Short-duration Contractions Research Articles

Thick filament activation is different in fast- and slow-twitch skeletal muscle.

The contractile properties of fast-twitch and slow-twitch skeletal muscles are primarily determined by the myosin isoform content and modulated by a variety of sarcomere proteins. X-ray diffraction studies of regulatory mechanisms in muscle contraction have focused predominately on fast- or mixed-fibre muscle with slow muscle being much less studied. Here, we used time-resolved X-ray diffraction to investigate the dynamic behaviour of the myofilament proteins in relatively pure slow-twitch-fibre rat soleus (SOL) and pure fast-twitch-fibre rat extensor digitorum longus (EDL) muscle during twitch and tetanic contractions at optimal length. During twitch contractions the diffraction signatures indicating a transition in the myosin heads from ordered OFF states, where heads are held close to the thick filament backbone, to disordered ON states, where heads are free to bind to thin filaments, were found in EDL and not in SOL muscle. During tetanic contraction, changes in the disposition of myosin heads as active tension develops is a quasi-stepwise process in EDL muscle whereas in SOL muscle this relationship appears to be linear. The observed reduced extensibility of the thick filaments in SOL muscle as compared to EDL muscles indicates a molecular basis for this behaviour. These data indicate that for the EDL, thick filament activation is a cooperative strain-induced mechano-sensing mechanism, whereas for the SOL, thick filament activation has a more graded response. These different approaches to thick filament regulation in fast- and slow-twitch muscles may be adaptations for short-duration, strong contractions versus sustained, finely controlled contractions, respectively. KEY POINTS: Fast-twitch muscle and slow-twitch muscle are optimized for strong, short-duration contractions and for tonic postural activity, respectively. Structural events (OFF to ON transitions) in the myosin-containing thick filaments in fast muscle help determine the timing and strength of contractions, but these have not been studied in slow-twitch muscle. The X-ray diffraction signatures of structural OFF to ON transitions are different in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle, being completely absent during twitches in soleus muscle and blunted during tetanic contractions SOL as compared to EDL Quasi-stepwise thick filament structural OFF to ON transitions in fast twitch muscle may be an adaptation for rapid, ballistic movements, whereas more graded OFF to ON structural transitions in slow-twitch muscle may be an adaptation for slower, finer motions.

Immediate and Short-Term Effects of Short- and Long-Duration Isometric Contractions in Patellar Tendinopathy.

Isometric muscle contractions are used in the management of patellar tendinopathy to manage pain and improve function. Little is known about whether long- or short-duration contractions are optimal to improve pain. This study examined the immediate and short-term (4 weeks) effects of long- and short-duration isometric contraction on patellar tendon pain, and tendon adaptation. Repeated measures within groups. Clinical primary care. Participants (n = 16, males) with patellar tendinopathy. Short-duration (24 sets of 10 seconds) or long-duration (6 sets of 40 seconds) isometric knee extension loading (85% maximal voluntary contraction), for 4 weeks. Immediate change in pain with single-leg decline squat (SLDS) and hop, as well as change in pain and tendon adaptation [within-session anterior-posterior (AP) strain] were assessed over 4 weeks. Pain was significantly reduced after isometric loading on both SLDS (P < 0.01) and hop tests (P < 0.01). Pain and quadriceps function improved over the 4 weeks (P < 0.05). There was significant AP strain at each measurement occasion (P < 0.01). Although transverse strain increased across the training period from ∼14% to 22%, this was not significant (P = 0.08). This is the first study to show that short-duration isometric contractions are as effective as longer duration contractions for relieving patellar tendon pain when total time under tension is equalized. This finding provides clinicians with greater options in prescription of isometric loading and may be particularly useful among patients who do not tolerate longer duration contractions. The trend for tendon adaptation over the short 4-week study period warrants further investigation.

The Surge in Short-Duration Contracts in Spain

Until the outbreak of the recent economic and financial crisis, Spain was leading the ranking of countries with the largest share of temporary employees. During the crisis this share has fallen to its lowest level in decades, but this does not mean that working conditions in Spain have improved. The flow of new temporary contracts is larger than ever before. A particularly striking feature is the steep growth in the volume of fixed-duration contracts lasting less than a week or a month. We document these trends and analyse how this phenomenon has affected the transition from temporary to permanent employment.

