Biomedical subjects

R C Woledge

Publications and source records attributed to R C Woledge.

At least 37 records · Page 2Linked to original sources

PubMed1998

Distinguishing metabolic heat from condensation heat during muscle recovery.

When a thermopile is used to measure the heat production of isolated muscle, the muscle is surrounded by gas saturated with water vapour, initially in equilibrium with the muscle. After contraction, the osmolarity of the muscle is raised so that it is no longer in equilibrium with the gas around it, and condensation will occur. When artificial muscles of known osmolarity were placed on a thermopile surrounded by gas in equilibrium with a solution of lower osmolarity, their temperature was found to be raised (by 102.7 mK osmol-1 l). This temperature increase was greatly reduced by covering the artificial muscle with a Teflon film. Experiments on living muscle from the dogfish Scyliorhinus canicula showed that muscle temperature was higher 2 min after a series of 20 twitches at 3 Hz if the muscle was not covered by Teflon than if it was covered. The Teflon covering did not diminish the muscle's contractile performance. We conclude that the condensation of water does contribute to the heat measured during the recovery period, but that when the muscle is covered by Teflon film condensation heat can largely be prevented so that only genuine metabolic recovery heat is produced.

Animals↗

PubMed1998

Muscle energetics during unfused tetanic contractions. Modelling the effects of series elasticity.

During an unfused tetanic contraction the contractile component stretches and then is stretched by the series elasticity in the muscle fibre during each tension oscillations. This causes the heat rate to increase, from increased metabolic rate, during the time when the contractile component is shortening. During the time when the contractile component is being stretched there is heat produced within the contractile component from dissipation of the work stored in the contractile component. A simulation is used to show that these effects are not negligible when the effects of shortening velocity on energy output rate is determined using unfused contraction. The overall effects resemble those that would be produced in a muscle if the effect of shortening velocity in accelerating the rate of cross-bridge cycling were reduced at low activation levels.

Animals↗

PubMed1998

Shortening during stimulation vs. during relaxation. How do the costs compare?

White muscle fibres from dogfish were used to investigate the energetic cost of shortening by fully active muscle and by relaxing muscle. The muscle preparation was tetanized for 0.6 s and shortened by 1 mm (about 15% L0) at 7 mm/s (about 30% V0) either during stimulation or during relaxation. Isometric tetani at L0 were also investigated. Mechanical work was calculated from force and length change. Work + heat was taken as a measure of energetic cost. Both work and energetic cost were higher for shortening during stimulation than during relaxation. We also evaluated separately the work and heat associated with the contractile component and with the series elastic component. Work stored in the series elasticity could be completely recovered as external work when the shortening occurred during relaxation.

Animals↗

PubMed1997

Mechanical and energy characteristics during shortening in isolated type-1 muscle fibres from Xenopus laevis studied at maximal and submaximal activation.

The mechanical and energy characteristics of isolated fast-twitch muscle fibres (type 1) of Xenopus laevis in isometric- and isovelocity contractions were measured at 20 degrees C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of activation. The high stimulation frequency gave fused contractions, while at the low stimulation frequency tension fluctuated. When maximum isometric force had been reached, the fibres were shortened by 10% of the fibre length at different velocities. At 60 Hz stimulation during shortening the rate of heat production increased above the isometric rate of heat production. At 20 Hz stimulation during shortening, however, the rate of heat production was not different from the isometric rate of heat production. Mechanical efficiency was the same at the high and low level of activation. The actomyosin efficiency (i.e. the mechanical efficiency corrected for "activation heat") was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activated muscle. From a comparison of the present results with those obtained from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow-twitch muscle fibres.

Actomyosin↗

PubMed1997

Interpreting the relation between force and cross-sectional area in human muscle.

The maximum force a muscle can produce depends on its cross-sectional area (CSA). However, the exact interpretation of this relationship has been a matter of controversy. Recently, the controversy has centered on whether the measurements are best correlated using regression analysis or ratio standards. Applying regression analysis to this problem implies that all the experimental error is in the measurement of force. Thus, confusion may arise by failure to take account of errors in the measurement of CSA. Using a statistical model, we show how regression analysis can be misleading as error is introduced into the measurement of CSA as well as that of force. Because neither the errors in force nor CSA can be quantified in the experimental situation, we conclude that ratio standards are less likely to mislead although the accuracy of the result depends on the degree of correlation between force and CSA in the muscle measured.

Biomechanical Phenomena↗

PubMed1997

Recovery after contraction of white muscle fibres from the dogfish Scyliorhinus canicula.

