐toluenesulphonamide (BTS), activation accounted for as much as 80% of total energy turnover during submaximal contractions. The purpose of this study was to compare EA measured by inhibiting force generation by: (1) the conventional method of reducing contractile filament overlap; and (2) pharmacological inhibition using BTS.

FATIGUE AND HEAT PRODUCTION IN REPEATED CONTRACTIONS OF MOUSE SKELETAL MUSCLE

1. This study tested the hypothesis that moderate fatigue of skeletal muscle arises from a mismatch between energy demand and energy supply. Fatigue was defined as the decline in isometric force. Energy supply and demand were assessed from measurements of muscle heat production. 2. Experiments were performed in vitro (21 degrees C) with bundles of muscle fibres from mouse fast‐twitch extensor digitorum longus muscle and slow‐twitch soleus muscle. Fibre bundles were fatigued using a series of thirty isometric tetani. Cycle duration (time between successive tetani) was 5 s. The amount of fatigue that occurred during a series of tetani was varied by varying contraction duty cycle (tetanus duration/cycle duration) by varying tetanus duration. 3. Peak isometric force and total heat production in each cycle were measured. For each cycle, the amounts of initial heat (H(i)) and recovery heat (Hr) produced were calculated and used as indices of energy use and supply, respectively. H(i) and Hr were used to estimate the net initial chemical breakdown (in energy units) in each cycle (H(i,net)). 4. The magnitude of H(i,net) was greatest in the early stages of the contraction protocol when Hr was still increasing towards a steady value. The magnitude of decline in force between successive tetani was proportional to H(i,net) for both muscles. 5. The results are consistent with the idea that the development of moderate levels of fatigue at the start of a series of contractions is due to the rate of energy supply being inadequate to match the rate of energy use.

Estimation of cross-bridge stiffness from maximum thermodynamic efficiency

In muscle, work is performed by myosin cross-bridges during interactions with actin filaments. The amount of work performed during each interaction can be related to the mechanical properties of the cross-bridge; work is the integral of the force produced with respect to the distance that the cross-bridge moves the actin filament, and force is determined by the stiffness of the attached cross-bridge. In this paper, cross-bridge stiffness in frog sartorius muscle was estimated from thermodynamic efficiency (work/free energy change) using a two-state cross-bridge model, assuming constant stiffness over the working range and tight-coupling between cross- bridge cycles and ATP use. This model accurately predicts mechanical efficiency (work/enthalpy output). A critical review of the literature indicates that a realistic value for maximum thermodynamic efficiency of frog sartorius is 0.45 under conditions commonly used in experiments on isolated muscle. Cross-bridge stiffness was estimated for a range of power stroke amplitudes. For realistic amplitudes (10–15nm), estimated cross-bridge stiffness was between 1 and 2.2pNnm−1. These values are similar to those estimated from quick-release experiments, taking into account compliance arising from structures other than cross-bridges, but are substantially higher than those from isolated protein studies. The effects on stiffness estimates of relaxing the tight-coupling requirement and of incorporating more force-producing cross-bridge states are also considered.