Twelve Weeks Of Explosive Strength Training Increases Both Maximal And Explosive Voluntary Torque Production

Muscle weakness, including low maximal & explosive strength, contribute to the functional limitations experienced by numerous patient groups, including osteoarthritis patients. Strength training can improve function in different populations. However, adaptations to different types of strength training are not well understood which limits the efficacy of training prescription. Explosive training (ET), involving rapid torque development with short duration contractions, provides a less fatiguing loading modality that may be preferably tolerated by patient groups. PURPOSE: The purpose of this study was to compare the functional, neural, & morphological adaptations to ET vs. conventional training (CT). METHODS: Healthy males completed ET (n=13) or CT (n=16) for 12-wks. Training involved 4 x 10 repetitions (x3/wk): contracting as fast & hard as possible for ~1 s (ET); or gradually increasing to 75% maximal voluntary torque (MVT) before holding for 3 s (CT). Torque & EMG during maximal (MVT & QEMGMVT) & explosive (T100 & EMG0-100) contractions, & total Quadriceps volume (QUADSVOL) were measured pre & post. Absolute changes were considered different between groups when: post hoc P-values were &lt 0.1 & effect size (ES) was &gt 0.5. RESULTS: MVT improved after both types of training (CT: +56 ± 23 Nm; ET: +40 ± 22 Nm), but increased more following CT (P=0.052; ES=0.69). There were similar EMGMVT changes after CT (+0.06 ± 0.03 mV) & ET (+0.04 ± 0.08 mV; P=0.449; ES=0.36), but QUADSVOL increases after CT (+148 ± 135 cm3) were greater than ET (+46 ± 137 cm3; P=0.074; ES=0.74). Improvements in T100 were greater after ET (+23 ± 25 Nm) vs CT (+7 ± 18 Nm; P=0.092; ES=0.72). Changes in EMG0-100 were greater with ET (+0.03 ± 0.04 mV) than CT (+0.01 ± 0.04 mV), but did not reach significance (P=0.253; ES=0.46). CONCLUSIONS: These results provide evidence for distinct neural & morphological adaptations specific to the training stimulus. However, given the lower contraction durations, fatigue & effort involved in ET, this type of training appears to provide an efficient means of increasing maximal & explosive function in previously untrained individuals.

Effects of Electrical Stimulation Pattern on Quadriceps Fatigue Development in Individuals with Spinal Cord Injury

Functional electrical stimulation (FES) has often been used to facilitate exercise in individuals with spinal cord injury (SCI). Unfortunately, rapid fatigue induced by constant frequency trains (CFT) limits the effectiveness of FES in those with SCI. Variable frequency trains (VFT) have therefore been suggested as useful strategies to offset the rapid fatigue during electrical stimulation. However, most results concern very short duration contractions with limited functional applications. PURPOSE: To test force and fatigue in response to VFT and CFT patterns in training-like conditions (2-s concentric contractions). METHODS: Seven SCI individuals underwent two testing sessions separated by at least 24 hours. During each testing session, muscle torque was measured during concentric leg extension (30°/s) under two sequential electrical stimulation train patterns presented in a randomized order. For the CFT pattern, this corresponded to 2 s of contraction generated by continuous 40 Hz stimulation with a pulse width of 450 μs; for the VFT pattern, this corresponded to 2 s of contraction generated by an 80 Hz doublet followed by continuous 20 Hz stimulation with a pulse width of 450 μs. Stimulation was applied every 5 s until the target torque (fixed at 50% of the maximal evoked force) was no longer produced. RESULTS: For an equivalent torque decrease (VFT: -24.0±0.6 vs CFT: -27.7±3.2 %), more contractions were performed when evoked with VFT as compared with CFT (VFT: 14.3±1.7 vs CFT: 9.3±2.5 contractions, P<0.05). In addition, the total work produced over sessions was significantly greater when VFT pattern was applied (1418.1 ± 115.7 Nm.s) as compared with CFT (879.8 ± 112.4 Nm.s). CONCLUSION: These results showed that paralyzed human skeletal muscle is more fatigue resistant to repetitive dynamic contractions evoked by a VFT stimulation pattern as compared with a CFT pattern. These data are of particular interest for training and rehabilitation after an SCI since they suggest that using VFT, people would be able to perform longer training sessions, potentially resulting in greater improvements.