Recovery after contraction of white muscle fibres of dogfish was investigated using 31P-NMR and measurements of heat production. The muscle fibres were stimulated to perform either a single isometric tetanus or a series of brief isometric tetani; the NMR measurements showed that approximately half of the phosphocreatine (PCr) was used. The period of activity was followed by a recovery period without stimulation. Both NMR and heat measurements agreed in showing that recovery was very slow, requiring at least 60 min for PCr resynthesis and for the production of recovery heat. The NMR results showed that changes in intracellular pH and in the concentrations of PCr and intracellular phosphate (Pi) had very similar time courses. Intracellular pH moved in the alkaline direction during the period of activity and then returned monotonically during recovery. The non-phosphate buffer power was 13.0 +/- 3.1 mmol l-1 intracellular water per pH unit (N = 4, mean +/- S.E.M.). The results are consistent with the view that oxidative processes resynthesize PCr during recovery, which is slow because of the low mitochondrial content of these muscle fibres.

Animals↗

PubMed1997

A comparison of the activation of muscles moving the patella in normal subjects and in patients with chronic patellofemoral problems.

The activation of the oblique fibres of vastus medialis, the postero-lateral fibres of vastus lateralis, and rectus femoris was studied by surface electromyography, during the force development of a maximal isometric contraction, performed near full extension (20 degrees of flexion), by 49 patients with chronic patellofemoral problems and 20 normal subjects. In the normal subjects activation of oblique portions of the vasti was in advance of force rise, during the time for 80% tension development. In the patient group, however, the activation of these lagged behind force rise. Force rise was slower in the patients even though the contraction was generally pain free. In all groups the activation of these two sections of the muscle remained approximately synchronous, suggesting that they have a reciprocal action in controlling patellar position, disruption of which might contribute to patellofemoral problems.

Adult↗

PubMed1996

Changes in maximal voluntary force of human adductor pollicis muscle during the menstrual cycle.

1. Muscle strength of the adductor pollicis (AP) was studied throughout the menstrual cycle to determine whether any variation in force is similar to the known cyclical changes in ovarian hormones. Three groups of young women were studied: trained regularly menstruating athletes (trained), untrained regularly menstruating (untrained) and trained oral contraceptive pill users (OCU). In addition a group of untrained young men was studied as controls. 2. Maximum voluntary force (MVF) of AP was measured over a maximum period of 6 months. Ovulation was detected by luteinizing hormone measurements or change in basal body temperature. There was a significant increase in MVF (about 10%) during the follicular phase of the menstrual cycle when oestrogen levels are rising, in both the trained and untrained groups. This was followed by a similar in MVF around the time of ovulation. Neither the OCU nor the male subjects showed cyclical changes in MVF.

Adolescent↗

PubMed1995

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

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 microns; temperature, approximately 1 degree C) 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 s-1), which corresponds to 1/3 the maximum shortening velocity (Vo). For the same average shortening velocity during staircase shortening (step size, approximately 0.5% Lo) the power output was 40-60% lower. The rate of heat production for the same average shortening velocity was approximately 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, approximately 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 approximately 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.

Acoustic Stimulation↗

PubMed1995

Energetics of shortening depend on stimulation frequency in single muscle fibres from Xenopus laevis at 20 degrees C.

Single intact slow-twitch (type 3) muscle fibres from the iliofibularis muscle of Xenopus laevis were shortened at a constant velocity (0.4 L0/S, where L0 is the initial length at different levels of activation (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38 +/- 0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33 +/- 0.03 W/g dry weight). The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40 +/- 3% of the isometric force at 40 Hz. In these unfused tetani during shortening the heat production rate (0.18 +/- 0.02 W/g dry weight) significantly decreased below the isometric heat production rate (0.25 +/- 0.02 W/g dry weight). At full activation the rate of total energy production (mechanical power plus heat production rate) during shortening was 1.88 +/- 0.32 times the isometric total energy production rate. This effect, i.e. an increase in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was only 1.07 +/- 0.08 times the isometric value. These results show that the Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37 +/- 0.06).

Animals↗

PubMed1993

Power and efficiency: how to get the most out of striated muscle.

White muscle fibres from dogfish were stimulated during sinusoidal cycles of shortening and lengthening that mimic the in vivo pattern of movement. The results show that the timing of the stimulation relative to movement and also the frequency of the movement affect mechanical power output and efficiency. Maximum mechanical power is produced at a higher frequency of movement than maximum efficiency. The value of maximum power for a cycle of movement is less than half that produced during ramp shortening of fully active fibres. Higher efficiency is achieved during cyclic movement than during ramps, probably because work can be done with lower "overhead" costs for activation and because little ATP is used during lengthening.