Efeito da estimulação abdominal sobre o contato plantar na lesão encefálica adquirida

Objetivo: Verificar a influência da estimulação dos músculos abdominais em pacientes com LEA sobre o contato plantar. Método: Participaram oito indivíduos com idade média de 46±15,9 anos. Para avaliação de pressão plantar, utilizou-se um baropodômetro, com coleta antes e após a estimulação abdominal, a qual foi realizada com os pacientes posicionados no divã com apoio na região sacral/quadril e joelhos flexionados em 90º, e pés, mãos e tronco superior sem apoio. Estes músculos foram estimulados a realizar contração por desequilíbrio provocado pela diminuição na base de suporte (apoiados somente em sacro). Assim, realizou-se contração isométrica para o posicionamento, intercalada por contrações isotônicas de curta amplitude, para recuperação do equilíbrio. Resultados: Na distribuição de pressão plantar, observou-se que o retropé e mediopé apresentam maior e menor pico de pressão, respectivamente (p

Oil Markets and Price Movements: A Survey of Determinants

The complexity of the world oil market has increased dramatically in recent years and new approaches are needed to understand, model, and forecast oil prices today. In addition to the commencement of the financialization era in oil markets, there have been structural changes in the global oil market. Financial instruments are communicating information about future conditions much more rapidly than in the past. Prices from long and short duration contracts have started moving more together. Sudden supply and demand adjustments, such as the financial crisis of 2008-2009, faster Chinese economic growth, the Libyan uprising, the Iranian nuclear standstill or the deepwater horizon oil spill, change expectations and current prices. The daily Brent spot price fluctuated between $30 and above $140 per barrel since the beginning of 2004. Both fundamental and financial explanations have been offered as explanatory factors. This paper selectively reviews the voluminous literature on oil price determinants since the early 1970s. It concludes that most researchers attribute the long-run oil price path to fundamental factors such as economic growth, resource depletion, technical advancements in both oil supply and demand, and the market organization of major oil petroleum exporting countries (OPEC). Short-run price movements are more difficult to explain. Many researchers attribute short-run price movements to fundamental supply and demand factors in a market with very little quantity response to price changes. Nevertheless, there appears to be some evidence of occasional financial bubbles particularly in months leading up to the financial collapse in 2008. These conflicting stories will not be properly integrated without a meeting of the minds between financial and energy economists.

Oil Price Drivers and Movements: The Challenge for Future Research

The complexity of the world oil market has increased dramatically in recent years and new approaches are needed to understand, model, and forecast oil prices today. In addition to the commencement of the financialization era in oil markets, there have been structural changes in the global oil market. Financial instruments are communicating information about future conditions much more rapidly than in the past. Prices from long and short duration contracts have started moving more together. Sudden supply and demand adjustments, such as the financial crisis of 2008-2009, faster Chinese economic growth, the Libyan uprising, the Iranian Nuclear standstill or the Deepwater Horizon oil spill, change expectations and current prices. Although volatility appears greater, financialization makes price discovery more robust. Most empirical economic studies suggest that fundamental values shaped expectations over 2004-2008, although financial bubbles may have emerged just prior to and during the summer of 2008. With increased price volatility, major exporters are considering ways and means to achieve more price stability to improve long-term production and consumption decisions. Managing excess capacity has historically been an important method for keeping world crude oil prices stable during periods of sharp demand or supply shifts. Building and maintaining excess capacity in current markets allow greater price stability when Asian economic growth suddenly accelerates or during periods of supply uncertainty in major producing regions. OPEC can contribute to price stability more easily when members agree on the best use of oil production capacity.Important structural changes have emerged in the global oil market after major price increases. Partially motivated by government policies major improvements in energy and oil efficiencies occurred after the oil price increases of the early and the late 70s such as the improved vehicle fuel efficiency, building codes, power grids and systems etc. On the supply side, seismic imaging and horizontal drilling as well as favorable tax regimes expanded production capacity in countries outside OPEC. After the oil price increases of 2004-2008, investments in oil sands, deep water, biofuels and other non-conventional sources accelerated. Recent improvements in shale gas production could well be transferred to oil-producing activities, resulting in expanded oil supplies in areas recently considered prohibitively expensive. The search for alternative transportation fuels continues with expanded research into compressed natural gas, biofuels, diesel made from natural gas, and electric vehicles.Still some aspects of the world oil market are not well understood. Despite numerous attempts to model the behavior of OPEC or its members, there exists no credible, verifiable theory about the behavior of the 50 years old organization. OPEC has not consistently acted like a monolithic cartel, constraining supplies to raise prices. Empirical evidence suggests that members sometimes coordinate supply responses and at other times compete with each other. Supply-restraint strategies include slower capacity expansions as well as curtailed production from existing capacity. Regional political considerations and broader economic goals (beyond oil) are influential factors in a country’s oil decisions. Furthermore, the economies of OPEC members as well as their financial needs have changed dramatically from 1970s and 1980s. This review represents a broad review of economic research and literature related to the structure and functioning of the world oil market. The theories and models of oil demand and supply reviewed here, although imperfect in many respects, offer a clear and well-defined perspective on the forces that are shaping the markets for crude oil and refined products. Much work remains to be done if we are to achieve a more complete understanding of these forces and the trends that lie ahead. The contents that follow represent an assessment of how far we have come and where we are headed. Of course, the entire world shares a vital interest in the many benefits that flow from an efficient, well-functioning oil market. It is intended and hoped, therefore, that the discussion in this review will find a broader audience.

Mitochondrial functional specialization in glycolytic and oxidative muscle fibers: tailoring the organelle for optimal function

In skeletal muscle, two major types of muscle fibers exist: slow-twitch oxidative (type I) fibers designed for low-intensity long-lasting contractions, and fast-twitch glycolytic (type II) fibers designed for high-intensity short-duration contractions. Such a wide range of capabilities has emerged through the selection across fiber types of a narrow set of molecular characteristics suitable to achieve a specific contractile phenotype. In this article we review evidence supporting the existence of distinct functional phenotypes in mitochondria from slow and fast fibers that may be required to ensure optimal muscle function. This includes differences with respect to energy substrate preferences, regulation of oxidative phosphorylation, dynamics of reactive oxygen species, handling of Ca2+, and regulation of cell death. The potential physiological implications on muscle function and the putative mechanisms responsible for establishing and maintaining distinct mitochondrial phenotype across fiber types are also discussed.

Does electrical stimulation enhance post-exercise performance recovery?

Elite sport requires high-volume and high-intensity training that inevitably induces neuromuscular fatigue detrimental for physical performance. Improving recovery processes is, therefore, fundamental and to this, a wide variety of recovery modalities could be proposed. Among them, neuromuscular electrical stimulation is largely adopted particularly by endurance-type and team sport athletes. This type of solicitation, when used with low stimulation frequencies, induces contractions of short duration and low intensity comparable to active recovery. This might be of interest to favour muscle blood flow and therefore metabolites washout to accelerate recovery kinetics during and after fatiguing exercises, training sessions or competition. However, although electrical stimulation is often used for recovery, limited evidence exists regarding its effects for an improvement of most physiological variables or reduced subjective rating of muscle soreness. Therefore, the main aim of this brief review is to present recent results from the literature to clarify the effectiveness of electrical stimulation as a recovery modality.

Skunked on the New York Cotton Exchange: What Really Happens to Jason Compson in The Sound and the Fury