Animals↗

PubMed1993

The efficiency of energy conversion by swimming muscles of fish.

The efficiency of energy conversion by fish myotomal muscle undergoing sinusoidal length changes with brief periods of stimulation in each cycle has been found to be greater than the efficiency of the same type of muscle when tetanically stimulated. This finding is reviewed in relation to the well-known energetic properties of muscle undergoing shortening. It is suggested that an adaptation of the ideas of Lombardi & Piazzesi which were proposed to explain the behaviour of muscle during lengthening could be used to provide one possible explanation of this finding. Their theory proposes that crossbridges, when they have sufficient energy because they are strained, can detach to a crossbridge state which can rapidly reattach without having to split ATP first. This type of theory might also provide an explanation of the findings on ventricular energetics which are expressed in the time varying elastance model of Suga.

Animals↗

PubMed1993

Muscle weakness in women occurs at an earlier age than in men, but strength is preserved by hormone replacement therapy.

1. The time-course of the age-related decline in specific muscle force (maximum voluntary force per cross-sectional area) in men and women was determined by measuring the maximum voluntary force and cross-sectional area of the adductor pollicis muscle in 273 subjects aged 17-90 years (176 men, 30 premenopausal women and 67 peri- or post-menopausal women who were not receiving hormone replacement therapy). 2. To determine whether the loss of specific muscle force is hormone-dependent in women, we studied a further 25 women, aged 42-72 years, who were receiving hormone replacement therapy. 3. There was no significant difference in specific force between young men and pre-menopausal women. Around the time of the menopause there was a dramatic decline in specific force in women which was prevented by the use of hormone replacement therapy. In men the weakness started later (around the age of 60 years) and the decline in specific force was more gradual, reaching the level seen in post-menopausal women after the age of 75 years. 4. The protective effect of hormone replacement therapy on muscle strength is likely to be an important contributory factor to its proven action in preventing osteoporotic fractures. The dramatic peri-menopausal decline in muscle strength is a likely explanation for the known increases in falls and Colles' fractures around the time of the menopause.

Adolescent↗

PubMed1993

The effect of metabolic fuel on force production and resting inorganic phosphate levels in mouse skeletal muscle.

1. The effect of different metabolic fuels (glucose, pyruvate and lactate) and no exogenous metabolic fuel on force production was studied in isolated mouse soleus and extensor digitorum longus (EDL) muscles. Force was measured, at 25 degrees C, during isometric tetanic contractions and during contractions with isovelocity stretching and shortening. In parallel experiments, measurements were made of the resting phosphorus metabolite levels using 31P NMR. 2. In soleus muscles, the isometric tetanic force was potentiated with pyruvate (20 mM) as metabolic fuel, compared with glucose (11 mM), by 17.8 +/- 3.6% (mean +/- S.E.M., n = 6). The force was the same with no exogenous metabolic fuel, with glucose, or with lactate as metabolic fuel. The force exerted during shortening was also potentiated by pyruvate and by the same proportion as isometric force. However, during rapid stretching there was no force enhancement with pyruvate. The changes in the force seen with pyruvate are qualitatively similar to those produced when inorganic phosphate (Pi) is lowered in skinned rabbit psoas muscle fibres. 3. We tested whether the Pi content decreased in the presence of pyruvate by measuring resting Pi using 31P NMR spectroscopy. We found that, in soleus muscles, resting Pi was present with glucose and absent with pyruvate as metabolic fuel, and the effect was reversible. 4. EDL muscles produced the same isometric force whether the metabolic fuel was glucose, pyruvate, lactate or if no exogenous metabolic fuel was supplied. EDL muscles already had Pi levels below detectability at rest in glucose. There were no changes in the 31P NMR spectrum with pyruvate as metabolic fuel. 5. It appears therefore that the force potentiation in soleus muscles with pyruvate is due to a lowering of Pi. EDL muscles, which have a very low resting Pi in glucose, therefore have very little potential for force enhancement by this mechanism.

Animals↗

PubMed1993

Neither changes in phosphorus metabolite levels nor myosin isoforms can explain the weakness in aged mouse muscle.