New York Cotton Exchange takes a large chunk out of Compson's day on April 6, 1928 in William Faulkner's Sound and the Fury. Speculation in cotton futures is just one thing more that addles and brings to ruin his nickel-and-dime schemes. For one who craves a profit on every deal, Compson gets a good drubbing in his cotton market trade. By the end of the day, he suffers a staggering loss from the short sale of one cotton futures contract, the total amount of which no one has yet put a specific dollar figure on. Nor is it clear that Faulkner himself did the math and realized how much actually lost in this trade. first commentator to understand that sold a contract of cotton short on the New York Cotton Exchange was William W. Cobau in Jason Compson and the Costs of Speculation. Cobau does not establish just how much the speculation cost him, however: How much money does lose? We do not know (258). essential fact is that sold short, something that had escaped critics such as Dorothy Tuck and Edmund L. Volpe, who, as Cobau notes, erroneously argued that he bought a futures contract. More recently, Rick Wallach, in The Compson Family Finances and the Economics of Tragic Farce, gets it wrong when he says that purchased a call option (80), a financial instrument not in existence in 1928 and that would not be available to commodity traders on the New York Cotton Exchange for another 56 years. (1) Wallach not only mixes up call option with a futures contract but also puts on the long rather than the short side of the market. Buying and selling of actual cotton on the New York Cotton Exchange is called the market, also known as the market because transactions take place right away, or on the spot. Spot cotton prices are determined by the price that buyers and sellers agree to in the cotton trading ring by means of an open outcry transaction. Cotton futures, however, also listed on the Exchange, are the opposite of the cash market. Futures are contracts that traders are willing to buy at a current price expecting that the contracts will rise in value at a certain time in the future. Compson sells short a futures contract, expecting that the contract will decline in value at a specific time in the months ahead when he can buy it back and pocket the difference between the greater proceeds from the short sale and the lesser cost of covering, or buying the contract back. Instead, suffers a loss when he has to buy back the contract at a higher price than what he sold it short for. Futures bring a high degree of risk because traders must assess a multitude of ever-changing variables and act on what they think may happen on or before a certain date in the future. New York Cotton Exchange Year Book 1927-1928 explains that the prices of futures are based on factors that affect the cotton crop such as acreage planted, rainfall and other weather conditions, boll weevil infestation or crop condition, ginning, probable production, the movement of the cotton crop, and a multiplicity of other data, all of which is available to the cotton trade and forms the basis for an appraisal of cotton at any time, as well as the future trend of prices (6). Buyers of cotton futures fall into one of two categories. first group consists of growers, producers, merchants, and processors who, in the months between planting and harvest, can assume a speculative hedge to reduce their price risk in the cash market against the possibility of an unsuccessful crop. Bad news for crops is bullish for futures prices. Futures allow this group to make up losses resulting from a poor crop with capital gains in the cotton market. On the other hand, good news for crops is bearish for futures prices. If the value of the crop rises, those in this group can eliminate the hedge book to gain full exposure to higher cotton prices. Hedgers, therefore, have an interest in the underlying commodity and seek to offset the risk of lower cash or spot prices for cotton at time of harvest by buying short-duration contracts in May or longer-duration contracts in October, November, or December, as conditions and their needs warrant. …

Blood flow response to muscle contractions is more closely related to metabolic rate than contractile work

The magnitude of the blood flow response to exercise has been linked to both the contractile work performed and the metabolic cost of the activity. Under certain conditions, contractile work and metabolic cost may be dissociated. This study examined the blood flow response to trains of contractions when contraction duration was manipulated under conditions of similar tension-time indexes (isometric analog of work). Previous investigations have shown that trains of short-duration contractions have a greater ATP utilization, which may result from an augmented ion transport required for muscle activation and relaxation. On the basis of these findings, we hypothesized that the blood flow response would be greater to a train of short-duration contractions than a train of long-duration contractions. Canine gastrocnemius-plantaris muscle (n = 8) was isolated, and blood flow assessed with an ultrasound flow probe placed around the popliteal artery. The sciatic nerve was stimulated to produce two contraction protocols that resulted in similar contraction-to-rest ratios: short duration: 0.25 s/0.75 s vs. long duration: 1 s /3 s. In accord with the design of the experiment, the tension-time indexes were identical for the two contraction protocols (short: 18.6 +/- 1.0 vs. long: 18.6 +/- 1.0 kN.s). Steady-state oxygen consumption was greater in the short-duration contractions (17.2 +/- 0.9 ml.100 g(-1).min(-1)) than in the long-duration contractions (11.7 +/- 0.7 ml.100 g(-1).min(-1)). Similarly, the steady-state blood flow was greater in contractions of short duration (125 +/- 7 ml/min) compared with long-duration contractions (92 +/- 7 ml/min). Contractions of short duration resulted in significantly higher oxygen consumptions and blood flows compared with contractions of long duration despite the same total contractile work. The blood flow response to muscle contraction appears to be more closely associated with muscle metabolism than contractile work performed.

Method for quantitatively assessing physical risk factors during variable noncyclic work.

Assessing exposure to physical risk factors during variable noncyclic work can be problematic. The purpose of this study was to modify an existing data reduction method for directly quantifying physical exposures during variable noncyclic work. Myoelectric activity of the finger flexors of two groups of workers, heavy equipment operators (N=25) and mechanics (N=25), was sampled to determine the intensity and duration of forceful exertions during normal tasks. Data were reduced with a modification of the exposure variation analysis (EVA), called clustered EVA (CEVA), using three intensity and two duration categories. A two-way, mixed-effects, repeated-measures analysis of variance evaluated the percentage of sampled work time in each CEVA category. CEVA was able to quantify variable noncyclic work, and it contrasted the work of the two groups, with simple effects significantly different for all the exposure categories. The mechanics performed high-intensity short-duration contractions 9.1% of the time, whereas the operators had similar contractions only 1.8% of the time. Conversely, the operators used low-intensity contractions of prolonged duration over 81% of the time, compared with only 43% for the mechanics. CEVA is a useful modification of EVA for contrasting the noncyclic work typical of understudied industries like construction. A simplification of electromyography with summary measures such as CEVA provides a comprehensible, yet accurate measure of forceful exertions during worktasks.

Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles.

1. The present study aimed to investigate the influence of isometric training protocols with long- and short-duration contractions on the elasticity of human tendon structures in vivo. The elasticity was assessed through in vivo determination of the elongation (L) of the tendons and aponeuroses using ultrasonography, while the subjects performed ramp isometric exercise up to maximum voluntary contraction (MVC). 2. Eight young males completed 12 weeks (4 days per week) of a unilateral isometric training programme on knee extensors, which consisted of two different combinations of contraction and relaxation times at 70 % MVC: one leg was trained using a short-duration protocol (3 sets of 50 repetitions of contraction for 1 s and relaxation for 2 s), and the other leg was trained using a long-duration protocol (4 sets of a combination of contraction for 20 s and relaxation for 1 min). The training volume per session, expressed as the integrated torque, was the same for the two protocols. 3. Both protocols resulted in a significant increase in MVC: 31.8 +/- 17.2 % for the short-duration protocol and 33.9 +/- 14.4 % for the long-duration protocol. Moreover, the training produced significant increases in the muscle volume of the constituents of the quadriceps femoris, with similar relative gains for the two protocols: 7.4 +/- 3.9 % for the short-duration protocol and 7.6 +/- 4.3 % for the long-duration protocol. 4. The short-duration protocol produced no significant change in L values at any of the force production levels. For the long-duration protocol, however, the L values above 550 N were significantly shorter after training. Analysis revealed that the group x test time interaction effect on tendon stiffness was significant. Stiffness increased significantly for the long-duration protocol, but not for the short-duration protocol. 5. The present study demonstrates a greater increase in stiffness of human tendon structures following isometric training using longer duration contractions compared to shorter contractions. This suggests that the changes in the elasticity of the tendon structures after resistance training may be affected by the duration of muscle contraction.

Effects of Propranolol on Human Postoperative Ileus

Background: The sympathetic nervous hyperactivity present in response to surgical stress has been implicated as an important component of the postoperative paralytic ileus. A randomized and prospective study was conducted, evaluating the effects of the preoperative beta-adrenergic blockade with propranolol in schistosomotic patients during the period of postoperative ileus. Methods: The study compared schistosomotic patients submitted, or not, to beta-adrenergic blockade. Basal cardiac frequency was determined and propranolol was used in a dose of 40 mg twice a day. The dose was adjusted weekly until a minimum decrease of 20% in cardiac frequency was achieved. Three coupled bipolar electrodes were placed in the left colon in both groups, and registration of myoelectric activity of the left colon was made twice a day during the period of postoperative ileus using a system of data collection (DATA Q Series 200). The electric signals were previously amplified, filtered and separated into Electric Control Activity (ECA) and Electric Response Activity (ERA). Results: The dose of propranolol varied from 80 to 160 mg/day. The proportional decrease in basal heart frequency varied from 20 to 33%, with an average of 25.4 ± 3.9% in the propranolol group, maintaining a mean of 24.3 ± 3.6% decrease in the postoperative period. Differences on clinical recovery of the postoperative ileus were not found. Significant differences on electromyographic patterns were not observed between the groups, except for the presence of a greater number of short-duration contractions in the second postoperative day in the beta-blocked group. Conclusion: The authors suggest that the preoperative beta-adrenergic blockade with propranolol does not determine myoelectric activity changes that could contribute to an earlier resolution of postoperative ileus.