1. The contractile force, phosphorus metabolite levels, intracellular pH and myosin isoforms were compared in isolated soleus and extensor digitorum longus (EDL) muscles from young (6 month old) and aged (28 month old) mice, at 23 degrees C. 2. The isometric force per unit cross-sectional area was significantly lower by 21 +/- 5% in soleus muscles from aged mice compared to those from young mice (mean +/- S.E.M., n = 11 and 9 respectively). 3. The EDL muscle contained twice as much total creatine and phosphocreatine as the soleus, 1.7 times as much ATP, and 0.4 times the inorganic phosphate (Pi) per unit weight. The intracellular pH and free ADP levels were not significantly different between these muscle types. 4. There was no significant difference in resting metabolite levels between young and old EDL or soleus despite the difference in mechanical strength. 5. Examination of the expression of myosin isoforms by non-denaturing gel electrophoresis has shown that the percentage of each isoform does not change with respect to age; thus, if there is an atrophic process occurring, it is not fibre type specific. 6. We have determined that neither the Pi levels nor the intracellular pH can explain the differences seen in muscle strength with age. There is also no correlation between muscle weakness and any of the other metabolites responsible for energy transduction (phosphocreatine, ATP or ADP).

Adenosine Diphosphate↗

PubMed1993

Changes in crossbridge and non-crossbridge energetics during moderate fatigue of frog muscle fibres.

1. The effect of sarcomere length (SL) during a fatiguing series of isometric tetani of frog muscle fibres was investigated. Tetani at 2.3 microns SL were more fatiguing than tetani at 3.2 microns SL, in that force declined twice as much as relaxation became much slower. 2. In a second set of experiments the force and heat production were measured during a series of fatiguing tetani. Heat was separated into two components: (a) crossbridge heat which is dependent on filament overlap and interaction, and (b) non-crossbridge heat which is independent of filament overlap and due to Ca2+ turnover. 3. In a series of fifty tetani, force, crossbridge heat and non-crossbridge heat each declined by 25-30% of its initial value. 4. The 25% reduction in non-crossbridge heat occurred completely during the first few tetani of the fatiguing series while force declined by less than 3%. This may be due to a reduction in Ca2+ binding to parvalbumin and to Ca2+ remaining bound during the remainder of the fatigue series. 5. After the first few tetani of the fatigue series the non-crossbridge heat hardly changed as force declined by a further 25% of its initial value. Continuing reduction of force with constant Ca2+ turnover indicates a reduction in the Ca2+ sensitivity of the filaments, and/or a reduction in the average force per attached crossbridge. 6. At the start of the fatiguing series, as force declines by about 7.5% there is a much larger decline of crossbridge heat (17%). The reason for this is unknown. Later in the series, force declined more rapidly than heat. This is probably due to a progressive accumulation of inorganic phosphate which acts by depressing force more than it depresses ATP breakdown.

Animals↗

PubMed1992

A comparison of isometric force, maximum power and isometric heat rate as a function of sarcomere length in mouse skeletal muscle.

The isometric force, maximum power and isometric heat rate have been measured at different sarcomere lengths (SL) between 1.40 and 3.63 microns in two types of mouse muscle, soleus and omohyoideus, at 25 degrees C. The SL force relationship is different in the two muscles. At a SL above optimum filament overlap, 2.44 microns in omohyoideus muscles, maximum power declined while isometric force remained high. In soleus muscles this occurred above a SL of 2.33 microns. In parallel experiments, the isometric heat rate declined linearly with increasing SL above 2.33 microns in soleus muscles, while isometric force remained closer to its maximum. At short SL, between 2.33-1.75 microns in soleus and 2.44-2.15 microns in omohyoideus, maximum power remained at or near its maximum value as did heat rate (soleus) while isometric force fell. In both muscles at SL greater than optimum for force development maximum power output (unlike force) is proportional to filament overlap. The variation in heat rate over this SL range can be described as the sum of constant rate and a rate proportional to filament overlap. These observations are compatible with the idea that maximum power and heat rate are less affected by non-uniformities in SL than is force.

Animals↗

PubMed1992

The weakness of old age is not due to failure of muscle activation.

Maximum voluntary force and cross-sectional area (MVF and CSA) of the human adductor pollicis muscle were compared in groups of young (19-55 years, mean = 28, n = 53) and elderly (74-90 years, mean = 80, n = 39) subjects, of both sexes. Despite the elderly subjects being in good health and active outdoors, they were considerably weaker than the young subjects, their MVF/CSA being 26 +/- 3% (mean +/- SEM) lower. It was found that both young and elderly subjects could fully activate their muscles. Therefore the muscle weakness of old age does not appear to be due to reduced activation and must be caused by another mechanism, possibly biochemical in nature.

Adult↗