Contraction duration affects metabolic energy cost and fatigue in skeletal muscle.

It has been suggested that during a skeletal muscle contraction the metabolic energy cost at the onset may be greater than the energy cost related to holding steady-state force. The purpose of the present study was to investigate the effect of contraction duration on the metabolic energy cost and fatigue process in fully perfused contracting muscle in situ. Canine gastrocnemius muscle (n = 6) was isolated, and two contractile periods (3 min of isometric, tetanic contractions with 45-min rest between) were conducted by each muscle in a balanced order design. The two contractile periods had stimulation patterns that resulted in a 1:3 contraction-to-rest ratio, with the difference in the two contractile periods being in the duration of each contraction: short duration 0.25-s stimulation/0.75-s rest vs. long duration 1-s stimulation/3-s rest. These stimulation patterns resulted in the same total time of stimulation, number of stimulation pulses, and total time in contraction for each 3-min period. Muscle O2 uptake, the fall in developed force (fatigue), the O2 cost of developed force, and the estimated total energy cost (ATP utilization) of developed force were significantly greater (P < 0.05) with contractions of short duration. Lactate efflux from the working muscle and muscle lactate concentration were significantly greater with contractions of short duration, such that the calculated energy derived from glycolysis was three times greater in this condition. These results demonstrate that contraction duration can significantly affect both the aerobic and anaerobic metabolic energy cost and fatigue in contracting muscle. In addition, it is likely that the greater rate of fatigue with more rapid contractions was a result of elevated glycolytic production of lactic acid.

EFFECT OF CONTRACTION DURATION ON ENERGY COST AND FATIGUE IN CONTRACTING MUSCLE 836

The purpose of this study was to investigate the extent to which altering contraction duration (and presumably ion pump ATPase activity) can affect energy cost and fatigue in isolated contracting muscle. Canine gastrocnemius muscle (n=6) was isolated and 3 contractile periods (3 min of isometric, tetanic contractions) were conducted by each muscle in a balanced order design. Each of the 3 contractile periods had stimulation patterns that resulted in a 1:1 work-to-rest ratio, with the difference in the 3 contractile periods being in the contraction duration (short duration = 0.25 sec stimulation/0.25 sec rest; moderate duration = 0.50 sec stimulation/0.50 sec rest; and long duration = 1 sec stimulation/1 sec rest) with the total time of stimulation and number of stimulation pulses being the same over each 3 min contraction period. Muscle O2 uptake, the O2 cost of developed force, and the fall in developed force (fatigue) was significantly greater(p<0.05) with contractions of short duration (stimulation = 0.25 sec). Muscle [ATP] did not fall significantly from resting values during any contraction period and was not different among any of the 3 contraction periods. Lactate efflux from the working muscle and muscle [lactate] were significantly greater with contractions of short duration (stimulation = 0.25 sec). These results suggest that contraction duration, through the demand of cellular ion pump ATPases, can significantly affect the energy cost and fatigue (possibly related to increased glycolytic flux) of contracting muscle.

Physiology and pathophysiology of colonic motor activity

The basic motor function of the colon is to mix and knead its contents, propel them slowly in the caudad direction, hold them in the distal colon until defecation, and provide a strong propulsive force during defecation. Infrequently, it also produces mass movements in the proximal colon. These motor functions are achieved in most species by three different types of contractions: the individual phasic contractions that include the short- and long-duration contractions, organized groups of contractions that include the migrating and nonmigrating motor complexes, and special propulsive contractions (giant migrating contractions). The spatial and temporal patterns of all of these contractions are controlled by myogenic, neural, and chemical control mechanisms. The individual phasic contractions are highly disorganized in time and space in the colon. For this reason, they are effective in mixing and kneading and slow distal propulsion. The underlying cause of the disorganization of short duration contractions is the irregularity in the frequency and waveshape of colonic electrical control activity and its phase unlocking throughout the colon. The individual contractions in many species occur in cyclic bursts called contractile states. At least in some species, these contractile states exhibit mostly caudad and sometimes orad migration. However, there are also nonmigrating or randomly migrating contractile states in the colon. These two patterns of contractile states are called colonic migrating motor complexes and colonic nonmigrating motor complexes, respectively. The giant migrating contractions provide the strong propulsive force for defecation and mass movements. The neural control of colonic contractions is organized at three levels--enteric, autonomic, and central. The enteric nervous system contains cholinergic and peptidergic neurons and plays a major role in the control of colonic contractions. The autonomic nerves, the vagi, pelvic, lumbar colonic, hypogastric, and splanchnic nerves, seem to continuously monitor the state of the colon and provide a modulatory input when necessary. These nerves play a major role in the reflexive control of colonic motor function. The voluntary input from the central nervous system coordinates the motor activity of the colon, rectum, anal canal and sphincters for orderly evacuation of feces during defecation. The role of acetylcholine, nonadrenaline, and the yet to be completely identified nonadrenergic, noncholinergic neurotransmitter, possibly VIP, in the control of contractions is fairly well established. Besides these, there are several other peptides and chemicals that are localized in the colonic wall; their physiological roles remain unknown. Colonic motor activity has been studied in several disease states. The findings have not always been consistent.(ABSTRACT TRUNCATED AT 400 WORDS)

Local effect of substance P on colonic motor activity in different experimental states

We studied the effects of close intra-arterial injections of substance P on colonic motor activity in the conscious state, during anesthesia, and during acute laparotomy. In the conscious state, with enteric nerves intact, substance P stimulated postsynaptic cholinergic neurons to induce a large amplitude and long duration contraction. This response was blocked by prior close intra-arterial injection of atropine and tetrodotoxin (TTX) but not hexamethonium. Hexamethonium and TTX given alone, close intra-arterially, induced a series of short-duration contractions. Prior close intra-arterial administration of hexamethonium significantly enhanced the colonic motor response to substance P. After blockade of nerve conduction by TTX, substance P induced a series of short-duration contractions with characteristics different from those when the nerves were functioning. Anesthesia alone had little effect on the colonic motor response to substance P, but laparotomy inhibited it significantly. Laparotomy similarly inhibited the contractile response to bethanechol. Gut handling had no further effect on this inhibition. We conclude that in the conscious state substance P acts preferentially on postsynaptic cholinergic neurons to contract colonic circular muscle. When the intrinsic nerves are blocked, substance P may act directly on the smooth muscle to induce circular muscle contractions with characteristics different from those induced when nerves are intact. Substance P also has a weak inhibitory motor effect by its action on presynaptic neurons that synapse on postganglionic intrinsic inhibitory neurons. Anesthetic doses of barbiturates have no major effects on the neuromuscular response to substance P, but laparotomy significantly inhibits the smooth muscle response and selectively blocks some neurons.

Energy cost and fatigue during intermittent electrical stimulation of human skeletal muscle.

Force generation and ATP utilization under anaerobic conditions were studied in the quadriceps femoris muscle of six volunteers. Electrical stimulation (20 Hz) was used to produce contractions with a duration of 0.8 s in one leg and contractions with a duration of 3.2 s in the other leg. The two procedures were designed to give the same total contraction time of 51 s and used the same number of stimulation pulses. Muscle biopsies were taken at rest and after 22 and 51 s of work and analyzed for ATP, phosphocreatine, and glucolytic intermediates. The results were compared with previous studies on continuous and intermittent stimulation. Fatigue developed significantly faster with contractions of short duration, and the energy cost was higher. Since force at the end of stimulation had a negative correlation to ATP utilization, there is no indication that the energy resources limit force generation. By comparison of stimulations producing the same amount of isometric work but with a different number of contractions, we estimate that the energy cost for activation and relaxation of a 1-s contraction is approximately 37% of the total ATP consumption.